Kalpana Katti North Dakota State University
Christian Hellmich Vienna University of Technology (TU Wien)
Ulrike G. K. Wegst Drexel University
Roger Narayan North Carolina State University
Z1: Tissue Mechanics I
Tuesday PM, December 02, 2008
Back Bay B (Sheraton)
9:00 AM - Z1:Tissue mechan
Chair comments in honor of Prof. Sidney Lee Show Abstract
9:15 AM - **Z1.1
Nature-Inspired Structural Materials.
Robert Ritchie 1 2 , Etienne Munch 2 , Max Launey 2 , Daan Hein Alsem 2 , Eduardo Saiz 2 , Antoni Tomsia 2 Show Abstract
1 Materials Science & Engineering, University of California, Berkeley, Berkeley, California, United States, 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
The structure of materials invariably defines the mechanical behavior. However, in most materials, specific mechanical properties are controlled by structure at widely differing length scales. Nowhere is this more apparent than with biological materials, which are invariably sophisticated composites whose unique combination of mechanical properties derives from an architectural design that spans nanoscale to macroscopic dimensions. Moreover, they are generally able to defeat the “law of mixtures” by devising such hierarchical structures with weak constituents into strong/tough hybrid materials that display superior properties to their individual constituents. The fracture resistance of such materials originates from toughening mechanisms at each dimension; few engineering composites have such a hierarchy of structure. However, the biomimetic approach has not been that successful because of the difficulty of synthesizing such materials. In this presentation we describe attempts to develop a range of bone- and nacre-like structural materials using a new freeze-casting technique, which utilizes the intricate structure of ice to create hybrid materials with complex lamellar and/or mortar and brick structures modeled across several length-scales. Our initial results show ceramic-polymer and ceramic-metal hybrid materials with toughness many times in excess of those expected from a rule of mixtures construction. The architecture and properties of the synthetic materials are compared to their natural counterparts in order to identify the mechanisms that control mechanical behavior over multiple dimensions and propose new design concepts to guide the synthesis of hybrid/hierarchical structural materials with unique mechanical responses.
9:45 AM - Z1.2
Effects of Obesity on Cortical Bone.
Sophi Ionova 1 2 , Sandy Do 3 , Holly Barth 1 2 , Joel Ager 2 , Alex Porter 5 , Christian Vaisse 3 , Tamara Alliston 4 , Robert Ritchie 1 2 Show Abstract
1 Materials Science and Engineering, UC Berkeley, Berkeley, California, United States, 2 Materials Science Division, Lawrence Berkeley Labs, Berkeley, California, United States, 3 Diabetes Center, University of California, San Francisco, San Francisco, California, United States, 5 Department of Materials, Imperial College London, London United Kingdom, 4 S/M Orthopaedic, University of California, San Francisco, San Francisco, California, United States
Obesity is associated with a host of biological and physiological changes, among which is a reduced risk of bone fracture in adults. While some studies have found obesity is associated with increased bone size and mass measures, it is still unclear whether the reduced risk stems from a change in bone quantity alone, or whether bone quality is affected as well. The objective of this study is to evaluate the changes in mechanical properties of cortical bone in response to diet-induced obesity in mice. 4 week old C57BL/6 male mice were fed a high fat diet (HFD) (N=15) or standard laboratory chow (N=15) for 16 weeks. All protocols were approved by the Institutional Animal Care and Use Committee and done according to federal guidelines for the care and use of animals in research. Blood was isolated immediately following sacrifice to evaluate levels of serum leptin, a hormone secreted by adipose tissue which impacts bone resorption and formation. Bone mass and body composition were evaluated with DEXA. The left femurs were tested in three-point bending to measure strength and bending stiffness. Right femurs were tested in notched three-point bending to measure fracture toughness (loading rate of 0.001mm/s). Bone geometry was evaluated in tomography at the Advanced Light Source and in an environmental scanning electron microscope. As expected, in the high-fat diet fed mice, body weight, fat mass, and leptin levels were significantly increased. In HFD mice, endostial and periostial diameters, as well as cortical wall thickness increased (all p<0.015), while bone mineral density was unchanged (p=.937). Strength, bending stiffness, and fracture toughness all were reduced in HFD (p<0.007, p=.010, p=.013, respectively), while failure load was slightly increased. Transmission electron microscope studies point to a qualitative reduction in collagenous organization in HFD versus control group. In summary, diet-induced obesity results in increased bone size (quantity), while reducing bone strength, bending stiffness, and fracture toughness as well as other indicators of bone quality. This study indicates that bone quantity and bone quality play important, albeit counteracting, roles in determining fracture risk.
10:00 AM - Z1.3
Micromechanics-based Conversionof CT Datainto Anisotropic Elasticity Tensors,applied toFE Simulations of a Mandible.
Christian Hellmich 1 , Cornelia Kober 2 , Bodo Erdmann 3 Show Abstract
1 , Vienna University of Technoloy (TU Wien), Vienna Austria, 2 , Hamburg University of Applied Sciences, Hamburg Germany, 3 , Zuse Institute, Berlin Germany
Computer Tomographic (CT) image data havebecome a standard basis for structuralanalyses of bony organs.In this context, regression functions betweenstiffness components andHounsfields units (HU) from Computer Tomography,related to X-ray attenuation coefficients,are widely used for the definition of the(actually inhomogeneous and anisotropic)material behavior inside the organ.Herein, we suggest to derive the functionaldependence of the fully orthotropicstiffness tensors on the Hounsfield unitsfrom the physical information containedin the X-ray attenuation coefficients:(i) Based on voxel average rulesfor the X-ray attenuation coefficients, we assign toeach voxel the volume fraction occupied bywater (marrow) and that occupied by solid bone matrix.(ii)By means of a continuum micromechanicsrepresentation for bone, which isbased onvoxel-invariant (species and whole bone-specific)stiffness properties of solid bone matrixand of water, we convert the aforementionedvolume fractions into voxel-specificorthotropic stiffness tensor components.The micromechanics model, in combinationwith the average rule for X-ray attenuation coefficients,predicts a quasi-linear relationship between axialYoung's modulus and HU,and highly nonlinear relationships for both circumferentialand radial Young's modulias well as for theshear moduli in all principal material directions.orresponding whole-organ Finite Element analyses of a partiallyedentulous human mandible characterized byatrophy of the alveolar ridgeshow that volumetric strain concentrations/peakswithin the organ are decreased when consideringmaterial anisotropy, and increased when consideringmaterial inhomogeneity.
10:15 AM - Z1.4
Fracture Mechanisms of Bone: A Comparative Study between Antler and Bovine Femur Bone.
Po-Yu Chen 1 , Joanna McKittrick 1 2 , Marc Meyers 1 2 Show Abstract
1 Materials Science and Engineering, University of California, San Diego, La Jolla, California, United States, 2 Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, California, United States
Deer antlers, one of the fastest growing tissues in the animal kingdom, have a primary function in intraspecific combat and have been designed for sustaining high impact loading and bending moment without fracture. Antlers have a similar microstructure as mammalian long bones, composed primarily of type-I collagen fibrils and carbonated apatite crystals, arranged in osteons in the compact bone and a lamellar structure in the cancellous bone. However, there are distinct differences between antler and bone. First, antlers have lower mineral content (~ 30 vol%) compared to bones (~ 40 vol%). Secondly, antlers consist mainly of young primary osteons whereas most adult limb bones consist of secondary osteons and older interstitial bone. In this study, fracture toughness of North American elk (Cervus canadensis) antler and bovine femur were measured using four-point bending tests on single notched compact bone samples (ASTM C1421). Bending tests were conducted under loading parallel and transverse to the long axis of antler and bone in both dry and re-hydrated conditions to study the effects of fiber orientation and hydration. Fracture toughness results in the longitudinal direction were much higher than that in the transverse direction and increased with degree of hydration for both antler and bovine femur. The fracture toughness of elk antler is ~ 50% higher than that of bovine femur. The highest fracture toughness value was obtained from the re-hydrated elk antler in the longitudinal orientation, which reached 10.3 MPa*m1/2 compared to that measured from bovine femur, which was 5.7 MPa*m1/2. The double-notched samples were also prepared and tested to examine the crack propagation using scanning electron microscopy. Toughening mechanisms, including crack deflection by osteons, uncracked ligament bridging, and microcracks formation, were observed and discussed. Comparisons between antler and bone were made. This research is supported by the National Science Foundation Grant DMR 0510138.
10:30 AM - Z1.5
Characterization of Local Dynamic Properties of Wet Cortical Bone using Nanoindentation.
Siddhartha Pathak 1 , Greg Swadener 3 , Surya Kalidindi 1 , Karl Jepsen 4 , Hayden Courtland 4 , Haviva Goldman 2 Show Abstract
1 Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania, United States, 3 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 4 Orthopaedics, Mount Sinai School of Medicine , New York, New York, United States, 2 Neurobiology and Anatomy, Drexel University College of Medicine, Philadelphia, Pennsylvania, United States
10:45 AM - Z1:Tissue mechan
11:00 AM - **Z1.6
Tissue-level Mechanical Property Heterogeneity in Mineralized Tissues.
Virginia Ferguson 1 Show Abstract
1 Mechanical Engineering, University of Colorado, Boulder, Colorado, United States
11:30 AM - Z1.7
Nanostructured Cellulosics Characterized by In-Situ Synchrotron X-ray Diffraction Coupled with Mechanical Tests.
Keckes Jozef 1 , Peter Boesecke 2 , Wolfgang Gindl 3 Show Abstract
1 Department of Materials Physics, Univeristy of Leoben, Leoben Austria, 2 , European Synchrotron Radiation Facility, Grenoble France, 3 Department of Materials Science and Process Engineering, University of Natural Resources and Applied Life Sciences, Vienna Austria
Cellulose provides the strength to the majority of organic structures on the Earth. Those structures are in most cases fibrous nanocomposites with complex hierarchical architecture whereby cellulose fibrils play a role of the reinforcing element. In order to understand the structure-property relationship in cellulosics, in-situ synchrotron diffraction studies on different types of tissues (e.g. wood, coir, bacterial cellulose) combined with tensile tests were performed at the ID01 beamline of the European synchrotron radiation facility (ESRF) in Grenoble, France. The tissues were cyclically strained in a tensile stage and X-ray diffraction patterns were collected using 2D CCD detector. By relating the mechanical data with the structural information, it was possible to analyse the deformation mechanisms in the cellulocics [1,2]. Upon straining, the tissues exhibited elastic and plastic behaviour depending on the original orientation of the fibrils. The deformation beyond the yield point did not reduce the stiffness of the fibres, since the tissues recovered their initial stiffness by every increase and decrease of the strain. As determined from WAXS data, the magnitude of the orientation factors of crystalline cellulose is only the function of the original texture and the applied strain in all cellulosics. The results indicate a presence of a dominant recovery mechanism occurring between the interfaces of crystalline fibrils. The interfaces play a role of slip planes filled by sacrificial bonds. The macroscopic plasticity occurs when the sites with the slip percolate through the whole tissue volume. Whenever the straining is interrupted or the strain is reduced, the sacrificial bonds are recovered and the cellulosics show original stiffness.  J. Keckes, I. Burgert, K. Frühmann, M. Müller, K. Kölln, M. Hamilton, M. Burghammer, S.V. Roth, S. Stanzl-Tschegg & P. Fratzl (2003), Cell-wall recovery after irreversible deformation of wood, Nature Materials 2, 810-814. W. Gindl, K.J. Martinschitz, P. Boesecke, J. Keckes (2006), Structural changes during tensile testing of an all-cellulose composite by in situ synchrotron X-ray diffraction, Composites Science and Technology 66, 2639–2647.
11:45 AM - Z1.8
Finite Element Simulation of Nanoindentation Tests on Cortical Bone Allowing for Tissue Anisotropic Elastic and Inelastic Behaviour.
Pasquale Vena 1 , Dario Gastaldi 1 , Valentina Sassi 1 , Davide Carnelli 1 , Roberto Contro 1 , Christine Ortiz 2 Show Abstract
1 Structural Engineering, Politecnico di Milano, Milano Italy, 2 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambrige, Massachusetts, United States
Nowadays, the nanoindentation technique is widely applied to the mechanical characterization of the bone tissue; it is particularly suited to this purpose because different length scales, relevant for the different micro/nanostructure arrangements of the tissue, might be investigated by an appropriate choice of the maximum load applied during the experiment.For cortical bone tissue, charcaterized by osteonic lamellae at the nano/microstructural level, anisotropic mechanical properties are expected. The estimation of anisotropic elastic properties from nanoindentation tests carried out along different orientations can be achieved by coupling the Oliver and Pharr theory (developed for isotropic materials) with the Swadener and Pharr theory that relates the anisotropic elastic tensor to the indentation moduli, for a given indentation direction. However, to identify all constants of an anisotropic elastic tensor from indentation data along three different directions, is still an open problem.Finite element simulations (FEM) of the experiment can certainly provide a deeper insight in the role played by the anisotropic material response on the results of nanoindentation tests; indeed numerical simulations account for all material constants (both elastic tensor components as well as the inelastic parameters defining the yield function). In this work, we present a finite element model of the nanoindentation tests conducted on the bone tissue, with particular reference to the cortical bone aimed at investigating the effects of elastic-plastic anisotropy on the nanoindentation experiments. Focus on the role of the anisotropic pressure-dependent yield function will be done. Transversely isotropic material properties are assumed; equivalent axysymmetric analyses simulate the indentation along the axial direction; whereas, full three dimensional models simulate indentation along the transverse direction. Results have shown that, on the basis of assumed anisotropic strength data taken from the literature, the FEM simulation of the nanoindentation test will provide results consistent with experimental results only in the case of pressure dependent yield criterion. Moreover, the FEM analyses results are consistent with direction dependent measurements of indentation modulus and hardness.
12:00 PM - Z1.9
Mechanical Anisotropy of Individual Osteons in Bone Tissue at High Spatial Resolutions.
Davide Carnelli 1 , Pasquale Vena 1 , Roberto Contro 1 , Christine Ortiz 2 Show Abstract
1 Department of Structural Engineering, Politecnico di Milano, Milano Italy, 2 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
The structural and mechanical anisotropy of bone is critical to its macroscopic biomechanical function. Secondary osteons, the fundamental micrometer-sized building blocks of cortical bone, are multilayered cylindrical composite structures of mineralized collagen fibrils arranged circumferentially in thick and thin lamellae. The anisotropy of individual osteons is hypothesized to provide a sufficient mechanical response to physiological (largely compressive, elastic) and accidental (multiaxial plastic, fracture) loading. In this research, instrumented nanoindentation was employed on adult compact bovine femoral bone to quantify the elastic and inelastic mechanical anisotropy in the longitudinal (parallel to the long bone axis) and transverse directions (both circumferential and radial) within a single osteon. The dual indentation technique, that is the use of indenters with different sizes and geometries on the same microstructural feature (e.g. an osteon), has been extended to the study of anisotropic materials with the objective of enhancing the analysis of the indentation results to extract more precise information from the experiments. Thus, pyramidal (Berkovich and Cube Corner), conical and spherical indenters (with different apex angle and end radius sizes, respectively), have been employed to evaluate the material response when subjected to loading under differing conditions. Moreover, the residual indent topography provided by atomic force microscopy imaging has also been used to provide meaningful experimental data, additional to those deduced from the force-depth indentation curves. Since residual displacements reflect constitutive anisotropy, the use of axial-symmetric indenters results in a mapped imprint that does not exhibit axial symmetry because of specimen anisotropy. Thus, the topography of the residual indents is a source of important quantitative information on the material anisotropic properties. The coupling of these tools allows for fundamental knowledge regarding the relationship existing between osteonal bone microstructure and anisotropic mechanical properties.
12:15 PM - Z1.10
Size-dependent Heterogeneity in Plasticity Promotes Energy Dissipation in Bone.
Haimin Yao 1 , Ming Dao 1 , Kuangshin Tai 1 , Timothy Imholt 2 , Subra Suresh 1 , Christine Ortiz 1 Show Abstract
1 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambirdge, Massachusetts, United States, 2 , Raytheon, Inc, Marlboro, Massachusetts, United States
It was recently discovered through analysis of nanoindentation data and computational modeling that nanoscale heterogeneity in the spatial distribution of the elastic and plastic mechanical properties of cortical bone predicted increased energy dissipation compared to homogeneous controls. Here, we investigate this interesting phenomenon further by isolating the contributions of heterogeneities in elastic moduli versus yield stress to energy dissipation. Two different elastic-perfectly plastic finite element analysis(FEA) models were formulated including a 2D four-point notched beam bent by displacement-controlled loads. In this model, heterogeneity maps were assigned to a 2µm×2µm area in the vicinity of the notch and outside of this region, the material was assumed to be homogeneous. The second FEA model involved a rigid indenter (included angle of 90o, tip radius of 141nm) penetrating a 10µm×10µm sample. Likewise, heterogeneity maps were assigned to a 2µm×2µm area directly beneath the indenter and outside of this region, material was assumed to be homogeneous. The heterogeneity maps of modulus were assigned values directly measured by nanoindentation and the heterogeneity maps of yield stress were calculated from experimentally-measured hardness, by assuming that the yield stress is proportional to the hardness. Next, the heterogeneity in elasticity (modulus) or plasticity (yield stress) or both was eliminated, resulting in additional map sets with different combinations of the heterogeneity in elasticity and plasticity. The results show that, for all cases, heterogeneities of plasticity cause up to 48% promotion of energy dissipation, whereas the spatial inhomogeneity of elasticity does not lead to considerable variation in energy dissipation. Hence, heterogeneity in plasticity, rather than elasticity, plays a dominant role in promoting energy dissipation. The experimentally-measured heterogeneity maps used in these simulations have a spatial resolution of 100nm. However, heterogeneity was found to be dependent on length scale in that the larger the probe size, the lower the degree of heterogeneity. Hence, the impact of heterogeneity on energy dissipation will decrease as the length scale is increased. These findings motivated further studies on the response of energy dissipation to the variations of standard deviation and mean value of the yield stress, which are two important quantities characterizing the plasticity heterogeneity. On one hand, it was found that the energy dissipation increases monotonically as the standard deviation of the yield stress is increased. On the other hand, the response of dissipation to the variation of the mean yield stress was found to be dependent on the stress status experienced by the material. Our simulations show that the plasticity heterogeneity in bone might be the optimization result for achieving higher energy dissipation and, therefore, higher mechanical resistance in typical loading circumstances.
12:30 PM - **Z1.11
Three-dimensional X-ray Microscopy of Biological Materials.
Peter Cloetens 1 , Oliver Betz 2 , Pierre Bleuet 1 , Sylvain Bohic 1 3 , Lukas Helfen 1 4 , Françoise Peyrin 1 5 , Ulrike Wegst 6 Show Abstract
1 , European Synchrotron Radiation Facility, Grenoble France, 2 Zoologisches Institut, Tübingen University, Tübingen Germany, 3 , Research Centre INSERM U-836, Grenoble France, 4 , ISS/ANKA, Forschungszentrum Karlsruhe, Karlsruhe Germany, 5 CREATIS, INSA Lyon, Lyon France, 6 Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania, United States
Z2: Tissue Mechanics II
Tuesday PM, December 02, 2008
Back Bay B (Sheraton)
2:30 PM - **Z2.1
Multilayered and Graded Biological Materials.
Christine Ortiz 1 , Subra Suresh 1 Show Abstract
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Multilayered and functionally graded materials are ubiquitous throughout biology. In addition to their biological function, their geometries and properties are designed to achieve important thermal and mechanical performance characteristics in a variety of protective and defensive exoskeletal structures. The diversity of structures and properties found in such systems is enormous presumably due to variability in the surrounding environment and predators. Here, we will discuss and compare a number of model systems in relation to their known "threats" including; 1) mineralized fish scales, 2) abalone shell, and 3) a gastropod mollusk shell from a deep-sea hydrothermal vent, and 4) teeth. The topics to be covered include; the spatial variation in mechanical properties through the cross-section from the outer to inner surfaces, the thickness and sequence of the layers, the interfacial geometry, confinement effects between the layers, structure and property gradation within and between layers, and anisotropy of the layers. Such studies provide valuable insights into rationalizing why natural materials have specific layered and graded architecture. They also help us to engineer synthetic materials with controlled gradients in composition, microstructure and properties to achieve enhancements in mechanical performance characteristics.
3:00 PM - Z2.2
Individual Collagen Fibrils with 100 nm-diameter Behave as Shear Piezoelectric Materials.
Majid Minary-Jolandan 1 , Min-Feng Yu 1 Show Abstract
1 Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Piezoelectricity, the ability to generate electrical potential in response to mechanical stress, is a well known phenomenon in certain crystalline materials. Interestingly, a number of biological materials, such as bone, skin, and tendons, also share such property. It has been postulated that the electrical charges generated under stress resulting from piezoelectricity may stimulate cellular responses in these biological materials and hence, result in growth and healing of these tissues. This phenomenon may provide an explanation for the Wolff’s law in bone. So far, studies have examined piezoelectricity in bulk samples of these tissues, which due to coexistence and interaction of piezo and non-piezo elements makes the explanation of the outcomes difficult. For example, in bone the collagen fibril is the piezoelectric components, while the hydroxyapatite mineral is non-piezoelectric element. In this study, we explore piezoelectricity in individual collagen fibrils using piezoelectric force microscopy (PFM). We show that an individual collagen fibril with a diameter of ~100 nm behaves predominantly as a shear piezoelectric material with a piezoelectric coefficient on the order of 1 pm/V. Such shear piezoelectric behavior could be explained by considering the structural organization of collagen molecules, tropocollagens, into a quasi-hexagonal symmetry conformation inside a collagen fibril. Using a computational model, it is estimated that under physiological shear stress, each collagen fibril is capable of generating an electrical potential up to several tens of millivolts. Such findings, confirm the nanoscale origin of piezoelectricity in bone and tendon, as well as the effective shear load-transfer mechanism among the collagen molecules in a collagen fibril.
3:15 PM - Z2.3
Hierarchical Modeling of the Elastic Properties of Lobster Cuticle via Ab Initio Calculations and Mean-field Homogenization.
S. Nikolov 1 , M. Petrov 1 , M. Friak 1 , Dierk Raabe 1 , C. Sachs 1 , H. Fabritius 1 , J. Neugebauer 1 , L. Lymperakis 1 Show Abstract
1 , Max-Planck-Institut fuer Eisenforschung, Duesseldorf Germany
We propose a hierarchical model for the prediction of the elastic properties of mineralized lobster cuticle using ab initio calculations to find the elastic properties of chitin and hierarchical homogenization performed in a bottom-up order to find the cuticle properties at all hierarchy levels. The mechanically relevant parts of lobster cuticle consist of planes reinforced with chitin-protein fibers embedded in a matrix consisting of calcium carbonate nanoparticles and proteins. The planes are stacked over each other and gradually rotate along the normal direction of the cuticle to form a twisted plywood structure. In addition, the cuticle has a canal pore system which pierces it through its thickness. The canals have the shape of twisted ribbons with elliptical cross section and are arranged in a hexagonal array so that the cuticle resembles a honeycomb-like structure. We compare the model predictions to experimental data for the Young moduli and the Poisson’s ratios of wet lobster endocuticle. It is found that the dominant factors determining the cuticle stiffness are the mineral content, the specific microstructure of the mineral-protein matrix and the in-plane area fraction of the pore canals. Our results suggest that the mineral-protein matrix consists of amorphous calcium carbonate spheres with varying diameters embedded in proteins and arranged in a microstructure with extremal properties in terms of stiffness. It is also found that most of the scattering in the experimentally measured Young moduli can be explained by the observed variation in the area fraction of the canals. We also discuss the role of chitin and the multifunctional optimization of the cuticle in terms of trade off between stiffness and transport capacity of the pore canal system. It is found that in lobster, the chitin-protein fibers increase the stiffness of a bulk endocuticle tissue in the fiber direction by about 50%.
3:30 PM - Z2.4
Parametric Modeling of the Mechanical Behavior of Multilayered Biological Exoskeletons.
Juha Song 1 , Mary Boyce 2 , Christine Ortiz 1 Show Abstract
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Natural exoskeletons are known to exhibit a huge diversity of structure and properties as they have adapted to environmental and predatory threats; typically balancing protection and mobility requirements to maximize survivability. Most exoskeletal materials are composed of different layers of materials where each layer possesses its own unique composite nanostructure, mechanical properties, and deformation mechanisms. The multilayered design of exoskeletons (i.e. number, sequence, thickness, geometry, and constitutive material of layers) and its relationship to the corresponding threats are largely unknown and are of great interest for the development of bioinspired human body and vehicle armor. Here, we focus on one model system, the quad-layered mineralized scales of the fish Polypterus senegalus, a living descendant of ancient palaeoniscoids. Computational methods (finite element analysis) were employed to predict the deformation under a penetrating load (simulating a predatory bite) for different multilayered structures. In particular, the following cases were considered: (i) the influence of the thickness of the outer enamel-like ganoine layer; (ii) the quad-layered structure compared to a simpler bilayer structure; and (iii) the sequence of the outer two layers (i.e. ganoine and dentin). To investigate the effect of the thickness of the outer ganoine layer, simulations of the microindentation of a ganoine-dentin bilayered model was carried out. It was determined that when the ganoine layer was 6 ~ 12 µm thick (the real thickness observed experimentally), the tensile radial stress field (S22) exceeds the circumferential stress field (S11), thereby promoting circumferential cracking upon penetration (observed experimentally), which locally confines the deformation at the indentation site and is highly advantageous. For too thin or too thick of a ganoine layer, S11 is large which promotes radial cracking and is undesirable as this can lead to catastrophic failure of the layer. In the second set of simulations, a ganoine-dentin bilayered model was compared to the quadlayered model representing the real scale microstructure. The effective indentation modulus, microhardness, and energy dissipation for both models were extremely similar, with the four layer model achieving a weight reduction up to ~20% of the bilayered system. Lastly, we have developed a "reverse" multilayered model which consists of four material layers, but where the order of two outer layers are reversed so the more compliant and softer dentin layer is located at the surface followed by the harder and stiffer ganoine layer underneath. As opposed to the actual multilayered design sequence which promotes advantageous circumferential cracking on the surface, the reversed layers magnified tensile normal and shear stresses around the junction thereby producing susceptibility to interfacial failure through delamination, which is highly undesirable during a penetrating attack.
3:45 PM - Z2.5
The Crustacean Cuticle: A Model to Study the Influence of Chemical Composition and Microstructure on the Mechanical Properties of a Biological Composite Material.
Sabine Hild 1 2 , Andreas Ziegler 2 , Frank Neues 3 , Matthias Epple 3 , Helge Fabritius 1 , Dierk Raabe 1 Show Abstract
1 Microstructure Physics and Metal Forming, Max-Planck-Institut für Eisenforschung, Düsseldorf Germany, 2 Central Facility for Electron Microscopy, University of Ulm, Ulm Germany, 3 Inorganic Chemistry, University of Duisburg-Essen, Essen Germany
The mineralized exoskeleton formed by the cuticle of crustaceans is an excellent model to study biological nano-composite materials. In spite of the diversity of crustacean species they share a similar structural principle for their cuticle: An organic matrix composed of chitin-protein fibers associated with various amounts of crystalline and amorphous calcium carbonate (ACC). Although this structural principle is ubiquitous the mechanical properties of the exoskeleton vary so that the cuticles of different species are well adapted to their different habitats and living conditions like their escape behavior. For isopods – a sub-group of the Crustacea- it is thought that the relative amounts of mineral modifications present in the cuticle are accountable for this. Besides that, the distribution of the various components should also affect the properties of the cuticle. The aim of this study was to show possible adaptations of the mechanical performance of different isopod cuticles to their specific biological requirements. Therefore we analyzed the chemical and structural cuticle composition of isopods adapted to various habitats and escape strategies using X-ray diffraction, scanning electron microscopy (SEM), scanning force microscopy (SFM) and confocal µ-Raman spectroscopic imaging. Latter reveals for all investigated species a layered arrangement of the mineral phases where calcite is restricted to the outer area of the cuticle providing a protective layer. ACC is localized in the middle having only little overlap with the crystalline layer and serves as transient calcium carbonate reservoir. SFM nano-indentation tests performed on cross sections of the cuticles of different isopods reveals higher mechanical strength for the crystalline than for the ACC rich phases. These results suggest that variations in the thickness of the calcite and ACC containing layers as well as the amount of organic material within the mineralized composite lead to variations in cuticle hardness and flexibility. High-resolution SEM investigations on freeze fractured samples show clearly that the macroscopic failure is additionally influenced by the microstructure and the interaction of the organic-inorganic interface.
4:00 PM - Z2:Tissue mecha
4:30 PM - **Z2.6
Structural Biomimetics for Mechanical Design of Functional Materials.
Mehmet Sarikaya 1 Show Abstract
1 GEMSEC, Materials Science and Engineering, University of Washington, Seattle, Washington, United States
Structural and compositional control of inorganic materials at the molecular-scale is the key in the synthesis of novel functional material systems. Biological hard tissues may serve as models for engineering materials as these biocomposites have excellent combination of functional properties due to their highly controlled chemistry, interfaces, structures, dimensions and morphologies leading to an efficient dynamic mechanical design. Biocomposites include bacterial nanoparticles and ordered films, amorphous architectures of spicules, crystalline structures of spines, layered and segmented organization of mollusk shells, nanoparticulate hybrid composites of mammalian bone and dental tissues. A common denominator in all hard tissues is the presence of biomacromolecules in addition to inorganic phases. The macromolecules, in particular proteins and polysaccharides, may be enzymes, nucleators, habit modifiers, molecular templates or scaffolds, or simply integrated structural components, that could control the intricate nano-and micro-structures of biocomposites from the molecular to the macro-levels through biochemical interactions with inorganic units where molecular recognition plays a crucial role. Here we present examples from our long-running work on the structure-property correlation of a variety of biological hard tissues from single celled and multicellular organisms, towards achieving functional material design lessons. Finally, we will propose biomechanical design rules and potential molecular biomimetic approaches to designing genetically engineered materials for practical technology and medicine. The research is supported by NSF-MRSEC, NSF-Biomat., and NIH.
5:00 PM - Z2.7
Nano- and Micro-Scale Mechanical Characterization of Biological Materials.
Jill Powell 1 Show Abstract
1 , CSM Instruments, Inc., Needham, Massachusetts, United States
Understanding the mechanical behavior of biological materials is essential to the development of biomedical devices. In order to maximize long-term performance of implanted devices, the mechanical behavior of biological tissue with changing environmental conditions must be fully investigated. In recent years, investigating these systems at a degree beyond the traditionally available macroscopic methods has become a great focus. This includes the use of micro- and nano-scale contact mechanical characterization such as indentation testing and scratch testing. CSM Instruments, through collaborative efforts with a number of well-recognized industry partners, has developed extensive experience in the varied analysis methods required by this quickly evolving field.The mechanical behavior of biomaterials (both biological and synthetic) span multiple magnitude levels, from the intracellular forces operating at the molecular level to macroscopic organization of multi-layer coating systems commonly employed. The purpose of this presentation is to introduce a number of theoretical and experimental studies, with the desire to create discussion that will lead to the further exploitation of mechanical characterization techniques.
5:15 PM - Z2.8
A Potentially Low Dissipation Mechanism for Load Support by Articular Cartilage.
Jeffrey Ruberti, 1 , Jeffrey Sokoloff 1 Show Abstract
1 Physics, Northeastern University, Boston, Massachusetts, United States
Articular cartilage comprises charged macromolecules, proteoglycans, trapped in the spaces within a network of collagen fibrils. When it is subjected to load, the collagen fibrils become more parallel to each other. The proteoglycan molecules each have a protein backbone, with 50-100 chondroitin sulphate chains (each with atomic weight of 20,000) attached. The proteoglycan backbone is about 300-400 nm long and each attached chondroitin sulphate chain is about 40nm long. The points of attachment of the chondroitin sulphate chains are between 3 and 8 nm apart, which is comparable to what one has for polyelectrolyte brush coated surfaces. A mechanism likely to lead to dissipation as load in the joints is applied and removed is entanglement and disentanglement of the chondroitin sulphate chains. We propose that this mechanism is suppressed by osmotic pressure due to counterions between the proteoglycan molecules, which keeps these molecules sufficiently far apart to prevent entanglement of the chains, which could lead to dissipation by the above entanglement mechanism. In order to support a load on the order of 2 MPa, typical of the static loads carried by articular cartilage, the counterion concentration between the proteoglycan macromolecules must be a factor of 5 times larger than typical salt concentrations in living matter (0.15M). A solution of the Poisson-Boltzmann equation shows that at such high counterion concentrations, the typical Debye-Huckel screening that often occurs when excess salt is present does not destroy this effect. Thus, the counterion osmotic pressure is able to support the load and keep the macromolecules sufficiently far apart to prevent dissipation due to the above entanglement mechanism.
5:30 PM - Z2.9
Lateral Migration and Structuration of Vesicles with Viscosity Contrast in Simple Shear and Poiseuille Flows.
Gwennou Coupier 1 , Natacha Callens 2 , Badr Kaoui 1 3 , Christophe Minetti 2 , Frank Dubois 2 , Chaouqi Misbah 1 , Thomas Podgorski 1 Show Abstract
1 Laboratoire de Spectrométrie Physique, Université Joseph Fourier (Grenoble 1) and CNRS, St Martin d'Hères France, 2 Microgravity Research Center, Université Libre de Bruxelles, Brussels Belgium, 3 Laboratoire de Physique de la Matière Condensée, Université Hassan II - Mohammedia, Casablanca Morocco
Flow of confined soft entities, such as vesicles, drops, capsules or blood cells in the circulatory system and in microfluidic devices, is a problem of a paramount importance with both fundamental and practical interests. The ability of these entities to adapt their shapes under non-equilibrium conditions allows them to migrate transversally even in theabsence of inertia (the Stokes limit). Transverse migrations induce non uniform lateral distributions of the suspended entities, which has important consequences on the rheology of a confined suspension. Likewise, the migration should impact on transport efficiency in various microfluidic devices that are now being developed for sorting different components out of a suspension.
In this contribution, we will present an overview of our recent experiments on biomimetic phospholipidic vesicles placed in two model flows : simple shear between two sliding walls and Poiseuille flow in a channel. Some of these experiments were run under microgravity conditions in order to get rid of the screening of the lift forces by the vesicle's weight.
In the simple shear flow, we focused on the lateral migration velocity , and also on the distribution of steady vesicles between the two walls under a balance of lift forces and hydrodynamic interactions . Both depend on vesicle size, deflation, and on the viscosity contrast between the inner and outer solutions.
When vesicles are placed in a Poiseuille flow, the lift force due to the presence of the wall is non-linearly coupled to the effects of the velocity profile curvature, that induces shape changes and also lateral migration . This results in a new migration law, that we confirmed through numerical simulations based on the boundary integral method; this law markedly differs from its analogue in the simple shear situation. Still, it depends strongly and non monotonously on the vesicle's deflation and viscosity contrast, which can be understood on the ground of general symmetry considerations .
 N. Callens, C. Minetti, G. Coupier et al., to appear in Europhys. Lett., arXiv:0804.0761
 T. Podgorski et al. , in preparation
 B. Kaoui, G. Ristow, I. Cantat et al., Phys. Rev. E 77, 021903, (2008)
 G. Coupier, B. Kaoui, T. Podgorski et al., Submitted to Phys. Rev. Lett., arXiv:0803.3153
5:45 PM - Z2.10
The Large Strain Hysteretic Behavior of Mussel Byssal Threads.
Brian Greviskes 1 , Mary Boyce 1 Show Abstract
1 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States
Mussels are known for their ability to remain adhered to the rocks of their aquatic habitat, even in the face of the large and repetitive forces of the pounding surf. To do this, they have evolved an attachment appendage (the byssal thread), which provides a resilient yet dissipative large strain behavior. The threads possess a multi domain protein-rich block copolymer architecture, which stores “excess” length in the form of folded protein domains. As the threads are stretched these domains unfold; unloading allows refolding, enabling repeated loadings. The thread’s macrostructure is also important in its deformation behavior. The threads are highly graded, with a soft compliant proximal section (attaches to mussel stem/body) transitioning to a harder and stiffer distal section (attaches to coastal rocks). The proximal section is larger in diameter, though it’s mainly composed of a jelly-like sheath, void of protein filaments, surrounding a thin protein-rich core. The distal section is protein-rich throughout. In the distal, these protein filaments lie parallel to the thread axis; in the proximal they are randomly oriented. This research examines, through an interplay between experiments and modeling, the mechanics of the mussel byssus from the components of the individual threads up to a multi-thread system level. We have conducted an extensive experimental characterization of the mechanical behavior of each thread region at different strain rates under monotonic and cyclic loading conditions, and have identified the material behavior with regard to rate and deformation effects.The two regions were observed separately in testing. The distal exhibits an initially stiff behavior followed by a “yield” and a transition into a more compliant large strain behavior. Upon unloading, the material reveals substantial recovery, indicating that the “yield” is not a plastic event, but rather the result of the microstructural evolution of the folded domains. More recovery occurred in slower rate tests and when the material was allowed to “rest” prior to subsequent loadings, demonstrating that the refolding is rate-dependent. The proximal region is found to be more compliant, demonstrating a “yield” that is more of a continuous unfolding event. The same type of recovery was observed in the proximal section.The data from these tests form the basis for the development of microstructurally-informed constitutive models of the stress-strain behavior of the proximal and distal thread regions. The models use the microstructural evolution of the protein filaments as the major deformation mechanism, and account for protein unfolding throughout the deformation process. Further, the models employ the rate-dependent refolding to predict the behavior of the material under cyclic loading conditions. Overall these models simulate the highly nonlinear, rate-dependent, softening, hysteretic, and resilient features of the material’s mechanical behavior quite well.
Kalpana Katti North Dakota State University
Christian Hellmich Vienna University of Technology (TU Wien)
Ulrike G. K. Wegst Drexel University
Roger Narayan North Carolina State University
Z3: Tissue Mechanics III
Wednesday AM, December 03, 2008
Back Bay B (Sheraton)
9:00 AM - **Z3.1
From Diffraction to Imaging - New Avenues in Studying Hierarchical Biological Tissues with X-ray Microbeams.
Oskar Paris 1 Show Abstract
1 Biomaterials, Max Planck Institute of Colloids and Interfaces, Potsdam Germany
Many biological materials such as vertebrate bone or invertebrate cuticles and shells have optimized mechanical properties which are determined by their hierarchical structure ranging from the atomic/molecular level up to macroscopic length scales. Structural investigations of such materials require new experimental techniques with position resolution ideally covering several length scales simultaneously. Beside light- and electron microscopy, synchrotron radiation based X-ray imaging techniques offer excellent possibilities in this respect, ranging from full field imaging with absorption- or phase contrast to X-ray microbeam scanning techniques. A particularly useful approach for the study of biological tissues is the combination of X-ray microbeam scanning with nanostructural and crystallographic information obtained from X-ray scattering (small- and wide-angle scattering, SAXS/WAXS). This allows constructing quantitative images of nanostructural parameters with micrometer scanning resolution, and hence, covers two length scales at once. Combining scanning SAXS/WAXS additionally with simultaneous X-ray fluorescence (XRF) mapping for chemical analysis provides a unique method combination for a comprehensive characterisation of complex biological tissues. The present paper reviews some of our recent scanning microbeam SAXS/WAXS/XRF work on bone and invertebrate tissues performed at the μ-Spot beamline at the synchrotron radiation source BESSY II in Berlin. The focus is on the imaging capability of the method, which turns out to be very useful for a better understanding of structure-function relationships in general and the complex mechanics of such systems in particular.
9:30 AM - Z3.2
Stress Profiling in Mollusk Shells with High Depth Resolution.
Emil Zolotoyabko 1 , Vladislav Demensky 1 , Boaz Pokroy 2 Show Abstract
1 Materials Engineering, Technion, Haifa Israel, 2 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States
Inorganic/organic composites produced by living organisms reveal superior properties, first of all the mechanical properties, which are strongly affected by the hierarchical nano- and micro-structures of these materials. The sophisticated arrangement of the structural blocks results in three-dimensional strain/stress fields within a material. These fields are the subject of intensive studies aimed at better understanding of the mechanics of these biogenic composite structures. The focus of numerous research groups is on the nacre structure of mollusk shells, which is built of oriented ceramic tablets of calcium carbonate, "glued" together by organic substance. This marvelous structure demonstrates greatly enhanced fracture toughness due to the crack arrest and deflection at the inorganic/organic interfaces. These interfaces are the source of internal forces and related inhomogeneous stress/strain fields. In this paper we report on the depth profiling of residual stresses in bivalve mollusk shells by using strain gauge measurements under controlled etching in the selected shell areas. A 30 nm depth resolution in stress profiling can be easily achieved. Gradual stress release under etching provides direct evidences that bivalve shells are, in fact, strained multilayered structures, which are elastically bent due to forces evolved at the organic/inorganic interfaces. The stresses are mainly concentrated in the nacre layer near the inner surface of the shell adjacent to the mollusk mantle. In this region, the stress release data reveal pronounced oscillations with a periodicity on a micrometer scale that reflects periodic stress variation across a multilayered structure. The obtained experimental results are well reproduced in the framework of the developed model, which describes the stress release in the strained multilayered structure subjected to selective etching.
9:45 AM - **Z3.3
Biomimetic Design of New Functional Materials.
Osamu Takai 1 , Chiya Numako 2 Show Abstract
1 , Nagoya University, Nagoya Japan, 2 , The University of Tokushima, Tokushima Japan
We are developing "biomimetic materials processing (BMMP)", which is defined as the design and synthesis of new functional materials by refining knowledge and understanding of related biological products, structures, functions and processes. The BMMP is not a simple imitation of biological materials processes, but is advanced materials processing for bionics, electronics, photonics, mechanics and so on. By means of the BMMP we can prepare “biomimetic materials” or more widely “bioinspired materials”. Chitons incorporate significant amounts of magnetite into their radular teeth and make tough teeth by using magnetite. The formation system of the teeth is like a belt conveyor. In this way the teeth of chitons have unique composition and structure. Mechanical properties of the radular teeth of a chiton Acanthopleura japonica were measured by nanoindentation and their crystal structures were determined by X-ray analyses. We discuss the biomimetic design of new functional materials from the mechanical viewpoint.
10:15 AM - Z3.4
Mechanical Properties Testing of Nacre: Combined In-Situ Tensile Testing with X-ray Diffraction.
Karen Magid 1 , Stephan Frank 1 , David Bronfenbrenner 2 , Apurva Mehta 3 , Ralph Spolenak 1 Show Abstract
1 Laboratory for Nanometallurgy, ETH-Zurich, Zurich Switzerland, 2 Materials Science and Engineering, University of California at Berkeley, Berkeley, California, United States, 3 Stanford Synchrotron Radiation Laboratory, Stanford University, Menlo Park, California, United States
Multilayered structures exhibit complex responses to external loads. Microstructured biomaterials such as nacre are presently receiving a great deal of attention due to their prospects as one of the keys in the elucidation of the next generation of advanced materials. The interwoven brick-and-mortar structure of nacre provides a particular combination of high strength, hardness, and toughness properties which is being utilized for its own merits and inspiring design of novel biomimetic materials. In order to move toward a more fundamental understanding of the strain response to load on a local level in nacre, development of a non-destructive, spatially resolved strain measurement technique using highly coherent, monochromatic synchrotron X-rays is underway. To fully capture the complex responses to strain, a technique is required which can measure and characterize the full second-rank strain tensor for the different components of these complex layered systems. The powder diffraction method developed at the Stanford Synchrotron Radiation Laboratory (SSRL) to map the 3D strain ellipsoidal surface by rotating the sample and thus sampling several locations on the surface presents an opportunity to gain information about the fundamental strain responses of the multilayered nacre materials. These samples have layered microstructures, which allow this multi-wavelength powder technique combined with the area detector to surpass the capabilities of traditional strain determination techniques. Sections from two species of nacre, Haliotis refescens and Pteria penguin, were prepared from larger specimens so that flat sections could be tested. Additionally, Digital Image Correlation (DIC) strain measurements were recorded during the tests for macroscale strain measurements. The combination of these techniques provides information across several length-scales. These preliminary tests demonstrate the viability of this experimental method. The ability to probe the second-rank strain tensor of the complex materials will provide valuable information about the mechanical properties of the systems undergoing tension. The direct measure of the elastic strain will enable us to determine either the fracture strength of the mineral component or the yield strength of the polymer component, depending on which is lower.
10:30 AM - Z3.5
Lipid Membrane Penetration Forces from AFM Force Spectroscopy.
Elizabeth Hager-Barnard 1 , Benjamin Almquist 1 , Nicholas Melosh 1 Show Abstract
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
Understanding how short peptide sequences are able to penetrate cell membranes is important in disease studies and engineering new peptides for drug delivery. While the energetics of membrane penetration has been well studied, the mechanical landscape during contact, translocation, and exit is largely unknown. We used atomic force spectroscopy (AFM) studies on lipid membrane stacks to map the force-distance profile during penetration of short peptides. These force curves reveal the spatial location and magnitudes of penetration barriers that can be related to peptide molecular structure and orientation. We studied the widely used cell penetrating peptide HIV-TAT, a positively charged 10-mer with six arginine groups. The peptides were attached in a single layer at the end of a flat AFM tip giving nanometer spatial resolution relative to the lipid bilayer. Using stacks of lipid membranes rather than individual supported membranes improves data quality by removing substrate effects and providing better statistics.
11:45 AM - Z3.7
Age-related Biomechanical Changes in Aortic Tissue Determined by Scanning Acoustic Microscopy.
Riaz Akhtar 1 , Michael Sherratt 2 , Rachel Watson 3 , Brian Derby 1 Show Abstract
1 School of Materials, The University of Manchester, Manchester United Kingdom, 2 Tissue Injury and Repair Group, Faculty of Medical and Human Sciences , The University of Manchester, Manchester United Kingdom, 3 Dermatological Sciences Research Group, Faculty of Medicine and Human Sciences, The University of Manchester, Manchester United Kingdom
There is extensive evidence that the gross-mechanical properties of connective tissues are determined by the micro-mechanical properties of their extracellular matrix (ECM) components. Within these matrices, collagen fibres confer tensile strength and elastic fibres allow tissues to deform and passively recoil. Age-related loss of this tissue elasticity has a profound effect on life-expectancy, principally via the mechanical failure of large arteries and reduced recoil in the lungs. Although loss of elasticity is a common feature across most ageing tissues, previous studies have failed to identify a consistent change in elastic fibre distribution or organisation with age. Large arteries, such as the aorta for example, which may undergo more than 2 x 109 extension and relaxation cycles during a human lifetime, rely on the mechanical properties of their elastic fibres to buffer variations in blood pressure. As a consequence of these physiological demands, these arteries are highly enriched in elastin and associated proteins such as fibrillin. Very little is known however about the relative mechanical contribution of these differing ECM matrix components to tissue elasticity in situ. Quantification of the micro-mechanical properties of the elastic fibre components is of great importance therefore to better understand not only the function of healthy tissues but also to shed light on the pathological processes underlying loss of elasticity in ageing tissues. In this study, the mechanical properties of unfixed aortic tissue sections were quantified for young (18 month old) and aged (≥ 8 years) sheep using scanning acoustic microscopy (SAM). In previous studies we have demonstrated that elastic fibre distributions in vascular tissue are clearly visible by SAM without the need to fix or stain the tissues. Micron scale variations in acoustic wave propagation speed (which is related to Young’s modulus) and ultrasonic attenuation were determined using SAM at ultra-high frequencies (960 MHz to 1.1 GHz) with the V(f) method (measurement of output voltage vs signal frequency). At these frequencies, a spatial resolution of approximately 1 μm was obtainable.Attenuation was significantly higher in aged as compared with young aorta samples and the age-related differences were far more pronounced than the observed changes in acoustic wave speed. Complementary immunohistochemistry studies suggest that ultrasonic attenuation at these high frequencies is positively correlated with the degree of molecular cross-linking. Uncontrolled protein crosslinking has been postulated as a potential mechanism in the age-related loss of tissue elasticity and our preliminary investigations appear to support this hypothesis.
12:00 PM - **Z3.8
Materials from the Seas: Characterization and Synthetic Mimics of Mussel and Barnacle Adhesives.
Cristina Matos-Perez 1 , James White 1 , Lauren Hight 1 , Jessica Wojtas 1 , Jeremy Burkett 1 , Joshua Cloud 1 , Jonathan Wilker 1 Show Abstract
1 Department of Chemistry, Purdue University, West Lafayette, Indiana, United States
12:30 PM - **Z3.9
Biomechanics of the Cytoskeleton: Cell Contractility and Mechanosensitivity of Cell Adhesion.
Vikram Deshpande 1 Show Abstract
1 Engineering Dept., Cambridge University, Cambridge United Kingdom
A variety of in vitro cellular systems that model in vivo physiologic and pathologic processes have recently been developed. These systems such as arrays of micro-needles and micro-patterned substrates are used to quantitatively probe mechanical responses of cells to a variety of bio-chemo-mechanical stimuli. These studies have begun to reveal that mechanical coupling of the cell to its environment has implications for cell development, differentiation, disease, and regeneration. Key roles in molecular pathways are played by adhesion complexes and the actin/myosin cytoskeleton, whose contractile forces are transmitted through transcellular structures. This talk will focus on the contractility of the cytoskeletal network and the “inside-out” mechanism whereby cytoskeletal tension drives focal adhesion assembly. The cytoskeletal contractility model accounts for the dynamic reorganization of the actin/myosin stress fibers and is motivated by three key bio-chemical processes: (i) an activation signal that triggers actin polymerization and myosin phosphorylation, (ii) the tension dependent assembly of the actin and myosin into stress fibers and (iii) the cross-bridge cycling between the actin and myosin filaments that generates the tension. Simple relations are proposed to model these coupled phenomena and a continuum model developed for simulating cell contractility. The mechanosensitivity of focal adhesion formation is motivated from thermodynamic considerations and a continuum framework developed in which the cytoskeletal forces drive the assembly of the focal adhesion multi-protein complexes. The coupled cytoskeletal and adhesion models are shown to be capable of predicting key experimentally established characteristics including (a) the decrease in the forces generated by the cell with increasing substrate compliance, (b) the influence of cell shape and boundary conditions on the development of structural anisotropy, (c) the high concentration of the stress fibers at the focal adhesions and the (d) the ability of cells to detect chemical and mechanical heterogeneities in their environment.
Z4: Cellular Mechanics
Wednesday PM, December 03, 2008
Back Bay B (Sheraton)
2:30 PM - **Z4.1
Chemo-Mechanical Stimulation of Cells by Nano-Digital Environments.
Joachim Spatz 1 Show Abstract
1 , MPI for Metals Research, Stuttgart Germany
Engineering of cellular environments has become a valuable tool for guiding cellular activity such as differentiation, spreading, motility, proliferation or apoptosis which altogether regulates tissue development in a complex manner. The adhesion of cells to its environment is involved in nearly every cellular decision in vivo and in vitro. Our approach to engineer cellular environments is based on self-organizing spatial positioning of single signaling molecules attached to inorganic or polymeric supports, which offers the highest spatial resolution with respect to the position of single signaling molecules. This approach allows tuning cellular material with respect to its most relevant properties, i.e., viscoelasticity, peptide composition, nanotopography and spatial nanopatterning of signaling molecule. Such materials are defined as “nano-digital materials” since they enable the counting of individual signaling molecules, separated by a biologically inert background. Within these materials, the regulation of cellular responses is based on a biologically inert background which does not trigger any cell activation, which is then patterned with specific signaling molecules such as peptide ligands in well defined nanoscopic geometries. This approach is very powerful, since it enables the testing of cellular responses to individual, specific signaling molecules and their spatial ordering. Detailed consideration is also given to the fact that protein clusters such as those found at focal adhesion sites represent, to a large extent, hierarchically-organized cooperativity among various proteins. Moreover, “nano-digital supports” are capable of involvement in such dynamic cellular processes as protein ordering at the cell’s periphery which in turn leads to programming cell responses.
3:00 PM - Z4.2
Constitutive Modeling of Cell Cytoskeleton.
Sitikantha Roy 1 , H. Jerry Qi 1 Show Abstract
1 Mechanical Engineering, University of Colorado, Boulder, Colorado, United States
In the present paper, we are proposing a constitutive model (stress-strain relation) for a 2D semiflexible random network, such as cell-cytoskeleton. We idealize a semiflexible network through a unit-cell representation, consisting of four semiflexible main chains and four equivalent springs. The unit cell representation captures the actual network’s distinct elastic energy storage mechanisms under the externally applied deformation field. The elasticity of a semiflexible network is governed by both enthalpic and entropic variation. In addition, the enthalpic effect shows two distinct regimes of energy storage mechanism: affine regime and non-affine regime. Most of the constitutive models existing in the current literature concentrate on the affine characteristics of the cytoskeleton deformation. Recently some computational models, based on finite element analysis of the actual network, such as Mikado Model, were used to demonstrate the basic physics involved in the mechanical deformation of semiflexible network. These models are computationally expensive and it is very difficult, if not impossible to develop a macroscopic constitutive relation based on them. In the present case, we develop a constitutive relation through an unit cell based homogenization approach, which also takes into account the basic physics of affine to nonaffine transition in the microscopic scale.
3:15 PM - Z4.3
Novel Scanning Probe Technique for Live Cell Imaging.
Chris Hassler 1 , Gaurav Singh 1 , Ravi Saraf 1 Show Abstract
1 Chemical and Biomolecular Engg., University of Nebraska-Lincoln, Lincoln, Nebraska, United States
We demonstrate a novel scanning probe technique to image the local ionic environment near the surface of a live yeast cell, Saccharomyces Cerevisiae, at sub 100nm spatial resolution. The technique is based on the ion charging and discharging of the Electrical Double Layer (EDL) formed at the platinum coated tip of a typical AFM cantilever. The charging and discharging is accomplished by alternating electric potential applied between the cantilever and a bottom gold electrode. Yeast cells are immobilized on the bottom gold electrode and exposed to oxidative stress by introduction of a menadione solution. The cells respond by pumping out menadione-GSH complex outside the cell. This affects the EDL dynamics at the Platinum tip positioned in close proximity to the cell wall and ultimately the cantilever vibration amplitude. The cantilever vibration amplitude is quantitatively recorded by a custom–designed heterodyne interferometer. Further applications relating to response of mammalian cells to external stress will be presented.
3:30 PM - Z4.4
Adhesion Between LHRH-Coated AFM Tips and Breast Cancer Cells.
Juan Meng 1 2 , Wole Soboyejo 1 2 Show Abstract
1 Princeton Institute for the Science and Technology of Materials , Princeton University, Princeton, New Jersey, United States, 2 Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey, United States
This paper presents the results of atomic force microscope (AFM) measurements of the adhesion force between breast cancer cells and Luteinizing Hormone Releasing Hormone(LHRH)–Coated AFM tips. As a control, the adhesive interactions are measured between normal breast cells and LHRH-coated AFM tips. The measurements show conclusively that the adhesive forces to breast cancer cells are almost five times bigger than those to normal breast cells. The increase is attributed largely to the interactions between LHRH and LHRH receptors that are over-expressed on breast cancer cells. The implications of the results are discussed for the localized targeting of cancer with ligand-conjugated nanoparticles.
3:45 PM - Z4.5
Confined Thin Film Adhesion Measurement in the Presence of Long-ranged Intersurface Forces.
Kai-tak Wan 1 Show Abstract
1 Mechanical Eng, Northeastern University, Boston, Massachusetts, United States
A membrane is clamped at the circular periphery before making contact with a planar surface of a flat-ended punch. External load applied to the punch delaminates the membrane with a shrinking contact circle. The interfacial adhesion as a result of the intersurface forces (or disjoining pressure) is investigated by a thermodynamic energy balance. When the surface force is short-ranged, the classical Johnson-Kendall-Roberts (JKR) limit applies, and "pull-off" at non-zero radius is expected. Should the disjoining pressure be long-ranged, the Derjaguin-Muller-Toporov (DMT) limit applies and a zero "pull-off" radius is expected. The two distinctly different behavior is linked by a DMT-JKR transition. The new model is essential in a number of applications in nano-technology (e.g. stability of micro- and nano- structures) and life-sciences (e.g. cell adhesion).
4:30 PM - Z4.6
Influence of the Anisotropic Mechanical Properties of Synthetic Liquid Crystalline Materials on Mammalian Cells.
Zhongqiang Yang 1 , Ankit Agarwal 1 , Sean Palecek 1 , Nicholas Abbott 1 Show Abstract
1 Chemical and Biological Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States
It is now widely appreciated that mammalian cells probe their mechanical environment through formation of focal adhesions, and that they respond to their biomechanical analysis in a variety ways. For example, in studies based on model materials such as polyacrylamide (PAAM) and poly(dimethylsiloxane) (PDMS), cellular processes such as cell adhesion, spreading, migration and differentiation can be modulated by mechanical environment. The mechanical environment of native tissue, however, is typically more complex than these model materials as native materials typically possess a hierarchy of structural organization, and they are often anisotropic in nature. This presentation will describe the use of synthetic liquid crystalline materials to create mechanical environments for cells that capture some of the complexity of native cellular environments. Two examples will be presented. In the first example, the mechanical properties of colloid-in-liquid crystal gels will be described, and their influence on the attachment, spreading and proliferation of fibroblasts will be reported. In contrast to model materials studied in the past, the networks of colloids that percolates through these materials lead to the creation of mechanical environments that are highly heterogeneous on the cellular scale. The second example will revolve around liquid crystalline elastomers that were synthesized to create anisotropic mechanical environments, and the results of a study of the influence of these anisotropic environments on cells. In each example, the mechanical properties of the liquid crystalline materials will be contrasted to widely studied model biomaterials such as hydrogels.
4:45 PM - Z4.7
Self-Assembled Fibrin Fiber and Membrane Air Bridges: Fluid Mechanical, Adhesive and Elastomeric Properties.
Santosh Pabba 1 , Mehdi Yazdanpanah 1 , Brigitte Fasciotto 1 , Vladimir Dobrokhotov 1 , Robert Cohn 1 Show Abstract
1 ElectroOptics Research Institute and Nanotechnology Center, University of Louisville, Louisville, Kentucky, United States
Fibrin is an interesting material in its ability to rapidly polymerize from solutions, in its adhesive strength, and in being a highly extensible elastomer. We have found that through simultaneous polymerization from pure extracts of fibrinogen and capillary driven thinning on corrugated solid substrates made by micromachining that fibrin will self-assemble into unique suspended nanofiber air bridges down to 20 nm in diameter. Note, that similarly small fibers have also been made from pure extracts of actin by this same process of brushing a solution of monomeric protein over micromachined solid substrates. At somewhat slower brushing speeds, rather than forming fibers, fibrin forms into regular arrays of suspended membranes of the appearance of diamond shaped trampolines. These are found anchored by nanofibers between each group of four circular vertical pillars on the substrate. With a micromachined array of square pillars, vertical, rather than horizontal membranes are found spanning between adjacent sidewalls of the pillars. Variously patterned micromachined arrays are being fabricated with the goal of better understanding the wetting and capillary thinning forces that result in stable assembly of the nanostructured fibrin air bridges. Additionally, the fibrin when maintained under buffer solution is strongly adhesive, so much so that it is possible to break a fibrin fiber and using its adhesiveness to attach it to another fibrin air bridge, thereby constructing a more complicated elastomeric structure. We specifically report on the measurements of extensibility and adhesiveness of single fibrin nanofibers as measured using an atomic force microscope and a micromanipulator. We also report on time-resolved AFM measurements of viscosity changes and single fiber formation on the AFM probe as the fibrin polymerizes. We are considering the self-assembly of arrays of fibrin air-bridges for application to construction of MEMS and BioMEMS devices having complex three-dimensional geometries. These could be used as microactuators and sensors, as well as microenvironments for the study of cell-cell communication. For instance each trampoline shaped membrane is large enough, around 10 microns, to gently support a single live cell well above a solid substrate. This system offers the possibility of an unusually well organized scaffold for biological and biophysical studies.
5:00 PM - Z4.8
Viscoelastic Properties of Healthy and P. Falciparum Infected Human Red Blood Cells.
Ming Dao 1 , Hedde van Hoorn 1 , Subra Suresh 1 Show Abstract
1 Materials Science & Engineering, MIT, Cambridge, Massachusetts, United States
A systematic study of healthy and Plasmodium falciparum infected red blood cells (RBCs) using an atomic force microscope (AFM) is carried out. Cell height and stiffness are measured for each individual cell. The initial contact point is carefully determined through an algorithm using Bayesian statistics. Different indentation rates at room and febrile temperatures are conducted to study the viscoelastic response of the RBC membrane. Experiments on P. falciparum infected RBCs are also performed. A stiffness increase for ring stage RBCs is measured to be a couple of times the stiffness of healthy RBCs. As the infected stage progresses a higher variation of single indentations is observed. The indentation measurements are modeled using a full 3D finite element model. The extracted membrane viscoelastic properties are found to be consistent with the results obtained using laser tweezers as we as those presented in the literature.
5:15 PM - Z4.9
Atomic Structure and Elastic Anisotropy of Crystalline α-chitin: An Ab-initio Based Conformational Analysis.
Michal Petrov 1 , Martin Friak 1 , Liverios Lymperakis 1 , Dierk Raabe 2 , Joerg Neugebauer 1 Show Abstract
1 Computational Materials Design, Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf Germany, 2 Microstructure Physics and Metal Forming, Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf Germany
5:45 PM - Z4.11
Inhomogeneous Dynamic Shear Properties of Articular Cartilage.
Mark Buckley 1 , Jonathan Fouchard 2 , Matthew Farrar 1 , Jason Gleghorn 1 , Lawrence Bonassar 1 , Itai Cohen 1 Show Abstract
1 , Cornell University, Ithaca, New York, United States, 2 , Université Paris-Diderot (Paris 7), Paris France
Articular cartilage is a specialized connective tissue that covers bones in joints and transmits load across them. Its complex and inhomogeneous structure endows it with a specific mechanical response that enables it to remain effective for 6-9 decades, or most of a human lifetime. However, damage to the structure of articular cartilage gives rise to disease by compromising proper functionality. Consequently, determining the complicated relationship between structure and function in this tissue is critical to understanding the origin of cartilage diseases. Here, we measure spatial variations in the shear modulus |G*(ω)| of bovine articular cartilage. In the steady state (ω=0), the shear modulus |G*(ω=0)| can vary by two orders of magnitude across a single sample and exhibits a global minimum 50-250 μm below the articular surface. Moreover, the shear modulus profile depends strongly on the applied shear and axial strains. The greatest change in |G*(ω=0)| occurs at the global minimum where the tissue is highly nonlinear, stiffening under increased shear strain and weakening under increased compressive strain. For 0.5 1/s<ω<1500 1/s, we find that the frequency dependence of |G*(ω)| at the global minimum and deeper into the tissue (d>500 μm) differ significantly. Comparisons of |G*(ω)| for samples sheared parallel and perpendicular to the split line help to elucidate our results, which can be explained through a simple thought model describing the observed behavior in terms of known spatial variations in the structure and composition of articular cartilage.
Kalpana Katti North Dakota State University
Christian Hellmich Vienna University of Technology (TU Wien)
Ulrike G. K. Wegst Drexel University
Roger Narayan North Carolina State University
Z5: Mechanics of Biomolecules
Thursday AM, December 04, 2008
Back Bay B (Sheraton)
9:30 AM - **Z5.1
Development of Multi Functional Nanostructures for Biomedical Imaging and Treatment.
Donglu Shi 1 , Hoon Sung Cho 1 , Christopher Huth 1 , Wei Wang 1 , Yan Chen 2 , Hong Xu 3 , Hongchen Gu 3 , Jie Lian 4 , Lumin Wang 4 , Rodney Ewing 4 , Guokui Liu 5 , Giovanni Pauletti 6 , Zhongyun Dong 7 Show Abstract
1 Chemical and Materials Engineering, University of Cincinnati, Cincinnati, Ohio, United States, 2 Institute for Nutritional Sciences, Chinese Academy of Sciences, Shanghai China, 3 Med - X Institute, Shanghai Jiao Tong University, Shanghai China, 4 Departments of Geological Sciences, Nuclear Engineering & Radiological Sciences and Materials Science & Engineering, University of Michigan, Ann Arbor, Michigan, United States, 5 Chemistry Division, Argonne National Laboratory, Argonne, Illinois, United States, 6 James L. Winkle College of Pharmacy, University of Cincinnati, Cincinnati, Ohio, United States, 7 Department of Internal Medicine, University of Cincinnati, Cincinnati, Ohio, United States
There is currently an increasing need for early detection and treatment of cancer before anatomic anomalies are visible. A major challenge in cancer diagnosis is to be able to locally biomark the cancer cells in clinical pathology for maximum therapeutic benefit. In cancer therapy, targeting and localized treatment are also the key challenges. One promising strategy for overcoming these challenges is to make use of highly fluorescent nanoparticles for qualitative or quantitative in vivo detection of tumor cells. However, an optimum nanostructure is yet to be developed with multiple functionalities including intensive fluorescence, effective drug storage capability, and therapeutic means. In this presentation, we report here a scheme of new nanostructure design that ideally satisfies these important requirements. By novel surface engineering, multiple functionalities are developed in a nano assembly for both in vivo imaging and treatment. Such a nano assembly possesses several uniquely combined functionalities including strong fluorescence, anti cancer drug storage, and hyperthermia. In this way, it is possible that early cancer diagnosis and local treatment can be achieved simultaneously. HRTEM, fluorescence spectroscopy, in vivo imaging experimental results will be presented on the surface nanostructures, optical properties, and cancer diagnosis.
10:00 AM - Z5.2
A Coarse-grained Molecular Dynamics Study of the Mechanical Properties of Single-layer and Double-layer Membranes.
Yong-Wei Zhang 1 2 , Ping Liu 2 Show Abstract
1 Materials Science and Engineering, National University of Singapore, Singapore Singapore, 2 , Institute of High Performance Computing, Singapore Singapore
10:15 AM - Z5.3
Mechanics of The Collagen-Hydroxyapatite Interface In Human Bone: Role of Interfacial Behavior.
Shashindra Pradhan 1 , Kalpana Katti 1 , Dinesh Katti 1 Show Abstract
1 Civil Engineering, North Dakota State University, Fargo, North Dakota, United States
10:30 AM - Z5.4
Combined Atomic Force Microscope and Atomistic Simulations Based Multiple Lengthscale Experiments and Atomistic Simulations of Bone Tissues.
Ming Gan 1 , Devendra Dubey 1 , Albert Cerrone 2 , Vikas Tomar 1 Show Abstract
1 Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, Indiana, United States, 2 Department of Civil Engineering, University of Notre Dame, Notre Dame, Indiana, United States
10:45 AM - Z5.5
Entropic-elasticity-controlled Dissociation and Energetic-elasticity-controlled Rupture Induce Catch to Slip Bonds in Cell-adhesion Molecules.
Yujie Wei 1 Show Abstract
1 Engineering Division, Brown University, Providence, Rhode Island, United States
The lifetime of biological bonds shortens exponentially with increasing tensile force, and such a bond behavior is usually termed as a ‘slip bond’ . In the last few years, experiments have further revealed that a small tensile force could strengthen bonds of adhesion molecules in the sense that bond lifetimes are prolonged. Such a binding behavior is termed as a ‘catch bond’ [2-5]. We develop a physical model to describe the kinetic behavior in cell-adhesion molecules. Unbinding of non-covalent biological bonds is decomposed into bond dissociation and bond rupture. Such a treatment on debonding processes is a space decomposition of bond breaking events. Dissociation under thermal fluctuation is non-directional in a 3-dimensional space, and its energy barrier to escape may be not influenced by a tensile force but the microstates which can lead to dissociation are changed by the tensile force; rupture happens along the tensile force direction. An applied force effectively lowers the energy barrier to escape along the loading direction. The lifetime of the biological bond, due to the superimposition of two concurrent off-rates, may grow with increasing tensile force to moderate amount and decrease with further increasing load. We hypothesize that a catch-to-slip bond transition is a generic feature in biological bonds. References Y.J. Wei, Entropic-elasticity-controlled dissociation and energetic-elasticity-controlled rupture induce catch-to-slip bonds in cell-adhesion molecules, Phys. Rev. E 77, 031910 (2008). G.I. Bell, Models for the Specific Adhesion of Cells to Cells, 200, 618-627, Science, 1978.  M. Dembo, D.C. Torney, K. Saxman, D. Hammer., The Reaction-Limited Kinetics of Membrane-to-surface Adhesion and Detachment. Proc. R. Soc. Lond. B. 234, 55-83, 1988. W. Thomas, M. Forero, V. Vogel, E.V. Sokurenko, Bacterial Adhesion to Target Cells Enhanced by Shear Force. Cell 109, 913-923, 2002. B.T. Marshall, M. Long, J.W. Piper, T. Yago, R.P. McEver, C. Zhu, Direct observation of catch bonds involving cell-adhesion molecules. Nature 423, 190-193, 2003.
11:30 AM - Z5.6
Fracture Mechanics of Beta-Structures in Protein Materials.
Sinan Keten 1 , Markus Buehler 1 Show Abstract
1 Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
The elasticity and strength behavior of proteins is crucial for their biological function. Under external forces, many proteins exhibit entropic elasticity with a characteristic stiffening behavior and unravel due to the rupture of inter-strand H-bonds. We develop a fracture mechanics based theoretical framework that considers the free energy competition between entropic elasticity of polypeptide chains and rupture of peptide hydrogen bonds, which we use here to provide an explanation for the intrinsic strength limit of protein domains at near-equilibrium deformation rates (Keten and Buehler, PRL, 2008). Our analysis predicts that individual protein domains stabilized only by hydrogen bonds cannot exhibit rupture forces larger than 100-300 pN in the quasi-static rate limit. This result explains an array of earlier experimental and computational observations that indicate such a universal, asymptotic strength limit at vanishing pulling rates. We discover that the rupture strength of H-bond assemblies is governed by geometric confinement effects, suggesting that clusters of at most 3-4 H-bonds break concurrently, even under uniform shear loading of a much larger number of H-bonds (Keten and Buehler, Nano Letters, 2008). This intrinsic limitation suggests that shorter strands with less H-bonds achieve the highest shear strength at a critical length scale. The hypothesis is confirmed by direct large-scale full-atomistic MD simulation studies of beta-sheet structures in explicit solvent. This finding explains how the intrinsic strength limitation of weak bonds can be overcome by the formation of a nanocomposite structure of cooperative H-bond clusters and covalent polypeptide chains, leading to much stronger assemblies as found in silks, amyloids and extracellular matrix. Our results explain recent experimental proteomics data, suggesting a correlation between the shear strength and the prevalence of beta-strand lengths in biology. We conclude based on this analysis that maximizing mechanical (and hence thermodynamical) stability may be a universal evolutionary design principle for beta-structures in protein materials. Our findings further illustrate that H-bond rupture events and entropic elasticity of the protein are intimately coupled and can’t be considered separately as previously believed. We validate our theoretical and computational predictions by showing close agreement with experimental AFM results. We conclude our analysis with a general discussion of elasticity and strength of key protein domains through a comparative study of the mechanical signatures of alpha-helices, beta-sheets and tropocollagen molecules (Buehler and Keten, NARE, 2008) based on MD simulation results. Our model confirms that fracture mechanics concepts, previously primarily used to explain macro-scale fracture phenomena, can also be directly applied to the nano-scale, and can be used to describe failure mechanisms in protein materials.
11:45 AM - Z5.7
Variations in Single Molecule Aggrecan Molecular Structure and Conformation after Removal of Selected GAG Constituents.
Hsu-Yi Lee 1 , Peter Roughley 2 , Alan Grodzinsky 1 3 4 , Christine Ortiz 5 Show Abstract
1 Electrical Engineering and Computer Science, MIT, Cambridge, Massachusetts, United States, 2 Genetics Unit, Shriner's Hospital for Children, Montreal, Quebec, Canada, 3 Biological Engineering, MIT, Cambridge, Massachusetts, United States, 4 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States, 5 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
The composition and spatial distribution of glycosaminoglycans (GAGs), for example keratan sulfate (KS) and chondroitin sulfate (CS), within the extracellular matrix proteoglycan aggrecan is thought to be an important determinant of the biomechanical function of cartilage tissue. Currently, the density, detailed location, and arrangement of KS-GAGs along the aggrecan core protein for various populations are not completely understood, and their relationship to aggrecan biophysical properties is largely unknown. Knowledge of such molecular structure-property relationships, in particular at the single macromolecular level, will provide an improved understanding of aggrecan function. This study utilizes high resolution atomic force microscopy (AFM) single molecule imaging to quantify the fine structure and conformation of aggrecan before and after removal of KS or CS GAGs via enzymatic degradation. Aggrecan was extracted and purified from 29-year-old human articular cartilage with no evidence of arthritic disease or joint damage. In order to remove CS or KS, aggrecan samples were incubated with protease-free Chondroitinase ABC or Keratanase II, respectively, then deposited on 3-aminopropyltriethoxysilane functionalized mica substrates and imaged via tapping mode AFM (tip radius < 10 nm, k = 42 N/m). The CS-digested aggrecan was observed to have a relatively long, GAG-free region of core protein, corresponding to the position of the CS1 and CS2 domains, which occupy ~70% of the total aggrecan contour length. However, shorter GAG chains were still, visible, located nearer the globular G1-G2 domains, corresponding to one of the putative locations of KS. The KS-digested aggrecan appeared globally similar to the untreated aggrecan, consistent with the presence of the many CS chains which are larger than KS. The aggrecan core protein profiles of all specimens were automatically traced from the AFM height images using a custom Matlab program. Full length aggrecan from treated and untreated groups were identified by the presence of N-terminal and C-terminal globular domains (G1 and G3, respectively). The average core protein contour length of the full length untreated aggrecan was measured as 474 ± 56 nm (n=19) and the average core protein contour length of full length aggrecan decreased 14% (407 ± 112 nm, n=16) after Keratanase II treatment and 28% (339 ± 71 nm, n=16) after Chondroitinase ABC treatment. The shrinkage of the core protein after Keratanase II and Chondroitinase ABC treatments suggests that the extension of untreated aggrecan results from the presence of the GAG chains and the associated repulsive and steric forces caused by these constituent GAG chains. The effective persistence length of the core protein also changed from 172±117 nm to 136±50 nm and 139±78 nm after the removal of KS and CS GAGs, respectively. The results show that both KS and CS GAGs contribute to core protein extension as well as the molecular stiffness of aggrecan.
12:00 PM - Z5.8
Mesoscale Modeling of Alpha-helical Protein Domains: Size Dependence of Strength and Elasticity.
Jeremie Bertaud 1 , Markus Buehler 1 Show Abstract
1 Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Alpha-helical protein domains are the key building blocks of cytoskeletal networks as well as hair, hoof and wool. Here we report the development of a coarse-grained model for alpha-helical protein domains with parameters derived from full atomistic simulations. The coarse-grained model enables modeling of the dynamics of large systems over a large range of length- and time-scales. The parameters in the coarse-grained model are determined to reproduce small and large deformation elasticity, energy dissipation and strain hardening of alpha-helical protein domains based on including atomistic details about the energy landscape of rupture of H-bonds. We present the validation of our mesoscale model by comparison with full atomistic results. We then apply our mesoscopic model to study size effects on strength, elasticity and effects of hierarchical arrangements of alpha-helical based protein domains.
12:15 PM - Z5.9
Poroviscoelastic Modeling of Cross-linked Actin Filament Networks.
Jeffrey Palmer 1 2 , Mary Boyce 2 Show Abstract
1 , MIT Lincoln Laboratory, Lexington, Massachusetts, United States, 2 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States
Actin microfilaments combine to form one of the predominant cytoskeletal filament networks for time-dependent, metamorphic cellular processes such as spreading, migration, and contraction. The elastic and poroviscoelastic stress-strain behavior of actin networks is modeled via a microstructurally-informed continuum mechanics approach. The force-extension behavior of the individual filaments is captured with an analytical expression of the MacKintosh worm-like chain relationship for semiflexible filaments. The filament expression is used in the Arruda-Boyce eight-chain network model to capture the 3D stress-strain behavior, quantifying the effects of isotropic network prestress and tracking microstructural stretch and orientation states during large deformations. The network model captures the initial stiffness of the network as well as the nonlinear strain stiffening observed at large stresses in shear rheological data of in vitro F-actin networks. The cytoskeletal network model has been extended to include the internal energy-based mechanical contributions at the filament and network levels from torsional cross-link deformations as well as from direct axial stretching of filaments. This enhanced model effectively captures the stress-strain behavior of F-actin networks cross-linked with two different types of actin binding proteins (filamin and streptavidin). The 3D constitutive network model provides a framework for simulating time-dependent spatial diffusion of cytosol within a porous, viscohyperelastic filament network. The poroelastic behavior is coupled with the hyperelastic network behavior through a 3D biphasic theory that includes network swelling effects for finite deformations. The mechanical response of the cytoskeletal network due to the localized swelling is incorporated by employing multiplicative decomposition of mechanical and swelling stretches. Microstructurally-based nonlinear shear viscoelasticity is also included to create a 3D poroviscohyperelastic network model capable of simulating the time-dependent response of cytoskeletal networks on short and long time scales. The model captures the nonlinear time-dependent behavior of in vitro actin-filamin and actin-avidin networks observed in shear rheological experiments. The constitutive models are evaluated in a finite element model with a cellular geometry (including membrane and nucleus submodels) and the ability to spatially vary network properties throughout the cell. These constitutive models can naturally be extended to other in vitro and in vivo cytoskeletal filament networks.
12:30 PM - **Z5.10
Mechanical Behavior of Biomimetic Foams and Aerogels Based on Cellulose Nnanofibers.
A. Svagan 1 , H. Sehaquin 1 , Q. Zhou 1 , Lars Berglund 1 Show Abstract
1 Dept. of Fiber and Polymer Technology, Royal Institute of Technology, Stockholm Sweden
Z6: Mechanics of Biomedical Materials I
Thursday PM, December 04, 2008
Back Bay B (Sheraton)
2:30 PM - **Z6.1
Enhanced Cellular Mobility and Growth on TiO2 Nanotube Surfaces.
Sungho Jin 1 , Karla Brammer 1 , Seunghan Oh 1 Show Abstract
1 Materials Science and Engineering, University of California, San Diego, La Jolla, California, United States
The mobility and growth behavior of osteoblast and endothelial cells cultured on vertically aligned, laterally spaced nanoscale TiO2 nanotubes have been studied. The adhesion/propagation of the osteoblast is substantially improved by the topography of the TiO2 nanotubes with the filopodia of growing cells actually going into the nanotube pores, producing an interlocked cell structure. The presence of the nanotube structure induced a significant acceleration in the profileration rate of adhered osteoblast cells by as much as ~300-400 %. It has also been found that the cell migration is influenced by the presence of such nanotube structures. The in-vitro endothelial response of primary bovine aortic endothelial cells (BAECs) indicates that the nanotopography of TiO2 nanotubes, as compated to flat Ti, provided nanoscale cues that facilitated cellular probing, cell sensing, and especially cell migration, by which more organized actin cytoskeletal filaments formed lamellipodia and locomotive morphologies. Motile cell protrusions were able to probe down into the nanotube pores for contact stimulation, and focal adhesions were formed and dissembled readily for enhanced advancement of cellular fronts, which was not observed on a flat substrate of titanium. . Seunghan Oh, Chiara Daraio, Li-Han Chen, Thomas R. Pisanic and Sungho Jin, "Significantly Accelerated Osteoblast Cell Growth on Aligned TiO2 Nanotubes", J. Biomed. Mater. Res. 78A, 97-103 (2006). . Karla S. Brammer, Seunghan Oh, John O. Gallagher, and Sungho Jin, "Enhanced Cellular Mobility Guided by TiO2 Nanotube Surfaces", Nano Lett. 8(3), 786–793 (2008).
3:00 PM - Z6.2
Characterization of Toughness in Photopolymerizable Acrylate Networks for Biomedical Applications.
Kathryn Smith 1 , Ken Gall 1 2 Show Abstract
1 Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Polymer networks formed through photopolymerization have emerged as candidate biomaterials for applications where it is advantageous to have in situ formation, quick synthesis rates, and easy processing into diverse geometries. However, these polymer networks lack the “mechanical integrity” or toughness for implementation in areas where a device or scaffold must function under rigorous loading regimes. The purpose of this study is to characterize how the toughness of polymer systems, specifically photopolymerizable acrylate networks, is affected by chemistry, network structure (i.e. crosslinking density), and environmental conditions including temperature and saline solution. Polymer systems consisting of at least one monofunctional acrylate and one difunctional crosslinker were copolymerized under UV light. The monofunctional acrylates chosen were methyl methacrylate (MMA), methacrylate (MA), and 2-hydroxyethyl methacrylate (2HEMA) while the crosslinker was poly(ethylene glycol) dimethacrylate (PEGDMA). Networks were developed with varying ratios of monomer components to create networks with different glass transition temperatures (Tg), crosslinking density, and hydrophilicity. Dynamic mechanical analysis was performed to determine the Tg and storage modulus as a function of temperature. In order to observe the influence of temperature on stress-strain properties, samples were strained to failure under isothermal conditions at temperatures both above and below each network’s Tg. Subsequently, networks were soaked in phosphate buffered saline (PBS) and strained under the same testing conditions in an environmental bath at different temperatures. A maximum in toughness was observed in all polymer networks at a temperature below the glass transition temperature. At “equivalent” test temperatures relative to Tg, networks with different crosslink densities demonstrate varying levels of toughness, with low crosslink density offering better toughness relative to highly crosslinked systems. When immersed in PBS, networks exhibited modulus and toughness values indicative of conditions above their Tg in air with the extent of toughness change related to the amount of water uptake by the network. These results suggest that the underlying mechanisms for toughness are tied to the chemical structure and macromolecular arrangement of polymers, in particularly the chemical components that interact strongly with PBS. From this study, important fundamental relationships between toughness and network structure are established that can be utilized in developing tough photopolymerizable polymer networks.
3:15 PM - Z6.3
An Experimentally Validated Micromechanical Model for Elasticity and Strength of Hydroxyapatite Biomaterials.
Andreas Fritsch 1 2 , Luc Dormieux 2 , Christian Hellmich 1 , Julien Sanahuja 3 Show Abstract
1 , Vienna University of Technoloy (TU Wien), Vienna Austria, 2 , Ecole Nationale des Ponts et Chaussess, Marne-la-Vallee France, 3 , Lafarge Research Center, Saint Quentier Fallavier France
Hydroxyapatite (HA) biomaterials production has been a major field in biomaterials science andbiomechanical engineering. As concerns prediction of their stiffness and strength, we propose to gobeyond statistical correlations with porosity or empirical structure-property relationships, as to resolve thematerial-immanent microstructures governing the overall mechanical behavior. The macroscopicmechanical properties are estimated from the microstructures of the materials and their composition, in ahomogenization process based on continuum micromechanics. Thereby, biomaterials are envisioned asporous polycrystals consisting of HA needles and spherical pores. Validation of respective micromechanicalmodels relies on two independent experimental sets: biomaterial-specific macroscopic (homogenized)stiffness and uniaxial (tensile and compressive) strength predicted from biomaterial-specific porosities, onthe basis of biomaterial-independent ( universal ) elastic and strength properties of HA, are comparedwith corresponding biomaterial-specific experimentally determined (acoustic and mechanical) stiffness andstrength values. The good agreement between model predictions and the corresponding experimentsunderlines the potential of micromechanical modeling in improving biomaterial design, through optimizationof key parameters such as porosities or geometries of microstructures, in order to reach the desiredvalues for biomaterial stiffness or strength.
3:30 PM - Z6.4
Thin Film Modulus based Wrinkling Metrology of Biomaterials Derived from Desaminotyrosyl-tyrosine Polycarbonates.
Khaled Aamer 1 , Christopher Stafford 1 , Matthew Becker 1 , Lee Richter 2 , Joachim Kohn 3 Show Abstract
1 Polymers Division, National Institute of Standards and Technology, Giathersburg, Maryland, United States, 2 Surface And Microanalysis Science Division, National Institute of Standards and Technology, Giathersburg, Maryland, United States, 3 Department of Chemistry & Chemical Biology, Rutgers University, Piscataway, New Jersey, United States
Poly(desaminotyrosyl-tyrosine alkyl ester) polycarbonates are a biocompatible, biodegradable class of materials that were developed recently to address the need for new biomedical applications. A number of the library members are currently in clinical trials for a variety of applications including one which was highlighted via recent FDA approval of a single component material used for hernia mesh. Applications in which these materials are found include deployable cardiovascular stents, drug delivery, and tissue engineering. Employing these materials as thin film coatings in applications such as cardiovascular stents requires them to possess specific mechanical properties to guarantee their integrity, function, and performance in biomedical devices. Among these mechanical properties is the thin film elastic modulus which can be measured using wrinkling based metrology. The scope of this work focuses on measuring the thin film elastic moduli of several tyrosine-derived polycarbonates, specifically desaminotyrosyl ethyl tyrosine polycarbonates p(DTE)c, an iodinated derivative p(I2-DTE)c, and several discrete blends as a function of film composition and thickness. The iodinated form, p(I2-DTE)c, was developed to increase the blended polymers radiopacity by increasing their x-ray scattering contrast. As a function of composition, the elastic modulus is statistically independent of the blend composition as the weight percentage of p(I2-DTE)c increases in the range of 67 nm to 200 nm for films of uniform thickness. As a function of film thickness, the observed elastic moduli of p(DTE)c shows a very slight increase, 17 %, down to 80 nm without substantial change down to 30 nm. The modulus for p(I2-DTE)c and the 50:50 with p(DTE)c by mass blend show statistically insignificant dependence with decreasing film thickness down to 80 nm.
3:45 PM - Z6.5
Grafing Copolymerization onto Poly(hydroxybutyrate-co-hydroxyvalerate)(PHBHV): Effect on Bacterial Adhesion and Biofilm Formation.
Isabelle Linossier 1 , Hoi Kuan Lao 1 , Karine Rehel 1 , Valérie Langlois 2 , Estelle Renard 2 Show Abstract
1 , Université de Bretagne Sud , Lorient France, 2 , ICMPE, SPC, UMR 7182, Thiais France
Bacteria tend to associate on surfaces to form a structured community known as biofilms. Due to their resistance to biocides biofilms have many harmful consequences on public health [1,2], biofouling , food-processing industries …. However, biofilms are very useful within the framework of wastewater treatment in bioreactors . In order to optimize these systems, studies are focused on enhancing the initial adhesion and promote the maturation of the biofilms to maintain their efficiency. The initial adhesion is strongly influenced by the physicochemical properties of the substrate, e.g., free surface energy, roughness, hydrophobicity and also the bacterial properties (cell wall charge, Lewis acid/base property…).The aim of this study is to modify Poly(hydroxybutyrate-co-hydroxyvalerate) (PHBHV) films by graft copolymerization of different vinylic monomers to change the hydrophilic/hydrophobic balance of the PHBHV and to evaluate the effect of the chemical function on the bacterial initial adhesion and the biofilm formation. The objective is to find the chemical function which promotes the bacterial adhesion and the biofilm formation.PHBHV films were prepared by graft copolymerization of acrylate or methacrylate monomer containing hydroxyl group (hydroxyethyl methacrylate), carboxylic acid group (acrylic acid), or dimethyl amine (dimethylamino ethyl methacrylate) functions. The grafting procedure was initiated by the benzoyl peroxide. To understand the effect of these chemical groups on the initial adhesion and the biofilm formation, three Gram-negative bacteria model were studied: Escherichia coli, Pseudomonas aeruginosa and Pseudomas putida. This work needed the use of analytical techniques: surface energy measurement by contact angle technique, confocal laser scanning microscopy, Karl-Fisher titration, scanning electron microscopy.References:W. Costerton, R. Veeh, M. Shirtliff, M. Pasmore, C. Post and G. Ehrlich, J. Clin. Invest., 112, 1466-1477, 2003.P. S. Stewart and J. William Costerton, The Lancet, 358, 135-138, 2001.S. Dobretsov, H. Xiong, Y. Xu, L. Levin and P.-Y. Qian, Marine Biotechnology, In PressJ. S. Dickson and M. Koohmaraie, Appl. Environ. Microbiol., 55, 832-836, 1989.A. Boley, W. R. Muller and G. Haider, Aquacultural Engineering, 22, 75-85, 2000.H. K. Lao, E. Renard, I. Linossier, V. Langlois and K. Vallee-Rehel, Biomacromolecules, 8, 416-423, 2007.
4:30 PM - **Z6.6
Fabrication, Microstructure and Mechanical Properties of an Osteochondral Scaffold.
Lorna Gibson 1 Show Abstract
1 Materials Science and Engineering, MIT, Cambridge MA, Massachusetts, United States
5:00 PM - Z6.7
Nanopatterned Biomimetic Surfaces for Spatial Keying and Binding Kinetics Studies.
Justin Abramson 1 2 , Matteo Palma 1 2 , Mark Schvartzman 4 , Shalom Wind 3 , Michael Sheetz 2 , James Hone 1 Show Abstract
1 Mechanical Engineering, Columbia University, New York, New York, United States, 2 Biological Sciences, Columbia University, New York, New York, United States, 4 Chemical Engineering, Columbia University, New York, New York, United States, 3 Applied Physics and Applied Mathematics, Columbia University, New York, New York, United States
Nanometer level spatial organization has recently been shown to play a crucial role in cell mechanics. One example of this is the adhesion of the cell to the extracellular matrix (ECM): it has been shown that there is a nanoscale periodicity critical to the formation of the focal adhesion complex. Ultra-high resolution nanofabrication methods borrowed from the semiconductor industry enable us to recapitulate this order in a deliberate and controlled way. Here we discuss our development of nanopatterned biomimetic surfaces on which the placement of single ligands is controlled at the nanometer scale. We have fabricated arrays of Au/Pd nanometer-sized dots using e-beam and nanoimprint lithography. Different chemical strategies at surfaces have been pursued both through the formation of mixed Self Assembled Monolayers (SAMs) as well as via chemical reactions at surfaces. The nanostructures so prepared are organized into hierarchical arrays in which structural parameters, such as spacing and orientation of specific functional groups, are systematically varied. We employ both Total Internal Reflectance Fluorescence (TIRF) and epifluoresence microscopy to monitor individual biomolecules at surfaces, and to detect differences in kinetics with single molecule sensitivity. We take advantage of the well established specificity of the biotin-avidin binding couple to demonstrate controlled arrangement of single avidin molecules. Further we are able to study the kinetics of individual mutant avidin molecules exhibiting a range of binding affinities by observing residence times on our surfaces via fluorescence microscopy. We will highlight the broader utility and application of such functional nanopatterned surfaces for nanoscopic control and studies. To this end the ability to key out bi-functional nano-rods in ordered arrays on surfaces will be presented. In conclusion, we have developed a substrate that mimics the spatial and chemical functionality of biological systems via precise control of chemically functionalized nanofabricated structures. The ability to probe and understand cellular mechanics at the resolution of individual macromolecules offers new insight into biomechanics and is a valuable tool in understanding pathologies of disease states at a fundamental level, perhaps opening up new avenues of therapy. M. Arnold, E.A. Cavalcanti-Adam, R. Glass, J. Blummel, W. Eck, M. Kantlenher, H. Kessler, J. P. Spatz ChemPhysChem 2004, 5, 383-388  O. Cherniavskaya C.J. Chen, E. Heller, E. Sun, J. Provenzano, L. Kam, J. Hone, M.P. Sheetz, S.J. Wind J. Vac. Sci. Technol. B 2005, 23(6), 2972-2978
5:15 PM - Z6.8
Adhesion Measurements using Layer-by-layer Assembled Hydrogel Films.
Adam Nolte 1 , Christopher Stafford 1 Show Abstract
1 Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
We demonstrate a contact mechanics approach for measuring the adhesion of biologically relevant thin film materials. Elastic poly(dimethylsiloxane) hemispheres are coated with alternating layers of polyelectrolytes to construct carboxylic acid-rich hydrogel films with controllable thickness and chemical functionality. We demonstrate pH-tunable adhesion of the functionalized hemispheres to amine-functionalized substrates under aqueous conditions, discussing the relevance of parameters such as film thickness, surface roughness, and the ambient environment in controlling adhesion. We will discuss future design strategies for broadening the testing capabilities to a wider array of mechanics- and adhesion-related biological phenomena.
5:30 PM - Z6.9
X-ray Microtomography Aids the Design of New Materials.
Philipp Hunger 1 , Amalie Donius 1 , Ulrike G.K. Wegst 1 Show Abstract
1 Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania, United States
Synchrotron-generated X-rays provide scientists with a multitude of investigative techniques well suited for the analysis of the structure, composition and interfaces of all classes of materials. Discussed will be the properties of synchrotron- versus desktop-generated X-rays and the opportunities and challenges that they provide for the characterization of complex biological and engineered materials. Case studies will be presented to illustrate the qualitative and quantitative techniques of absorption and phase contrast tomography available and the challenges and opportunities that each of these provide for materials research in general and the design of new materials in particular.
5:45 PM - Z6.10
Thermodynamics of Receptor-Mediated Endocytosis of Nanoparticles.
Sulin Zhang 1 , Hongyan Yuan 1 , Ju Li 2 , George Lykotrafitis 3 , Gang Bao 4 , Subra Suresh 3 Show Abstract
1 Department of Engineering Science and Mechanics, Pennsylvania State University, University Park, Pennsylvania, United States, 2 Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 3 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 4 Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, Georgia, United States
Owing to their small size, ligand-coated nanoparticles (NPs) can be efficiently directed toward, and subsequently endocytosed by tumor cells through ligand-receptor recognition and interaction, thereby offering an effective approach for specific targeting of tumor cells. In this work, we present a thermodynamic model for receptor-mediated endocytosis of ligand-coated NPs. The model reveals that cellular uptake of NPs can be elaborately controlled by particle size and the ligand density, characterized by a two-dimensional phase-diagram. Two phase boundaries, governed by the local energetics of NP wrapping and tension-mediated receptor depletion, respectively, are found, which meet at a triple point. Beyond the phase boundaries, endocytosis is inhibited, while within the boundaries, an optimal condition for maximized cellular uptake can be identified. We found that for saturated ligand density, the optimal particle radius is about 25-30nm with a maximized cellular uptake of several thousand. Both the findings agree well with prior experimental data. A lower ligand density shifts both the optimal particle radius and the cellular uptake to smaller values. The predicted phase-diagram of NP endocytosis provides valuable guidance to the rational design of NPs for site-specific targeting.
Z7: Poster Session: Tissue Mechanics IV
Friday AM, December 05, 2008
Exhibition Hall D (Hynes)
9:00 PM - Z7.1
Some Implications of the Microstructure on the Mechanical Behavior of a Popillia Janponica Cuticle.
Liang Cheng 1 , Liyun Wang 1 , Anette Karlsson 1 Show Abstract
1 Mechanical Engineering, University of Delaware, Newark, Delaware, United States
The exoskeleton of a Popillia japonica (Japanese beetle) is an important part of the arthropod body. It has multi-functional capabilities, including supporting the body weight, filtering chemicals, and resisting external load (e.g., enemy attack). The exoskeleton consists mainly of high stiffness chitin fibers and connective proteins, complimented with limited amounts of lipids and inorganic materials. We investigate how the excellent properties are achieved in the insect exoskeletons with these relatively simple construction materials.The complex hierarchical structure of the insect exoskeleton may be the reason for its exceptional properties and functions. On the nanometer level, the chitin and connected protein form “microfibrils,” which in turn bundle together to form “macrofibrils.” The chitin-protein macrofibrils are assembled into a complex hierarchical structure of the insect exoskeleton, which is similar but distinct for each species. The goals of this study are (i) to investigate the hierachical structure of the chitin-protein macrofibrils at three anatomic locations of the Popillia japonica (pronotum, elytrum and legs); and (ii) to elucidate the implications of the structure on the mechanical responses of the exoskeleton. Our electron microscopy studies revealed that the exoskeleton (cuticle) from the investigated anatomic locations share similar overall structural morphology. The thickness of the cuticle ranges from 20-30 μm. The pronotum has the thickest cuticle, whereas the elytrum and leg cuticles are approximately of the same in thickness. The cuticle is divided into four regions parallel to the surface (in the order of increasing depth from the surface): epicuticle, exocuticle, mesocuticle and endocuticle. The epicuticle is a very thin (less than 1 μm), waxy layer and is not load bearing. The three regions beneath the epicuticle are the major load-bearing structure with dominant chitin-protein constituents. The exocuticle (~4-5 μm thick) and mesocuticle (~6-7 μm thick) consist of multiple layers, composed of parallel aligned chitin-protein macrofibrils. The layers stack onto each other successively forming a helicoidal structure, where each layer is gradually rotated relative to its adjacent layer about their normal direction. The endocuticle is the thickest region of the cuticle (11-13 μm thick), composed with a so-called pseudo-orthogonal structure. In addition, through-the-thickness structural features including the pore canals and associated pore canal fibers are identified. Mechanics based models are developed to explore the implications of the multiscale morphology on the mechanical responses of the exoskeleton. We show that the helicoidal structure results in approximately in-plane isotropy. Further, our model suggests that similar to the stitching strategy to prevent delamination in man-made laminated composites, the transverse pore canal fibers are able to increase the interlaminar strength of exoskeleton.
9:00 PM - Z7.10
A Surfactant-Based Argument for Quasi-Plastic Deformation of the Lung under Indentation.
Maricris Silva 1 , Andrew Gouldstone 1 Show Abstract
1 Mechanical and Industrial Engineering, Northeastern University, Boston, Massachusetts, United States
Of all the internal organs, the lung has arguably the strongest link between mechanical behavior and physiologic function. It is generally agreed that lung mechanics are dominated by activity of alveolar surfactant, but the connection between surfactant chemistry and macroscopic behavior has not been shown for different gaseous environments. Using indentation of excised lungs, we have induced atelectasis (alveolar collapse) under controlled loading, and in previous abstracts have imaged this using optical coherence tomography (OCT). In this work, we present surfactant-based arguments for the differences in mechanical behavior arising from different inflation gases. Specifically, inflation with pure oxygen or oxygen-0.2% isoflurane leads to increased and decreased plasticity, respectively, relative to inflation with air. However, the mechanisms by which this occurs are different. In addition, we quantitatively link macroscopic stress-strain behavior of lung to in-plane compression experiments of surfactant monolayers.
9:00 PM - Z7.11
Nanomechanical Studies On Bovine Cortical Bone.
Bedabibhas Mohanty 1 , Kalpana Katti 1 , Dinesh Katti 1 Show Abstract
1 Civil Engineering, North Dakota State University, Fargo, North Dakota, United States
9:00 PM - Z7.12
The Effects of Whitening and Etching Agents on the Mechanical Properties of Dentin and Enamel.
Bonnie Zimmerman 1 , L. Datko 1 , M. Kennedy 1 , D. Dean 1 Show Abstract
1 , Clemson University, Clemson, South Carolina, United States
Fracture mechanisms seen within human teeth are controlled by many factors including the enamel and dentin mechanical properties. By understanding their unique mechanical properties, better predictive models of tooth fracture can be made. The complex composite nature of dentin, enamel and the dentin-enamel junction make measuring their properties difficult. This study will look at both the variation of enamel and dentine hardness and modulus from different teeth and also the effects of etching and whitening on the mechanical properties. In this study, the properties were examined by quasi-static nanoindentation using a two-micron, 60 degree conical tip. Initial results showed slightly higher modulus values for dentin and enamel to transition from 19 GPa to 137 GPa. Decrease of these values was seen when the samples were preserved through saturation in HBSS and decreasing time between slicing and testing of tooth cross sections. Changes due to whitening and etching will be compared to unaltered tooth samples. These procedures were applied to teeth of various ages and degrees of mineralization.
9:00 PM - Z7.13
The Compliance of Human Trabecular Meshwork and Meshwork Cells of the Eye.
Julie Last 1 , Paul Russell 1 , Ted Acott 3 , Michael Fautsch 4 , Paul Nealey 2 , Christopher Murphy 1 Show Abstract
1 School of Veterinary Medicine, University of Wisconsin, Madison, Wisconsin, United States, 3 , Oregon Health & Science University, Portland, Oregon, United States, 4 Department of Ophthalmology, Mayo Clinic, Rochester, Minnesota, United States, 2 Department of Chemical Engineering, University of Wisconsin, Madison, Wisconsin, United States
The human trabecular meshwork is a tissue involved in the regulation of intraocular pressure via its impact on drainage of aqueous humor out of the anterior chamber. The extracellular matrix associated with the trabecular meshwork cells is altered in glaucoma and the changes seen with electron microscopy suggest that the matrix is becoming less compliant with disease. We are interested in measuring the compliance of normal human trabecular meshwork and determining if the compliance of this tissue changes with glaucoma. We are also interested in determining the effect of changing the substrate compliance on the compliance of the human trabecular meshwork cells. In order to determine the compliance of these soft tissues and cells, we have employed atomic force microscopy (AFM) nanoindentation to measure the elastic modulus. AFM nanoindentation allows the determination of the local elastic modulus at the surface of the tissue. In samples of trabecular meshwork from normal human donor tissue, the elastic modulus was approximately 3.0 kPa. This is in contrast to the elastic modulus obtained for glaucomatous meshwork, which was approximately 70 kPa. Cells cultured from normal meshwork had modulus values that varied according to the modulus of the substrate on which they were growing. The values for cell elastic modulus ranged from 1.2 kPa for cells cultured on a polyacrylamide gel (polyacrylamide modulus ~ 4 kPa) to a modulus of approximately 5 kPa for cells grown on glass (glass modulus ~ 50 GPa). These results suggest that the decreased compliance of the meshwork found in glaucomatous tissue would directly be reflected by a decreased compliance of the overlying trabecular meshwork cells. Cells in the glaucomatous meshwork may become more rigid with disease. Drugs currently being investigated that decrease actin stress fibers in trabecular meshwork cells may act by preventing the cell from developing the decreased compliance expected to occur in response to an extracellular matrix of decreased compliance.
9:00 PM - Z7.14
Adhesion and Interfacial Fracture Toughness Between Hard and Soft Materials.
Nima Rahbar 1 , Kurt Wolf 2 , Argjenta Orana 2 , Roy Fennimore 2 , Zong Zong 3 , Juan Meng 3 , George Papandreou 2 , Cynthia Maryanoff 2 , Wole Soboyejo 3 Show Abstract
1 Civil and Environmental Engineering, University of Massachusetts, North Dartmouth, Massachusetts, United States, 2 , Cordis Corporation, Spring House, Pennsylvania, United States, 3 Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey, United States
This paper presents the results of a combined experimental and theoretical study of adhesion between hard and soft layers that are relevant to medical devices, such as drug-eluting stents, as well as other applications such as semiconductors. Brazil disk specimens were used to measure the interfacial fracture energies between model parylene C and 316L stainless steel over a wide range of mode mixities. The trends in the overall fracture energies are predicted using a combination of adhesion theories and fracture mechanics concepts. Measured interfacial fracture energies are shown to be in good agreement with predictions.
9:00 PM - Z7.15
Computational Mechanics in Silico.
Ashkan Vaziri 1 Show Abstract
1 , Northeastern University, Boston, Massachusetts, United States
Deciphering the relationship between cellular processes and the structure of living cells is a key step towards understanding and predicting cell functions with direct implications in understanding human health and disease. The active nature of these cellular processes, which span several decades in spatial and temporal scales pose significant challenges in unraveling this complex structure-function paradigm. Complementing novel experimental techniques with robust computational approaches capable of modeling mechanical response at varying scales provides new avenues to resolve this paradigm. We provide an overview of continuum based computational approaches used in studying and interpreting responses of individual cells and nuclei as well as in measuring the mechanical characteristics of living cells and discuss some of the key insights provided by these approaches.
9:00 PM - Z7.16
Correlation Between 2-D and 3-D Mesoscale Dynamic Fracture Analyses of Trabecular Bone.
Albert Cerrone 2 , Vikas Tomar 1 Show Abstract
2 Civil Engineering, University of Notre Dame, Notre Dame, Indiana, United States, 1 Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, Indiana, United States
9:00 PM - Z7.17
Superhydrophobic Nanocrystalline Nickel Films Inspired by Lotus Leaf.
Mehdi Shafiei 1 , Ahmet Alpas 1 Show Abstract
1 Mechanical, Automotive and Materials Engineering, University of Windsor , Windsor, Ontario, Canada
New engineering surfaces with enhanced hydrophobicity have been fabricated by replicating lotus leaf texture on an acetate film, on which a nanocrystalline (NC) Ni film with a grain size of 30 ± 4 nm was electrodeposited to produce self-sustaining, hard replicas (i.e., H = 4.42 ± 0.14 GPa). A secondary short electrodeposition process was applied to the NC Ni surfaces to modify the morphology of the microscale protuberances. Ni spheres formed around protuberance peaks in the resulting texture increased the radius of curvature of the peaks, which reduced their contact pressure on water drops. This enabled the protuberances to support larger water drops on their peaks without allowing water to wet the entire surface. To further enhance hydrophobicity of the NC Ni replicas, the samples were coated with a PFPE derived solution. While a flat NC Ni sample had a water contact angle of 64 deg., the NC Ni replica treated with a 2-minute secondary electrodeposition and the PFPE solution provided a high contact angle of 156 deg., which is comparable to that of the lotus leaf. One important conclusion of this study was that the contact angle did not exceed 94 deg. by optimizing surface texture or surface chemistry alone. However using these two effects together boosted the contact angle to the superhydrophobic range.
9:00 PM - Z7.18
Morphological Evolution of Lipid Bilayer Membranes Using a Coupled Composition-Deformation Phase-Field Method
Chloe Funkhouser 1 2 , Francisco Solis 3 , Katsuyo Thornton 2 Show Abstract
1 Biomedical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Materials Science & Engineering, University of Michigan, Ann Arbor, Michigan, United States, 3 Integrated Natural Sciences, Arizona State University West, Glendale, Arizona, United States
The plasma membrane, a lipid bilayer membrane surrounding all mammalian cells, is not homogeneous, but rather contains domains termed ‘rafts,’ defined as regions enriched with cholesterol and saturated lipids. We present a continuum-level method for modeling phase separation (raft formation) and morphological evolution of multicomponent lipid membranes. The model applies to binary membranes with planar and spherical background geometries, simulating a nearly planar portion of membrane or entire vesicle, respectively. The model treats the individual composition of each leaflet of the lipid bilayer, which determines the spontaneous curvature of the bilayer. The compositions and shape of the membrane are in turn coupled through a modified Helfrich free energy, which includes a term that couples the compositions of the two leaflets. The evolution of the compositions is modeled using a phase-field method with the evolution described by a Cahn-Hilliard-type equation, while the shape changes are described by relaxation dynamics. We use this model to numerically simulate the dynamics of phase separation in spherical systems and planar systems initialized with a variety of compositional and geometrical configurations. For nearly planar bilayer systems with each leaflet having the same phase fraction, we find that domains align to reduce the interaction energy, as expected. However, when the coupling effect is stronger, this alignment occurs more quickly and more precisely, showing that the coupling affects the dynamics. For bilayer systems with leaflets having different phase fractions, domains still show a tendency to align, though the phase boundaries on the two leaflets are forced to be offset as a result. The coupling between leaflets is found to influence morphological evolution; in some cases the equilibrium morphological phase observed using this bilayer model is very different from what was observed with our simpler monolayer model using similar parameters and initialization. The results for vesicles using spherical background will also be discussed.
9:00 PM - Z7.2
Erythrocyte Membrane Deformability in Arteriole-Sized Microfluidic Channels.
Alison Forsyth 1 , Jiandi Wan 1 , William Ristenpart 2 , Howard Stone 1 Show Abstract
1 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States, 2 Chemical Engineering and Materials Science, University of California Davis, Davis, California, United States
The deformability of erythrocytes plays a major role in the pathology of several diseases, including malaria, sickle cell anemia and spherocytosis. Moreover, deformations are believed to trigger the release of adenosine triphosphate, which helps regulate vascular tone and is consequently an important factor in various vascular diseases. Understanding erythrocyte deformability is therefore crucial for designing effective therapeutic strategies, and because of the fast time scales involved little is known about deformations under flow conditions in arterioles, where the largest pressure drop occurs. Here we investigate single-cell viscoelastic responses in these arteriole-sized vessels, which we mimic hydrodynamically by using poly(dimethylsiloxane) channels with a constriction 2-4 times larger than a typical erythrocyte. High-speed video and image analysis were used to quantify the stretch, trajectory, projected area and orientation for cells exposed to varied doses of glutaraldahyde or diamide, which are chemicals that are known to ‘rigidify’ erythrocytes. Our results show that (i) the deformability of diamide-treated cells is signficantly greater than that of glutaraldahyde-treated cells and (ii) the magnitude of deformation is similar for diamide-treated and untreated cells, but the diamide-treated cells relax more slowly. These results indicate that diamide affects the membrane relaxation time, rather than the membrane shear modulus, while glutaraldahyde directly stiffens the cells. We expect that the experimental procedure described here will be useful for characterizing the effect of different therapeutic agents on cellular deformability.
9:00 PM - Z7.20
Anisotropic Design of Multilayered Exoskeletons in Biology.
Lifeng Wang 1 , Juha Song 2 , Christine Ortiz 2 , Mary Boyce 1 Show Abstract
1 Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Various organisms in biology have evolved outstanding exterior materials and structures to protect them from predatory and environmental threats, for example, mollusc shells, mineralized fish scales, and insect integument. Most exoskeletal materials exhibit a multilayered design where each layer possesses its own unique nanostructure that enables it to undergo unique deformation mechanisms that contribute to its overall protective mechanical function. The goal of this study is firstly, to use microstructurally-based computational modeling to predict how the nanoscale morphology of individual layers results in mechanical anisotropy of the layer and secondly, to understand how mechanical anisotropy of individual layers contributes to the overall biomechanical properties of the exoskeleton, in particular penetration resistance. The model system chosen for this study was the outer ganoine layer of the mineralized scales of the armored fish Polypterus senegalus, which is known to undergo anisotropic fracture mechanism. Ganoine possesses a prismatic nanostructure composed of hydroxyapatite nanocrystals with 50-100 nm diameter surrounded by an organic matrix 3-5 nm thick. A micromechanical finite element analysis (FEA) model of prismatic microstructure of ganoine was created. The 3D representative volume element (RVE) considered periodic staggered arrangement of mineral nanocrystals bonded by thin adhesive layers. Several multi-axial loading conditions were applied on the periodic RVE to determine the anisotropic elastic constants and the anisotropic yield and corresponding underlying deformation mechanisms. Next, a full 3D FEA nanoindentation model was carried out which compared the load-depth behavior of the anisotropic material to a corresponding isotropic material determined from fits of an elastic-perfectly plastic model to nanoindentation data. The results showed less than 10% difference in load-depth behaviors between isotropic and anisotropic model, but anisotropy led to a more focused, localized, and deeper depth of plasticity beneath the indenter when compared to the isotropic material. A 3D discrete nanoindentation model was also created which modeled the discrete prismatic nature of the ganoine layers by locally implanting nanostructured crystals surrounded by thin organic layers within the volume under the indenter. The results demonstrated that the effective continuum-level anisotropic model was able to remarkably capture the essence of the behavior of the discrete prismatic structure material. Finally, in order to explore the role of the anisotropic ganoine layer within the larger scale multilayered shell, microindentation was carried out on a quad-layered FEA model that incorporated an anisotropic ganoine layer. The anisotropy of the ganoine layer was found to have a significantly advantageous effect on increasing energy dissipation, resisting radial crack propagation, and suppressing interfacial failure in multilayered structure.
9:00 PM - Z7.21
Analysis of Nanoindentation in Biological Specimens.
Joseph Jakes 1 2 , Chuck Frihart 1 , James Beecher 1 , Robert Moon 1 , Donald Stone 2 3 Show Abstract
1 , USDA Forest Products Laboratory, Madison, Wisconsin, United States, 2 Materials Science Program, University of Wisconsin, Madison, Wisconsin, United States, 3 Materials Science and Engineering, University of Wisconsin, Madison, Wisconsin, United States
In the past decade researchers have increasingly relied on nanoindentation to probe the local mechanical properties in biological materials and structures. Nanoindentation is capable of characterizing sub-micrometer sized features, which helps when one is trying to unravel the relationship between function and structure at the microscopic scale. However, the theory of nanoindentation, as it is applied in the conventional Oliver-Pharr analysis used to calculate hardness from depth and load, was developed from studies of bulk, homogenous, rigidly-supported specimens. In contrast to those kinds of “ideal” specimens, biological specimens typically possess a multitude of heterogeneities and free edges (e.g., lumens, or holes running along the lengths of cells in wood) that are present at length scales comparable to the size of the nanoindents. These heterogeneities complicate the interpretation of nanoindentation data, and up until now, it has not been possible to obtain nanoindentation free of artifacts caused by them. In addition, artifacts arise when the specimen or structural feature of interest is not rigidly supported, such as occurs in the long range flexing of an open cellular structure during indentation. In this work we demonstrate experimentally that the predominate effects of free edges, heterophase interfaces, and large-scale specimen flexing are to introduce a structural compliance (Cs) into the measurement. If this structural compliance, which can be positive or negative, is accounted for, then the local properties can be measured free from artifacts. We develop an experimental approach to account for Cs in nanoindentation measurements so that the hardness and modulus can be determined with a minimum of error. The approach is validated by performing experiments on model systems including a silicon beam supported only at the ends, and sites near the free edge of a fused silica calibration standard; and on more complex systems including the tracheid walls in unembedded loblolly pine (Pinus taeda), and the polypropylene matrix in a polypropylene–wood composite. Results from experiments studying the edge effects are in good agreement with elasticity theory. The method works even when there are multiple interfaces, such as with the cell walls in wood or with multiple components, including voids, in the polypropylene composite. In addition, we have developed an experimental technique called broadband nanoindentation creep (BNC) from which we are capable of obtaining hardness strain rate sensitivity data over 4 orders of strain rate. Combining BNC with our methods to account for Cs, the differences in strain rate sensitivity between the cell walls and middle lamella of both untreated and ethylene glycol plasticized wood are also determined. These results provide important information about the interactions between the three biopolymers (cellulose, hemicellulos and lignin) which compose wood.
9:00 PM - Z7.22
Modeling of Gecko Toe Adhesion.
Bin Chen 1 , Peidong Wu 1 , Huajian Gao 2 Show Abstract
1 , McMaster Univ., Hamilton, Ontario, Canada, 2 , Brown Univ., Providence, Rhode Island, United States
To understand underlying mechancisms of gecko toe adehsion, we have employed a hierarchical model for the microstructures under geck toe. At the bottom of hierarchy, we show that the peeling strength of a spatula pad for attachment can be 10 times larger than that for detachment. At the intermediate level of hierarchy, we show that the 10 times difference in the peeling strength of a spatula pad for attachment and detachment is magnified up to a 100 times difference in adhesion energy at the level of seta. At the top of hierarchy, the attachment of a gecko toe is modeled as a pad under displacement controlled pulling, yielding an adhesive force much larger than gecko’s body weight, while the detachment is modeled as a pad under peeling, leading to a negligible peel-off force. Our work reveals that the hierarchical microstructures can provide gecko with robust attachment and easy detachment.
9:00 PM - Z7.23
Leg Joints of the Lobster Homarus americanus as an Example of Cuticle Modification for specific Functions through Variations in Structure, Composition and Properties.
Helge Fabritius 1 , Sabine Hild 1 , Dierk Raabe 1 Show Abstract
1 , Max-Planck-Institut für Eisenforschung, Düsseldorf Germany
Arthropoda are characterized by the possession of an exoskeleton formed by the integument or cuticle. This evolutionary design concept has proven to be very successful regarding their sheer quantity, the number of different taxa, their evolutionary age, and the variety of niches they occupy. Despite the enormous morphological diversity found in Arthropoda, the basic organization of the cuticle is always the same, a hierarchically organized nano-composite material consisting of a matrix of chitin, a fibrous polysaccharide which is associated with various proteins. In some arthropod groups including Crustacea like our model organism, the American lobster Homarus americanus, the cuticle also contains variable amounts of nanoscopic biominerals, the most widespread being crystalline and amorphous calcium carbonate. The fibres are organized in the form of a twisted plywood. The exoskeleton is a structural entity which has to be replaced frequently by the organisms in order to grow. Its various morphologically distinct parts have to fulfill a multitude of different functions like providing mechanical stability to the body, acting as a barrier to the environment, enable movement through the formation of joints and bearing both external loads as well as internal loads caused by attached muscles. To adjust the mechanical properties to the required task, the animals vary the basic cuticle structure through modifications in microstructure like number and thickness of the chitin-protein fibre layers and the amount of incorporated biomaterials as well as the use of different proteins with distinct properties. This study focuses on the articulations in various limps of H. americanus, where elaborate joint structures between segments provide mobility to enable locomotion. Joint structures require different mechanical properties than simple load bearing cuticle parts or the soft arthrodial membranes. We chose hinge joints between the segments of the thoracopods and pivot joints in the claws of the lobster to investigate their microstructure, composition and mechanical properties using electron microscopy, Energy-Dispersive X-Ray Analysis, Raman spectroscopy and atomic force microscopy. The results are compared to previous studies conducted on mineralized cuticle and arthrodial membranes of H. americanus.
9:00 PM - Z7.25
Control of Secondary Structure and Nanomechanical Properties of Silk Fibroin Films.
Maneesh Gupta 1 , Srikanth Singamaneni 1 , Eugenia Kharlampieva 1 , Lawrence Drummy 2 , Rajesh Naik 2 , Vladimir Tsukruk 1 Show Abstract
1 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Materials and Manufacturing Directorate, Air Force Research Laboratories, Wright-Patterson AFB, Ohio, United States
Silk fibroin proteins isolated from the cocoon of the silkworm Bombyx mori have been heavily researched for applications such as tissue engineering scaffolds, biosensors, bioelectronics, and structural materials. The mechanical and chemical properties of materials made from fibroin depend heavily on the secondary structure of the protein. Fibroin has been shown to readily transform from random coil to β-sheet conformation in response to external stimuli which include heat, organic solvent, and mechanical stress. In this work, we develop a method to pattern the secondary structure of silk films by first masking portions of the film with polystyrene (by capillary transfer lithography) and then exposing the film to methanol vapor. Areas that are exposed to the vapor undergo a transition from random coil to β-sheet secondary structure. Using this method we have been able to pattern micron size features. The secondary structure of the protein in the films was mapped using confocal Raman microscopy by monitoring the peak positions of the amide I, II, and III peaks. AFM nanomechanical probing was performed to map the local mechanical properties of the random coil and β-sheet regions. The effect of pattern geometry on large scale mechanical properties of the film was measured by buckling and tensile measurements.
9:00 PM - Z7.3
Structural Characterization of Iron in Human Spleen.
Martin Kopani 1 , Marcel Miglierini 2 , Adriana Lancok 3 , Julius Dekan 2 , Milan Melnik 4 , Milan Mikula 5 , Jan Manka 6 , Martin Skratek 6 , Jan Jakubovsky 1 Show Abstract
1 Department of pathology, Comenius University, School of Medicine, Bratislava Slovakia, 2 Department of Nuclear Physics, Slovak University of Technology, Faculty of Electrical Engineering and Information Technology, Bratislava Slovakia, 3 Institute of Inorganic Chemistry, Academy of Science of Czech Republic, Husinec-Rez near Praque Czechia, 4 Department of Inorganic Chemistry, Slovak University of Technology, Faculty of Chemical and Food Technology, Bratislava Slovakia, 5 Department of Graphics Arts Technology and Applied Photochemistry, Slovak University of Technology, Faculty of Chemical and Food Technology, Bratislava Slovakia, 6 Institute of Measurement Science, Slovak Academy of Science, Bratislava Slovakia
9:00 PM - Z7.4
The Role of GAGs in the Nanomechanical Properties of the Human Dentin-Enamel-Junction (DEJ).
Marta Baldassarri 1 2 , Elia Beniash 1 3 Show Abstract
1 , Forsyth Institute, Boston, Massachusetts, United States, 2 , Harvard Medical School, Boston, Massachusetts, United States, 3 , University of Pittsburgh, Pittsburgh, Pennsylvania, United States
Teeth are comprised of an outer hard enamel layer and an underlying softer dentin tissue, coupled at the dentin-enamel junction (DEJ). This junction contributes to the mechanical integrity of the tooth by stopping cracks generated in enamel and preventing their propagation through dentin. It has been shown that the dentin organic matrix, specifically collagen fibers, is primarily responsible for the mechanical integrity of the DEJ. However, the functional role of other matrix components, such as glycosaminoglycans (GAGs), is still unclear. The aim of this study was to determine how different dentin GAGs affect the mechanical properties of the DEJ.Twelve 500 µm thick sections of human incisors were cut in the sagittal plane with a low-speed saw (Buehler, IL, US). Samples were polished down to 0.25 um with a Minimet 1000 polisher (Brucker, Lake Bluff, IL). The sections were randomly divided into four groups of three samples each. One group was used as a control, while in the other three groups, GAGs were proteolytically digested. Specifically, chondroitin sulfate A and C were digested with Chondroitinase AC (group AC), chondroitin sulfate B was digested with Chondroitinase B (group B), and keratin sulfate was digested with Keratanase (group KS). Mechanical tests of polished dentin surfaces near the DEJ were carried out in air, using Atomic Force Microscope (AFM) (Nanoscope III, Veeco Metrology, Santa Barbara, CA, US). Nanoindentations were performed in dentin with a Berkovich tip with a spring constant of 231 N/m at a distance of approximately 50 um from enamel. In each sample, an average of 24 loci were indented at a maximum load of 10 µN and a scanning frequency of 1 Hz. From the unloading-displa