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.