Symposium Organizers
Ferenc Horkay National Institutes of Health
Noshir Langrana Rutgers University
Walter Richtering RWTH Aachen University
QQ1: Design and Characterization of Responsive Gel Systems
Session Chairs
Ferenc Horkay
Noshir Langrana
Monday PM, November 30, 2009
Room 208 (Hynes)
9:30 AM - **QQ1.1
Covalent Aadaptable Networks for Smart Materials Applications.
Christopher Bowman 1 , Brian Adzima 1 , Heeyoung Park 1 , Christopher Kloxin 1 , Timothy Scott 2
1 Chemical and Biological Engineering , University of Colorado, Boulder, Colorado, United States, 2 Center for Bioengineering, Mechanical Engineering, University of Colorado, Boulder, Colorado, United States
Show AbstractTypical covalently-bound polymer hydrogels, such as PEG diacrylate-based materials, have permanent network connectivity; however, the permanent nature of these materials renders them intractable to post-fabrication manipulation. Photolytic or thermally degradable linkages can extend the utility of a gel, but ordinarily at the expense of permanent destruction of the material. Here, we describe the incorporation of photo- and thermo-reversible crosslinks into a polymer network, allowing for readily and repeated post-polymerization manipulation. Unlike conventional gels, these covalent adaptable networks (CANs) respond to external stimuli without being destroyed. This novel class of material represents a paradigm shift in chemical design, possessing an entirely new set of mechanical properties and behavior such as mechanical actuation, stress relief, and a gel-to-sol transition.
10:00 AM - **QQ1.2
Confined Smart Hydrogels for Applications in Chemomechanical Sensors for Physiological Monitoring.
Jules Magda 1 , Genyao Lin 1 , Prashant Tathireddy 1 , Florian Solzbacher 1 , Volker Schulz 2 , Margarita Guenther 2 , Gerald Gerlach 2
1 , University of Utah, Salt Lake City, Utah, United States, 2 , Technische Universität Dresden, Dresden Germany
Show AbstractMany sensing approaches have been proposed that employ smart hydrogels, but perhaps the simplest sensing scheme can be obtained by confining a microscopically-thin smart hydrogel between a porous membrane and the diaphragm of a miniature pressure transducer. In such a scheme, a change in the environmental analyte concentration, as sensed through the pores of the membrane, changes the hydrogel osmotic swelling pressure, thereby changing the mechanical pressure measured by the pressure transducer. Sensor selectivity can be enhanced by attaching moieties to the hydrogel that selectively bind the analyte of interest. In order to illustrate this versatile sensing approach, we discuss its use in the development of a promising real-time blood glucose sensor for diabetic patients. For this application, we employ recently-developed enzyme-free hydrogels that are reversibly crosslinked by glucose but not by fructose. These novel glucose-respsonive gels allow us to construct a sensitive and selective implantable glucose sensor that avoids the “oxygen deficit” problem that currently plagues commercial electrochemical glucose sensors.
11:00 AM - **QQ1.3
High Resolution Monitoring of Swelling of Biospecific Hydrogel Materials.
Sven Tierney 1 , Bjorn Stokke 1
1 Dep. of Physics, The Norwegian University of Science and Technology, Trondheim Norway
Show AbstractVarious hydrogel material designed to adopt an equilibrium swelling state selectively depending on a biological relevant molecule can be utilized for label-free biosensing. This requires sufficiently sensitive readout-technology for the monitoring changes in the hydrogel swelling. In this paper, we describe determination of hydrogel swelling employing an interferometric technique. Thus, 50-60 μm radius, hemispherical hydrogels manufactured at the end of an optical fiber constituting the biospecific sensing element makes up a Fabry-Perot cavity. The interference wave of the reflected light at the fiber-gel and gel-solution interfaces enables detection of the optical pathlength within the gel and thus the gel swelling. This interference technique supports detection of changes in optical length of the hydrogels with 2 nanometer resolution. The biological specific sensing group in the hydrogel can alter the conditions within the hydrogel materials by changes in the charge density (e.g., glucose oxidase catalyzing glucose oxidation) or crosslinking density that display sensitivity to a biological relevant molecule. A glucose sensitive hydrogel material was realized on this platform utilizing a boronic-acid moities with additionally cationic, tertiary amines incorporated to tune the carbohydrate selectivity. The resulting hydrogel was found to be applicable for continuous monitoring of glucose levels in the relevant physiological range, and at physiological temperatures. Glucose induced, reversible crosslinking of the incorporated boronic acid groups attached to topologically separated network chains is the possible molecular mechanism for this material. A hybid hydrogel material with hybridized dioligonucleotides grafted to the polymer network as network junctions in addition to the covalent crosslinks supports detection of complementary oligonucleotides or other biological molecules based on specific aptamers. The signal transduction principle for altered swelling state of the hydrogel is based on destabilizing the junction point by displacement hybridization thereby yielding altered cross-link density. Concentration sensitivity applied as specific label-free detection of oligonucleotide is estimated to be in the nanomolar region. The current design support detection in excess of 1x1012 sequences. The results show that a wide array of bioresponsive hydogels can be manufactured at the end of the optical fiber for high resolution readout of the specific signal, thus supporting development of biosensors.
11:30 AM - QQ1.4
Dissociation of Thermally Reversible Polypeptide Hydrogels via Near Infrared Light.
Manoj Charati 1 , Ian Lee 1 , Kolin Hribar 1 , Jason Burdick 1
1 Bioengineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractStimuli-responsive hydrogels are highly valued in fields ranging from controlled drug release, tissue repair and in microdevices. These hydrogels exhibit structural changes or dissociate based on changes in temperature, pH, and even with light exposure. With this in mind, polypeptide-based hydrogels, formed from a genetically engineered multi-block polypeptide have been previously developed by Tirrell and coworkers that exhibit a temperature dependent transition from a solid to liquid state (~40-45 C). The polypeptide consists of two associative leucine zipper end-blocks and a random coil midblock; self-assembly of the leucine zipper domains of the polypeptide leads to network formation, in which oligomer bundles serve as crosslinking points. For additional and triggered functionality, we formulated a composite of the thermo-reversible polypeptide gel with gold nanorods, where exposure to infrared light induces heating of the nanorods and consequently, melting of the gel. The gold nanorods were synthesized through a seed mediated growth process, imaged on a transmission electron microscope, and exhibited a characteristic morphology with an average length, width, and aspect ratio of 31.08 ± 4.52 nm, 9.24 ± 2.75 nm, and 3.64 ± 0.82, respectively, with the majority having elongated shapes and a small fraction that were rounded. The nanorods exhibited typical absorbances for both the longitudinal and transverse plasmon peaks and have a peak absorbance of ~800 nm. Gelation occurs within ~5 minutes (indicated by rheology plateau of ~1kPa) and is not hindered with the incorporation of the nanorods, encapsulated at 2.7×10-13 mol/ml. When exposed to near infrared light (~1W, 808 nm), the samples increase in temperature and dissociation occurs almost instantaneously, evident both macroscopically and through rheological studies. The dissociation of the thermally reversible network can be controlled effectively by the concentration of nanorods in the gel and the intensity and time of exposure to the near infrared light. The infrared light controlled dissociation of these hydrogels offers unique opportunities such as for delivery of drugs and growth factors at a precise dosage and a specific time or for incorporation of hydrogel actuators in microdevices. One additional benefit to this system is that near infrared light penetrates biological tissues quite well, as compared to other wavelengths, and this approach allows for triggered dissociation with transdermal light.
11:45 AM - QQ1.5
Thermal Responsive Hydrogels Formed by Triblock Copolymers with PEG and Oligomeric End-Blocks Bearing Cholesterol Moieties.
Yuxiang Zhou 1 , Nitin Sharma 2 , Rajeswari Kasi 1 2
1 Chemistry Department, University of Connecticut, Storrs, Connecticut, United States, 2 The Institute of Materials Science , University of Connecticut, Storrs, Connecticut, United States
Show Abstract Physical hydrogels self-assembled through no-covalent intermolecular interactions have become a subject of great interest because they can be developed to smart materials which are responsive to external stimuli such as temperature variation, light, electric or magnetic field, pH, etc. These materials have found applications including controlled drug release, tissue engineering scaffolds, biosensors, actuators and so on. In the present study, three types of oligomeric triblock copolymers have been prepared, namely, oligo(cholesteryl methacrylate)n-PEG-oligo(cholesteryl methacrylate)n (OCMA-PEO-OCMA), oligo(5-cholesteryloxypencyl methacrylate)n-PEG-oligo(5-cholesteryloxypencyl methacrylate)n (OC5MA-PEO-OC5MA)and oligo(10-cholesteryloxydecyl methacrylate)n-PEG-oligo(10-cholesteryloxydecyl methacrylate)n (OC10MA-PEO-OC10MA). These three copolymers have the same PEG mid-block and different end-blocks containing side-on cholesterols via different lengths of methylene spacers (0, 5, 10 CH2). It is been found that these polymers can form hydrogels when dissolved in water with enough concentrations. And these hydrogels exhibit gel-sol transitions upon heating. Rheological studies show that by changing the spacer lengths and polymer concentrations, the modulus and the gel-sol transition temperatures (Tgel-sol) of the formed hydrogels can be tailored. Thus the hydrogel with desired Tgel-sol (~37 oC) and elastic modulus have been achieved by selecting the right polymer with the proper concentration. Scanning electron microscope images of these hydrogels show porous structures. The driving force of the gelation is presumed to be the stacking of cholesterol moieties that form the junctions of the polymer network. More work including X-ray diffraction and dynamic light scattering is under going to confirm this speculation. Since these triblock copolymers also feature the biocompatibility of PEO and the good cell affinity of cholesterol moieties, the prepared hydrogels can be good candidates for tissue engineering scaffolds or drug delivery systems.
12:00 PM - QQ1.6
Swelling and Diffusion Characteristics of Modified Poly (N-isopropylacrylamide) Hydrogels.
Guoguang Fu 1 , W. Soboyejo 1
1 Mechanical Engineering, Princeton Univ., Princeton, New Jersey, United States
Show AbstractThermo-responsive hydrogels are capable of swelling changes to external temperature. A series of modified poly (N-isopropylacrylamide) (PNIPA) hydrogels were synthesized by free radical polymerization in aqueous solution. Acrylamide (AAm) was used to increase the lower critical solution temperature (LCST), while sodium alginate (SA) was used to improve the swelling performance of the hydrogels. Experiments show that 5.5% mass ratio of AAm increased the LCST by about 9oC above that of conventional PNIPA. Also, SA significantly improved the equilibrium swelling ratio associate with temperature change. Trypan blue diffusion revealed significant differences in the fluid release obtained from hydrogels with modified LCST and swelling properties. The implications of the modified fluid release and swelling characteristics are also discussed for the device design of thermo-sensitive hydrogels for localized drug delivery.
12:15 PM - QQ1.7
Thermal and Mechanical Properties of Double-Gelling Thermosensitive Polymers.
Hanin Bearat 1 , Jorge Valdez 1 , Brent Vernon 1
1 Harrington Department of Bioengineering, Arizona State University, Tempe, Arizona, United States
Show AbstractStimuli-responsive materials have been widely used in various fields of biomedical research, with thermosensitive materials being of interest as hydrogels. Poly(N-isopropyl acrylamide) (poly(NIPAAm)) is an interesting polymer since it has temperature sensitivity around physiological conditions, with its lower critical solution temperature (LCST) being around 32°C. Its LCST can be altered with addition of different monomers; an addition of a hydrophobic monomer lowers its LCST while a hydrophilic monomer increases it. We have investigated two different Michael-type addition systems by conjugation of poly(NIPAAm) with hydroxyethylmethacrylate-acrylate (HEMA-acrylate) and cysteamine for system 1, as well as cysteamine-vinylsulfone and cysteamine for system 2, synthesized through free radical polymerization. Upon combination, the thiol group on the cysteamine attacks the olefin groups on either the HEMA-acrylate or the cysteamine-vinylsulfone, resulting in a chemical crosslink. Our system not only undergoes chemical crosslinking, but physical crosslinking due to the presence of NIPAAm. A comparison study was conducted on the properties of both systems. The polymers were characterized through HNMR, FTIR and HPLC for chemical composition verification and molecular weight distribution, as well as DSC and rheology for the thermosensitive and mechanical properties of the gels. Samples for DSC and rheology were prepared in PBS at pH 7.4, at 5wt% and 30wt%, respectively. All polymers demonstrated LCST points lower than 37°C, thus inducing physical gelation at body temperature. Various mixing times were tested for mechanical properties of both hydrogel systems using rheology. The polymers were mechanically mixed for 5 seconds, 2 or 4 minutes. A time sweep was conducted for each mixing time at 37°C for 75 minutes at a frequency of 1Hz and an oscillation stress of 10Pa. Results show that after mixing the two components for 5 seconds, system 1 did not form a gel whereas system 2 underwent gel formation within 30 seconds. However, after 2 minutes of mixing, system 1 formed a gel after 54 minutes and system 2 had formed a gel prior to its deposition on the rheometer plate. As the mixing time increased to 4 minutes, the time to gelation decreased to 48.5 minutes for system 1, indicating that longer duration of mechanical mixing allows for more chemical crosslinks to form prior to physical gelation at 37°C. Mechanical and thermal properties of the two systems demonstrate qualifications for potential use as in situ gels for endovascular embolization.
12:30 PM - QQ1.8
Responsive Polymer Scrolls Made by Strain Engineering.
Brian Simpson 1 , Kyriaki Kalaitzidou 1
1 G. W. W. School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractResponsive polymer scrolls have the ability to change geometry, flow characteristics, and adsorption properties upon the stimulation of an environmental change, such as temperature. These scrolls are fabricated by utilizing residual stress that is developed at the interface of a bi-layer structure. The focus of this work is to demonstrate the reversible response of polymer scrolls through the use of rheology as a means of capturing the transition from 3-D cylindrical to 2-D flat structures upon application of a trigger. The material system under investigation is poly dimethylsiloxane (PDMS)-gold (Au) bilayers. Temperature is used as the trigger to tune the residual stress locked at the interface of the bilayer. A theoretical equation that relates the material properties with the processing conditions and the residual strain is provided and used to estimate the residual strain. The experimental analysis involves determining the material modulus, thickness of the two layers, and the radius of curvature of the scrolls , This analysis can provide the scheme for optimizing the size, shape, and behavior of responsive polymer scrolls so that they can be utilized for numerous applications within the electronics, biomedical, and material science fields.
12:45 PM - QQ1.9
Development of a Flexible Conductive Polymer Membrane on Electroactive Hydrogel Microfibers.
Maria Bassil 1 2 , Mario El Tahchi 1 , Michael Ibrahim 1 4 , Eddy Souaid 1 , Georges El Haj Moussa 1 , Gisele Boiteux 2 , Joel Davenas 2 , Senentxu Lanceros-Mendez 3 , Joseph Farah 1
1 LPA-GBMI, Department of Physics, Lebanese University, Beirut, Jdeidet, Lebanon, 2 IMP/LMPB Laboratoire des Matériaux Polymères et Biomatériaux, CNRS, UMR5223, Ingénierie des Matériaux Polymères, Claude Bernard University -Lyon I, , Lyon, Villeurbanne, France, 4 IMP-LMI, Claude Bernard University -Lyon I, , Lyon, Villeurbanne, France, 3 Department of Physics, , University of Minho, Braga Portugal
Show AbstractPolyacrylamide (PAAM) hydrogel is wet and soft material like the tissue of living organism and presents a remarkable stimuli response [1-2]. In addition, it is biocompatible and not biodegradable. Recently, we have presented a new artificial muscle design [3-5] based on this hydrogel. This electro-bio-active device consists on a fiber like elements of hydrolyzed PAAM, working in parallel, embedded in a thin conducting gel membrane that plays the role of electrodes [5].The first step in realizing the design is achieved by the production of PAAM microfibers. The influence of surface properties on the rate of shrinking of hydrogel microfibers was studied. It is shown that the geometrical distribution of microfibers influences their response to electrical and chemical stimuli. While the number of microfibers placed in parallel increases the rate of shrinkage decreases but it stays much higher than that of bulky gel [6].The second step is the development of the gel membrane which holdsthe electroactive hydrogel microfibers together. In fact, the main task in artificial muscle development is the control of the volumetric changes. Since the volume variation of the gel structure is under the kinetic control of the driving electrical power; developing a thin, flexible and conductive membrane which plays the role of electrodes can:- ensure a large surface area to stimulate the gel with an electrical input while improving the gel’s time of response.- hold the hydrogel microfibers together and enhance the mechanical properties of the microfibrous structure while moving smoothly with it.In this study, a thin gel membrane presenting such properties has been integrated in the PAAM structure. The geometrical properties of the layers and their composition are systematically modified and the effect of these modifications on the actuation rate of the structure has been investigated.Polymers presenting electrically controlled properties could lead to a revolution in a number of applications especially in the field of artificial muscle fabrication.[1] Y.Bar-Cohen (2001) Electroactive Polymer Actuators as Artificial Muscles: Reality, Potential, and Challenges Second Edition, Bellingham, SPIE Press Monograph Vol. PM136.[2] P. Calvert, Hydrogels for Soft Machines, Adv. Mater. 21 (2009) 743–756[3] M.Bassil, J.Davenas and M.El Tahchi, Sensors and Actuators B: Chemical 134 (2008) 496–501.[4] M.Bassil, M.El Tahchi and J.Davenas, Advances in Science and Technology 61 (2008) 85-90.[5] M.Bassil, M.Ibrahim, M.El Tahchi, J.Farah and J.Davenas, Mater. Res. Soc. Symp. Proc. 1134 (2009) BB01-10.[6] M.Bassil, M.Ibrahim, M.El Tahchi, S.Lanceros-Mendez, G.Boiteux, J.Davenas and J.Farah, Proc. of the 2009 E-MRS Spring meeting, June 8–12, Strasbourg, France.
QQ2: Modeling and Simulation
Session Chairs
Jules Magda
Walter Richtering
Monday PM, November 30, 2009
Room 208 (Hynes)
2:30 PM - **QQ2.1
Molecular Dynamics Simulations of Polyleucine and Polyleucine-Modified HA.
Grant Smith 1 , Dmitry Bedrov 1 , Ben Hanson 1
1 Materials Science and Engineering, University of Utah, Salt Lake City, Utah, United States
Show AbstractThere is growing interest in exploiting the self-assembly of biocompatible amphiphilic polymers in aqueous solution to create novel structures and materials for a wide variety of biomedical applications. We are exploring the utilization of polyleucine as a hydrophobic modifier in order to control the structure and rheological properties of HA networks. Here we hope to exploit the fact that hydrophobic polyleucine side chains covalently attached to HA at the appropriate frequency and molecular weight will self-assembly to form physical crosslinks. We have utilized atomistic molecular dynamics simulations to study the conformations and interactions of polyleucine in aqueous solution, and, utilizing knowledge of polyleucine self-assembly gleaned from these studies, have conducted molecular dynamics simulations of polyleucine-modified HA in order to understand the role of side chain frequency and molecular weight on network properties.
3:00 PM - **QQ2.2
Gelation in Semi-Flexible Polymer Solutions.
Sanat Kumar 1
1 , Columbia U, New York, New York, United States
Show AbstractComputer simulations have been employed to study the formation of a physical gel by semi-flexible polymer chains. The formation of a geometrically connected network of these chains is investigated as a function of temperature. As the temperature is lowered, a percolated homogeneous solution phase separates to form a non-percolated nematic fluid and upon further decrease in the temperature, it goes back to a percolated gel state. The gelation, at lower temperatures, is due to the dynamic arrest of chains, preventing them from completing the phase separation process. The effect of cooling rate is also studied. Quenching the system to the final temperature at a faster rate yields gelation while slower quenches result in phase separation. Depending on the rate of cooling, we either get a percolated gel or a single nematic bundle, both with the same local density.
4:00 PM - QQ2.3
The Effect of Nucleic Acid Length and Sequence on Effectiveness of the DNA Film Growth: A Molecular Dynamics Study.
Yaroslava Yingling 1 , Stacy Snyder 1
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractNucleic acids, such as RNA and DNA molecules, are especially appealing for nanobiotechnological applications due to their versatility in function and structure and molecular recognition properties of base pairing. We investigate the effectiveness of self-assembly of nucleic acids into thin, multilayered films, which have been prepared using single-stranded DNA deposited via a layer-by-layer technique. These thin films can form novel hollow multilayer capsules that posses unique engineered features such as size, shape, composition, porosity, stability, surface functionality, and biodegradability. Using extensive molecular dynamics simulations we explored the effects of nucleic acid strand length and nucleotide sequence on the efficiency of film growth. We monitor the dynamics and conformation of successive DNA oligonucleotides as film is grown in order to explain experimental results, including anomalous changes in growth efficiency with strand length and sequence. Our simulations predict the effective structural modifications that the film undergoes as a function of length and sequence and explains the experimental observation of a required minimum length of 20 nucleotides for film growth and a saturation limit at higher length or change in a sequence. Clearer understanding of the self-assembly process is expected to make possible the algorithmic self-assembly of nucleic acid thin films for applications in drug delivery and biological sensing.
4:15 PM - QQ2.4
Atomistic Modeling of the Structural and Mechanical Properties of Silk Nanocrystals and Nanocomposites.
Sinan Keten 1 , Britni Ihle 1 , Markus Buehler 1
1 Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractSpider silk is a unique material that can blend disparate properties such as extensibility and high strength, making it one of the toughest materials known. Despite significant developments in our understanding of secondary structures in silk and their link to mechanical behavior, the molecular mechanisms of silk assembly and fracture are have not yet been elucidated. Through full-atomistic molecular dynamics simulations in explicit and implicit solvent as well as simplistic continuum formulations, we present a study that rides the gap between macro and nano-scale descriptions of silk by providing a first-principles based model for the constitutive behavior of silk nanocrystals. Replica exchange simulations on short segments of spider silk proteins predict a multi-phase representative model structure for the material that contains orderly beta-sheet regions dispersed within less orderly domains. Simulations elucidate the influence of mechanical tensile and shear forces on structure formation and fracture. The model illustrates the size, geometry and sequence dependent mechanical properties of spider silk, shedding light on the validity of earlier hypotheses and lower resolution models on silk mechanics.
4:30 PM - QQ2.5
Multiscale Mechanics of Large Random Fiber Networks.
Catalin Picu 1 , Hamed Hatami-Marbini 1
1 , Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractRandom fiber networks are ubiquitous in biological and non-biological systems. The cytoskeleton, on which the cell integrity and structure depend, is a typical example of such network. We investigate the mechanics of such ensembles on multiple scales and observe that the mechanical fields (stress, strain, stiffness) exhibit power law scaling over a range of scales bounded below by the mean fiber segment length and above by the fiber persistence length. This conclusion results from the analysis of spatial correlations of local elastic moduli evaluated over network sub-domains at multiple probing scales. Therefore, the network deforms similarly with a stochastic fractal heterogeneous elastic object. Based on this observation, a multiscale method is developed to solve boundary value problems defined on large fiber network domains, without accounting for every fiber in the system.
4:45 PM - QQ2.6
Local Measurements of Phase Transitions in Biological Polymers (Lambda DNA and Bacteriorhodopsin Membrane).
Maxim Nikiforov 1 , Roger Proksch 2 , Sophia Hohlbauch 2 , Stephen Jesse 1 , Sergei Kalinin 1
1 CNMS, ORNL, Oak Ridge, Tennessee, United States, 2 , Asylum Research Co., Santa Barbara, California, United States
Show AbstractPhase transitions play an important role in biology. Specifically the thermodynamic stability of internal membrane proteins and bio-polymers (DNA) is an important issue of biophysics. Purple membranes from Halobacterium halobium contain bacteriorhodopsin, an integral protein 70-80% of whole mass is intramembraneous. There are heated debates in the field about the parameters of thermal denaturation of bacteriorhodopsin, such as the denaturation temperature, enthalpy etc. Recently, bacteriorhodopsin and DNA are materials proposed as a components of biomolecular electronics. Thus, reliable information about the phase transitions of supported smaples of bacteriorhodopsin membranes and DNA is necessary.Phase transitions in polymer/biopolymer materials are associated with the large changes in mechanical properties of the samples. We developed the technique for the measurements of the temperature dependence of the mechanical properties with high spatial resolution. This technique is based on the measurements of the contact stiffness of the atomic force microscopy tip – sample system as a function of temperature. The contact stiffness was monitored using dual frequency resonance tracking methodology developed by Asylum Research Co.We measured softening temperature of the bacteriorhodopsin membranes deposited on mica substrate using the developed technique. It was found that extracellular and intracellular membranes have similar softening temperatures. The tip – sample contact are had 20 nm radius meaning that the response from less than 20 molecules was measured. The temperature dependence of mechanical properties of single stranded DNA was also measured and used for the calculations of softening temperature of the single biological molecule.A portion of this research at Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. A portion of this research was sponsored by Asylum Research Co.
5:00 PM - QQ2.7
Nanoscale Pattern Formation in Polyelectrolyte Gels.
Prateek Jha 1 , Francisco Solis 2 , Juan de Pablo 3 , Monica Olvera de la Cruz 1
1 , Northwestern University, Evanston, Illinois, United States, 2 , Arizona State University, Glendale, Arizona, United States, 3 , University of Wisconsin-Madison, Madison, Wisconsin, United States
Show AbstractPolyelectrolyte (PE) gels with hydrophobic backbones exhibit complex phase behavior that includes the formation of nanophases, that is, the local segregation of the monomers. The formation of these inhomogeneous phases is possible due to the presence of different length scales for interactions; in this case with electrostatic effects providing long length-scales. We demonstrate the presence of these nanophases in a model for a PE gel that incorporates entropic, elastic, electrostatic, and solvent interactions. We analyze the model with both linear and non-linear methods and show that the linear approximation properly identifies the region of instability against nanophase formation. Nonlinear effects lead, in addition, to features not accessible through the linear approach, such as the formation of very sharp interfaces between the nano-segregated regions. We discuss the dependence of the periodicity and monomer and charge distributions for the gel as functions of the gel physical parameters. We find that the higher the charge fraction and the lower the crosslink density, the larger the range of solvent quality for nanophase segregation.
5:15 PM - QQ2.8
Artificial Polymers Mimic Bacteriophage Capsid Proteins and Encapsulate Nucleic Acids.
David Robinson 1 , Michael Kent 1 , George Buffleben 1 , Ronald Zuckermann 2
1 Energy Nanomaterials, Sandia National Laboratories, Livermore, California, United States, 2 Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractThe filamentous bacteriophage m13 and related viruses encapsulate DNA with protein, forming an organic wire about 1 micrometer long and less then 10 nanometers wide. The length of the wire is formed from many copies of a single protein, which is a single alpha helix formed from about 50 amino acids. It can be viewed as a very sophisticated surfactant, with hydrophilic regions that interact with the DNA and form the outer surface, and hydrophobic regions that pack against each other. We have implemented these design principles in peptoids (sequence-specific N-functional glycine oligomers) and have found that they form well-defined aggregates with DNA. This approach may prove applicable to the design of hydrogels with tailored filament length, stiffness, and functionality. It may complement phage display methods, providing new approaches to gene transfection and nanofabrication that do not involve bacterial intermediates. We have adjusted the peptoid-DNA interaction by tuning the sequence and length of the peptoids and observing by gel electrophoresis. We have observed evidence of filamentous assemblies by neutron and light scattering and electron microscopy. Molecular dynamics simulations, circular dichroism, and NMR provide confidence that the peptoids have a helical conformation.This work was performed under the Laboratory-Directed Research and Development Program at Sandia National Laboratories, a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
5:30 PM - QQ2.9
The Effect of Friction on the Structure and Dynamics of Unbonded Random Fiber Networks.
Gopinath Subramanian 1 , Catalin Picu 2 1
1 Scientific Computation Research Center, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Insitute, Troy, New York, United States
Show AbstractThe role of interfiber friction in defining the structure and dynamics of entangled fiber networks is studied. The fibers are not bonded to each other and the excluded volume condition is imposed in all simulations. When subjected to isostatic compression, the fiber mass undergoes a transition from the sparse network structure to a dense state. The critical density at which this transition occurs decreases with increasing fiber aspect ratio. This transition, in the case of frictionless fibers is sharp and occurs over a small range of densities. In the presence of friction, the transition is more gradual. In the network state, frictionless fibers were observed to display an exponential distribution of elastic energy at fiber-fiber contacts. Fibers with friction displayed a power law distribution. Cyclic compression/relaxation shows hysterisis behaviour both in the presence and absence of friction. However, in the presence of friction, multiple cycles of compression/relaxation result in the fiber network forming one single entangled mass, held together solely by friction, and this phenomenon is not observed in frictionless fibers. The density of this resultant mass is seen to increase with the coefficient of friction.
5:45 PM - QQ2.10
A Large Deformation Theory for Thermo-chemo-mechanically Coupled Polymeric Gels.
Shawn Chester 1 , Lallit Anand 1
1 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractMany stimulus responsive polymer gels operate in non-isothermal, chemically saturated environments. Such situations require not only thermo-mechanical coupling, but thermo-chemo-mechanical coupling. Thermo-chemo-mechanical coupling is a coupling between the temperature, ``chemical'', and deformation fields. All fields may be inhomogeneous and evolve with the system over time. The mostfamiliar type of material coupling is thermo-mechanical coupling, between the temperature and deformation fields. What is non-standard is the chemical coupling. In the specific case considered in this talk, the chemical coupling is the diffusion of fluid into the polymer network causing large volume changes. Many continuum level theories in the literature do not fully couple the temperature, chemical, and deformation fields. Instead, they take the limit of isothermal, or having a constant fluid concentration field. We have developed a general, thermodynamically consistent, continuum level thermo-chemo-mechanically coupled theory for large deformations of polymer gels. In discussing special constitutive equations, we limit our attention to isotropic materials, and a special model for the free energy based on standard Gaussian statistical mechanics considerations of changes in configurational entropy due the stretching of the polymer chains, along with a simple Flory-Huggins model for the free energy change due to mixing of the fluid with the polymer. Furthermore, current commercial finite element codes do not provide for a coupling between the large deformation, temperature, and the chemical fields. Therefore, the corresponding numerical procedure in the context of standard finite element methods is also developed. The numerical simulation capability is then implemented for some representative examples of swelling in polymer gels due to fluid absorption.
QQ3: Poster Session I
Session Chairs
Ferenc Horkay
Walter Richtering
Tuesday AM, December 01, 2009
Exhibit Hall D (Hynes)
9:00 PM - QQ3.1
Controlled Porous Structure of DNA Hydrogel by Salt and pH.
Sun Hee Lee 1 , Chang Kee Lee 1 , Su Ryon Shin 1 , Seon Jeong Kim 1
1 , Hanyang University, Seoul Korea (the Republic of)
Show AbstractDNA hydrogels are currently of considerable interest in bio-applications due to the remarkable properties of DNA. DNA has a high sensitivity by salt and pH because of electrostatic repulsion of phosphate group and hydrophobic interaction of base pair. These DNA properties have influence on the characteristic of DNA hydrogel consisted of native DNA in which DNA strands were formed random entanglements to provide physically crosslinked networks. DNA hydrogel have response to change the volume transition by external stimuli such as temperature, pH, ionic strength, chemicals, surfactants, etc. Because of their significant swelling and deswelling in response to external stimuli, these polymeric networks are used for a variety of applications in biological such as system drug delivery and tissue engineering. In this work, DNA hydrogel have been fabricated without crosslinker using wet-spinning method. The DNA hydrogel showed interesting as response behavior, since drastic volume changes can be induced by change such as salt and pH. When salt concentration increased, DNA hydrogels appeared to be decreased. In pH condition, as pH value decreased, DNA hydrogels showed shrinking behavior. The swelling of the DNA hydrogel appeared to be reversible by controlling change of pH and concentration of salt. Thus DNA hydrogel is useful for bio-application.
9:00 PM - QQ3.10
Time-Resolved Fourier Transform Infrared Spectroscopy of Diblock-Copolymers Inspired by Spider Silk.
Wenwen Huang 1 , Sreevidhya Krishnaji 2 , David Kaplan 3 , Peggy Cebe 1
1 Physics, Tufts University, Medford, Massachusetts, United States, 2 Chemistry, Tufts University, Medford, Massachusetts, United States, 3 Biomedical Engineering Department, Tufts University, Medford, Massachusetts, United States
Show AbstractWe report the characterization of a new family of silk-based block copolymers. The block copolymers HAB3, HAB2, HBA, HBA2, HBA3 and HBA6 utilize protein sequences, A and B, found in native spider dragline silk (Nephila clavipes), where B = hydrophilic block, A = hydrophobic block, and H is a histidine tag. To assess structure we used time- resolved Fourier transform infrared spectroscopy and X-ray scattering. Thermal properties were determined by differential scanning calorimetry and thermogravimetic analysis. Results indicate that as the size of the hydrophobic A-block increases, the content of beta sheets increases, reaching a crystallinity of 30% in the block copolymer design that had the greatest hydrophobic content (A6 content). As temperature was slowly increased from room temperature to 280 °C (at 2 °C/min), bound water which served as a plasticizer evaporated during heating and the conformation of the block copolymers also changed. This process was monitored by Fourier transform infrared spectroscopy. In the Amide I region, a similar trend was shown in all block copolymers: as temperature increased to 180 °C which is above the copolymer glass transition temperature, Tg ~ 150 °C, content of Beta sheet and Alpha helices was reduced, and content of Beta turns was increased.Support was provided from the National Science Foundation, Division of Chemical, Bioengineering, Environmental, and Transport Systems, through CBET-0828028 and the MRI Program under DMR-0520655 for thermal analysis instrumentation.
9:00 PM - QQ3.11
The Synthesis of Poly(N-isopropylacrylamide) / Polyacrylamide Anisometric Hydrogel Particles by Photocrosslinking and Temperature Responsive Release Behavior.
Sona Lee 1 , Jonghwi Lee 1
1 Chemical Engineering, Chung-Ang University, Seoul Korea (the Republic of)
Show AbstractPoly(N-isopropykacrylamide) (PNIPAAm) formed interpenetrating network (IPN) hydrogels with polyacrylamide (PAAm) in the preparation of anisometric particles by Ultra Violet (UV) irradiation under nitrogen. The PNIPAAm-based hydrogels were prepared by using N,N’-methylenebisacrylamide and clay of various concentrations as crosslinkers. 2-Hydroxy-1-[4-(hydroxyethoxy)-phenyl]-2-methyl-1-propanone (Irgacure2959) was used as a water soluble initiator. A freeze drying step was inserted into the two photocrosslinking steps of anisometric particles of IPN in order to photocrosslink PAAm in pre-crosslinked PNIPAAm hydrogels phases. Water soluble dyes and Green Tea Polyphenol (GTP) were infiltrated into the resultant IPN hydrogels as a model active pharmaceutical ingredient. The swelling ratio of PNIPAAm/PAAm hydrogels was examined with loaded model drugs at various temperatures. The anisometric properties related with drug release from the hydrogels were measured by optical microscopy, Fourier transform infrared spectroscopy (FTIR), UV-vis spectroscopy, and differential scanning calorimeter (DSC). Uniaxial compression tests revealed that the concentration of PNIPAAm and crosslinkers could control the morphological changes of the hydrogels as a function of temperature.
9:00 PM - QQ3.2
A New Bio-inorganic Nanocomposite Membrane for Glucose-modulated Release of Insulin.
Claudia Gordijo 1 , Adam Shuhendler 1 , Xiao Wu 1
1 , University of Toronto, Toronto, Ontario, Canada
Show AbstractDiabetes is a serious condition characterized by the body’s inability to adequately metabolize glucose. It affects more than 246 million people worldwide. The conventional way of controlling glycemia is the frequent self-administration of insulin, which often results in hypoglycemia. A more effective approach to delivering insulin in direct response to blood glucose levels mimicking a healthy human pancreas is thus highly desirable. Smart materials have been investigated for glucose-modulated insulin delivery. Herein, we propose the application of a new bio-inorganic nanohybrid glucose-responsive membrane as a single self-regulated platform for self-regulated insulin deliver. In preparation of such materials we have coordinated functionalities coming from organic, inorganic and biological components. We have conjugated multifunctional MnO2 nanoparticles with bovine serum albumin and the enzymes glucose oxidase (GOx) and catalase (CAT). A membrane was prepared by crosslinking these biomolecule-MnO2 conjugates in the presence of pH-responsive poly(N-isopropylacrylamide-co-methacrylic acid) (PNIPAM/MAA) hydrogel nanoparticles.The membrane acts as a glucose sensor by the action of GOx, which catalyzes glucose oxidation and production of gluconic acid. This reaction creates a low pH microenvironment in the membrane, causing the hydrogel nanoparticles to shrink, leading to the formation of an interconnected porous framework that increases insulin permeability. In this system GOx turnover cycles produces hydrogen peroxide, an undesirable product which can leads to enzymes inactivation and unwanted toxicity. We demonstrated that the combination of CAT and MnO2 nanoparticles yields better results in terms of quenching of H2O2 and long-term GOx stability as compared to the membranes in which CAT or MnO2 were separately employed. SEM images of the nanohybrid membranes revealed a complex sponge-like structure with MnO2 and hydrogel nanoparticles homogeneously distributed in the thin cavity wall of the base membrane. The in vitro release of insulin was evaluated over time at glucose concentrations relevant to diabetic patients. The self-regulated system released very small amounts of insulin at normal blood glucose levels, while a four-fold increase in insulin release was observed when glucose concentration was increased to hyperglycemic levels. When the glucose level drops to a normal level the rate of insulin release is decreased. The reliability and reproducibility of the glucose-responsive insulin release profile of the system allows for self-regulated insulin release in response to glucose concentrations analogous to a healthy pancreas.
9:00 PM - QQ3.4
Chemically-Responsive Gels Prepared from Microspheres Dispersed in Liquid Crystals.
Santanu Pal 1 , Ankit Agarwal 1 , Nicholas Abbott 1
1 Chemical & Biological Engineering, University of Wisconsin-Madison, Madiosn, Wisconsin, United States
Show AbstractLiquid crystalline materials are a promising class of stimuli-responsive materials that have been demonstrated to undergo surface-induced orientational ordering transitions that can be highly sensitive and specific to chemical species. However, past studies demonstrating surface-induced transitions in LC have employed thin films of low molecular weight liquid crystals (LCs) that are difficult to stabilize (due to dewetting of the LC on a surface). In this presentation, we report that it is possible to prepare liquid crystalline gels using mixture of polystyrene (PS) microspheres and nematic LCs which undergo changes in orientational order, and thus optical appearance, in response to exposure to specific chemical compounds. These colloid-in-liquid-crystal (CLC) gels are mechanically stable and can be molded on chemically functionalized surfaces into thin films containing micrometer-sized LC rich domains that span the two interfaces of the gels. In contrast to other reports of LC gels, where the presence of a polymeric or self-assembled small-molecule gelator network within a nematic LC frustrates ordering transitions from propagating through the gels over distances, we demonstrate that thin films of CLC gels, when supported on chemically functionalized surfaces, do undergo easily visualized ordering transitions upon exposure to organophosphonate compounds. Because these optically responsive CLC gels are mechanically robust and can be molded, we believe this class of composite LC material may be broadly useful for the design of chemically-responsive LC devices.
9:00 PM - QQ3.5
Microfluidics-assisted Synthesis of Micrometer Scale Heparinazed Hydrogels.
W. Jeong 1 2 , J. Lim 1 2 , J. Choi 1 2 , S. Yang 1 2
1 Chemical & Biomolecular Eng., KAIST, Daejeon Korea (the Republic of), 2 National Creative Research Initiative Center for Integrated Optofluidic Systems, KAIST, Daejeon Korea (the Republic of)
Show AbstractOver the past decades, considerable efforts have been devoted for the fabrication of polymeric gels at nano-to-micrometer scale with uniform size and functional property. Micrometer scale gels, cross-linked and swellable hydrophilic polymer particles, especially offer significant advantages as drug delivery vehicles compare to conventional submillimeter scale gels. The small size of the micrometer scale gels can improve drug or gene effect in target tissue, and the stability of therapeutic agents against chemical/enzymatic degradation. Microfluidic flow-focusing devices made of PDMS provide a versatile approach to prepare monodisperse droplets at high frequencies. However, they exhibit significant problems as the size of channel decreases. Leakage can occur at the interface between PDMS and glass due to the high pressure originated from the fluid injection. Continuous-flow microfluidics tends to undergo chaotic oscillations in which flows vary uncontrollably. In addition, the dust and debris often block the channels after only a few uses. To overcome these problems, we introduce additional sheath flow in the flow-focusing regime. The dispersed and continuous phases are hydrodynamically focused into a narrow stream, and the width of stream determines the size of the droplets. Three-inlet streams are controlled pneumatically to offer fast response time and stable flow. Few-micron sized emulsion droplets can be generated at steady state by using the pneumatic pumping system. The evaporation of solvent leads to further shrinkage of droplets, therefore submicron-sized hydrogels can be generated. We use biodegradable heparin, a highly sulfated natural glycoaminoglycan, as model polymeric materials. The photo-crosslinkable heparin is prepared by substituting the carboxylic groups of heparin with acrylate groups using N-(3-aminopropyl) methacrylamide hydrochloride. Aqueous emulsion droplets comprising of heparin, photoinitiator, crosslinker and growth factor are generated via hydrodynamic focusing and in-situ crosslinking by UV light. Hydrogels via these strategies suggest opportunity for two-stage degradation. Non-covalent heparin-growth factor interaction is capable of receptor-mediated gel erosion for targeted drug delivery. The left fragments of covalently bonded hydrogel can degrade via hydrolytic processes under physiological condition. To determine optimum conditions for growth factor delivery, we investigate swelling and degradation behavior for hydrogel at various heparin concentrations.
9:00 PM - QQ3.6
Depth Dependence of the Osmotic Properties of Bovine Cartilage.
Candida Silva 1 , Iren Horkayne-Szakaly 1 , David Lin 1 , Peter Basser 1 , Ferenc Horkay 1
1 NICHD, NIH, Bethesda, Maryland, United States
Show AbstractArticular cartilage is composed of highly hydrated extracellular matrix synthesized by chondrocytes. Cartilage is an inhomogeneous and anisotropic tissue that covers the ends of bones. Healthy cartilage allows bones to glide over one another and absorbs energy from the shock of physical movement. Osteoarthritis is one of the most frequent causes of physical disability among adults. It is a joint disease that mostly affects the cartilage. In osteoarthritis the surface layer of cartilage breaks down and wears away.The major structural elements of cartilage (collagen fibers, proteoglycan assemblies and chondrocytes) are not randomly organized but divided into zones that differ in molecular organization. The orientation of collagen fibers within the tissue differs between the zones. They exhibit a tangential orientation at the surface, random configuration in the middle layer, and radial orientation close to the bone surface. The superficial layer contains flattened chondrocytes aligned parallel to the surface. In the middle layer the chondrocytes are spherical, while in the deep layer they form columns oriented in the vertical direction. The proteoglycan aggregates are composed of aggrecan subunits bound into very large aggregates with hyaluronic acid. (This interaction is stabilized by a third component, the link protein.) The proteoglycan aggregates occupy the pores of the collagen matrix. Articular cartilage exhibits high water content, ranging from 65% to 90% of the tissue weight, depending on age and location. This complex morphology provides the basic structural framework for the cartilage matrix, and the load-bearing ability of the tissue strongly depends on this structure. The collagen fibers contribute to the tensile properties and the proteoglycans provide resistance and resilience to deformation. When cartilage is compressed, the interstitial fluid is pressurized and supports the applied contact stress.Although several biomechanical models have been proposed to explain the biomechanical properties of cartilage, none of these models provides a complete understanding of the relationship between the structure and the functional properties. In this work we use osmotic stress techniques to investigate the osmotic response of cartilage layers as a function of the depth from the articular surface. The hydration of thin cartilage slices from different zones is measured under controlled osmotic conditions. The osmotic compression modulus, which quantifies the load-bearing capacity of the tissue, is determined from measurement of the osmotic swelling pressure as a function of the swelling degree. The atomic force microscope is used to map the local elastic properties of the extracellular matrix and the cells. The experimental results reveal significant depth-dependent variations in the compressive properties.
9:00 PM - QQ3.8
Rheology of Highly Packed Ionic Microgels.
Paul Menut 1 2 , David A. Weitz 1
1 , Harvard University, Cambridge, Massachusetts, United States, 2 , Montpellier SupAgro, Montpellier France
Show AbstractMicrogels based systems exhibited a various range of rheological behavior depending on their particles volume fraction, the internal cross-linked density of each particle, and the properties of the solvent. One of their main interest for application purposes is their ability to respond quite rapidly and dramatically to the evolution of their surrounding medium. They can then swell or shrink in response to change in temperature, osmotic pressure, pH or ionic force, depending on their own chemical structure.At low volumic fraction, the viscosity of a microgel suspension increase with the concentration, following the usual Einstein’s rules for hard spheres.However, when the concentration increases, their ability to deform and to shrink allows the preparation of materials with microgel volume fraction higher than one, if taking their hydrodynamic diameter as equal to the one measured in dilute state. This behavior underline the ability of those particles to deform and to shrink. The study of the rheological properties of those highly packed systems has shown a paste-like behavior, characterized by aging and rejuvenation. In this study, we explore the rheological properties of charged microgel systems (polyNIPAM-co-AAc), in presence of counter-ions, while increasing packing ratio. Their behavior was characterized by a series of creep/relaxation procedures, on a stress-controlled rheometer (AR-G2). The effect of the electrostatic interaction evolution between particles during packing is describe, as the monitoring of counter-ion concentration allows the controlled screening of the charges.
9:00 PM - QQ3.9
Hybrid Gels from Self-Assembling Peptide Networks.
Sameer Sathaye 1 , Radhika Nagarkar 2 , Joel Schneider 2 , Darrin Pochan 1
1 Department of Materials Science and Engineering, University of Delaware, Newark, Delaware, United States, 2 Department of Chemistry and Biochemistry, University Delaware, Newark, Delaware, United States
Show AbstractHybrid Gels constitute a novel class of polymeric materials developed with an aim of combining and/or enhancing the diverse and complementary properties of their individual constituent networks. Self-assembling peptide hydrogels formed from aqueous solutions of β-hairpin forming peptides have been extensively reported. These hydrogels are interesting candidates as part components of hybrid gels due to their ability to retain their inherent physical properties in the presence of other hydrogel networks and other added functionality (e.g. an inorganic coating of the gel fibrillar nanostructure). Synergistic interactions of these peptidic networks with other added polymer co-networks with a range of tunable synthetic characteristics and properties have been explored by various characterization techniques such as Dynamic Mechanical Analysis (DMA), Transmission Electron Microscopy (TEM) and Small Angle Neutron Scattering (SANS). The ease of producing co-networks between a wide array of target polymer networks and β-hairpin peptides as the fundamental, core network will be discussed.
Symposium Organizers
Ferenc Horkay National Institutes of Health
Noshir Langrana Rutgers University
Walter Richtering RWTH Aachen University
QQ4: Mechanical Properties of Gels and Tissues
Session Chairs
Tuesday AM, December 01, 2009
Room 208 (Hynes)
9:30 AM - **QQ4.1
Some Minimal Models of Network Elasticity.
Jack Douglas 1
1 Polymers Division, NIST, Gaithersburg, Maryland, United States
Show AbstractClassical network elasticity theories are based on the conception of flexible volumeless network chains fixed into a network in which there are no excluded volume interactions between the chains and where the chains explore accessible configurations by thermal fluctuations. The limitations of this approach are clear from the observation that unswollen rubbery materials are nearly incompressible, reflecting the existence of strong intermolecular interactions that restrict the polymer chains to an exploration of their local molecular environments. The imposition of a deformation to these solid rubbery materials then necessitates a consideration of how local molecular packing constraints become modified under deformation and the impact of these changes on the macroscopic elasticity of the material as a whole. The simple ‘localization model’ of rubber elasticity, introduced by Gaylord and Douglas (GD), provides an attractive minimal model for the network elasticity of rubbers having strong intermolecular interactions in the dense polymer state. The properties of this model are summarized and compared to observations on rubbery materials where both the cross-linking density and swelling are varied. The model is extended to describe networks of stiff chains and networks having junctions formed through reversible association.
10:00 AM - **QQ4.2
Mechanics of Active Biopolymer Networks.
David Weitz 1
1 Physics & SEAS, Harvard University, Cambridge, Massachusetts, United States
Show AbstractThis talk will present the results of measurements of the mechanics of reconstituted biopolymer networks, and will explore the effects of molecular motors within the networks.
11:00 AM - **QQ4.3
Nonlinear Elastic Response of Extracellular Matrices to Cell-generated Stresses.
Jessamine Winer 1 , Shaina Oake 1 , Paul Janmey 1
1 Institute for Medicine and Engineering, Univ. Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractMost native extracellular matrices composed of collagen or fibrin fibers exhibit nonlinear rheology characterized by strain stiffening and negative normal stress in response to simple shear deformation. Tension between matrix and cell membrane adhesion complexes can initiate signals that alter cell structure and function. Many cell types modulate their spread area, stiffness, motility, and protein expression in response to the substrate stiffness. Studies of stiffness sensing typically employ linear elastic materials whose stiffness is independent of the applied strain, whereas biological gels often stiffen in response to increasing strain. Fibroblasts and mesenchymal stem cells adherent to linearly elastic gels typically display a small, round phenotype on soft substrates and increase spread area as the elastic modulus of their substrate increases. On the strain-stiffening biopolymer gel fibrin, the same cell types are maximally spread even when the gel's low strain elastic modulus would predict a round morphology. Traction microscopy reveals that cells apply active displacements of several microns up to five cell lengths away, and atomic force microscopy shows that these displacements locally stiffen the gel by deforming it beyond its linear range. The magnitude of cell-applied strains is inversely related to the gel's low strain elastic modulus and results in long distance cell-cell communication and alignment. Non linear elasticity of both intracellular and extracellular biomaterials allows cells to alter their own stiffness as well as that of the extracellular matrix by applying tensions that locally strain the networks and cells appear to exploit local stiffness chances for long range mechanical communication.
11:30 AM - QQ4.4
Theory of Load Support by Articular Cartilage.
Jeffrey Sokoloff 1 , J. Ruberti 2
1 Physics, Northeastern University, Boston, Massachusetts, United States, 2 Department of Mechanical and Industrial Engineering, Northeastern University, Boston, Massachusetts, United States
Show AbstractArticular cartilage is comprised of charged macromolecules, proteoglycans, trapped within a network of type II collagen fibrils. A proteoglycan can be modeled as a collection of charged polymers (in our case the chondroitin sulphate chains) each anchored to a straight line (to represent the backbone of the proteoclycan), which is essentially a cylindrical symmetry polymer brush. Analytic mean field, commonly used in the study of polyelectrolyte brushes, and scaling theory are used to determine the load support that arises as a proteoglycan is compressed. The calculations show that under a pressure of a MPa, the radius of each of these molecules is compressed to 14 % of its zero load value, which is in agreement with observation. The load carrying ability of cartilage based on this model for proteoglycan is studied as a function of degree of compression of the cartilage and salt concentration, and compared to experiment. Dissipation due to polymer chains protruding from these macromolecules getting entangled with polymer chains attached to neighboring proteoglycans is shown to be unlikely to occur because the number of monomers, belonging to a given polymer which gets entangled in a neighboring proteoglycan, is likely to be too small to produce a blob of radius as large as a mesh length.
11:45 AM - QQ4.5
Robustness-strength Performance of Alpha-helical Protein Filaments with Hierarchical Structures.
Zhao Qin 1 2 , Steven Cranford 1 2 , Theodor Ackbarow 1 3 , Markus Buehler 1 2 4
1 Laboratory for Atomistic and Molecular Mechanics, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Theory Division, Max Planck Institute of Colloids and Interfaces, Potsdam Germany, 4 Center for Computational Engineering, Massachusetts Institute of Technology, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractHierarchical nanostructures are common for biological materials, ranging through atomistic, molecular to macroscopic scales. By utilizing the recently developed Hierarchical Bell Model (HBM), here we show that the use of hierarchical structures leads to an extended physical dimension in the material design space that resolves the conflict between strength and robustness which is a limitation faced by many synthetic materials. We report a case study in which we combine a large number of alpha-helical elements in various hierarchical combinations and measuring each performance in the strength-robustness space. We find that for a large number of constitutive elements, more than 98% random structural combinations of elements lead to either high strength or high robustness, reflecting the banana-curve performance in which strength and robustness are mutually exclusive properties. This banana-curve type behavior is common to most engineered materials. In contrast, only few (<2%) specific types of combinations of the elements in hierarchies is possible to maintain high strength at high robustness levels. We found that alpha-helical protein filaments with certain arrangement of two level hierarchies are optimal for strength and robustness. This behavior corresponds to the naturally observed material performance in biological materials, suggesting that some particular hierarchical structures enable to fundamentally change the material performance. The results suggest that biological materials may have developed under evolutionary pressure to yield materials with multiple objectives, such as high strength and high robustness, a trait that can be achieved by utilization of hierarchical structures. Our study indicates that both the formation hierarchies and the assembly of specific hierarchical structures play a crucial role in achieving these mechanical traits. Our findings may enable the development of self-assembled de novo bioinspired nanomaterials based on peptide and protein building blocks.
12:00 PM - QQ4.6
Physically Crosslinked Hyaluronic Acid-based Novel Hydrogels.
Divya Bhatnagar 1 , Miriam Rafailovich 1
1 Materials Science and Engineering, Stony Brook University, Stony Brook, New York, United States
Show AbstractThis work investigates the rheological properties of Hyaluronic acid(HA)-Clay hydrogels prepared by physically crosslinking non-modified HA with inorganic clay. Resulting hydrogels were transparent and mechanically stable. To investigate their mechanical properties the hydrogels were subjected to oscialltory shear rheometry which allows the evaluation and comparison of the shear storage moduli (G'), an index of the stiffness of the hydrogels. While the temperature sweep monitored the effect of temperature on G', the stress and frequency sweeps measured G' as a function of stress and frequency respectively. Results from frequency sweeps suggested the formation of a stable,three dimensional network while the stress sweep revealed the linear viscoelastic region and the breaking stress for the HA-clay hydrogels. Results from temperature sweep tests suggested the stability of the crosslinks before 130°C and that the effect of increasing temperature makes the hydrogels less rigid. Cell cultivation on the surface of a novel HA(Hyaluronic Acid)-clay hydrogel was studied using human dermal fibroblasts. It was found that the cells could becultured on the surfaces of HA-clay hydrogels for upto 4 days. Such a detailed rheological characterization of our HA-clay hydrogels with no chemical additives will aid in the design of biomaterials targeted for biomedical or pharmaceutical purposes, including rigid cell scaffoldstructures.
12:15 PM - QQ4.7
Ionic Crosslinking for Enhancing Mechanical Performance of Acrylic Triblock Copolymer Hydrogels.
Kevin Henderson 1 , Tian Zhou 1 , Kenneth Shull 1
1 Materials Science & Engineering, Northwestern University, Evanston, Illinois, United States
Show AbstractSymmetric, amphiphilic triblock copolymers provide a facile, self-assembled route toward the formation of physically crosslinked polymer hydrogels. Due to a well-defined network architecture and homogeneity of crosslinks, these materials are capable of withstanding much larger strains than chemically crosslinked materials of comparable stiffness. However, the mechanical properties of these materials are still incomparable to the robust characteristics of tough biological materials. The ability to tailor the molecular-scale dissipation mechanism of crosslinked polymer hydrogels is of significant interest for the fabrication of tough, synthetic alternatives to biological materials.Here, we present one such route through the extension of ionic crosslinking in an acrylic triblock copolymer hydrogel. The polymer architecture consists of symmetric, hydrophobic poly(methyl methacrylate) endblocks and a hydrophilic poly(methacrylic acid) midblock with a total polydispersity of 1.13. These materials spontaneously form bridged, spherical micelles upon appropriate solvent exchange with Young’s moduli on the order of 1 kPa. Upon ionic crosslinking with divalent cations (Zn, Ca), the solvated midblock of the polymer chain forms metallic complexes that serve as dynamic, pseudo-covalent linkages that significantly alter the mechanical response of the material. The modulus increases by 2-3 orders of magnitude depending on pH, solution concentration, and cation identity. Tensile testing and compressive indentation tests indicate not only a favorable increase in material toughness but also a considerable degree of recoverable energy dissipation upon successive loading not witnessed in many other toughening mechanisms.
12:30 PM - QQ4.8
Development of PEG-Based Hydrogels with Tunable Physical Crosslinks Mediated by Collagen Mimetic Peptide’s Triple Helical Propensity.
Patrick Stahl 1 2 , Nicole Romano 1 , Denis Wirtz 3 2 , S. Michael Yu 1 2
1 Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland, United States, 2 Institute for NanoBioTechnology, Johns Hopkins University, Baltimore, Maryland, United States, 3 Chemical and Biomolecular Engineering, Johns Hopkins University, Columbia, South Carolina, United States
Show AbstractHydrogels are widely used in biomedical applications including tissue engineering and drug delivery due to their biocompatibility and biomimetic qualities. The most effective tissue engineering hydrogels need to mimic the natural extracellular matrix’s (ECM) dynamic mechanical properties. The complexity of the ECM suggests that engineering ideal scaffolds for specific tissue is an exceptionally challenging task, and the current inability to present mechanical signals in a spatially and temporally defined manner is considered one of the major obstacles to engineering complex tissue for organ replacement therapy. Our group has designed synthetic hydrogels featuring complexes of star-shaped poly(ethylene glycol) (PEG) and collagen mimetic peptides (CMPs). These PEG-CMP complexes form hydrogels via physical crosslinks mediated by the thermally reversible triple helical assembly of CMPs due to their characteristic (ProHypGly)x repeat sequence. Furthermore, by tailoring the peptides’ terminal bifunctionality, the PEG-CMP hydrogels can also feature covalently bonded network structure in addition to the triple helix physical crosslinks. Here we present the relative rheological properties of multiple PEG-peptide hydrogel architectures determined by particle tracking microrheology and circular dichroism studies that enable analysis of the temperature-sensitive triple helix crosslinks in the hydrogels. We are able to disrupt CMP-mediated physical crosslinks by altering the temperature of the gel or adding unbound CMPs that compete for triple helix formation. These results give the PEG-CMP systems potential as novel scaffolds with tunable mechanical properties.
12:45 PM - QQ4.9
Characterizing the Mechanical Response of DNA Crosslinked Hydrogels Under Physiological Conditions.
Uday Chippada 1 , F. Jiang 2 , D. Varma 2 , B. Yurke 3 , N. Langrana 1 2
1 Department of Mechanical and Aerospace Engineering, Rutgers University, Piscataway, New Jersey, United States, 2 Department of Biomedical Engineering, Rutgers University, Piscataway, New Jersey, United States, 3 Department of Material Science and Engineering, Boise State University , Boise, Idaho, United States
Show AbstractRecent studies have shown that mechanical response of extracellular matrices significantly influences cellular properties, and hence, is an important parameter in designing biomaterials. Our group developed a DNA crosslinked hydrogel to examine cellular responses of spinal cord neurons to substrate compliances. Using DNA as crosslinker in polymeric hydrogel formation has given rise to a new class of hydrogels with a number of attractive properties (e.g., reversible gelation and controlled crosslinking). Here, it was demonstrated that by varying length of crosslinker, monomer concentration, and level of crosslinking, DNA gel properties can be controlled from ~100 Pa to 30 kPa, which is in the range of stiffness of spinal cord. Our studies on neurite outgrowth on functionalized DNA gels showed that although primary dendrite length is not significantly affected, spinal cord neurons extend more primary dendrites and shorter axons on stiffer gels. Additionally, a greater proportion of neurons had more primary dendrites and shorter axons on stiffer gels. In this work, we further characterize the mechanical response of DNA cross-linked gels under varying conditions i.e. temperature and crosslink density. It has been observed for 14-14-40 DNA gels, that the stiffness goes down from 4.9 kPa to 1.87 kPa when the temperature is increased from the room temperature to the incubator temperature (37°C), while for 20-20-40 gels, the stiffness goes down from 12.5 kPa to 9 kPa. The same gels when tested after 24 hours of swelling have yielded a stiffness which is almost 9 times less. This indicates that under physiological conditions, DNA hydrogels behave differently as compared to normal conditions. Thus characterizing the mechanical properties of the hydrogels at different conditions would give us a better understanding of the material to cell interactions.
QQ5: Dynamic Response and Pattern Formation
Session Chairs
Erik Geissler
Grant Smith
Tuesday PM, December 01, 2009
Room 208 (Hynes)
2:30 PM - **QQ5.1
Dynamics of Long-Chain Tracers Enclosed in Hydrogels.
Sebastian Seiffert 1 , Julia Gansel 1 , Wilhelm Oppermann 1
1 , Clausthal University of Technology, Clausthal-Zellerfeld Germany
Show AbstractWe investigate the dynamics of fluorescently labeled linear macromolecules that are enclosed in semidilute polymer matrixes and gels. The experiments were designed such that the transition from a semidilute solution to a permanent network could be covered. This was achieved by employing a matrix polymer, polyacrylamide, carrying pendent dimethylmaleimide groups. Stepwise irradiation of such samples in the presence of a triplet sensitizer causes successive dimerization of the maleimides leading to progressive cross-linking. The translational diffusion coefficients of the tracers were estimated by fluorescence recovery after photobleaching (FRAP), while fluorescence correlation spectroscopy (FCS) gave additional information on the tracer dynamics on a length scale shorter than the size of the tracers. Parameters varied were the concentration of matrix polymer (20–80 g/L) as well as the molar masses (200,000–1,300,000 g/mol) of the labeled probes. The experiments show that the chemical cross-linking of a semi-dilute polymer solution causes a moderate decrease of the translational diffusion of enclosed linear chains. When intramolecular dynamics are analyzed, the degree of labeling of the tracer chains has a significant impact on the particular features of subdiffusion observed on the sub-µm scale: Lower degrees of labeling amplify the visibility of intrachain dynamics. The effect of cross-linking of the matrix polymer on subdiffusion can be quite prominent, depending on the concentration range under study. Cross-linking can even result in an apparent increase of chain dynamics.
3:00 PM - **QQ5.2
Probing the Dynamics of Biomacromolecules in Polymeric Systems.
Hacene Boukari 1 , Candida Silva 2 , Ariel Michelman-Ribeiro 3 , Ralph Nossal 2 , Ferenc Horkay 2
1 Dpt of Anatomy and Structural Biology, Albert Einstein College of Medicine, Bronx, New York, United States, 2 Lab. of Integrative Medical Biophysics, National Institutes of Health, Bethesda, Maryland, United States, 3 , National Institute Standards and Technology, Gaithersburg, Maryland, United States
Show AbstractMeasurement of the transport properties of biomacromolecules moving within complex biopolymeric matrices is relevant for understanding many biological processes and for developing appropriate structures for biomedical applications related to drug delivery and tissue engineering. Major effort is being made to identify the basic biophysical and biochemical properties of the host matrix which control the behavior of dispersed molecules (e.g., drugs, proteins, DNA). We recently demonstrated how fluorescence correlation spectroscopy (FCS) can be used to extract information about the diffusion and polymer binding of various nanoparticles and biomacromolecules (~1-100 nm) in various polymeric solutions and hydrogels. We particularly focused on two model polymer systems: Poly(vinyl-alcohol) (PVA, MW=85 kDa) and Ficoll 70 (MW=70 kDa). PVA is a neutral, water-soluble, linear polymer used as a component of tissue engineering matrices. Ficoll70 is a water-soluble, highly-branched sucrose-polymer used in perfusion experiments and studies of the effects of crowding on, for example, protein stability. In this talk, we will review FCS measurements of the diffusion of various globular, linear, and branched fluorescent probes moving in non-fluorescent PVA or Ficoll70 solutions. Two basic lengthscales are relevant to the analysis of the data : the probe size and the characteristic mesh size of the host systems. Here we find that the data can be readily described by a model proposed by de Gennes et al. For small probes the decrease of the diffusion coefficient D(c) with increasing polymer concentration (c) can be readily fit with a stretched exponential exp(-Bcn), where n is related to the solvent quality. In the case of PVA solutions we find 0.73 ≤ n ≤ 0.84, showing that water is a good solvent. Moreover, there is a roughly linear relation between B and the size of the globular probes. In contrast, n=1 for the Ficoll samples, suggesting a theta-like behavior of the Ficoll solutions. Cross-linking of PVA chains to form gels can further slow down the diffusion of some small probes such as TAMRA and R6G. The more that the polymer chains are cross-linked, the slower the nanoparticles diffuse. Moreover, we observed a simple linear relation between the elastic modulus and the diffusion time. We also studied the effects of dehydration on the diffusion of probes in PVA samples, using a specially-designed optical chamber to measure in-situ changes of the diffusion of the probes while the samples were dehydrating. With our FCS setup we were able to monitor, continuously and simultaneously, changes in the concentration and diffusion of the probes in the dehydrating samples. Further, we used the systematic changes of the fluorescence in the dehydrating samples to calculate the volumetric changes of the samples with time, and related the changes in the diffusion of the probes with the volumetric changes of the host PVA system.
4:00 PM - QQ5.3
Dynamic and Reversible Switching of Micro- and Nano-scale Patterns by Smart Soft Materials: Toward Adaptable Nanoscale Architecture.
Philseok Kim 1 2 3 , Lauren Zarzar 1 2 , Xuanhe Zhao 1 , Joanna Aizenberg 1 2 3
1 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States, 2 Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, United States, 3 Wyss Institute for Biologically Inspired Engineering, Harvard University, Cambridge, Massachusetts, United States
Show AbstractThe fabrication and control of surface structures at a molecular, nanometer and micrometer length scale are of increasing interest in materials science; such research encompasses superhydrophobicity and wetting, biomimetic surface fabrication, microfluidics, tunable optics and photonic devices, controlled material deposition, drug delivery, tissue engineering, and the understanding of cell-surface interactions and biofilm formation.1-3 Recently, we have demonstrated nanoscale reversible pattern formation based on tensegrity nanostructures consisting of arrays of isolated, high-aspect-ratio rigid silicon structures (AIRS) and polyacrylamide hydrogel that exhibit switching of hydrophilicity of the surface upon humidity change.4,5 The aforementioned various applications require flexibility in the choice of materials with tunable mechanical properties, as well as chemical and biological functionalities. To achieve this goal, silicon-based AIRS can be easily replicated into a soft material by using an unconventional lithography method, in which a negative replica of the silicon AIRS is molded by polydimethylsiloxane (PDMS) and then back-filled with polymeric materials.6 Our preliminary results show that reversibly switchable soft polymeric micro/nanoscale surface patterns actuated by responsive materials such as hydrogels, electroactive polymers, and dielectric elastomers can be readily achieved. The movement and orientation of these soft actuator materials can be controlled and patterned by synthesizing them in contact with a pre-patterned, confining substrate. Such procedures enabled cooperative actuation of soft segments into complex patterns including microflorets and nanotraps. Modeling of the actuation based on a finite element method will be presented along with the experimental results to provide insights and understanding of the dynamic behavior of these materials. Fabrication of a wide variety of switchable nanoscale patterns and integrated architectures are currently under way using combinations of different geometric elements and different responsive soft materials to make these structures adaptable to various applications.1.C. Dorrer and J. Rühe, Adv. Mater. 20 159, 20082.M. T. Yang, N. J. Sniadecki, and C. S. Chen, Adv. Mater. 19 3119, 20073.X. Zhang, F. Shi, J. Niu, Y. Jiang, and Z. Wang, J. Mat. Chem. 18 621, 20084.A. Sidorenko, T. Krupenkin, A. Taylor, P. Fratzl, and J. Aizenberg, Science 315 487, 20075.A. Sidorenko, T. Krupenkin, and J. Aizenberg, J. Mat. Chem. 18 3841, 20086.B. Pokroy, A. K. Epstein, M. C. M. Persson-Gulda, and J. Aizenberg, Adv. Mater. 21 463, 2009
4:15 PM - QQ5.4
Self-regulated Folding Instability in Highly Confined Polymer Gel.
Srikanth Singamaneni 1 , Michael McConney 1 , Vladimir Tsukruk 1
1 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractFolding is an important and one of the most complicated events in organogenesis such as neurulation which involves in the folding of the notochord to from neural tube, and the folding of the brain in developed animals to increase the surface area. The mechanics involved in the folding of various biological structures during the growth in confined environment remains intriguing. We report a homogenous, soft polymer gel, much like the biological tissues, in a confined state that exhibits self-regulatory and scale-invariant folding upon swelling (growth). The system demonstrates long-range spatial self-awareness and pattern placement control under simple boundary conditions. These topological boundary conditions have a dramatic effect on the newly forming topological patterns, which indicates a reinforcing property available to mechanical stress-strain based systems. This robust demonstration of intricate pattern formation in polymer gel might serve as a model system to understand the mechanically-mediated morphogenesis in complex biological systems.
4:30 PM - QQ5.5
Active Surface Topographies in Constrained Hydrogel Films.
Ophir Ortiz 1 , Ajay Vidyasagar 2 , Ryan Toomey 2
1 Electrical Engineering, University of South Florida, Tampa, Florida, United States, 2 Chemical and Biomedical Engineering, University of South Florida, Tampa, Florida, United States
Show AbstractPeriodic wrinkles or corrugations can appear on the free surfaces of constrained hydrogel films. The constraint generates a residual compressive stress that is partially relieved through deviation of free surfaces from a planar configuration. The morphology, amplitude and wavelength of the surface instability are generally well-defined and depend on the strength of the constraint. If swelling of the hydrogel overcomes the constraint, the film can slip along its substrate, altering the resultant compressive stress and surface topography. By tuning this slippage, we aim to direct the surface features presented by thin hydrogel films. Active control of surface morphology could have important implications in several applications ranging from sensors to controlled cell attachment and detachment. In this work, we discuss a method of tuning the amount of slippage and surface topography of patterned poly(N-isopropylacrylamide) (poly-NIPAAm) films. The films were fabricated from photo-crosslinkable polymers comprising NIPAAm and photo-active methacroyloxy-benzophenone (MaBP) monomers. The patterned films were anchored to a substrate through a hydrogen-bonding self-assembled monolayer. The relationship between pattern geometry, pattern dimensions, and solvent were investigated with respect to the mechanical instability generated at the free surface. We observed that the wavelength, width, and amplitude of the instability always increased with the thickness of the polymer pattern. The morphology of the surface instability, however, was especially sensitive to thickness of the pattern, the solvent, and the spatial position with respect to the edge of polymer pattern. If the slippage of the film against the substrate is minimal, a wrinkle pattern in the form of bicusps was observed. If the compressive stress exceeded the hydrogen-bonding anchors, slippage of the film generated either a blister pattern or a honeycomb pattern.
4:45 PM - QQ5.6
r-WETS for Patterning of Polyelectrolyte Multilayers.
Nicole Zacharia 1 , Geoffrey Ozin 2
1 Mechanical Engineering, Texas A&M University, College Station, Texas, United States, 2 Chemistry, University of Toronto, Toronto, Ontario, Canada
Show AbstractPatterning structure variation and functionality (such as gradients) into soft materials and gel is essential for sophisticated applications as functional biomaterials. Reactive wet stamping (r-WETS), developed by Grzybowski and others, is a technique similar to microcontact printing, except that the stamp material is a swollen hydrogel with the ability to continuously apply aqueous chemical reagents. Although this method has previously been applied to numerous subtrates, we report using this method for the creation of patterns in and etching of polyelectrolyte multilayers (PEMs), which are essentially ionically crosslinked hydrogels. Although standard microcontact printing allows for some modification of PEMs (such as surface application of dyes or silanes), PDMS stamps cannot deliver local changes in ionic concentration or pH, which is possible with the r-WETS method using a hydrogel stamp made of a material such as agarose. PEMs composed of polyethyleneimine (PEI) and polyacrylic acid (PAA) as well as polyallylamine hydrochloride (PAH) and PAA are known to undergo phase transitions from continuous to porous morphologies when exposed to low pH solutions. We are able to locally apply changes in pH to PEM films, patterning porous regions, something not previously demonstrated in these PEM systems. This has implications for drug delivery, as the porous/nonporous PEM films are known to have different uptake and release properties. We have created a type of compartmentalized film with different release properties throughout. We are also able to control the depth of phase transformation in these films by applying the hydrogel stamp for different amounts of time. In addition to phase transformations, it has been demonstrated that pH treatment can drastically affect swelling in PEMs, and that the films have a memory for the treatment they have received. We are able to create films with patterned areas that respond differently to the stimulus of pH. With r-WETS we are also able to locally etch films using high concentrations of salt, or etch hydrogen bonded films with changes in pH. This enables the fabrication of LbL morphologies that were previously difficult to make, or required very specific materials and expensive techniques (e.g. the use of photoactive polymers and photolithography to create similar porous patterns similar to those we are able to create cheaply in the lab).
5:00 PM - QQ5.7
Poly(ethylene glycol) as a Bioactive Electron Beam Resist.
Yi Wang 1 , Emre Firlar 1 , Matthew Libera 1
1 Chemical Engineering and Materials Science, Stevens Institute of Technology, Hoboken, New Jersey, United States
Show AbstractPoly(ethylene glycol) [PEG] is well known for its biocompatible antifouling properties, and it has been used in a variety of different forms–monolayers, gels, films, etc –to help mediate the interactions of proteins and cells with synthetic surfaces. In addition to soft and/or photo- lithographic approaches to pattern PEG gels on surfaces, electron-beam methods have been exploited to flexibly create nano and micro gels both with and without chemically active groups for post-patterning functionalization. E-beam patterned gels have been used to control protein adsorption, to display biospecific proteins, and to create surfaces which control and confine various bacterial and tissue cell types. In contrast to traditional e-beam resists which typically are used to impart a pattern in a different material and are rarely present in the final device, a PEG-based resist can play a central role in a biomaterials application. Understanding how surface-patterned gels form in response to e-beam irradiation is thus of particular importance for achieving both non-specific and bio-specific interactions between patterned surfaces and various physiological systems. Here we present the results to systematically characterize the effects of incident electron dose and energy on gel formation in PEG thin films. Substrates used were glass and silicon. Glass brings the additional challenge of substrate charging during irradiation. We are thus particularly interested in the interactions of low-energy (~2 keV) electrons with both the PEG film and the underlying substrate. We used a model pattern consisting of arrays of 5x5 micron subarrays with each point in the sub-array receiving an exposure within the range of 10-18 C to 10-13 C. After development by multiple THF/water washes, the resulting patterns were assessed by optical and scanning electron microscopy as well as by dry/wet AFM imaging. We find that gel-formation is a strong function of both incident dose and incident electron energy. There is a minimum dose required for gelation beyond which the crosslink density increases and the resulting swell ratio decreases with increasing dose. Importantly, the minimum dose required to form a stable substrate-bonded gel at 10 keV incident electron energy is greater than it is at 2 keV. But, over a certain dose range, 10 keV electrons can form gels with better resolution – i.e. smaller gels – than can 2 keV electrons. We have used Monte Carlo methods to explore this effect by simulating electron trajectories and energy deposition due to both incident electrons and backscattered electrons. These simulations show that the incident energy significantly influences the proximity effect, where exposure occurs due to backscattered electrons beyond the region irradiated by the incident electrons. Proximity effects are particularly important for biointeractive applications because even a monolayer of PEG can substantially affect the interactions of proteins and cells with the surface.
5:15 PM - QQ5.8
Tunable Thermo-responsive Hydrogel Patterns at the Nanoscale for Biomedical Applications.
Hariharasudhan Chirra Dinakar 1 , J. Zach Hilt 1
1 Chemical and Materials Engg, University of Kentucky, Lexington, Kentucky, United States
Show AbstractIntelligent hydrogels have gained great interest as functional components in various biomedical micro- and nanodevices due to their unique ability to respond to environmental stimuli in a pre-programmed, intelligent manner. In particular poly(N-isopropyl acrylamide) (PNIPAAm), which has a lower critical solution temperature (LCST) of around 32oC, has been widely studied for various thermo-responsive studies. The unique property to respond due to its LCST has enabled PNIPAAm to be used widely for controlled drug delivery and other biomedical applications.In prior art, soft lithography techniques along with surface initiated polymerization methods have enabled researchers to grow polymer brushes over surfaces. specifically microcontact printing (µCP) and atom transfer radical polymerization (ATRP) has been successfully shown to amplify patterned monolayers of surface tethered initiators into responsive polymeric brushes.The majority of the prior brush studies have focused on the synthesis and the response behavior by elucidating how the brush thickness is dictated by ATRP and the change in structural conformation by the surrounding environment. However, the synthesis of crosslinked polymeric systems as spatially confined, responsive hydrogel thin films, over surfaces has been relatively unexplored, thereby providing a diverse scope for applications in the biomedical field. Herein, we report the synthesis of patterns of poly(ethylene glycol) n dimethacrylate (PEGnDMA; n=200/400) crosslinked PNIPAAm hydrogel over gold via microcontact printing (µCP) followed by ATRP. Conceptually, the advantage of a nanoscale hydrogel thin film used for response applications is addressed by the diffusion equation where the response time is directly proportional to the square of the length scale. We achieved controlled hydrogel pattern growth to the nanoscale using ATRP. We also studied the thermo-responsive behavior of ATRP grown hydrogel thin films using atomic force microscopy (AFM) measurements for direct visualization of the phase transitions of the hydrogel. By controlling the mesh size of the hydrogel matrix using different molecular weights crosslinker and/or with the amount of crosslinker employed, we controlled the hydrogel volume swelling ratio for response applications. Thermo-responsive patterns exhibited bulk effects of broad transition at the nanoscale while, the hydrogel surface showed sharp LCST transition. Property changes were also monitored using quartz crystal microbalance with dissipation (QCM-D)which provided information on the ‘instantaneous’ behavior of the thin film hydrogel and its ability to reach the thermodynamic equilibrium response state. The tunable response behavior along with the controlled growth of the hydrogel achieved via ATRP at the nanoscale hold promise as functional components in point of care diagnostic sensing and therapeutic drug delivery devices.
5:30 PM - QQ5.9
Assemblage of Neural Networks on a Microfabricated Poly-amidoamine Hydrogel: A Novel Smart Material Interfacing Living Systems.
Cristina Lenardi 1 , Gabriel Dos Reis 2 , Fabio Fenili 3 , Antonella Gianfelice 4 , Elisabetta Ranucci 3 , Paolo Ferruti 3 , Paolo Milani 3
1 C.I.Ma.I.Na. and DSMAB, Universita' di Milano, Milano Italy, 2 C.I.Ma.I.Na. and SEMM , European School of Molecular Medicine, IFOM, , Milano Italy, 3 C.I.Ma.I.Na. and Dipartimento di Chimica Organica e Industriale, Universita’ di Milano, Milano Italy, 4 C.I.Ma.I.Na. and Dipartimento di Fisica, Universita’ di Milano, Milano Italy
Show AbstractPoly(amidoamine)s (PAA) are highly hydrophilic synthetic polymers obtained by Michael-type poly-addition of amines to bisacrylamides. Crosslinked PAA in aqueous media form optically transparent hydrogels that in many cases proved to be fully biodegradable and biocompatible, warranting a definite potential for biomedical applications1. There is an increasing demand for new generation bio-hydrogels to evolve from the present substrates to highly physiologic 3D environments adapted to live complexity2. Accordingly, we present here the possibility of micro-designing and engineering versatile PAA hydrogels to enhance and control major cell processes such as adhesion, proliferation, and tissue organization. In particular, we report the development of direct-write electron beam lithography (EBL) method to pattern the surface of PAA hydrogels. The process is performed on the dry hydrogels before swelling with water. The pattern design and the dimensional aspect ratio are maintained after swelling. EBL enables to a fine control of physical and biochemical features of the patterns, down to a lateral resolution of 500 nm. The surfaces exposed to the e-beam can be coated with proteins or other biomolecules whose amount depends on the exposure dose. PC12 cells, able to differentiate into neuronal cells, are plated on microfabricated hydrogels. Cells and diffentiated neurites show a strong preference of growing on the electron beam modified areas. In particular the microfabrication allows to precisely control the guidance of neurite outgrowth from single cell through microchannels, thus reconstituting neural networks. This technique applied to PAA hydrogels has then the potential of creating low-cost biocompatible platforms accurately reproducing the physiological characteristics of cellular microenvironments or neuronal networks2,3.1 Ferruti P, Bianchi S, Ranucci E, Chiellini F, Piras AM. Biomacromolecules 2005; 6, 2229–2235.2Mitragotri S, Lahann J. Nat Mater. 2009, 8,15-23.3Laura M.Y. Yu, Nic D. Leipzig, Molly S. Shoichet. Materials Today 2008, 11, 36-43
5:45 PM - QQ5.10
Wide Angle X-Ray Scattering Studies of Guided Assembled Organoclay Electrorheological Suspensions.
Baoxiang Wang 1 , Zbigniew Rozynek 1 , Tomas S. Plivelic 2 , Jon Otto Fossum 1
1 Department of Physics, Norwegian University of Science and Technology (NTNU), Trondheim Norway, 2 MAX-lab, Lund University, Lund Sweden
Show AbstractAbstract: Electrorheological (ER) fluids, as complex fluids, composed of semiconducting dielectric particles dispersed in non-conducting liquids, are smart materials whose structure and rheological properties are quickly and reversibly changed under application of external DC/AC electric fields. This effect occurs due to particle polarization and consequent formation of a chain-like or columnar structure oriented parallel to the direction of the electric field. We have studied surfactant modified clay in silicone oil using wide angle x-ray scattering (WAXS). Organic modified clay electrorheological suspension was synthesized by the combination of ion exchange and liquid-liquid phase transfer method. Firstly, Cethyl-Trimethyl-Ammonium Bromide (CTAB) modified different clays (Na-fluorohectorite, kaolinite, laponite and montmorillonite) were synthesized by the ion exchange method. Through the formation of organoclay, the properties of clay change from hydrophilic to hydrophobic which is suitable for the transfer from the aqueous phase to the organic phase. Secondly a liquid–liquid phase transfer method was employed in order to prepare novel organic modified clay/silicone oil suspensions. The transfer process was identified by the rapid change of colour for the aqueous and organic phase. For different clay and CTAB substitution, the CTAB molucules are thought to lay either parallel to the host layers forming lateral mono- or bilayers or radiate away from the surface forming extended (paraffin-type) mono- or bimolecular arrangements or adsorp onto the surface of clay layers. The three-dimensional WAXS images from chains of CTAB modified clay under a DC external electric field exhibit marked anisotropic patterns. The dynamic formation of the chain for organoclay ER suspensions and unmodified clay ER suspensions respectively have also been investigated, which may provide new evidence for the enhancement of ER effect.
QQ6: Poster Session II
Session Chairs
Noshir Langrana
Candida Silva
Wednesday AM, December 02, 2009
Exhibit Hall D (Hynes)
9:00 PM - QQ6.1
Glucose-Responsive Hydrogel Sensors based on Single Walled Carbon Nanotube Near Infrared Fluorescence.
Hyeonseok Yoon 1 , Paul Barone 1 , Rene Ortiz-Garcia 1 , Jingqing Zhang 1 , Jin-Ho Ahn 1 , Jong Ho Kim 1 , Michael Strano 1
1 Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractThis presentation includes useful information on glucose detection using a single walled carbon nanotube (SWNT)-based optical sensor. SWNT/poly(vinyl alcohol) (PVA) hydrogels were made via the dialysis of SWNT/PVA solution, followed by the cross-linking reaction. The hydrogels were further modified to incorporate carboxyl groups and glucose binding receptors were covalently attached to PVA hydrogel matrices. Several glucose binding receptors were tested for efficacy within the PVA hydrogel system, eventually settling on apo-glucose oxidase as a means to reversibility and stably modulating the dielectric environment in response to glucose. The change in the dielectric environment affects strongly the SWNT fluorescence, which allows a rapid and reversible glucose recognition. This is the first demonstration of a glucose receptor as a selective binding site to modify the dielectric environment in this type of material. SWNT fluoresce in the near infrared where human tissue penetration is maximum and biological autofluorescence is minimal. In addition, PVA matrix is biocompatible. Therefore, these new platforms have high potentials toward tissue implantable sensors.
9:00 PM - QQ6.10
Double-layer Mediated Electromechanical Response of Biological Systems in a Liquid Environment.
Maxim Nikiforov 1 , Senli Guo 1 , Gary Thompson 2 , Vladimir Reukov 2 , Stephen Jesse 1 , Alexei Vertegel 2 , Sergei Kalinin 1
1 CNMS, ORNL, Oak Ridge, Tennessee, United States, 2 , Clemson University, Clemson, South Carolina, United States
Show AbstractDevelopment of nanoscale systems capable of not only “thinking”, i.e. calculating, but also “acting” on the nanoscale is rapidly emerging as one of the prominent directions in the nanoscience. While silicon-based electronics is now projected to scale down to sub-10 nm level, control of mechanical motion on these length scales presents an unresolved problem. Among all applications of these concepts, electrically-driven molecular machines are emerging as primary candidates. Understanding of the electromechanical coupling is vital for implementation of this concept. Despite the universally recognized role of electromechanical coupling in nanoscale systems, the mechanisms of those until now have been inaccessible to experimental studies. The reason for this dearth of understanding is threefold and stems from (a) the need to establish a good electrical contact to molecule or molecular assembly, (b) the need to detect resulting displacements and (c) the need for imaging in liquid environment.In the last decade, significant progress in understanding of electromechanical coupling on the nanoscale has been achieved with the development of Piezoresponse Force Microscopy (PFM). PFM has been broadly used to image domain structures down to ~3-10 nm resolution, study the mechanisms for domain nucleation and growth etc. Recently PFM imaging of calcified tissues in ambient environment with ~sub 10 nm resolution has been demonstrated. Similarly, PFM imaging and switching of ferroelectrics in liquids has been demonstrated. However, much lower electromechanical response of biological materials (1-3 pm/V, as compared to 10-1000 pm/V for perovskites) combined with (a) much lower sensitivity of PFM in liquids due to viscous damping and added mass effects and (b) limitation on large voltage amplitudes have made the electromechanical imaging of biological systems in liquids a challenging task.Here we report measurements of electromechanical coupling in amyloid fibrils. In this study we show that the double layer at the amyloid fibril water interface is responsible for the electromechanical response. In all studies to date, electromechanical response can not be unambiguously deconvoluted from the contrast variations due to the electrostatic interactions, the variations of elastic constants and the frequency dispersion of cantilever transfer function (i.e. topographic cross-talk). Here we use band excitation PFM (BE-PFM) technique to measure electromechanical response of amyloid fibril on mica and correlate it with the mechanical properties of the materials. The correlations between electromechanical activity and mechanical properties are also established for various biologically significant objects such as bacteriorhodopsin membranes and lambda-DNA.This research at Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.
9:00 PM - QQ6.11
Dental Composite Materials Based on Visible Light Photopolymerisation.
Astrid Gugg 1 , Beate Ganster 1 , Norbert Moszner 2 , Robert Liska 1
1 Institute of Applied Synthetic Chemistry, Vienna University of Technology, Vienna Austria, 2 , Ivoclar Vivadent AG, Schaan Liechtenstein
Show AbstractImportant kinds of currently used dental filling materials are dental composites, which are based on radiation curable methacrylates and suitable fillers. The state of the art photoinitiator system, consisting of camphorquinone (CQ) and ethyl 4-dimethylaminobenzoate suffers from low reactivity, especially in water based formulations. Bisacylphosphine oxides can be modified with hydrophilic substituents to increase their poor solubility in water. They feature higher reactivity especially in the water-based formulations, but their absorption spectra do not overlap adequately with the emission spectra of the novel LED based dental lamps (430 - 490 nm), which are adapted to the absorption spectra of CQ. Acylgermanium compounds feature a bathochromic shift of the important n-π* transition, which means that they meet the requirements for the LED lamps. It has been found that bisacylgermanes possess a high level of photoreactivity in aquous systems. Furthermore, they show an excellent photobleaching effect, sufficient storage stability in dental formulations and a high quantum yield. Mechanistic investigations such as steady state photolysis experiments, time resolved EPR spectroscopy and the CIDNP technique confirmed an α-cleavage process and the initially formed radicals and theit primary follow-up products were investigated. In visible light curing dental filling composites, acylgermanes showed excellent performance. Therefore, this new generation of initiators will hit the market within this year.
9:00 PM - QQ6.12
A Novel Route for Preparation of Transparent and Superhydrophobic Silica Aerogels.
Xuan Cheng 1 , Guoyou Wu 1 , Yuxi Yu 1 , Ying Zhang 1
1 , Xiamen University, Xiamen China
Show AbstractSilica aerogels were synthesized by a two-step surface modification using methlytrimethoxysilane (MTMS) followed by trimethylchlorosilane (TMCS) via ambient pressure drying. The transparent silica aerogels prepared through this novel route possessed the porosities of 88~93% and BET specific surface areas of 850~1100 m2/g. The HRTEM analysis revealed three-dimensional nanoporous structures with the mean particle sizes ranging from 5 to 15 nm and the distributions of pore size from 2 to 50 nm. The presence of –CH3 functional groups at the surface of silica aerogels as indicated by the FTIR spectra was further confirmed by two visible exothermic peaks at 310 oC and 450~500 oC from the DTA curves. In addition, the silica aerogels were superhydrophobic with the contact angle as high as 160 degree. Possible mechanism is discussed based on then experimental data.
9:00 PM - QQ6.13
Structural Changes in Polymer/Clay Melted Nanocomposites Induced by Electric Field: Synchrotron X-ray Studies of Structure Dynamics and Rheology.
Tomas Plivelic 2 , Suedina Silva 3 , Eduardo Azevedo 4 , Geraldo Silva 5 , Zbigniew Rozynek 1 , Baoxiang Wang 1 , Jon Fossum 1
2 Max-Lab, Lund University, Lund Sweden, 3 , Fed.Univ.Campina Grande, Campina Grande Brazil, 4 , Fed.Univ.Pernambuco, Recife Brazil, 5 , University of Brasilia, Brasilia Brazil, 1 Physics, NTNU, Trondheim Norway
Show AbstractIn this work, different nanocomposites were produced and characterized by synchrotron X-ray scattering techniques at room temperature and in the melted state under an electric field. The time evolution of the alignment of the layered silicates under different E-fields, as well as, the final degree of its orientation is discussed here.Several chemical approaches and diverse polymeric matrices/silicates have been utilized to prepare polymer-clay nanocomposites (PCN). However, one key issue still lies in how to control the microstructure of these materials and achieve a good dispersion of the clays [1]. A novel idea was recently explored by Kim et al. [2] using the effect of an external electric field to assist the penetration the polymer chains into the silicate galleries. The polymer clay nanocomposites were produced by meltextrusion using sodium bentonite clay as filler (Argel 35 from BUN-CG, Brazil) and high density polyethylene (HDPE) or polypropylene base blend (PP-blend) as matrix. Results show that the X-ray intensity data depend strongly on polymeric matrix, degree of organic clay and the E-field characteristics, such as strength or a frequency of the AC field. The original polymer/clay intercalated structures show modified orientation and organization after E-field treatments. 1.Collister J. In Polymer nanocomposites: synthesis, characterization, and modeling. Vaia RA, Krishnamoorti R, London: Oxford university press; 2002. Chapter 22.Kim DH, Park JU, Ahn KH, Lee SJ. Macromol. Rapid 2003; 24:388.
9:00 PM - QQ6.14
Fast gelling Biodegradable in situ Injectable Hydrogel based on Chitosan and Oxidized Hyaluronic Acid.
Sandhya Nair 1 2 , Remya Nair 2 , Prabaha Nair 2
1 , Indian Institute of Space Science & Technology, Thiruvananthapuram, kerala, India, 2 , Sree Chitra Institute of Biomedical Science and Technology, Thiruvananthapuram, kerala, India
Show Abstract Hydrogels derived from natural polymers are suitable materials as tissue engineering scaffolds on account of their resemblance to the extracellular matrices of cells. We, in this paper discuss the synthesis and characterization of an in situ biodegradable, non-cytotoxic injectable gel consisting of chitosan, glycerol phosphate disodium salt (GP) and oxidized hyaluronic acid/hyaluronic dialdehyde (HDA) with varying amount of oxidation. Combinations of the solutions of chitosan, GP and HDA resulted in an instantaneous gel (less than 10 seconds gelling time) irrespective of temperature with the amount of HDA required being dependent on the oxidation percent. Higher the oxidation percent of HDA lesser the volume of HDA solution required for gel formation. The hydrogel formed is stable in water at 37 °C for more than 6 weeks. To our knowledge this is a first time report of a thermally independent in situ forming gel containing unmodified chitosan, GP and hyaluronic acid. The gels were found to be non-cytotoxic to L929 cells. The mouse L929 fibroblast cells encapsulated within the gels remained 100% viable for more than 4 weeks. Gels could be freeze dried and lyophilized to form porous scaffolds that supported the adhesion of L929 cells. The L929 cells could further proliferate and secrete ECM components when seeded on the porous freeze dried scaffolds. The novel gel material is hence proposed as a scaffold and encapsulating material for growing cells for tissue engineering applications.
9:00 PM - QQ6.2
Electrochemically Tunable Photonic Gels with Bistability via Ionic Crosslinking.
Kyosung Hwang 1 , Youngjong Kang 1
1 , Hanyang Univ., Seoul Korea (the Republic of)
Show AbstractResponsive photonic crystals have been of great interest due to their potential applications in sensors, displays and optoelectronics. Recently our group developed photonic gels which are responsive to various chemical stimuli. Extending the work, we report electrically tunable photonic gels which exhibit bistability against applied electric signals. Electrically tunable photonic gels were prepared from self-assembly of polystyrene-b-poly(2-vinyl pyridine) (PS-b-P2VP) block copolymers. The position of photonic band gap (PBG) was tuned by controlling swelling/deswelling of P2VP layers via modulating the extent of ionic crosslinking with electrical signal. We anticipate that our photonic gels are applicable for full color electronic papers.
9:00 PM - QQ6.3
Autonomic Materials: Establishing the Foundation to Design Dynamically, Self-Responsive Materials
Kevin Heitfeld 1 , Richard Vaia 1
1 , Air Force Research Laboratory, Wright Patterson AFB, Ohio, United States
Show AbstractThis work investigates the quantitative correlation between experimental parameters and recent theoretical models of autonomous Belousov-Zhabotinski (BZ) self-oscillating gels. These self oscillating gels integrate an auto-catalytic (BZ) reaction into a chemically responsive hydrogel. When the catalyst is incorporated into the gel, the gel periodically expands and contracts in synchronicity with the redox reaction, transforming the local chemical concentration and chemical gradients into mechanical oscillations. Current models estimate the strength of coupling between the BZ reaction and the gel dynamics; and for simplicity, ignore the details of the polymer morphology and its spatial variation. Here in, we will quantify the impact of polymer characteristics, such as crosslink density, network architecture, and chemical structure, on the presence and period of kinematic oscillations. These insights provide a framework to validate theoretical models of how the spatial-temporal patterns depend on gel geometry, and catalyst patterning. Designing and building materials that transport information through chemical waves, convert chemical energy to mechanical motion, or respond to mechanical stimuli with a chemical wave analogous to the nervous system will provide unprecedented autonomic, responsive materials for future adaptive systems.
9:00 PM - QQ6.4
Swelling Behavior of a Chemically Crosslinked PVA Gel and its Possible Phase Transition in Response to External Stimuli.
Atsushi Suzuki 1 , Emiko Otsuka 1 , Shuhei Kudo 1 , Keiichiro Kamemaru 2 , Shintaro Usui 1 , Yumiko Hirashima 2
1 Graduate School of Environment and Information Sciences, Yokohama National University, Yokohama Japan, 2 Faculty of Education and Human Sciences, Yokohama National University, Yokohama Japan
Show AbstractPolymer gels are known to exhibit the unique properties, particularly with respect to the volume phase transition and critical behavior. Among a variety of hydrogels, polyvinyl alcohol (PVA) or PVA-based hydrogels have been extensively studied for practical applications in a variety of fields, since PVA has many advantages due to low toxicity and high biocompatibility. Although there are many reports on the stimuli-responsive PVA-based hydrogels, the volume change in the pure PVA gels has not been extensively studied.We report the swelling behavior of a chemically crosslinked PVA gel with different degrees of hydrolysis in water, several organic solvents, and their mixed solvents. The gels were dehydrated after gelation and were put into the respective solvent. The gel volume in pure water decreased with increasing the temperature, and the decrement increased with decreasing the degrees of hydrolysis. The swelling ratio depended on the organic solvent (alcohols, aceton, dioxane, and dimethyl sulfoxide (DMSO)) and its concentration. In the cases of the mixed solvents of alcohol-water, aceton-water, dioxane-water, the gel shrank continuously with increasing the concentrations of the organic solvents and reached the collapsed state in the respective pure organic solvent. In the case of DMSO, on the other hand, the gel first shrunk, swelled and finally reached the swollen state in the pure DMSO. With the use of the mixed solvent of methanol-DMSO, the swelling ratio discontinuously changed at a specific concentration. The measurements using a Fourier Transform Infrared Spectroscopy and an X-ray diffraction suggested that the cross-links due to hydrogen bonds and micro-crystals were formed during the dehydration process after gelation. The additional crosslinks were destroyed in a rich solvent, and they have an essential role to determine the swelling behavior in a poor solvent. The swelling behavior could be understood in terms of the solubility between polymers with different degrees of hydrolysis and organic solvents, and the formation and destruction of physical crosslinks in the gels.From these results, we will present the possible phase transition of pure PVA gel in response to the additional external stimulus, such as temperature, and its mechanism on the basis of the solubility between polymers and solvents.
9:00 PM - QQ6.5
Morphology, Dynamic Viscoelasticity and Concentration Dependence of Micelle-Gel Transition of polystyrene-b-poly(N-tert-butylacrylamide) Copolymer Solution.
Nitin Sharma 1 , Rajeswari Kasi 1
1 Polymer Program,Institute of Materials Science, University of Connecticut, Storrs, Connecticut, United States
Show AbstractSolution self-assembly of block copolymers has been a subject of research emphasis for last few decades due to their numerous applications such as responsive materials, adhesives, rheology modifiers, drug delivery, gel actuators and microfluidic devices. In this contribution we present here morphology and dynamic viscoelastic studies of polystyrene-b-poly(N-tert-butylacrylamide) (S278NtBAM517 and S93NtBAM252) diblock copolymers in 1-octanol as a function of copolymer concentration. These diblock copolymers in 1-octanol, a selective solvent for N-tert-butylacrylamide block, self assemble to form spherical micelle which is evidenced by transmission electron microscopy (TEM). At higher concentrations, these diblock copolymers in 1-octanol form glass-clear gels. Scaling relations for the micelles and gels have been established in the terminal frequency range. We have investigated the formation and properties of critical states during micelle-gel transition, taking copolymer concentration as the gelation variable. Rheological measurements have shown that these systems exhibit power law behavior at the gel point (G’ (ω) ~ G” (ω) ~ ωn). Critical gelation concentration (cg) was determined by frequency independence of loss tangent in the vicinity of gel point. The material parameters, Sc (gel stiffness) and n (viscoelastic exponent), which describe the critical gel properties are also obtained from power law behavior and gel point equation. Order-disorder transition temperatures (TODT) of the block copolymer solutions were determined by performing dynamic temperature ramps. Work in progress focuses on investigating the mechanism of micelle-gel transition by using techniques such as static and dynamic light scattering (SLS, DLS), small angle neutron scattering (SANS) and other rheological experiments.
9:00 PM - QQ6.6
Rheological Characterization of Polymer-clay Nanocomposite Hydrogels and the Effect of Glucose on Them.
Divya Bhatnagar 1 , Jack Lombardi 2 , Miriam Rafailovich 1
1 Materials Science and Engineering, Stony Brook University, Stony Brook, New York, United States, 2 , Estee Lauder , Melville, New York, United States
Show AbstractHydrogels, which consist of three-dimensional polymer networks and large amounts of water, have long been believed to be interesting but mechanically fragile materials limited to specific uses. Recently,important breakthroughs have been made as a result of the creation of nanocomposite hydrogels(NC gels), and most of the traditional limitations of hydrogels have been overcome. Nanocomposite gels (NC gels) consisting of organic (polymer)/inorganic(clay) networks were prepared by in-situ, free radical polymerization of N-(isopropylacrylamide) (NIPA) in the prsesence of inorganic clay in aqueous solution at high yield. The composition of the NC gels was controlled directly by altering the composition of the initial reaction mixture. The resulting NIPA-Clay NC hydrogels were uniform and transparent, irrespective of thier clay and polymer contents.The effects of the composition , such as different amounts of clay in the NC gels on the mechanical properties were investigated in detail. The hydrogels were subjected to oscillatory shear rheometry to investigate the change of storage modulus (G') with increasing content of clay. With the increasing clay content from C[clay]=1 to 25 (C[clay] is proportional to the weight of clay per unit volume of water, and a value of 1 corresponds to 0.762 g clay per 100mL of water), the modulus and the breaking stress (corresponding to the point of dip on the Linear viscoelastic range) of the gel increased proportionaly to the clay content. Results of frequency sweep suggested the formation of highly stable and mechanically tough hydrogels and Temperature sweep showed that the modulus remains constant over a range of temperature from 15°C to 150°C.In addition, the effect of glucose concentration on NC hydrogels with different clay content was also investigated by adding glucose into the polymer-clay solution. Glucose concentrations from 0.1%(w/v) to 1%(w/v) showed noticeable change in shear storage modulus G' of the NC gel with low clay content as compared to NC gel with higher clay content. Glucose and clay compete for water with the hydrogel and hence glucose has more effect on the stiffness of the hydrogel when the clay content is low.
9:00 PM - QQ6.7
Theory of the Load Carrying Ability of Polymer Hydro-Gels and its Consequences for Lubrication.
Jeffrey Sokoloff 1
1 Physics, Northeastern University, Boston, Massachusetts, United States
Show AbstractA problem of great biological and biomedical importance is that of the role of polysaccharides in producing smooth lubrication of tissues and organs and protecting cell surfaces from damage. There is good reason to believe that the fascinating tribological properties of biological systems originate from the soft and wet nature of tissues and organs. Therefore, a solvated polymer gel is an important state of matter which one must understand, in order to understand the mechanisms for low friction in biological surfaces. It is argued on the basis of solutions of the Poisson-Boltzmann equation for both low and high salt concentration (relative to the counterion concentration) and scaling arguments that two hydrogels in contact should always be separated by a thin fluid layer until the gels are under sufficient load to reduce the gels’ thicknesses down to the order of the product of the persistence length of a polymer segment separating two neighboring links and the number of polymer links across the thicknesses of the gels.
9:00 PM - QQ6.8
Highly Ordered Nematic Phases and Emerging Positional Ordering of Nanosized Clay Platelets in Water.
Elisabeth Hansen 1 , Jon Otto Fossum 1 , Henrik Hemmen 1 , Yves Meheust 4 , Mario Engelsberg 2 , Eduardo de Azevedo 3
1 Department of Physics, Norwegian University of Science and Technology, Trondheim Norway, 4 Geosciences Rennes, Université de Rennes 1, Rennes France, 2 Departamento de Física, Universidade Federal de Pernambuco, Recife Brazil, 3 Programa de Pós-Graduação em Ciência de Materiais and Departamento de Física, Universidade Federal de Pernambuco, Recife Brazil
Show AbstractWe study liquid crystalline phases in dispersions of synthetic sodium-fluorohectorite clay nanoplatelets suspended in saline water. Sedimentation in Earth’s gravitational field size-separates the initially isotropic dispersions into several strata [1], of which some display anisotropic properties attributable to the presence of liquid crystalline ordering. In the current work, size-selecting gravitational sample sedimentation procedures [1] were combined with a slow partial evaporation of the water, and we observe that these combined processes lead to a striking increase in the attainable order. This is evidenced both by high optical birefringence orders of monodomains and by measured values for the order parameter S2 reaching positive values as high as 0.91, based on intermediate-angle x-ray data and our previously developed method [2]. Strongly anisotropic small angle x-ray scattering profiles indicate that this new and highly ordered phase is a true uniaxial nematic, where the clay platelets are lying horizontally face down, and where the preferred direction of order corresponds to the average direction of the platelets’ face normals. Relatively broad and marked peaks in the small angle x-ray scattering profiles of the partially evaporated samples demonstrate the emergence of a positional ordering between platelet faces characterized by a lattice with a repetition distance in the range of a few nanometers. The positional correlations may be identified as possible pre-transitional features of a nematic-to-columnar phase transition [3]. [1] J.O Fossum, E. Gudding, D.d.M. Fonseca, Y. Meheust, E. DiMasi, T. Gog and C. Venkataraman. Observations of orientational ordering in aqueous suspensions of a nano-layered silicate. Energy 30, 873-883 (2005).[2]Y. Meheust, K.D. Knudsen and J.O. Fossum. Inferring orientation distributions in anisotropic powders of nano-layered crystallites from a single two-dimensional WAXS image. J.Appl.Cryst. 39, 661 (2006).[3]A.V. Petukhov, D. van der Beek, R.P.A. Dullens, I.P. Dolbnya, G.J. Vroege and H.N.W. Lekkerkerker. Observation of a Hexatic Columnar Liquid Crystal of Polydisperse Colloidal Disks, Phys. Rev. Lett. 95, 077801 (2005).
9:00 PM - QQ6.9
X-ray Scattering Study of a Clay/Gelatine Hybrid Electrorheological Elastomer.
Baoxiang Wang 1 , Zbigniew Rozynek 1 , Tomas S. Plivelic 2 , Jon Otto Fossum 1
1 Department of Physics, Norwegian University of Science and Technology (NTNU), , Trondheim Norway, 2 MAX-lab, Lund University, Lund Sweden
Show AbstractAbstract: The physical properties of stimuli-responsive soft materials can dramatically change when subjected to external stimuli such as temperature, pH, electric and magnetic fields. Recently, Electrorheological (ER) elastomers, cross-linked polymer gels with dispersed polarizable particles–have attracted attention. Dispersed phases filled with particles responsive to an applied electric or magnetic field include intercalated or exfoliated platelets obtained from clays, from mica, or from porous particles etc. In the present work, elastomer nanocomposites are prepared using dispersed phases filled with particles that respond to an applied magnetic or electric field. We have studied kaolinite as well as Na-fluorohectorite clay particle as they assemble into chainlike structures in gelatin hydrogel (forming an electrorheological elastomer). One of our aims is to produce a water-based, crude organic ER elastomers, a potential bionic intelligent material, which will be environmentally friendly and low-cost. The experimental techniques used in the present studies include synchrotron X-ray scattering techniques, atomic force microscopy, and optical microscopy. Wide Angle X-ray Scattering (WAXS) patterns observed are highly anisotropic and show differences compared to the patterns obtained from clay ER elastomers in zero electric field. Both the clay nanolayered particles and the gelatine matrix have preferential orientation in the direction of the electric field. Such self-organization may have practical relevance for nano-patterning, for properties of nanocomposites, for macroscopically anisotropic gels, etc.
Symposium Organizers
Ferenc Horkay National Institutes of Health
Noshir Langrana Rutgers University
Walter Richtering RWTH Aachen University
QQ7: Self-assembly and Structure Formation
Session Chairs
Hacene Boukari
Jack Douglas
Wednesday AM, December 02, 2009
Room 208 (Hynes)
9:30 AM - **QQ7.1
Tissue Microelasticity Directs Stem Cell Lineage – Better (polymeric) Materials for Better Biology.
Dennis Discher 1
1 , University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractCells make a number of key decisions by actively applying forces to their microenvironment, which that they ‘touch’. Naive mesenchymal stem cells (MSCs) from human bone marrow specify lineage and commit to phenotypes with extreme sensitivity to tissue micro-elasticity (1). AFM and other methods are being used to measure tissue micro-elasticity, if not already known, and the tissue scale relevant to tissue generation is E ~ 0.1 - 100 kPa. Soft matrices that mimic brain appear neurogenic, stiffer matrices that mimic muscle are myogenic, and comparatively rigid matrices that mimic pre-calcified bone prove osteogenic. Inhibition of the force-generating motor myosin blocks all elasticity directed lineage specification. We will compare and calibrate these results with a single, clinically-used glucocorticoid that is seen to maintain Muscle, promote Fat, and lead to Bone loss. To clarify the underlying molecular mechanics of remodeling, a “Cysteine Shotgun”, Mass Spectrometry-based proteomic method is being developed (2). Cysteine is hydrophobic and buried in proteins, and so tagging of 'cryptic' Cys reveals structural differences attributable to unfolding and/or dissociation of cellular proteins. The 'foldome' results have significant implications for understanding physical effects of the in vivo microenvironment around cells and also for use of materials in biological studies and therapeutic applications of stem cells (3). REFERENCES: (1) A. Engler, S. Sen, H.L. Sweeney, and D.E. Discher. Matrix elasticity directs stem cell lineage specification. Cell (2006). (2) C.P. Johnson, H-Y. Tang, C. Carag, D.W. Speicher, and D.E. Discher. Forced unfolding of proteins within cells. Science (2007). (3) D.E. Discher, D. Mooney, P. Zandstra. Growth factors, matrices, and forces combine and control stem cell. Science (2009).
10:00 AM - **QQ7.2
Bio-organic Armor in Nature.
Christine Ortiz 1
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractBiological exoskeletons or "natural armor" are multilayered, hierarchical structures that serve many functions, in particular protective mechanical roles such as; penetration, wear, and scratch resistance, minimization of back deflection and potential blunt trauma, damage detection and sensing, self-repair and regeneration, and, in certain cases, flexibility and mobility. In many natural armor systems, purely organic layers exists either externally to mineralized layers (e.g. the periostracum in molluscs), alternating with mineral layers internally (e.g. the chintophosphatic shell of brachiopods), or more rarely between two mineralized layers forming a stiff(hard)–compliant(ductile)-stiff(hard) sandwich design (Crysomallon squamiferum, a hydrothermal vent mollusc). These organic conchiolin layers are cross-linked, proteinaceous, and possess a diversity of morphologies (e.g. multilayered, lamellar, fibrous, hairy, etc.). They are known to act as a template for shell mineralization and possibly serve as protection from harsh corrosive and dissolutive marine environments (e.g. brackish, cold-water, low pH conditions), as well as chemical protection from boring secretions. It is hypothesized that bioorganic layers may serve a role in thermal regulation and are also mechanically advantageous. This talk will discuss the structure-property relationships of such layers and focus on their contribution to the mechanical performance of an entire biological exoskeletal structure, an effect that will be significant, for example, for thick periostraca such as those found in the antarctic snail, Torellia mirabilis and the hydrothermal vent mollusc, Alvinoconcha hessleri. The mechanical properties of selected bioorganic layers will be presented, as measured by instrumented nanoindentation and microhardness experiments. It was determined that the bioorganic armor layers of selected model systems were approximately an order of magnitude more compliant and more ductile than neighboring mineralized layers, and did not undergo fracture as mineralized layers did for the same maximum load. In addition, bioorganic layers consistently arrest cracks propagating from the neighboring mineralized layers. Studies of bioorganic coatings with such a unique combination of mechanical, thermal, and chemical properties are relevant to such industries as; semiconductor, oil and gas, textiles, defense, and construction.
11:00 AM - **QQ7.3
NMR Studies of Fluid Transport in Articulate Cartilage.
Olle Soderman 1 , George Greene 2 , Jacob Israelachvili 2 3 , Eveliina Lammentausta 4 , Daniel Topgaard 1 , Bruno Zappone 3
1 Physical Chemistry, Lund University, Lund Sweden, 2 Materials Department, University of California at Santa Barbara, Santa Barbara, California, United States, 3 Department of Chemical Engineering, University of California at Santa Barbara, Santa Barbara, California, United States, 4 Department of Clinical Sciences, Lund University, Lund Sweden
Show AbstractArticulate cartilage is a complex material, composed of collagenous fibers, and cells called chondrocytes, all of which are embedded in a firm gel-like material. The chondrocyte cells produce proteoglycans which bind to hyaluronic acid, forming large highly hydrophilic aggregates. These aggregates are entangled with each other and with the collagen fibrils forming what can best be described as a hydrogel that sits within the collagen network. As a colloidal scientist, one would describe cartilage as a complex porous material which anisotropic orientation. Clearly, the flow of fluids within the matrix plays a crucial role in determining the response to stress. Here we present an in vitro NMR pulsed field gradient study of the flow in articulate cartilage. With a specially designed pressure cell it was possible to investigate the flow of water under varying pressure. Together with experiments based on Fluorescence Recovery After Photobleaching (FRAP) our results show that the highly anisotropic structure of cartilage has a strong effect on the way fluid diffuses laterally and normally at different stages of compression.In separate experiments, the permeation of a Gd-chelate as a function of time and space is determined. The background for these experiments is to find a way to detect the first degenerative changes in cartilage by using a negatively charged contrast agent, which distributes itself inversely compared to the proteoglycans (which are negatively charged) concentrations. The loss of proteoglycans is considered the first step in osteoarthritis. By imaging the spin-lattice relaxation as function of time and position in samples from human hip cartilage the flow distribution of the contrast agent can be investigated for cartilage in different state of disease.
11:30 AM - QQ7.4
Single Molecule Structure and Properties of Human Intervertebral Disc Aggrecan.
Fei Liang 1 , Peter Roughley 6 , Alan Grodzinsky 2 3 4 , Christine Ortiz 5
1 Chemical Engineering, M.I.T., Cambridge, Massachusetts, United States, 6 , Genetics Unit, Shriner's Hospital for Children, Montreal, Quebec, Canada, 2 Biological Engineering, M.I.T., Cambridge, Massachusetts, United States, 3 Electrical Engineering and Computer Science, M.I.T., Cambridge, Massachusetts, United States, 4 Mechanical Engineering, M.I.T., Cambridge, Massachusetts, United States, 5 Materials Science and Engineering, M.I.T., Cambridge, Massachusetts, United States
Show AbstractThe intervertebral disc (IVD) is a fibrocartilaginous tissue located between the vertebrae in the spinal column. The large self-assembling proteoglycan, aggrecan, within the IVD extracellular matrix (ECM) is essential for resisting multiaxial compressive loads during physiological activity. Aggrecan degradation, resulting from abnormal and/or reduced cellular synthesis as well as proteolytic cleavage, leads to a reduction in disc biomechanical functionality and clinical pathology. Knowledge of the molecular-level structure and properties of aggrecan at different stages of degradation can help provide a fundamental mechanistic understanding of the disc degeneration process. This study utilizes high resolution tapping mode atomic force microscopy (AFM) to directly visualize the single molecule structure of aggrecan before and after removal of keratan sulfate (KS) or chondroitin sulfate (CS) glycosaminoglycans (GAGs) constituents via enzymatic treatment with Chondroitinase ABC (CSase) and Keratanase II (KSase), respectively. Aggrecan was extracted and purified from the healthy intervertebral disc of a 24-year old human and separated into two aliquots as follows: 1) a pool composed of aggrecan that were originally attached to hyaluronic acid via their G1 end domains in vivo ("aggregated"); and 2) a pool consisting of aggrecan that lack G1 domains and were not associated with hyaluronic acid in vivo and instead free in the ECM ("non-aggregated"). For the aggregated aggrecan samples, AFM height images showed that, on average, the contour length of the core protein (Lc) for the CSase treated molecules (Lc = 118 ± 65 nm, n (number of molecules) = 98) was 26% shorter than the untreated (Lc =184 ± 125 nm, n = 176).The aggregated KSase treated Lc aggrecan (166 ±106 nm, n =161) was found to be 10% shorter compared to the untreated aggregated aggrecan. Similar trends were observed for the non-aggregated pool. These data suggest that the core protein of the aggrecan is extended by the presence of the GAG chains, especially the CS-GAGs, presumably due to repulsive electrostatic double layer and steric intra- and intermolecular forces. Aggregated aggrecan was found to be relatively longer (20%) then that in the non-aggregated pool, indicating that molecules in the latter pool are likely degradation products entrapped in the extracellular matrix. Compared to human articular cartilage (Lc = 216 ± 10 nm, n = 193, obtained from 29-year old human being), the aggrecan in human IVD appeared to be more degraded. In addition, very few full-length aggrecan (possessing both G1 and G3 domains) was observed in the disc specimens, which fact further confirms that the disc aggrecan is comparatively more degraded then articular cartilage aggrecan. A higher degree of degradation in the disc aggrecan poses a larger challenge for the disc repair through clinical methodologies such as tissue engineering.
11:45 AM - QQ7.5
Molecular-level Structure of Aggregated Cartilage Proteoglycans Reconstituted in Vitro.
Hsu-Yi Lee 1 , Sangwon Byun 1 , John Sandy 2 , Christine Ortiz 1 , Alan Grodzinsky 1
1 , MIT, Cambridge, Massachusetts, United States, 2 , Rush University, Chicago, Illinois, United States
Show AbstractThe extracellular matrix of cartilage tissue consists of a self-assembled proteoglycan network of aggrecan noncovalently bound by its G1 N-terminal domain to hyaluronic acid (HA), which is further stabilized by link protein. With increasing age, disease, and injury, the size of proteoglycan aggregates decreases correspondingly with a reduction of aggrecan size, as well as the number of aggrecan molecule per aggregate, resulting from proteolytic degradation and/or reduced and abnormal biosynthesis. It is hypothesized that the degradation of the self-assembled proteoglycan aggregate structure plays a critical role in the loss of biomechanical integrity and clinical pathology of the tissue. In the present study, we utilize atomic force microscopy (AFM) to image the spatial organization and structure of cartilage proteoglycan aggregates reconstituted in vitro at extremely high resolutions, i.e. down to the single molecule (GAG) level. Aggrecan monomers (A1A1D1) or fragments (A1A1D6) were extracted and purified from bovine cartilage by CsCl extraction and GuHCl density gradient centrifugation. Two molecular weights of HA (Healon ~4MDa and SelectHA ~250KDa) were employed separately. Aggregates were formed by combining aggrecan with 0.1% (w/w) HA in DI water for 2 hrs. Reconstituted aggregate or pure aggrecan samples were fractionated on a chromatography column and the elution profile showed that the reconstituted A1A1D1-Healon sample has larger molecular weight (>15MDa) than the pure aggrecan sample (~3MDa), indicating that aggregates were formed. The high molecular weight fractions of the A1A1D1-Healon sample were pooled, washed with DI water, concentrated with microconcentrators and adsorbed on mica for tapping mode AFM imaging (spring constant ≈ 45 N/m, tip end radius<10 nm). AFM height images revealed the direct visualization of aggregates, as well as some unassembled aggrecan and HA. Star-like aggregates consisted of 5~12 aggrecan monomers were observed with their G1 domains localized in the center and the long axes radiating outwards. This unique astral shape of aggregates might be due to a collapsed HA central filament or aggrecan self-assembly through their G1 domains without HA. Individual aggrecan were observed with their G1 domains bound to HA as well. This methodology will be able to reveal the detailed structure of reconstituted aggregates composed of aggrecan or HA with variant molecular weights, as well as the structural changes of native aggregates as a function of age and disease, and thus shed light on the mechanistic origins of age and disease related cartilage degradation in mechanical properties.
12:00 PM - QQ7.6
Responsive Hydrogels from Elastomeric Gluten-Mimetic Proteins.
Scott Dick 1 , Shane Scott 1 , Fan Wan 1 , James Harden 1 2
1 Physics, University of Ottawa, Ottawa, Ontario, Canada, 2 Ottawa Institute of Systems Biology, University of Ottawa, Ottawa, Ontario, Canada
Show AbstractElastomeric proteins are ubiquitous in both animal and plant tissues, where they are responsible for the elastic response and mechanical resilience of tissues. Examples include elastin variants in animal tissues, resilin from insect cuticle, abductin from the hinge ligament of mollusks, and gluten from wheat. In addition to fundamental interest in the molecular origins of their elastic behavior, this class of proteins has great potential for use in biomaterials for drug delivery and tissue engineering applications. The structural and elastomeric properties of these proteins are thought to be controlled by a subtle balance between hydrophobic interactions and entropic effects, and in many cases their physical properties can be recapitulated by multi-block protein polymers formed from repeats of short, characteristic polypeptide motifs. We have recently developed new biomimetic multi-block protein polymers based on variants of several elastomeric gluten consensus sequences. Our proteins include majority block constituents designed to maximize their solubility in aqueous solution, minimize the formation of extended secondary structure, and to undergo a transition between compact and extended conformational states in response to changes in temperature and solution pH. In addition, the proteins have distributed tyrosine residues or short associating blocks, each of which allow for inter-molecular crosslinking to form hydrogel networks. These proteins are biologically synthesized in bacterial hosts and purified using standard molecular biology methods. In this talk, we present studies of the phase behavior and rheological properties of these protein hydrogels as a function of solution conditions (temperature and pH), and discuss the relation of these material properties to the molecular behavior (secondary & tertiary structure and phase behavior) of non-crosslinked gluten-mimetic proteins.
12:15 PM - QQ7.7
The Dynamic Alterations In Stiffness Of The Substrate Affect Cell Growth.
Frank Jiang 1 , Uday Chippada 2 , Lulu Li 3 , Bernard Yurke 4 , Rene Schloss 1 , Bonnie Firestein 5 , Noshir Langrana 2 1
1 Biomedical Engineering, Rutgers University, Piscataway, New Jersey, United States, 2 Mechanical and Aerospace Engineering, Rutgers University, Piscataway, New Jersey, United States, 3 Chemical & Biochemical Engineering, Rutgers University, Piscataway, New Jersey, United States, 4 Materials Science & Engineering and Electrical & Computer Engineering, Rutgers University, Piscataway, New Jersey, United States, 5 Cell Biology and Neuroscience, Rutgers University, Piscataway, New Jersey, United States
Show AbstractThe micro-environment that cells reside in is dynamic, and is composed of extracellular matrix (ECM) and other cells. Cells take a variety of cues, of which mechanical stresses and strains are an important subset. From an engineering viewpoint, it is beneficial to be able to modify the physical properties of the implants. All of these reasons make it desirable to have a dynamic culture system with controlled property changes. Previous work from our group showed that by supplying DNA crosslinker, changes in the mechanical stiffnesses, together with de-gelation are possible. In the current work, we set to explore the potential of DNA gel systems to examine cellular responses to such changes. At Day 0, the L929 fibroblasts were seeded on DNA gels and allowed to adhere and grow for two days. Different amount of crosslinker DNA is delivered at Day 2. Fibroblast will be exposed to the changing stiffness of the DNA gel, and were fixed at Day 4.Characterization of the DNA gels shows that increase in crosslinking level leads to rise in mechanical stiffness. Following the aforementioned protocol, cells were characterized. Firstly, the results confirmed that the exogenous DNA delivered to the cell culture did not give rise to significant modification to the cellular behavior due to potential cellular intake of these short ssDNA. Secondly, there is significant cell contraction on D50_80 gels, while interestingly, cells became more polarized. This is similar to the comparison between D50 and D50_100 groups, except that on the D50_100 gel group, cells maintain their projection area while becoming more polarized. Thirdly, delivery of 20% crosslinker DNA to the gel group of initial 80% crosslinking density resulted in a similar change as seen between D50 and D50_100, whereas there is no apparent difference in projection area while polarity of the cells strengthens. Quantification of FAK expression suggests that there may be a threshold in the cellular mechano-sensing of the substrate stiffness. Taken together, these results clearly show that cells can sense dynamics in mechanical stiffness and the response is specific to the range of the stiffness. This can facilitate design of biomaterials to engineer cell growth.
12:30 PM - QQ7.8
Spontaneous Formation of Two-dimensional Crystalline Materials from Periodic Amphiphilic Peptoid Polymers.
Ki Tae Nam 1 , Amanda Marciel 1 , Sarah Shelby 1 , Ritchie Chen 1 , Philip Choi 1 , Ryan Mesch 1 , Tammy Chu 1 , Byoung-Chul Lee 1 , Michael Connolly 1 , Christian Kiesielowski 2 , Ronald Zuckermann 1
1 Molecular Foundry, Lawrence Berkeley National laboratory, Berkeley, California, United States, 2 National Center for Electron Microscopy, Lawrence Berkeley National laboratory, Berkeley, California, United States
Show AbstractProteins are biopolymer chains which fold hierarchically into precise three-dimensional functional structures encoded by their specific, information-rich monomer sequences. Despite ongoing advances, it is still a major challenge for synthetic polymers to form atomically defined structures and reach the same level of complexity and functionality that proteins can attain. Although various strategies using macromolecules and multi-block copolymer have been developed for the structural organization, the synthetic flexibility to functionalize precise position and to incorporate delicate sequence patterns into current platforms is limited. However, the robustness of synthetic polymers and their capability to form diverse nanostructured materials over large length scales have advantages over peptide-based approaches. In spite of the intrinsic potential for hierarchical ordering, peptides engineered for assembly into nanomaterials tend to form one-dimensional nanostructures which are often twisted due to structural factors such as chirality. In an effort to extend the fundamental understanding of protein structure to non-natural systems, we have designed a class of peptoid polymers to fold like proteins and assemble into a nanostructure. Peptoids are a novel class of non-natural polymers based on a repeated oligo-N-substituted glycine, and designed to mimic peptides and proteins. Their side chains are appended to the amide nitrogen rather than to the alpha-carbon as in a peptide. Like proteins, peptoids are sequence-specific heteropolymers and have been shown to exhibit potent biological activities and fold into specific structures, but exhibit resistance to proteolysis that peptides lack.In this talk, we will talk about the sequence-specific assembly of peptoids into atomically-defined two-dimensional nanostructures. Mixing two complementary charged peptoids in aqueous solution results in the formation of giant, free-floating sheets with only 2.7nm thickness. Fluorescence microscopy analysis has shown that peptoid sheets are free-floating and robust in aqueous solution. Typical sheet edge length is between tens to hundreds of micrometers and in some cases up to two millimeters. The peptoid chains in the sheet are aligned with well defined orientation and form a robust bilayer. It is one of the thinnest known two-dimensional organic crystalline materials as determined by XRD and AFM. Direct observation of ordered peptoid chains was achieved with aberration-corrected TEAM 0.5 microscope. The sheet forming kinetics and structure analysis will be discussed in the talk. We have uncovered surprising similarities and informative differences in structural features and sequence pattern between these peptoids and beta sheet-forming polypeptides.
12:45 PM - QQ7.9
Spatially Defined Enzyme Catalysed Peptide Self Assembly.
Richard Williams 1 , Patrick Hartley 1
1 CMHT, CSIRO, Melbourne, Victoria, Australia
Show AbstractNature elegantly exploits the controlled assembly of nanoscale structures by manipulation of properties on a molecular level. Typically peptides and proteins are central to this process, with their flexible chemistries and adaptable structures. Inspired by motifs elucidated from these systems, supramolecular hydrogels of small peptides have been the focus of considerable study. These are formed from molecules which self assemble to yield higher-order assembled structures. In order to successfully mimic nature, it will be of enormous benefit to control where and how self-assembly is initiated; by ensuring that the formation of structures is limited to a defined spatial region, the potential for improved order of and control within the self assembled structures is possible. This ambitious target can be achieved by molecular biocatalsts known as enzymes. These molecules have defined activity and are typically bulky molecules with topographic properties that can be manipulated. Here we show that by confining a protease, the formation of self-assembled fibrils can be limited to the site of biocatalysis, producing building blocks on demand for the assembly process.
QQ8: Microgels and Functional Nanostructures
Session Chairs
Dennis Discher
Darrin Pochan
Wednesday PM, December 02, 2009
Room 208 (Hynes)
2:30 PM - **QQ8.1
Microgels as Versatile Building Blocks to Explore Self-Assembly and Phase Transitions: Crystals, Glasses and Squeezed States.
Peter Schurtenberger 1
1 Adolphe Merkle Institute, University of Fribourg, Marly 1 Switzerland
Show AbstractThermosresponsive charged microgel particles serve as ideal model systems in order to investigate the influence of charges, ionic strength, volume fraction and particle softness on the phase behavior and the associated structural and dynamic properties. Here we present a study of the phase behaviour of PNIPAM (N-isopropylacrylamide) micro-gel particles by static and dynamic light scattering, Diffusing wave spectroscopy (DWS) and confocal laser scanning microscopy (CLSM). The combination of DWS and CLSM not only provides us with dynamic information over a very large range of time scales, it also gives access to detailed and spatially resolved structural data such as the correlation functions through multiple particle tracking in different regions of the phase diagram. We investigate different ordered an disordered states and demonstrate the enormous variations in viscoelastic properties that can be achieved with these systems. Moreover, we demonstrate that oppositely charged microgels allow us to monitor heteroaggregation and cluster formation in a highly controlled manner.
3:00 PM - **QQ8.2
Response of Physical Gels to External Forces.
Stefan Egelhaaf 1
1 Condensed Matter Physics Laboratory, Heinrich-Heine-University, Duesseldorf Germany
Show AbstractWe have studied the behavior of colloidal gels in the quiescent state and under shear. The samples contained a hard-sphere suspension of intermediate colloid concentration in which a depletion attraction was induced by adding nonadsorbing polymer.When approaching the gelation boundary, but still in the liquid phase, the structural, dynamic and viscoelastic properties significantly changed, indicating the formation of clusters and transient networks. Upon increasing the polymer concentration beyond the gelation boundary, the rheological properties changed qualitatively, now they are consistent with the formation of colloidal gels. Our experimental results, namely the location of the gelation boundary as well as the elastic (storage) and viscous (loss) moduli, are compared to different theoretical models [1].Furthermore, progressively larger amplitude oscillatory shear was applied to the samples [2]. To within experimental uncertainties, the point at which irreversible particle rearrangements or yielding occurs was found to coincide with the observation of crystallization. These findings are summarized in a ‘shear state diagram’ in the strain amplitude-oscillation frequency-plane and are in quantitative agreement with predictions based on the probability for a particle to escape from the attractive potential of its neighbor using a Kramers approach.[1] M.Laurati, G.Petekidis, N.Koumakis, F.Cardinaux, A.B.Schofield, J.M.Brader, M.Fuchs, S.U.Egelhaaf (2009) J.Chem.Phys. 130, 134907.[2] P.A.Smith, G.Petekidis, S.U.Egelhaaf, W.C.K.Poon (2007) Phys.Rev.E 76, 041402.
4:00 PM - QQ8.3
Radiofrequency Actuation of Hydrogel Nanocomposites.
Nitin Satarkar 1 , Wenli Zhang 1 , Samantha Meenach 1 , Christopher Barton 1 , Richard Eitel 1 , Kimberly Anderson 1 , J. Zach Hilt 1
1 Chemical and Materials Engineering, University of Kentucky, Lexington, Kentucky, United States
Show AbstractThere is significant interest in development of hydrogels and hydrogel nanocomposites for a variety of biological applications including drug delivery, sensors and actuators, and hyperthermia cancer treatment. Incorporation of nanoparticulates into hydrogel matrix can result into unique material characteristics such as enhanced mechanical properties, swelling response, and capability of remote actuation. Nanocomposites containing magnetic nanoparticles/carbon nanotubes can be remotely actuated with radiofrequency electromagnetic fields (RF). In this work, development of hydrogel nanocomposites containing magnetic nanoparticles, remote actuation with RF, and some of their applications are highlighted.Magnetic hydrogel nanocomposites were synthesized by incorporation of magnetic nanoparticles into temperature responsive N-isopropylacrylamide (NIPAAm) matrix. Different nanocomposite properties such as dispersion, temperature responsive swelling, and heating response to RF of 295 kHz were characterized. Nanoparticle loadings and hydrogel composition were tailored to obtain a nanocomposite system that exhibited significant change in volume when exposed to RF, as temperatures increased above lower critical solution temperature (LCST). The nanocomposites were loaded with model drugs of varying molecular weights and RF controlled pulsatile release of imbibed drug was demonstrated. This method has a potential for a controlled in vivo drug release applications of therapeutics.Additionally, a microfluidic device was fabricated using low temperature co-fired ceramic (LTCC) processing technique. A Y-shaped microfluidic channel was obtained with the magnetic hydrogel nanocomposite as a valve in one of the channels. Application of 295 kHz RF resulted into selective heating of the nanocomposite followed by collapse and hence opening of the hydrogel valve, leading to flow of liquid. When RF was turned off, swelling of hydrogel valve stopped the flow through channel. The remote control on flow of liquid with RF was observed for multiple on-off cycles, and valve kinetics were quantified by pressure measurements. Effect of hydrogel geometry on collapse and recovery in response to RF was studied using nanocomposites of different thicknesses.In order to design the nanocomposite system for a specific application, it is very important to understand heating when subjected to RF and resultant changes in the nanocomposite properties. A variety of poly (ethylene glycol) nanocomposites were synthesized and temperatures on RF heating were recorded. A semi-empirical heat transfer model was developed to account for heat generation due to RF as well as loss to surroundings. The model successfully predicted the temperature rise as well as equilibrium temperatures for different hydrogel dimensions, swelling properties, nanoparticles loadings, and RF power. The heat transfer analysis was extended in vivo for potential hyperthermia cancer treatment applications.
4:15 PM - QQ8.4
Thermosensitive Core-Shell Microgel as a ``Nanoreactor" for Metal Nanoparticles.
Yan Lu 1 , Matthias Ballauff 1
1 Physical Chemistry I, University of Bayreuth, Bayreuth Germany
Show AbstractEnvironmentally responsive microgels have been subjects of great interest in the last two decades due to their versatile applications in fields like drug delivery, chemical separation and catalysis [1-2]. In our study, thermosensitive core-shell microgel particles have been used as the carrier system for the deposition of metal nanoparticles, in which the core consists of polystyrene (PS) whereas the shell consists of poly(N-isopropylacrylamide) (PNIPA) network crosslinked by N, N’-methylenebisacrylamide (BIS) [3]. Immersed in water the shell of these particles is swollen. Heating the suspension above 32°C leads to a volume transition within the shell, which is followed by a marked shrinking of the network. Silver, gold and palladium nanoparticles have been homogeneously embedded into thermosensitive PNIPA-networks, respectively [4-5]. We demonstrate that the catalytic activity of the microgel-metal nanocomposites can be tuned by the volume transition within the microgel of these systems by using the catalytic reduction of 4-nitrophenol as the model reaction. Moreover, following the concept of a “green chemistry”, the oxidation of alcohols to the corresponding aldehydes or ketones can be carried out in aqueous solution under aerobic conditions at room temperature by using microgel-metal nanocomposites as the catalyst [6-7]. The influence of temperature on the catalytic activity has been also investigated, which will be affected both by the volume transition of the microgel and by the change of polarity of the microgel in this case.References:[1] S. Nayak, L.A. Lyon, Angew. Chem. Int. Ed. Engl. 2005, 44, 7686.[2] R. Pelton, Adv. Colloid Interface Sci. 2000, 85, 1.[3] M. Ballauff, Y. Lu, Polymer 2007, 48, 1815.[4] Y. Lu, Y. Mei, M. Drechsler, M. Ballauff, Angew. Chem. 2006, 45, 813.[5] Y. Mei, Y. Lu, F. Polzer, M. Ballauff, M. Drechsler, Chem. Mater. 2007, 19, 10623. [6] M. Schrinner, S. Proch, Y. Mei, R. Kempe, N. Miyajima, M. Ballauff, Adv. Mater. 2008, 20, 1928.[7] Y. Lu, S. Proch, M. Schrinner, M. Drechsler, R. Kempe, M. Ballauff, J. Mater. Chem. 2009, 19, 3955.
4:30 PM - QQ8.5
pH-Responsive, Reversibly Swellable Nanotube Arrays.
Khek-Khiang Chia 1 , Michael Rubner 2 3 , Robert Cohen 1 3
1 Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractWe demonstrate a technique for synthesizing substrate-bound arrays of submicron-sized, reversibly swellable tubes by using porous templates. The sacrificial template approach allows straightforward control over the tube length, diameter and lateral arrangement of the resultant surface-bound nanotubes. A specific polyelectrolyte multilayer (PEM) system composed of poly(allylamine hydrochloride) and poly(acrylic acid) was chosen as the building block for the nanotube arrays due to its ability to undergo pH-triggered swelling-deswelling transitions. Furthermore, the morphology in the dry state can be reversibly controlled into a dense or nanoporous state by changing the degree of swelling of the PEM in the wet state. Activation of the swelling-deswelling transitions results in dramatic changes in the length and diameter of the nanotubes (approaching 520% volumetric expansion) as characterized in situ via confocal laser scanning microscopy. The pH-driven reversible swelling-deswelling and nanoporosity behavior observed with planar films and nanotubes of this PEM system is a direct consequence of the breaking and reforming of ionic crosslinks. The pH-triggered changes in the dimensions of the nanotubes are utilized as mechanical actuators for surface-bound colloidal particles.
4:45 PM - QQ8.6
Development of Bio-nanocomposite Polymer Hydrogels for Drug Delivery and Tissue Engineering Applications.
Gudrun Schmidt 1 , Chia-Jung Wu 1 , Patrick Schexnailder 1 , Akhilesh Gaharwar 1 , Qu Jin 1
1 Biomedical Engineering, Purdue University, West Lafayette, Indiana, United States
Show AbstractThe design of bio-nanocomposite hydrogel networks from polymer and nanoparticles successfully exploits and applies synergistic properties that result from the combination of chemical, physical, and biological characteristics of the individual components. Here, the compositions and the multi phase structures of bio-nanocomposite hydrogels made from silicate cross-linked PEO and polysaccharide are related to some of their properties. The physical cross-linked hydrogels are injectable and self-healing because the cross-linking is reversible under deformation. The chemically cross-linked hydrogels are elastomeric-like with high elongations. The presence of polysaccharide affects the viscoelastic properties and reinforces the hydrogel network. The polysaccharide adds advantageous biological properties to the hydrogel such as enhanced cell spreading and adhesion.
5:00 PM - QQ8.7
Shape Shifting Polymeric Nanoparticles.
Nathan Gianneschi 1
1 Chemistry & Biochemistry, University of California, San Diego, La Jolla, California, United States
Show AbstractNanoscale particles capable of undergoing dramatic changes in morphology in response to stimuli are expected to have broad utility in a range of important applications including targeted drug delivery and detection strategies. To date, development of stimuli responsive systems of this type have largely focussed on a range of triggers including pH, temperature, and the action of enzymes. Nanoparticles capable of undergoing reversible changes in morphology in a programmable manner remain relatively unexplored. The sequence selective recognition properties of DNA, and its performance as a selective substrate for enzymes make it ideal as an informational element in the synthesis of stimuli-responsive nanoscale particles. Herein, an approach to shape shifting soft polymeric materials is presented that unites the special encoding, and electrostatic properties of the DNA polymer as a construction tool and as an element in the logical manipulation of an artificial chemical system.
5:15 PM - QQ8.8
pH-Responsive Nanogels as Cargo System for Loading and Release Inorganic Nanoparticles and DNA.
Teresa Pellegrino 1
1 , Italian Institute of Technology, Genova Italy
Show AbstractStimuli responsive polymers are three dimensional polymer networks able to change their structural properties as a function of a defined applied stimulus. Acidic pH-responsive polymers exhibit a change in the ionisation state upon lowering the pH which, in turn results in a swelling of the polymer matrix. The different permeability of the polymer as a function of the pH could be exploited for the incorporation and subsequently the release of previously trapped molecules.In this work we have prepared pH-responsive polymer (P2VP nanogels) based on 2-vinylpyridine and divinybenzene with a narrow size distribution and a tunable control over the spherical diameter of the nanogel below 200 nm. P2VP nanogels exhibit swelling at pH lower than 4.5. We here exploit the use of acidic pH-responsive nanogels as cargo systems for the loading and the release of magnetic nanoparticles, short oligonucleotide sequences and for a combination of both of them, mediated by the external pH. The loading and the release processes have been investigated by dynamic light scattering (DLS), transmission electron microscopy (TEM) and confocal microscopy. We here provide a proof of concept of a novel use of pH responsive P2VP nanogels as cargo system not only for oligonucleotide sequences but also for magnetic nanoparticles and their controlled released under a pH stimulus. This kind of nanocarriers might serve as a new tool for multivalent approach to cancer therapy based on the concomitant effects of hyperthermia and of the gene therapy (or drug release).
5:30 PM - QQ8.9
A Comparative Study of Characteristics of Ferrogels Prepared using Coated and Uncoated Fe3O4 Nanoparticles.
Kamlesh Suthar 1 2 , Muralidhar Ghantasala 1 2 , Derrick Mancini 2 1
1 Mechanical Engineering, Western Michigan University, Kalamazoo, Michigan, United States, 2 Center for Nanoscale Material, Argonne National Laboratory, Chicago, Illinois, United States
Show AbstractThis paper presents the results of our analysis of ferrogels prepared using uncoated and polyvinylpyrrolidone (PVP)-coated magnetite (Fe3O4) nanoparticles. The N-isopropylacrylamide (NIPAM) based gel is prepared using N,N’-methylenebisacrylamide (BIS), ammonium persulfate (APS) and N,N,N',N'-tetramethylethylenediamine (TEMED) along with ~20 nm Fe3O4 nanoparticles. Ferrogel samples were prepared using uncoated and PVP-coated Fe3O4 nanoparticles separately at three different concentrations namely 1.25%, 2.5% and 8.75%. These samples were analyzed using ultra small angle x-ray scattering (USAXS), Transmission Electron Microscopy (TEM) and DC SQUID magnetometery. The ferrogel samples prepared with coated nanoparticles demonstrated significantly different agglomeration characteristics compared to the ferrogels prepared using uncoated particles. Samples prepared with PVP coated nanoparticles showed a predominantly single particle distribution compared to uncoated samples. USAXS data indicated that the gels prepared using uncoated nanoparticles have a large two particle agglomerations. In both cases, the volume fraction of the particles in the gel is linearly proportional to the initial particle concentrations. In both systems, the agglomerated particles appear to be spherical, with few of those indicating chain like structures. TEM examination of the samples showed the formation of flake or fiber like structures in the gels prepared using coated particles, which were not seen in the gels prepared with uncoated particles. The DC SQUID magnetometry analysis indicated that the magnetic moment of the gel samples prepared with uncoated particles is around 2 emu/gram compared to 1.2 emu/gram of those having coated particles. Details of our results and analysis are presented.
5:45 PM - QQ8.10
Optofluidic Generation of Magnetic Color/Shape Encoded Particle in Microfluidic Environment for Multiplexed Bioassay.
Hyoki Kim 1 , Jianping Ge 2 , Yadong Yin 2 , Sunghoon Kwon 1
1 , Seoul National University, Seoul Korea (the Republic of), 2 Department of Chemistry, University of California, Riverside, Riverside, California, United States
Show AbstractIn the field of analytical chemistry and bioscience, magnetic particle allows effective assay handling by removal of particles from carrier solution via application of external magnetic field. Also, encoded particles for multiplexed assays in microfluidic environments have attracted much attention due to their capability of high throughput screening for drug discovery and gene expression profiling with precise controllability of a small volume of reactants. Production of magnetic particle functionalized with effective coding scheme would greatly reduce the time required for multiplexed assay. In this report, we present a novel method of in-situ synthesis of magnetic color and shape encoded particles for multiplexed biochemical assay in microfluidic environment using single material by assembly of superparamagnetic nanocrystal clusters (CNCs) and photochemical fixation of assembled CNCs instantaneously in polymeric gel matrix. Precursor for synthesis multifunctional particle is composed of superparamagnetic CNCs, solvation liquid, and photocurable prepolymer. Under external magnetic field, CNCs form to chain-like structure along the magnetic field line in liquid phase, and diffracted color result from the periodicity of chain-like structure. Thus, diffracted color can be tuned through entire visible range simply by varying field intensity. Once desired diffracted color is obtained, corresponding chain-like ordered CNCs structures can be immobilized to the arbitrary morphology by solidifying the photocurable prepolymer through instantaneous exposure of UV whose illumination pattern can be modulated with digital micromirror device (DMD). By virtue of oxygen inhibition layer inside the PDMS channel, generated particles can move along the flow stream without being stuck to the channel walls. Using this property, color/shape coded particles can be produced under distinct levels of magnetic field intensity with multiple patterned UV light. Various color/shape encoded particles are demonstrated by this sequential color tuning and fixing process. Note that only a single material was used for the generation of various color/shape coded particles, which greatly simplifies manufacturing process.Also, synthesized color/shape coded magnetic supraparticles show magnetic property so that magnetic separation from carrier liquid can be easily applicable via application of external magnetic field. Magnetic separation of produced encoded particle is completed within seconds from carrier fluid due to the large effective magnetization of the coded particle. By combining novel material system and special instrument enables generation of limitless number of codes and greatly simplify the manufacturing process of encoded particles, and allows magnetic separation from carrier fluid effectively. We believe that the novel material system with special instrumentation open a new door to the multiplexed biochemical assay platform.
Symposium Organizers
Ferenc Horkay National Institutes of Health
Noshir Langrana Rutgers University
Walter Richtering RWTH Aachen University
QQ9: Structure and Dynamics of Gels and Biopolymer Solutions
Session Chairs
Thursday AM, December 03, 2009
Room 208 (Hynes)
10:00 AM - **QQ9.2
Shear Thinning and Rehealing Hydrogels from Peptide Folding and Self-assembly for Biomedical Materials and Hybrid Material Construction.
Darrin Pochan 1
1 Materials Science and Eng, University of Delaware, Newark, Delaware, United States
Show AbstractSelf-assembly of molecules is an attractive materials construction strategy due to its simplicity in application. By considering peptidic or charged synthetic polymer molecules in the bottom-up materials self-assembly design process, one can take advantage of inherently biomolecular attributes; intramolecular folding events, secondary structure, and electrostatic interactions; in addition to more traditional self-assembling molecular attributes such as amphiphilicty, to define hierarchical material structure and consequent properties. The local nano- and overall network structure, and resultant viscoelastic and cell-level biological properties, of hydrogels that are formed via beta-hairpin self-assembly will be presented. These peptide hydrogels are potentially excellent scaffolds for tissue repair and regeneration due to inherent cytocompatibility, porous morphology, and shear-thinning but instant recovery viscoelastic properties. The 20 amino acid parent peptide MAX1 (H2N-VKVKVKVKVDPPTKVKVKVKV-CONH2), has been shown to fold and self-assemble into a rigid hydrogel based on environmental cues such as pH, salt, and temperature including physiological conditions. The hydrogel is composed of a network of fibrils that are 3 nm wide and heavily branched and entangled with no covalent crosslinking required for gel stiffness. In addition, slight design variations of the MAX1 sequence allow for tunability of the self-assembly/hydrogelation kinetics as well as the tunability of the local peptide nanostructure and hierarchical network structure. In turn, by controlling hydrogel self-assembly kinetics, one dictates the ultimate stiffness of the resultant network and the kinetics through which gelation occurs. Importantly, once formed into a solid, self-supporting gel the network can be disrupted by the introduction of a shear stress. The system can shear thin but immediately reheal to preshear stiffness on the cessation of the shear stress. This shear thinning behavior of these physical networks makes them interesting candidates for injectable delivery in vivo where no post injection chemistry is required to set up the network. Peptide structure for folding and self-assembly, self-assembly characterization, gel shear thinning properties, cell-level biological properties and hybrid material production with inorganic functionalization of the peptides network will be discussed.
11:00 AM - **QQ9.3
Fast Dynamics of Semiflexible Chain Networks of Self Assembled Peptides.
Monica Branco 1 , Norman Wagner 1 , Florian Nettesheim 1 , Darrin Pochan 1 , Joel Schneider 1
1 Chemical Engineering, Univ. Delaware, Newark, Delaware, United States
Show AbstractWe present the first neutron spin echo (NSE) measurements of self-assembling peptide hydrogel networks to study the fibril dynamics on the nanometer and nanosecond length and time scales. MAX1 and MAX8 are synthetic beta-hairpin peptides that undergo triggered self-assembly at the nanoscale to form a physically cross-linked network of fibrils with a defined cross-section. When subjected to physiological pH and ionic strength (pH 7.4, 150 mM NaCl), the soluble peptides fold into a beta-hairpin and, subsequently, self-assemble to form a structurally rigid hydrogel stabilized by noncovalent cross-links. The sequence of MAX8 is identical to MAX1 with the exception of one single amino acid substitution that reduces the net charge on the peptide. As a result, faster folding and self-assembly kinetics are observed for MAX8 at the same peptide concentration and identical buffer conditions, and gels with a larger storage modulus are formed. NSE measurements of the peptide hydrogels demonstrate that the self-assembled peptide fibrils can be described as semiflexible chains on nanolength and time scales. Alteration of the peptide sequence affected the nanoscale dynamics of the hydrogels but not to an extent comparable to the large difference observed in the bulk viscoelasticity. Small angle neutron scattering (SANS) of the hydrogels reveals increased scattering for MAX8 at low wavevectors, an indication of a heterogeneous network with a tighter mesh size. Therefore, we conjecture that the difference in elastic modulus arises from differences in assembly kinetics that result in increased fibrillar branching and physical cross-links rather than a change in the fibril nanostructure or persistence length.
11:30 AM - QQ9.4
Influence of Architecture of Negatively Charged Multisensitive Microgels on Interaction with a Positively Charged Polycation.
Jochen Kleinen 1 , Walter Richtering 1
1 Institute of Physical Chemistry, RWTH Aachen University, Aachen Germany
Show AbstractMicrogels are internally crosslinked spherical particles of colloidal dimensions and are used for different applications. PNiPAM (Poly-N-Isopropylacrylamide) has a volume phase transition temperature (VPTT) of 32°C in water. Besides temperature also the pH can be used to trigger the properties of microgels if pH-dependent monomers are incorporated in the microgel. Incorporation of charges into a microgel can be realized by copolymerisation of neutral and charged monomers. Core-shell microgels can be obtained by stepwise polymerisation of first the core and in a second step the shell. This leads to microgels where charged monomers are either incorporated in the core or in the shell. The influence of different architectures can be monitored by pH-dependent size and zeta-potential measurements. Charged microgels interact with oppositely charged surfactants and small drug molecules as well as with native polyelectrolytes (e.g. DNA and proteins). This loading process leads to stimuli-sensitive drug delivery system or to carriers for enzymes. The microgel properties can be modified by loading of synthetic polyelectrolytes (as e.g. polystyrenesulfonate and polyallylamine).Two different batches of the polycation PDADMAC (polydiallyldimethylammonium chloride) were used to study the interacting amount of polycation. The batches differ in the molecular weight (low and high MW). The interacting amount of PDADMAC was normalised to the amount of MAA (negative charges) of the investigated microgel, allowing a comparison of different microgels. We could show that the amount of interacting PDADMAC depends both on the architecture of the microgel and on the MW of the interacting PDADMAC. The size and the zeta-potential of the resulting microgel-PDADMAC complexes depend strongly on the combination of the different microgels and the two PDADMAC batches.
11:45 AM - QQ9.5
Structure and Dynamics of Self-Assembled Thermoresponsive Polymer Gels Based on Poly(N-isopropylacrylamide) of Different Architecture.
Christine Papadakis 1 , Joseph Adelsberger 1 , Amit Kulkarni 1 , Abhinav Jain 1 , Andreas Meier-Koll 1 , Weinan Wang 1 , Achille Bivigou Koumba 2 , Andre Laschewsky 2 , Thomas Hellweg 3 , Peter Mueller-Buschbaum 1
1 Physikdepartment E13, Technische Universität München, Garching Germany, 2 Institut für Chemie, Universität Potsdam, Golm Germany, 3 Physikalische Chemie I, Universität Bayreuth, Bayreuth Germany
Show AbstractThermoresponsive polymer gels display strong changes in volume when heated above the lower critical solution temperature (LCST). They are thus attractive candidates as sensors or actuators or for controlled ultrafiltration [1]. We investigate thermoresponsive polymer gels based on poly(N-isopropyl-acrylamide) (PNIPAM) which has an LCST of 32°C. The focus is on the influence of polymer architecture on the morphology and on the local dynamics of the thermoresponsive polymer block. PNIPAM homopolymers as well as diblock and triblock copolymers with a longer PNIPAM block and hydrophobic polystyrene (PS) end blocks are investigated [2]. In aqueous solution, the homopolymers form a transient gel, whereas the diblock copolymers form a jammed solution of core-shell micelles and the triblock copolymers physically cross-linked micellar networks. The latter constitute an attractive alternative to chemically crosslinked networks.Di- and triblock copolymers were synthesized using RAFT [3]. We have carried out structural studies using small-angle neutron scattering (SANS) at KWS 2, JCNS, FRM-II, Garching, Germany, and small-angle X-ray scattering (SAXS) at beamline A2, HASYLAB at DESY, Hamburg, Germany. The collapse transition was characterized using dynamic light scattering. The local dynamics of the PNIPAM block was investigated using neutron spin echo spectroscopy at J-NSE, JCNS, FRM-II. PNIPAM homopolymers form homogeneous solutions below the LCST, which are described by an Ornstein-Zernike structure factor, whereas above the LCST, polymer-rich and water-rich domains are created, which grow with time. In micellar solutions formed by the P(S-b-NIPAM) diblock copolymers, the micellar shell collapses at the LCST. The collapse transition is rather broad. 10-15 K above the LCST, the collapsed micelles form large aggregates. The P(S-b-NIPAM-b-S) triblock copolymers form micellar solutions as well, however, the collapse transition is very sharp with immediate formation of large aggregates. Below the LCST, the averaged local diffusion coefficient of the PNIPAM block is temperature-independent and decreases with increasing polymer concentration. Above the LCST, the diffusion coefficient of segmental dynamics is higher than below the LCST and independent of concentration. The combination of different scattering methods thus allows a detailed characterization of self-assembled thermoresponsive polymer networks. 1.A. Nykänen, J. Ruokolainen, et al., Macromolecules 2007, 40, 5827.2.K. Troll, C. M. Papadakis, et al., Colloid Polym. Sci. 2008, 286, 1079. W. Wang, C.M. Papadakis, P. Müller-Buschbaum, et al. Macromolecules 2008, 41, 3209. W. Wang, C.M. Papadakis, P. Müller-Buschbaum, et al., Macromol. Rapid Commun. 2009, 30, 114. A. Jain, C. M. Papadakis et al., Macromol. Symp., submitted.3.A.M. Bivigou Koumba, C.M. Papadakis, et al., Macromol. Chem. Phys. 2009, 210, 565.
12:00 PM - QQ9.6
Reversibly Cross-linking Gels as Healing Agents for Epoxy-amine Thermosets.
Amy Peterson 1 , Giuseppe Palmese 1 , Robert Jensen 2
1 Chemical & Biological Engineering, Drexel University, Philadelphia, Pennsylvania, United States, 2 , U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland, United States
Show AbstractMaterials that can repair cracks and recover from mechanical failure are desirable. Because remendable materials both repair and prevent the propagation of cracks, they offer the potential for increased durability, safety, and cost efficiency for many applications. We report the development of a traditional epoxy-amine thermoset made to heal with the addition of a reversibly cross-linked gel healing agent based on the Diels-Alder (DA) reaction of furan and maleimide. Reversibly bonded networks using this chemistry have been previously reported; however, none has been used as a secondary healing phase.The thermoset is healed by a reversibly cross-linked gel that exists as a secondary particulate phase within the matrix. This “composite” approach allows for crack healing while maintaining the desirable physical and mechanical properties of the base thermoset. Reversible bonding of the healing agent results in a material that exhibits the mobility of a linear polymer at increased temperatures and the mechanical properties of a crosslinked polymer at ambient conditions as well as the ability to heal multiple times. The healing agent for this system is a thermoreversible gel in which the cross-links are based on the DA reaction between maleimide groups on a bismaleimide and pendant furans on a linear polymer. At room temperature the cross-linked network is a DMF-swollen gel. However, after approximately 20 minutes at 90°C a sufficient number of cross-links are reversed so that the system liquefies and forms a polymer-bismaleimide solution in DMF. Upon cooling to ambient temperature a gel was observed to form after approximately 13 hours.Direct application of the reversibly cross-linking network to a crack surface in an epoxy-amine thermoset resulted in recovery of 37% of the initial epoxy-amine network’s strength. Composites in which the reversibly crosslinking gel was incorporated as a secondary particulate phase recovered 21% of the initial composite strength after the first healing cycle, with healing possible up to five times.Although the Diels-Alder reaction was discovered almost 70 years ago, there has not been an exhaustive study of reaction kinetics and thermodynamic parameters for furan and maleimide. We report reaction kinetic values and equilibrium concentration values for a number of model compound systems. Future work will include reactive polymer systems and relating mechanical properties to the Diels-Alder reaction. Such studies are necessary to optimize the healing ability of Diels-Alder-based systems.
12:15 PM - QQ9.7
Reversible Buckling of Patterned Micro-Scale poly(N-isopropylacrylamide) Structures Due to Surface Confinement.
Samuel DuPont 1 2 , Peter Stroot 1 , Ryan Toomey 2
1 Civil and Environmental Engineering, University of South Florida, Tampa, Florida, United States, 2 Chemical and Biomedical Engineering, University of South Florida, Tampa, Florida, United States
Show AbstractComplex polymer gels with unique and tunable environmental responses have led to extensive exploration with respect to feasible applications. Surface-confined polymer materials may have far-reaching prospects in the fields of biomaterials, bio-MEMS, and scaffold-aided cell culture, especially in laboratory-based stem cell differentiation. In light of this, it is important to understand how surface confinement affects environmental responsive gels as surface confinement is the dominant mode by which these materials are present on a device, implant, or culture plate. Herein we investigate the swelling behavior of temperature responsive poly(N-isopropylacrylamide) (PNIPAAm) micostructures confined to a rigid surface by covalent linkages. We have observed unique surface instabilities that arise upon swelling of surface confined structures and remain after the gel has equilibrated with the aqueous environment. Due to the reversible nature of PNIPAAm swelling, these instabilities are reversible in nature and can be “turned” on or off with small changes in temperature. Lateral swelling, peripheral undulation, and bulk structural buckling are the three key modes of swelling instability that have been identified and are believed to be the result of a unified phenomenon. We have also shown how internal orientation of the polymer gel changes with swelling and have concluded that the formation of instabilities is a phenomenon which evolves from the gel/water interface. By use of confocal microscopy, we have made detailed three-dimensional measurements of some of the key aspects of each type of instability and have isolated some geometry dependant variables which dictate occurrence and scale of the instability. Structure height, aspect ratio, and gel chemistry appear to be key variables which affect the emergence of various confinement-induced swelling instabilities and can be presumably used to guide design strategies aimed at the formation of complex three-dimensional structures. The formation of complex geometries from simple two-dimensional patterning strategies opens an entire gamut of design possibilities and may be capable of addressing some recent problems facing current limitations in micro/nano-scale device design.
12:30 PM - QQ9.8
Responsive and Self-healable Hydrogels for Drug Delivery.
Xuanhe Zhao 1 , Nathaniel Huebsch 1 , David Mooney 1 , Zhigang Suo 1
1 School of Engineering and Applied Science, Harvard University, Cambridge, Massachusetts, United States
Show AbstractHydrogels that display a physicochemical response to stimuli are widely explored as drug-delivery systems. In order to release large biomolecules(e.g. proteins and DNAs), the hydrogels usually degrade or rupture. This irreversible approach has drawbacks and limitations. For example, once a hydrogel begin to degrade, it becomes difficult to control the degradation rate and thus the drug-release rate. Some drugs may also bind with degraded polymers and lose their bioactivity. In addition, the degradation and rupture of hydrogels may reduce their integrity and stiffness. This will sacrifice their function as mechanical support for tissue growth.It is common that minor injuries in human tissues may self-heal without medical attention. Inspired by this behavior, we propose to use a self-healable hydrogel for controlled drug delivery. The hydrogel is based on biocompatible polymers that can be reversibly crosslinked by divalent ions (e.g. Ca2+, Mg2+). Drugs are encapsulated in the hydrogel, which is then delivered to an in-vivo locus including the site of a wound, trauma or disease. Then, we irradiate the hydrogel with therapeutic ultrasound to cause cavities of micrometer-size in it. The cavities may connect and enhance the release of drugs by convection. Meanwhile, physiological fluid containing divalent ions transports into the cavities. The divalent ions re-crosslink the polymers in the cavities, and dynamically re-heal the hydrogel. Once the ultrasound is off, the enhanced release of drugs stops. The hydrogel eventually degrades after drug delivery. In this way, we construct an on-demand and reversible drug delivery system based on the ultrasound-responsive and self-healable hydrogel.
12:45 PM - QQ9.9
Anomalous Phase Behavior of Hydrogen Bonded Polymer-Surfactant Complexes.
Manesh Gopinadhan 1 , Evan Beach 2 , Paul Anastas 2 , Chinedum Osuji 2
1 Department of Chemical Engineering, Yale University, New Haven, Connecticut, United States, 2 Department of Chemistry, Yale University, New Haven, Connecticut, United States
Show AbstractSpecific interactions between small molecules and polymer chains bearing complementary binding sites can be used to engineer supramolecular complexes which display liquid crystalline order. In particular, hydrogen-bonding interactions offer a flexible platform for the self-assembly of graft copolymer-like structures by reversible association. The surfactant is designed such that specific environmental stimuli can induce changes in the polymer chain-level structure. In a system using an imidazole-based surfactant and poly(styrene-b-methacrylic acid), this strategy has led to the development of a thermally-switchable light gate. The imidazole/triazole-based surfactants contain a rigid, mesogenic moiety that promotes formation of a liquid crystalline phase depending on temperature. The same principle could be used to develop polymers that change morphology depending on electrical current, magnetic field, pressure, fluid flow, ionic strength, or chemical environment. In this work, we report self-assembly of liquid crystalline materials in the melt state, via hydrogen bonding between an imidazole/triazole-terminated mesogenic species and the carboxylic acid groups of poly(acrylic acid), in a side chain motif. These systems show mesophase formation even for 0.07 molar ratio of mesogen to acrylic acid repeat units and they display unusual thermal stability below 0.2 molar ratio. Anomalous swelling behavior has been found for these systems in contrast to similar hydrogen bonded systems. The binding equilibria can be manipulated via temperature, making these materials an interesting class of stimuli-responsive materials both in solution and in the melt state.
QQ10: Biointeractive Gels and Tissue Engineering
Session Chairs
Olle Soderman
Norman Wagner
Thursday PM, December 03, 2009
Room 208 (Hynes)
2:30 PM - **QQ10.1
Anisotropic Collagen Materials for Contact Cell Guidance.
Gerald Fuller 1
1 Gerald Fuller, Gerald Fuller, Stanford, California, United States
Show AbstractThis research on collagen explores the phenomena of "contact guidance" where oriented substrates are known to produced anisotropy and alignment of cells onto which they are attached. This can be important to the regeneration of tissues such as nerves and muscle. Several strategies are being explored. A method has been devised to spread collagen in molecular form onto the surface of water. This step allows for the control of the kinetics of fiber formation. Subsequent interfacial flows and Langmuir-Blodgett deposition are used to arrive at highly oriented, thin films of oriented collagen fibers. A second method takes advantage of the cholesteric liquid crystalline response of collagen when in highly concentrated solution. Manipulating the cholesteric banding structure of collagen also leads to anisotropic films and remarkable contact guidance. Finally, a method is described wherein liquid crystalline collagen materials can be extruded into a variety of shapes, including thin tubes, which can be fashioned into nerve guides and other conduits.
3:00 PM - **QQ10.2
Development of Freestanding Agarose Membranes within Microfluidic Channels.
Jeffrey Zahn 1 , Mercedes Morales 1
1 , Rutgers University, Piscataway, New Jersey, United States
Show AbstractAgarose gels have long been used to separate linear DNA fragments via gel electrophoresis. Agarose is an ideal material for creating semipermeable membranes due to its excellent biocompatibility, ability to tailor transport properties through changing agarose concentration and rapid transition from fluid to gel phases by changing temperature with the agarose gelling at ~60 C. In this work, compartmentalized microfluidic devices separated by free standing agarose gel membranes are fabricated. The gel membranes are fabricated using a three inlet device converging to a single daughter channel with the agarose solution in the center channel and two side supporting sheathing flows consisting of water. The membrane properties are tailored by the agarose solid percentage while the thickness is modulated by the relative flow rates between the water and agarose inlet channels. The flow is established at high temperature (>80 C) and the gel is set by reducing the temperature and fluid flow rates to gel the agarose membrane. These devices may be used for two compartment molecular exchange, or DNA fragment separation and recovery via electrophoresis across the membrane material.
4:00 PM - QQ10.3
Photodegradable Hydrogels to Investigate the Effect of Network Structure on Encapsulated Cell Function.
Mark Tibbitt 1 , April Kloxin 1 2 , Kiran Dyamenahalli 3 , Kristi Anseth 1 2
1 Chemical and Biological Engineering, University of Colorado, Boulder, Colorado, United States, 2 Howard Hughes Medical Institute, University of Colorado, Boulder, Colorado, United States, 3 Medical Scientist Training Program, University of Colorado, Denver, Colorado, United States
Show AbstractCell-extracellular matrix (ECM) interactions have a profound influence on cell morphology and cytoskeletal organization, which ultimately direct critical cell functions such as proliferation, differentiation, and migration. Cells are routinely seeded on gel surfaces as a means to understand how defined interactions with the ECM direct cell processes; however, these 2D culture systems confine cells to an unnatural planar morphology and polarize their interactions with the material substrate and neighboring cells. Further, most of the hydrogel systems used to date are static in nature and fail to capture the dynamic properties of the native ECM. Thus, a hydrogel that allows spatial and temporal modulation of extracellular mechanics and structure in three-dimensions is needed. Here, we present a photodegradable poly(ethylene glycol) (PEG) based hydrogel whose 3D structure can be controlled predictably in both space and time via light exposure, which we exploit to investigate the effect of ECM structure on cell morphology and cytoskeletal organization in 3D. Specifically, ethyl 4-(4-(1-hydroxyethyl)-2-methoxy-5-nitrophenoxy)butanoic acid (photodegradable group, PD) was incorporated into a PEG-based macromer to form a photocleavable crosslinker (PD-b-PEG-b-PD). Copolymer gels of PD-b-PEG-b-PD diacrylate (PEGdiPDA, Mn ~4070 g/mol) and PEG monoacrylate (PEGA, Mn ~400 g/mol) were synthesized under redox initiated free-radical chain polymerization (15 wt% in water) in the presence of cells. Upon exposure to cytocompatible single-photon (365nm and 405nm) or two-photon (740nm) irradiation the PD group cleaves, releasing PEG from the network backbone. In this manner, single-photon and two-photon irradiation can be used to modulate, predictably and in real time, the local polymer density that a cell experiences as well as pattern defined structures within the gel. Human mesenchymal stem cells (hMSCs) were encapsulated in these photodegradable PEGdiPDA-co-PEGA gels. Prior to encapsulation, MSCs were transfected with a GFP-actin plasmid (BD Biosciences) to allow the imaging of dynamic changes in cell morphology and cytoskeletal organization in real-time with confocal microscopy. Post-encapsulation, a gradient in the crosslinking density was introduced using flood irradiation (365nm at 10 mW/cm2) to investigate the effect of crosslinking density on cell morphology. Decreased polymer density led to an increase in cell spreading and average cell area as compared to non-degraded gels. To precisely direct cell morphology and cytoskeletal organization, two-photon excitation (740nm, Zeiss LSM 710) was used to erode the gel selectively in the pericellular region of individual MSCs. Ultimately, this platform will allow for more sophisticated studies of the effect of spatially and temporally defined cell-ECM interactions on cell function as it pertains to cell physiology within the 3D niche.
4:15 PM - QQ10.4
Fiber-reinforced Hydogels for Soft Tissue Replacement.
Animesh Agrawal 1 , Nandula Wanasekara 1 , Vijaya Chalivendra 1 , Paul Calvert 1
1 , University of Massachusetts Dartmouth, North Dartmouth, Massachusetts, United States
Show AbstractSynthetic hydrogels have poor mechanical properties that limit their use in load bearing applications. In contrast, biological hydrogels are tough and strong due to reinforcement with nano to micron size fibers. Our work is focused on designing a new class of hydrogel assemblies based on fiber reinforced hydrogel composites. Analogous to spinning a spider web, a pultrusion system was developed to spin micron-diameter polymer fibers from solution in order to build predefined three dimensional patterned fiber-reinforced hydrogel structures. The gel chemistry is based on epoxy-amine crosslinking. Various epoxy-amine gels were formulated with swellabilitiy ranging 0% to 1000% in water. The fibrous reinforcement affects the swelling and mechanical properties of the gel. For mechanical characterization, nano-indentation and flat punch probe indentation techniques were performed. The effect of fiber density and extent of swelling on the reinforcement of epoxy matrix is investigated. Preliminary results show that Young’s modulus value of gel changes locally with reinforcement. Young’s modulus values gradually increase from the epoxy region to fiber region due to local constrain provided by the fibrous structure.
4:30 PM - QQ10.5
Reconstitution of Actin Cytoskeleton in Artificial Cells.
Tianzhi Luo 1 , Douglas Robinson 1 2
1 Cell Biology, Johns Hopkins School of Medicine, Baltimore, Maryland, United States, 2 Chemical and Biomolecular Engineering, Johns Hopkins Whiting School of Engineering, Baltimore, Maryland, United States
Show AbstractThe actin cytoskeleton is composed of actin filaments, myosin II motors, and various actin cross-linking proteins (ACLP), plays a major role in regulating cell shape changes and cellular force propagation, and participates in many biological signaling pathways. In the past decades, many in vitro experiments have been performed to understand the mechanical properties of actin network that consists of actin filaments, actin-based motors and ACLPs. However, in real cells, actin filaments are anchored to the plasma membrane, forming the actin cortex which is linked to the cytoplasmic actin network, and in vitro studies of how the actin network responds to mechanical force in a confined cell shape environment are still lacking. We used purified proteins (including G-actin, myosin II motors and several kinds of ACLPs) to form actin cytoskeleton inside lipid bilayer vesicles in which the actin network is anchored to the inner membrane through natural protein-lipid interactions. Micropipette measurements were performed to measure the mechanical properties of these artificial cells at different protein concentrations and lipid compositions. The microstructures and dynamics of the network were revealed by monitoring fluorophore-tagged proteins with fluorescence microscopy. It was found that membrane-cytoskeleton stiffness and membrane undulation are highly dependent on the anchoring protein concentration, actin filament length, and the inner membrane lipid composition.
4:45 PM - QQ10.6
A Peptide Hydrogel Scaffold Functionalized with Hydroxyapatite-Binding Peptide for Hard Tissue Engineering.
Mustafa Gungormus 1 , Monica Branco 2 , Hanson Fong 1 , Candan Tamerler 1 , Joel Schneider 2 , Mehmet Sarikaya 1
1 Department of Materials Science and Engineering, University of Washington, Seattle, Washington, United States, 2 Chemistry and Biochemistry, University of Delaware, Newark, Delaware, United States
Show AbstractA major ongoing challenge in the field of reconstructive and regenerative medicine is successful repair or replacement of hard tissue, which has been lost due to disease, congenital defects or trauma. To overcome the inadequacies of current tissue repair and reconstruction strategies, tissue engineering approaches have been developed and expanded with focus on utilizing synthetic tissue scaffolds to replace or regenerate the lost tissue. We have developed an in situ forming, self assembling peptide hydrogel that possesses a genetically engineered hydroxyapatite binding peptide (HABP) sequence with a high, inherent propensity for calcium phosphate mineralization. In vitro solution mineralization studies demonstrated that the peptide hydrogels containing the HABPs retain the morphogenetic capability of the HABP resulting in a hydrogel scaffold with an inherent ability to regulate the HA mineral deposited on it. The mineral deposited on the native hydrogel was poorly crystalline while elongated crystalline biological apatite was deposited on the HABP containing hydrogel. In vitro cellular mineralization studies using cementoblast (OCCM-30) cells showed that the resulting gel is biocompatible and can support cellular growth and mineral deposition. This study demonstrates that inorganic binding peptides conjugated to self assembling peptide hydrogels participate in the biomineralization process in a fashion resembling an extra cellular matrix (ECM). The ability to engineer such three-dimensional assemblies with spatial control and programmed functions may find substantial use in tissue engineering applications for successful restoration and regeneration of the hard tissues. The research supported by NSF-MRSEC and -IRES programs at the University of Washington.
5:00 PM - QQ10.7
Environmentally Responsive Hydrogels for Cell Culture Applications.
Smruti Patil 1 , Pulkit Chaudhury 2 , Lisa Clarizia 3 , Peter Gaines 2 , Melisenda McDonald 3 , Emmanuelle Reynaud 4 , Daniel Schmidt 5
1 Biomedical Engineering and Biotechnology, University of Massachusetts Lowell, Lowell, Massachusetts, United States, 2 Biological Sciences, University of Massachusetts Lowell, Lowell, Massachusetts, United States, 3 Chemistry, University of Massachusetts Lowell, Lowell, Massachusetts, United States, 4 Mechanical Engineering, University of Massachusetts Lowell, Lowell, Massachusetts, United States, 5 Plastics Engineering, University of Massachusetts Lowell, Lowell, Massachusetts, United States
Show AbstractAs interest in stem cells and biopharmaceuticals grows, cell culture is becoming progressively more important both in academia and industry. A major obstacle to cell culture at any scale is the stress imposed by decreases in nutrient concentration and accumulation of acidic waste in media as a result of cellular catabolism. Frequent media changes provide a simple solution to this problem, but this imposes enormous costs (labor and media/culture-ware expenses) and increased risks of contamination. Automatic media exchange systems can help alleviate these stresses as well, but localized fluctuations in nutrient concentrations and pH levels can still be deleterious to cell growth and productivity, and the complication and expense of these systems is prohibitive for a wide range of critical lab-scale research.With this problem in mind, we have applied organic sol-gel chemistry to the production of environmentally responsive hydrogels capable of stabilizing pH while releasing glucose in both a simple aqueous setting and in mammalian cell culture. Materials formulations have been developed based on the formation of aliphatic triisocyanate-based networks in solution, using a variety of minimally toxic hydrophilic prepolymers based on poly(ethylene oxide) (PEO), poly(propylene oxide) (PPO), and poly(ethylene imine) (PEI) segments. Depending on the exact prepolymers selected, the materials formed are either polyurea or polyurethane networks. Polyurea network formation is rapid (seconds to hours) and requires no catalyst. Polyurethane network formation is somewhat slower (hours) but has been demonstrated successfully as well using catalysts specifically selected for low toxicity / biocompatibility.Network formation in solution results in a solvogel that is thoroughly dried to remove all solvent. The resultant xerogels are dense, tough, non-porous bodies that are then characterized via pH-dependent swelling and extractables measurements. Glucose loading is accomplished by immersing the dried xerogels in an aqueous glucose solution that causes swelling and glucose absorption. Simultaneously, this process removes any extractables, helping to ensure the biocompatibility of the final materials. An additional advantage of this approach is that glucose loading times and concentrations (as well as the addition of a subsequent glucose leaching step) may be altered to tune the glucose release profiles of the hydrogels thus formed.Following glucose loading, we report glucose release data as a function of formulation and conditions, as well as the ability of these materials to maintain appropriate pH and glucose levels in mammalian cell culture using the SA-13 human-human hybridoma cell line. In particular, we observe that our optimized formulation maintains a pH of 7.05 ± 0.02 in this particular system, and can substantially extend the period over which no addition of glucose is required with no evidence of cytotoxicity or retardation of cell growth.
5:15 PM - QQ10.8
Harnessing a Protein Conformation Change to Control Growth Factor Release from Dynamic Hydrogels.
William King 1 , William Murphy 1 2 3
1 Biomedical Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States, 2 Materials Science, University of Wisconsin-Madison, Madison, Wisconsin, United States, 3 Pharmacology, University of Wisconsin-Madison, Madison, Wisconsin, United States
Show AbstractDynamic hydrogels have been a central research area for controlled growth factor delivery devices. We have recently formed and characterized a novel class of hydrogels in which a protein’s nanometer-scale conformation change is translated to a macroscopic hydrogel volume change. Specifically, the protein calmodulin was combined with acrylate-terminated polymer chains to form conjugates, which were photocrosslinked to form hydrogels. Calmodulin’s ligand-induced conformation change resulted in reversible hydrogel volume changes of ~0-80%. The magnitude of the ligand-induced hydrogel volume change was tuned by varying the initial calmodulin concentration in the hydrogel. Based on these findings, we hypothesized that the ligand-induced hydrogel volume decrease would lead to release of absorbed vascular endothelial growth factor (VEGF). In these materials, the mass of VEGF absorbed into the hydrogels was controlled by the calmodulin concentration, which can be attributed to electrostatic calmodulin-VEGF interactions. Significantly, the hydrogels released VEGF at a faster rate when exposed to a specific ligand than when they were maintained in ligand-free buffer. The total mass of VEGF released from the hydrogels was regulated by varying the protein concentration in the hydrogel and the magnitude of the ligand-induced volume change. Also, VEGF release could be “triggered” by exposing the hydrogels to a calmodulin-binding ligand at specified times. Taken together, these results indicate that a protein conformational change may provide a broadly applicable mechanism to control growth factor release from dynamic hydrogels.
5:30 PM - QQ10.9
Internal Composition and Mechanical Properties of Exponentially Growing Polyelectrolyte multilayer Films: Influence on Cell Adhesion.
Thomas Boudou 1 , Thomas Crouzier 2 , Claire Nicolas 1 , Catherine Picart 1 2
1 , CNRS-UMR 5628, LMGP, Grenoble France, 2 , CNRS-UMR 5235, Montpellier France
Show AbstractIt is now well established that most of cellular processes strongly depend on the reciprocal interactions of cells with their surrounding microenvironment, including topographical, biochemical and mechanical stimuli. Polyelectrolyte multilayer (PEM) films have recently emerged as a new kind of surface coating with a potentiality to independently tune several film properties. Here we used different types of PEM films made of either synthetic or natural polyelectrolytes, namely poly(L-lysine)/hyaluronan (PLL/HA), chitosan/hyaluronan (CHI/HA) and poly(allylamine hydrochloride)/poly(L-glutamic acid) (PAH/PGA) for investigating the interplay between film stiffness and cell adhesion. In a first step, film buildup in physiological conditions was followed by quartz crystal microbalance with dissipation monitoring and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), which allows an unambiguous quantification of the different groups present in the polyelectrolytes. The formation of amide bonds upon film cross-linking using a water soluble carbodiimide EDC was then achieved in a tunable manner. The film internal structure, group pairing and cross-links densities were quantified by ATR-FTIR. (PAH/PGA) films emerge as the most dense films with the lowest hydration, highest COO- molar density as well as the steepest increase in PGA and PAH molar densities as a function of number of deposited layers. In addition, the monomer ratio of PAH over PGA was lower for these films at ~0.3 as compared to ~0.5 for (PLL/HA) and (CHI/HA) films. All the films could be cross-linked in a tunable manner but PAH/PGA exhibited the highest absolute number of amide bonds created, ~5 times more than (PLL/HA) and ~10 times more than (CHI/HA) films. The Young’s modulus E of the films was also measured using AFM nano-indentations. E was found to vary over a large range, from ~10 to 200 kPa for (CHI/HA) films, up to 400 kPa for (PLL/HA) films and between 120 and 800 kPa in the case of (PAH/PGA) films, depending on the initial EDC concentration. Interestingly, a linear relationship between E and the density of covalent cross-links created was observed for (PLL/HA) and (CHI/HA) films whereas (PGA/PAH) films exhibited a bi-phasic behavior. We finally investigate the influence of the film mechanical properties on adhesion, spreading and proliferation of C2C12 myoblasts and 3T3 fibroblasts. Few cells attached on the most compliant films, whereas the cells cultured on the stiffer films adhered, spread and proliferated. A significant increase in proliferation was measured at 4H and 24H for both cell types on the stiffest films. Immuno-staining of the cytoskeleton for actin and vinculin, a component of focal adhesions, revealed that cells cultured on the soft films remained round and showed neither F-actin stress fibers nor vinculin plaques. In contrary, cells spread on stiff films and showed numerous, elongated focal adhesions and well organized stress fibers.
Symposium Organizers
Ferenc Horkay National Institutes of Health
Noshir Langrana Rutgers University
Walter Richtering RWTH Aachen University
QQ11: Functional Gels and Composites
Session Chairs
Ferenc Horkay
Walter Richtering
Friday AM, December 04, 2009
Room 208 (Hynes)
9:45 AM - QQ11.2
Multiresponsive Biopolyelectrolyte Hydrogel Membranes.
Ihor Tokarev 1 , Venkateshwarlu Gopishetty 1 , Yuri Roiter 1 , Sergiy Minko 1
1 Chemistry and Biomolecular Science, Clarkson University, Postdam, New York, United States
Show AbstractWe developed a novel method for the fabrication of biodegradable and chemically erasable membranes with multiple responsive functions. The pore size, permeability, adhesive, and mechanical properties of the membrane can be regulated by the pH and ionic strength of the surrounding aqueous solution. In addition to those functions, the membrane can store nanoparticles and enzymes that extend their functions toward an antimicrobial and optical coating and a reactor that can metabolize and excrete chemicals. The proposed membrane is suggested as a universal platform for biosensors, functional biocoatings, and drug delivery systems.
10:00 AM - QQ11.3
Photo-Induced Release From Porphyrin-Dextran Composite Polymersomes.
Neha Kamat 1 , Gregory Robbins 1 , Michael Therien 2 , Daniel Hammer 1
1 Bioengineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 2 Chemistry, Duke University, Durham, North Carolina, United States
Show AbstractPolymersomes, vesicles made from diblock copolymers, have recently been shown to degrade in response to light when select proteins in the aqueous core and a porphyrin dye in the membrane are co-encapsulated [1]. In order to better understand the mechanism of degradation, the phenomenon was reproduced when replacing the luminal protein with dextran, which is low cost, biocompatible, and has properties that can be systematically modified. Co-encapsulation of dextran and porphyrin dimer (PZn2) into polymersomes results in a variety photo activated degradation responses, from membrane budding to complete membrane rupture, when exposed to focused light of visible to near-IR wavelengths. The mechanism of membrane instability was studied through systematically adjusting the loading of both dextran and porphyrin and monitoring the effect of these changes on the polymersome response to light. Dextran induced instability was also studied by varying dextran molecular weight (MW) and concentration. Increasing dextran molecular weight correlated directly with the percentage of vesicles in a population that respond to light stimulation, and polymersome populations became completely unresponsive to light stimulation at very low molecular weights or very low concentrations. These results suggest that both amphiphilicity and aggregation of the hydrophilic encapsulant are required for vesicles to become light responsive. In dextran containing vesicles, a direct correlation between percentage of light responsive vesicles and a decrease of the mechanical strength of the membrane was established using micropipette aspiration. This result is unexpected because membrane strength increased with likelihood of responsiveness when proteins were encapsulated in previously published results. This decrease in mechanical rigidity was reproduced for vesicles made of diblock copolymers of varied strength and molecular weight; these results suggest intercalation of dextran into the membrane. The interaction of dextran with the membrane was verified through Flourescence Recovery After Photobleaching (FRAP) experiments with flourescein-labeled dextran. These experiments showed very slow diffusion or complete immobility of dextran at the membrane. Finally, we hypothesized that this effect is due to the low quantum yield of porphyrins; heat generated during illumination causes a thermal expansion of the membrane and a differential stress that can be relaxed only through membrane destabilization. This hypothesis will be tested with different membrane dyes. In summary, these results suggest that a generalizable system of membrane induced instability exacerbated by local heat production should be reproducible with different hydrophilic encapsulated molecules and membrane-encapsulated, hydrophobic dyes.[1] Robbins, G.P., et al., Photo-initiated destruction of composite pophyrin-protein polymersomes. In review, 2008.
10:15 AM - QQ11.4
Effects of the Degree of Crosslinks on the Adhesion Properties Between the Crosslinked Polymers.
Daisuke Sakasegawa 1 , Takeshi Tsuduki 1 , Motoaki Goto 2 , Atsushi Suzuki 1
1 Graduate School of Environment and Information Sciences, Yokohama National University, Yokohama Japan, 2 , NIPROPATCH Co., Ltd., Kasukabe Japan
Show AbstractThe effects of the degree of crosslinks on the adhesion properties between polydimethylsiloxane (PDMS) were investigated with the use of a point-contact method in the crossed-cylinder geometry in air at room temperature. The apparatus of adhesion measurement used here was developed to measure the adhesion force by the bending spring, and the restoring force of the spring correlated with the deformation force of the sample. The unique adhesion curve was obtained using this apparatus, which have not been obtained by the other techniques. In this study, the degree of crosslinks (ρ) was changed in a wide range from polymer solution to elastic gel of PDMS, and the adhesion properties were examined using the apparatus under constant experimental parameters, i.e., separation velocity, normal load, and waiting period prior to separation. As a result, the maximum adhesion force (FA) strongly depended on ρ and FA exhibited a maximum at a characteristic ρ around the gelation point (tanδ = 1 in the result of the dynamic viscoelastic measurement). This result could be understood in terms of the increment of the bulk stiffness and the decrement of the real contact area with increasing ρ. In addition to this finding, the shapes of the adhesion curves were found to depend on ρ, which were categorized into three types. The respective type corresponded to the bulk properties, i.e., elastic gel, weak gel, and polymer solution. In the case of the weak gel, the characteristic adhesion curve was obtained, which resulted from the fibrils and cavitations near the surface. In order to understand the change in the adhesion curve with changing ρ, the contact surface was directly observed with the use of a point-contact method between two PDMS samples applied thin to a glass plate and a glass sphere. With decreasing ρ, the shape of the contact area during the separation evidently changed; in the elastic gel the contact area continuously decreased with keeping a circle and disappeared at the separation, while the adhesion fingering was found in the weak gel and its fingering shape of the contact area strongly depended on ρ. We found that the shape of the adhesion curve corresponded to the shape of the contact area during the separation process.From these results, we will discuss the factors that determine the adhesion curve and clarified the ρ–dependence of the separation mechanism of the present evaluation method.
11:00 AM - QQ11.5
Self-assembly of pH-responsive Fluorinated Dendrimer-based Particulates for Drug Delivery and Noninvasive Imaging.
Jason Criscione 1 , Bonaire Le 2 , Eric Stern 1 , Matthew Brennan 3 , Christoph Rahner 4 , Xenophon Papademetris 1 5 , Tarek Fahmy 1 2
1 Biomedical Engineering, Yale University, New Haven, Connecticut, United States, 2 Chemical Engineering, Yale University, New Haven, Connecticut, United States, 3 Surgery, Yale University, New Haven, Connecticut, United States, 4 Cell Biology, Yale University, New Haven, Connecticut, United States, 5 Diagnostic Radiology, Yale University, New Haven, Connecticut, United States
Show AbstractDendrimers are nanoscale macromolecules with well-defined branching chemical structures. Control over the architecture and function of these structures has enabled many advances in materials science and biomedical applications. Though dendrimers are directly synthesized by iteration of simple repetitive steps, generation of the larger, more complex structures required for many biomedical applications by covalent synthetic methods has been challenging. Here we demonstrate a spontaneous self-assembly of poly(amidoamine) dendrimers into complex nanoscopic and microscopic particulates following partial fluorination of the constituent dendrimer subunits. These dense particulates exhibit a stimulus-induced response to low external pH that causes their disassembly over time, enabling controlled release of encapsulated agents. In addition, we show that these assemblies offer a sufficiently high density of fluorine spins to enable detection of their site-specific accumulation in vivo by 19F magnetic resonance imaging (19F MRI). Fluorinated dendrimer-based particulates present new features and capabilities important for a wide variety of emerging biomedical applications.
11:30 AM - QQ11.7
Characterization of Yield Stress and Slip Behavior of Skin/Hair Care Gels and Correlation with Sensory Attributes.
Seher Ozkan 1 , Tim Gillece 1 , David Moore 1
1 Material Science Group, Global R&D, International Specialty Products Inc. , Wayne, New Jersey, United States
Show AbstractDetermination of yield stress and rheological characterization of hydrogels and gel-like percolated suspensions/emulsions present special challenges associated with thixotropy, viscoplasticity and the wall slip, which renders the application of generally accepted rheological characterization methodologies difficult. Generally, yield stress is determined from shear stress versus shear rate data by fitting the Bingham or Herschel-Bulkley equations to the data. However these methods prone to experimental errors and wall slip effect and not all materials comply with these common methods. Other methods to determine yield stress was also given by several authors such as utilization of dynamic strain sweep data at a constant frequency and stress controlled creep tests to determine the yield stress. In this study we have characterized rheological behavior of three selected hydrogels, which are common ingredients of skin/hair care formulations, in order to correlate rheological parameters of the products with sensory attributes. Selected hydrogels were crosslinked poly(acrylic acid) (Carbopol 980), crosslinked methyl vinyl ether/maleic anhydride copolymer (StabilezeQM) and crpsslinked vinyl pyrrolidone/acrylic acid copolymer (UltrathixP100).The straight-line marker technique was applied to investigate the slip behavior during steady torsional flow at different gap openings and dynamic strain sweep tests. Yield stress, consistency index and power law index of hydrogels were obtained by fitting slip corrected shear stress versus shear rate data to Herschel-Bulkley model. Stress controlled tests were also conducted to investigate thixotropy and creep behavior and determination of yield stress. Yield stress values were compared with the ones that were determined from dynamic and strain controlled steady experiments. Overall results were correlated with sensory panel test results.
11:45 AM - QQ11.8
Fabrication and Synthesis of CdTe Quantum Dots Incorporated Hydrogel based Multi-bandgap Solar Energy Storing Electric Double Layer Photo Capacitors.
Chi-Wei Lo 1 , Hongrui Jiang 2 1
1 Material Science Program, Univ. of Wisconsin Madison, Madison, Wisconsin, United States, 2 Electrical and Computer Engineering, Univ. of Wisconsin Madison, Madison, Wisconsin, United States
Show AbstractMicrofluidics refers to devices and methods for controlling and manipulating fluid flows with length scales less than a millimeter via many kinds of functional elements including valves, pumps, actuators, switches, sensors, dispensers, mixers, filters, separators, heaters, etc. However, there has not yet been a viable method to integrate a renewable power source into microfluidic systems to drive these components. A photovoltaically rechargeable capacitor seems to be excellent candidate for being a clean and renewable power source for lab-on-a-chip systems. We here present a novel hydrogel electric double layer photo capacitor based on poly(acrylamide) (PAAm) membranes incorporating cadmium tulleride (CdTe) quantum dots (QDs) with controllable bandgaps to absorb multiple wavelength in the solar spectrum. CdTe QDs of 4 different sizes was encapsulated in N,N-methylenebisacrylamide (NMBA) cross-linked PAAm membranes. The QDs-PAAm hydrogel as then filled with I-/I3- redox electrolyte solution and placed between carbon anode and ITO cathode. The QDs-PAAm hydrogel membrane served as a photo-responsive gel electrolyte. The device was capable of harvesting and store solar energy while easy to be integrated with current fabrication process.The CdTe QDs were synthesized by using colloidal synthesis in a non-coordinating solvent and the sizes of the QDs were controlled by temperature. We precisely control the reaction temperature at 125°C, 150°C, 175°C, and 200°C to obtain four different sizes of CdTe QDs. Ligand exchange with mercaptoacetic acid was performed to make CdTe QDs dispersed in dimethyl sulfoxide (DMSO) and stored in dark. UV-Vis spectrum of CdTe QDs DMSO solution of 4 different temperatures confirmed different optical absorption indicating controllable bandgaps. The sizes of the QDs were determined by high-resolution transmission electron micro spectroscopy (HRTEM). The QDs-PAAm hydrogel was formed by using CdTe QDs solution as solvent to prepare PAAm pregel solution and cured under UV light. The resulting QDs-PAAm hydrogel exhibits partially transparent red in color. The QDs-PAAm hydrogel was then soaked with redox electrolyte in 5 mins and followed by sandwiched between anode and cathode for further characterization.The photocurrent response of the fabricated device was investigated. The maximum current output Imax can be as large as 0.38μA when exposed to 100mW/cm2 light source with the max voltage Vmax of 0.5V. The current increased dramatically when exposed to UV light. The current dropped as the light was removed. The cycles can be repeated with good reproducibility. It is also interesting to note that the device is also capable of storing small amount of the solar energy. In addition to the power output, the gel was capable of photopatterning which shows a ready integration with microfluidic application.
12:00 PM - QQ11.9
Microliter Liquid Calorimetry with Fast Heating Rate for Protein Analysis.
Lito de la Rama 1 , Liang Hu 1 , Mike Efremov 2 , Eric Olson 1 , Mark McLean 3 , Leslie Allen 1
1 Materials Science and Engineering, University of Illinois - Urbana Champaign, Urbana, Illinois, United States, 2 Chemical and Biological Engineering and Center for Nanotechnology, University of Wisconsin-Madison, Madison, Wisconsin, United States, 3 Biochemistry, University of Illinois-Urbana-Champaign, Urbana, Illinois, United States
Show AbstractWe present results of calorimetry analysis on small volumes of fluids using a capillary-based microcalorimeter. The calorimetric cell consists of a glass capillary with a thin film metal heater deposited on the surface. During device operation, the metal thin film heats the sample through joule heating when a current is applied on the film. It also acts as a thermometer since the resistance vs temperature profile of the film can be calibrated using a standard resistance temperature detector. The heated volume is 30uL with a heating rate of up to 10C/min. Using this device, we have studied the gel-liquid crystal phase transition of an aqueous solution of dimethyl-dioctadecyl ammonium bromide (DODAB). It has a known phase transition at 45C and this is used as a test material to verify the device operation. Different concentrations of DODAB were tested. Furthermore, we have demonstrated the use of this device in the study of the folding/unfolding transition of proteins with Lysozyme from hen egg white and Rnase A as test systems.