Ferenc Horkay National Institutes of Health
Noshir Langrana Rutgers University
Walter Richtering RWTH Aachen University
QQ1: Design and Characterization of Responsive Gel Systems
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 Show Abstract
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
Typical 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 Show Abstract
1 , University of Utah, Salt Lake City, Utah, United States, 2 , Technische Universität Dresden, Dresden Germany
Many 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 Show Abstract
1 Dep. of Physics, The Norwegian University of Science and Technology, Trondheim Norway
Various 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 Show Abstract
1 Bioengineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Stimuli-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 Show Abstract
1 Chemistry Department, University of Connecticut, Storrs, Connecticut, United States, 2 The Institute of Materials Science , University of Connecticut, Storrs, Connecticut, United States
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 Show Abstract
1 Mechanical Engineering, Princeton Univ., Princeton, New Jersey, United States
Thermo-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 Show Abstract
1 Harrington Department of Bioengineering, Arizona State University, Tempe, Arizona, United States
Stimuli-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 Show Abstract
1 G. W. W. School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Responsive 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 Show Abstract
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
Polyacrylamide (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 .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 .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. Y.Bar-Cohen (2001) Electroactive Polymer Actuators as Artificial Muscles: Reality, Potential, and Challenges Second Edition, Bellingham, SPIE Press Monograph Vol. PM136. P. Calvert, Hydrogels for Soft Machines, Adv. Mater. 21 (2009) 743–756 M.Bassil, J.Davenas and M.El Tahchi, Sensors and Actuators B: Chemical 134 (2008) 496–501. M.Bassil, M.El Tahchi and J.Davenas, Advances in Science and Technology 61 (2008) 85-90. M.Bassil, M.Ibrahim, M.El Tahchi, J.Farah and J.Davenas, Mater. Res. Soc. Symp. Proc. 1134 (2009) BB01-10. 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
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 Show Abstract
1 Materials Science and Engineering, University of Utah, Salt Lake City, Utah, United States
There 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 Show Abstract
1 , Columbia U, New York, New York, United States
Computer 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 Show Abstract
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States
Nucleic 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 Show Abstract
1 Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Spider 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 Show Abstract
1 , Rensselaer Polytechnic Institute, Troy, New York, United States
Random 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 Show Abstract
1 CNMS, ORNL, Oak Ridge, Tennessee, United States, 2 , Asylum Research Co., Santa Barbara, California, United States
Phase 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 Show Abstract
1 , Northwestern University, Evanston, Illinois, United States, 2 , Arizona State University, Glendale, Arizona, United States, 3 , University of Wisconsin-Madison, Madison, Wisconsin, United States
Polyelectrolyte (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 Show Abstract
1 Energy Nanomaterials, Sandia National Laboratories, Livermore, California, United States, 2 Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, United States
The 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 Show Abstract
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
The 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 Show Abstract
1 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States
Many 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
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 Show Abstract
1 , Hanyang University, Seoul Korea (the Republic of)
DNA 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, T