Markus J. Buehler Massachusetts Institute of Technology
Horacio D. Espinosa Northwestern University
Tia B. Tolle U. S. Air Force Research Laboratory
Mark R. Van Landingham U. S. Army Research Laboratory
Ulrike Wegst Drexel University
OO1: Living Systems I
Monday PM, November 28, 2011
Room 208 (Hynes)
9:30 AM - **OO1.1
Structure-Function Relationships in Damage-Tolerant Structural Glasses: Lessons from Nature.
Joanna Aizenberg 1 2 , James Weaver 2 Show Abstract
1 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States, 2 Wyss Institute for Biologically Inspired Engineering, Harvard University, Cambridge, Massachusetts, United States
Hexactinellid sponges are known for their ability to synthesize unusually long and highly flexible fibrous spicules, which serve as the building blocks of their skeletal systems. These spicules consist of a central core of monolithic hydrated silica, surrounded by alternating layers of hydrated silica and organic material. Following loading, fracture of this laminated structure involves cracking of the constituent silica and crack deflection through the intervening thin organic layers, leading to a distinctive stair step-like fracture pattern. Crack deflection mitigates the high stress concentration that would otherwise be present at the crack tip, resulting in high spicule strength and toughness. This design strategy thus prevents the structure from failing catastrophically as one would expect for a non-laminated glass rod. In addition to the remarkable mechanical properties of their individual spicules, hexactinellid sponges are also known for their ability to form remarkably complex hierarchically-ordered skeletal systems. Using information gained from the study of these structures, we are developing new design strategies for the synthesis of robust lightweight scaffolds for load bearing applications. Using a combination of direct mechanical testing and simulation-based strategies, we are beginning to identify specific design elements that contribute to the robustness of these unique structural materials. Architectural designs based on the lessons learned from these studies could ultimately result in the development of more cost effective and energy efficient buildings.
10:00 AM - **OO1.2
Multiscale Toughness Amplification in Natural Composites.
Reza Rabiei 1 , Sacheen Bekah 1 , Francois Barthelat 1 Show Abstract
1 , McGill University, Montreal, Quebec, Canada
Natural structural materials such as bone and seashells are made of relatively weak “building blocks”, yet they exhibit remarkable and vital combinations of mechanical properties. This performance can be largely explained by the staggered microstructure of these materials: stiff inclusions of high aspect ratio are laid parallel to each other with some overlap, and bonded by a softer matrix. In this work we have explored, through experiments and finite element models, how this seemingly simple microstructure generates high stiffness, high strength and attractive post-yielding behaviors. For example, we found that the arrangement of the inclusions have a profound effect on the deformation and failure mechanism, and that a raking stretcher structure similar to that of tropocollagen in collagen fibrils delays localization and enhances energy dissipation. Fracture toughness is the most impressive feature of these materials and we have successfully captured, through experiments and microstructure-based models, the salient multiscale fracture mechanisms of staggered composites. Crack bridging by the inclusions amplifies the toughness of the interfaces, and the bridging toughness is amplified a second time by a more powerful dissipative mechanism from a large process zone. Under specific conditions which are met by nacre and possibly bone, steady state cracking is never achieved, and toughness increases indefinitely with crack advance. In terms of design, the highest toughness amplification can be achieved with high aspect ratio, high volume concentration and small tablet size. The tablets themselves are subjected to high stress concentration and to stress singularities, and tablet fracture must be prevented by small tablet size, and also by the “soft wrap” effect provided by the soft interfaces, which greatly reduce the stress intensity factor in the tablets. Based on these findings we have designed, fabricated and tested novel biomimetic composites with unusual and attractive combinations of properties.
10:30 AM - OO1.3
Molecular Insight on Spider Webs: Nanomechanics of Silk Key to Robustness.
Steven Cranford 1 2 , Anna Tarakanova 1 2 , Nicola Pugno 3 , Markus Buehler 1 2 Show Abstract
1 Department of Civil and Environmental Engineeirng, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Laboratory of Atomistic and Molecular Mechanics, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Department of Structural Engineering and Geotechnics, Politecnico di Torino, Torino Italy
Nature provides an array of building materials and aptly chooses suitable means for a multitude of natural functions. The elasticity of blood vessels, the toughness of bone or the protection of nacre illustrate the apropos of Nature’s material selection. In particular, the behavior of spider silk and webs, ranging from the constituent protein sequence to the spiral geometry, have intrigued materials scientists as an example of a highly adapted natural system. It is well known that silk displays exemplary mechanical properties, but less clear is the benefit with respect to the mechanical behavior and robustness of a complete spider web structure. As a result, the role and expression of the unique nonlinear material behavior of silk – which initially softens and then becomes dramatically stiffer when stretched to large deformation – in its natural application within a web, as well as its function in structural failure, is unknown. Here we show, via a holistic nano-to-macro-to-nano approach, that the particular nonlinear (hyperelastic) material behavior of silk leads to a localization of deformation upon loading and consequently minimal web degradation upon failure, as well as an extreme resistance to defects. In a series of simulations we demonstrate that silk’s inherent nonlinear stiffening behavior is responsible for its advantage over other linear-elastic or elastic-plastic materials, making it a preferred building material for robust web-structures. Our findings demonstrate that the superior functionality of spider silk in webs is based on its unique yielding-stiffening behavior, rather than the relatively high values of ultimate strength and strain that are also considered exceptional. In its natural environment, localization of load bearing enhances a web’s ability to sustain unforeseen loading, such as falling debris, and thus avoid catastrophic failure. While studies of other biomaterials (e.g. bone or nacre) have shown that their great mechanical robustness is due to the formation of large plastic regions that facilitate the redistribution of mechanical energy our findings show that the opposite is true at the scale of spider webs, where extreme localization of failure is the key to explain its overall performance. Our findings reveal a general design paradigm in which nonlinear material behavior can be exploited in a synthetic structure to minimize damage and thus greatly enhance structural robustness.
10:45 AM - OO1.4
Self-Assembly in Freeze-Cast, Hierarchically-Structured Composite Scaffolds.
Philipp Hunger 1 , Amalie Donius 1 , Ulrike G. Wegst 1 Show Abstract
1 Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania, United States
Natural composites like bone, antler and nacre possess exceptional sets of mechanical properties that are thought to be intricately linked to the multi-level hierarchical composite structure present in these material systems. Several attempts to mimic nature's design and to produce composite materials with equally attractive property profiles have been presented in the literature. Their success has, however, been limited to fully dense composite films with thicknesses of several tens or hundreds of micrometers; frequently, components were used that did not match the length scales of their natural counterparts. The materials presented in this contribution add the third dimension to nacre by forming a highly porous bulk material, a hierarchically-structured composite scaffold, with fully dense, nacre-like cell walls. They are formed by freeze casting of, in this case, water-based ceramic slurries, and the self-assembly of the slurry components that occurs during the materials' directional solidification. Reported are structure-property-processing correlations observed in these materials which are unusual, because they are, like nacre, solely glued by a polymeric phase and not processed further by sintering. While keeping the overall porosity constant, the effect of different freezing rates on pore size and geometry were studied at the first hierarchical level of the honeycomb-like structure. While keeping the overall cell wall material composition constant, the influences of particle size and shape on the mechanical properties were investigated at the second level of the hierarchical structure.The results illustrate several pathways to control both structure and mechanical properties in freeze-cast composites and highlight the considerable gain in high stiffness, strength and toughness that can be achieved with a nacre-like cell wall design also in highly porous materials such as those used in tissue engineering and other biomedical applications.
11:30 AM - OO1.5
Structure and Mechanical Performance of Teleost Fish Scales.
Deju Zhu 1 , Lawrence Szewciw 1 , Franck Vernerey 2 , Francois Barthelat 1 Show Abstract
1 Mechanical Engineering, McGill University, Montreal, Quebec, Canada, 2 Civil Engineering, University of Colorado, Boulder, Colorado, United States
Protective materials and structures found in natural organisms may inspire new armors with improved resistance to penetration, flexibility, light weight and other interesting properties such as transparency and breathability. All these attributes can be found in teleost fish scales, which are the most common types of scales in modern fish species. In this work, we have studied the structure and mechanics of fish scales from striped bass (Morone saxatilis). This scale is about 200-300 microns thick and consists of a hard outer bony layer supported by a softer cross ply of collagen fibrils. The tensile properties of the collagen and bony layers of individual scales were determined along three different directions using a miniature tensile stage. Fish scales are constructed to resist puncture from a predator’s bite. Perforation tests with a sharp needle indicated that a single fish scale provides a high resistance to penetration which is superior to polystyrene and polycarbonate, two engineering polymers that are typically used for light transparent packaging or protective equipment. Under puncture, the scale undergoes a sequence of two distinct failure events: First, the outer bony layer cracks following a well defined cross-like pattern which generates four “flaps” of bony material. The deflection of the flaps by the needle is resisted by the collagen layer, which in biaxial tension acts as a retaining membrane. Remarkably this second stage of the penetration process is highly stable, so that an additional 50% penetration force is required to eventually puncture the collagen layer and defeat the scale. The combination of a hard layer that can fail in a controlled fashion with a soft and extensible backing layer is the key to the resistance to penetration of individual scales. On the actual fish skin several scales overlap, resulting in alternating soft and hard layers similar to bulletproof glass.
11:45 AM - OO1.6
Structural and Mechanical Studies of the Individual Building Block of a Transparent Natural Armor of Placuna Placenta.
Ling Li 1 , Christine Ortiz 1 Show Abstract
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
A number of species of mollusks possess transparent and highly mineralized exoskeletons which combine both optical and protective mechanical functionalities. These biomaterials are composed of nanoscale building blocks which minimize the scattering and absorption of light, as well as resist penetration, dissipate energy and localize fracture. Here, we investigate the nanostructure and mechanical behavior of the transparent shell of the mollusk, Placuna placenta (Linnaeus 1785). The shell is an organic-inorganic nanocomposite composted of ~99 wt% calcite and 1 wt% organic, and consists of ~1750 individual layers. Individual building block within each layer is an elongated diamond-shaped plate with a length of 141.8 ±43.4 μm, width of 5.54±1.36 μm, thickness of 294±84 nm, and tip angle of 10.45±2.95°. Transmission electron microscopy analysis show that spherical-shaped organic inclusions (diameter: 47±25nm) uniformly distributed within the crystalline building block (estimated volume fraction of intracrystalline organics: 2.8%). The mechanical behavior of the individual plates was quantified using an atomic force microscopy (AFM)-based nano-/micro-three-point bending experiments. The bending samples were prepared using a Dual-Beam Focused Ion Beam and the dimensions of each individual sample were quantified using scanning electron microscopy (SEM, typical dimensions, length: 10 μm, width: 0.8 μm, height: 0.3 μm). Customized AFM probes with a well-defined wedge geometry (angle: 60°, length: 1.1 μm, and height: 5 μm) were used for the bending experiment. Force/displacement curves were recorded using a 3D Molecular Force Probe (Asylum Research, CA), which maintains a closed loop X/Y/Z function on loading and unloading (maximum force: ~ 18 μN) in ambient conditions (number of samples, n=5) at displacement rates 0.1-5.0 μm/s. The lack of any loading-unloading hysteresis and permanent deformation (verified by SEM) indicated a purely elastic response. The predictions of an isotropic, elastic 3D finite element model of the beam exhibited an excellent fit to the experimental data (typical R-square values > 0.999), and allowed for estimation of Young’s modulus as found to be 86±9 GPa. This relatively high stiffness may be attributed to low organic content, which is also believed to enhance transparency by reducing scattering and absorption. The nanoscale material design principles found in this study might be applied for high-performance synthetic transparent armor materials.
12:00 PM - OO1.7
Theoretical Analysis and Tunable Design of the Failure of Synthetic and Biological Suture Joints.
Yaning Li 1 2 , Christine Ortiz 2 , Mary Boyce 1 Show Abstract
1 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States, 2 Material Science and Engineering, MIT, Cambridge, Massachusetts, United States
Suture joints are composite mechanical structures that are most often composed of compliant interdigitating seams connecting stiffer components. Suture joints with different types of interfacial geometries (e.g. triangle, sinusoid, trapezoid and fractal) are found throughout nature and serve to bear and transmit loads, absorb energy, and provide flexibility to accommodate growth, respiration and/or locomotion. One fascinating feature of suture joints is tunable failure over a large range depending on the interfacial geometry. To investigate this topic, an analytical composite model for a triangular “sawtooth” suture joint was developed which assumes perfect bonding between the stiffer “teeth” and the more compliant interfacial layer. Additionally, analogous 2D finite element analysis (FEA) numerical models (ABAQUS 6.9) were formulated that possessed cohesive bonding between the tooth components and interfacial layer. To capture the effects of evolution of nano-/micro cracks in the post-failure process, both linear and exponential damage evolution laws were used to quantitatively describe the degradation rate of material stiffness during crack propagation. Parametric studies were carried out to investigate the role of sawtooth angle (5~150 degrees), tooth/interface volume fraction (0.6/0.4~0.95/0.05), and material properties of the tooth/interface (stiffness ratio in a range of 100~10,000) on the failure mechanisms and strength of the suture joints. For most simulations, suture joints were observed to fail either within the stiffer “tooth” components or by interfacial failure, although for a few cases, the two mechanisms occurred simultaneously. The mechanism of tooth failure exhibited the maximum failure strength. Through this parametric analysis, it was found that when the tooth and interface fail simultaneously, the fracture energy locus reaches a cusp at an optimal tip angle. Thus, it is concluded that an optimal geometry with the maximum failure strength and energy corresponds to the transition of the two failure mechanisms. Consequently, substantial advantages in stiffness, strength and energy absorption for a given volume and weight are achieved. The conclusions from this investigation provide a guideline for optimal design of suture joints.
12:15 PM - OO1.8
Function-Related Variations in Structure and Composition of Crustacean Cuticle.
Helge Fabritius 1 , Svetoslav Nikolov 2 , Simone Karsten 1 , Andreas Ziegler 3 , Bastian Seidl 3 , Martin Friak 1 , Pavlina Elstnerova 1 , Sabine Hild 4 , Katja Huemer 4 , Dierk Raabe 1 , Joerg Neugebauer 1 Show Abstract
1 , Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf Germany, 2 , Bulgarian Academy of Sciences, Sofia Bulgaria, 3 , University of Ulm, Ulm Germany, 4 , Johannes Kepler University Linz, Linz Austria
The exoskeleton of crustaceans is formed by the cuticle which covers the whole animal and forms a structural entity based on fibrils of protein coated chitin crystallites that assemble to fibres which are organized in the form of twisted plywood. The cuticle material has to fulfil all vital functions for the organism which require very diverse physical properties like mechanical stability, elasticity, wear and friction resistance, transparency or differential permeability depending on the specific function of a certain skeletal element. These variations in physical properties are generated by modifications of the microstructure and chemical composition of the cuticle. How local modifications affect the mechanical properties and at which level within the structural hierarchy of the cuticle they occur is not yet understood. Therefore, we investigated the microstructure, chemical composition and the resulting mechanical properties of load-bearing cuticle, arthrodial membranes, joint structures and mandibles of different crustacean species on different length scales in a comparative approach. The results show that cuticle design is conservative on the low levels of structural hierarchy but of surprising variability in the organization of both organic matrix and mineral phase on higher levels. The experimental data are used as input for a hierarchical model for cuticle that uses ab initio calculations for the properties of basic components like chitin and calcite and hierarchical homogenization performed in a bottom-up order for the higher hierarchical levels. The model is used to evaluate changes of the overall elastic properties of cuticles from different species with the observed variations in properties of constituents and volume fractions of key structural elements. It is found that among the possible variations in cuticle ingredients and volume fractions, the experimental data reflect an optimal use of the structural variations regarding the best possible performance for a given composition. Since this approach can only predict the average elastic properties of cuticle, we have started to develop a model for the elasto-viscoplastic properties using a new Fast Fourier Transforms approach. Our goal is to obtain detailed information about the local stresses and strains in the phases and at the interfaces at large deformations and the mechanisms of damage initiation and failure.
12:30 PM - OO1.9
Hierarchical Composites Built from Cellular Architectures.
Bryan Kaehr 1 2 , Jason Townson 2 , Jeff Brinker 1 2 Show Abstract
1 Advanced Materials Laboratory, Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 Department of Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico, United States
A long standing goal of materials science is the synthesis of nanomaterials that mirror the function and complexity of biological systems. As synthetic strategies to generate functional nanomaterials with true 3D control remain elusive, routes have been explored that simply employ biological structures (proteins, peptides, DNA, viruses, cells, plant tissues, etc.) as templates for inorganic materials resulting, in general, in encapsulation/coating of the template. The ability to develop an inorganic support for intracellular architectures would greatly expand the opportunities for biocomposites by increasing the otherwise poor chemical and thermal stability of cellular structures and functions. To these ends, we have developed a process wherein mammalian cells direct their imprint in silico. Here substrate bound and solution cultured mammalian cells direct silica deposition using a general procedure that has been shown to direct silica condensation to protein-based materials of diverse properties (ACS Nano, 2011, 5, 1401-1409). Preservation of nm- to macro-scale cellular features and dimensions is observed on both the cell surface and interior after drying at room temperature - and largely after calcination to 550C. Cell/silica composites can template artificial lipid bilayers and be loaded with both small and macromolecular cargo. Further, we find evidence of intrinsic enzyme activity within these biocomposites. This combination of retained activity and supported catalysis in situ provides enormous opportunities to develop hierarchical nanomaterials for sensing and biocatalysis applications. Additionally, this approach represents a new platform with which to capture and preserve fundamental cellular interactions and responses, providing a simple means to freeze and image the complete heterogeneity of cell populations with nm-scale precision.
12:45 PM - OO1.10
Deformation Mechanisms in a Polymeric Composite with Combined Stochastic and Periodic Architectures.
Juliana Bernal Ostos 1 , Renaud Rinaldi 1 , Luke Miller 3 , Chris Hammetter 2 , Alan Jacobsen 4 , Galen Stucky 3 1 , Frank Zok 1 Show Abstract
1 Materials Department, University of California, Santa Barbara, Santa Barbara, California, United States, 3 Chemistry and Biochemistry Department, University of California, Santa Barbara, Santa Barbara, California, United States, 2 Mechanical Engineering Department, University of California, Santa Barbara, Santa Barbara, California, United States, 4 , HRL Laboratories, LLC, Malibu, California, United States
Polymeric cellular materials are lightweight and have the ability of absorb energy under compression. There are two main types of cellular architectures, stochastic foams and periodic lattices. Stochastic foams are characterized by a long plateau stress that allows them to absorb energy efficiently. However, polymeric foams have low strength, and this hinders their ability to absorb large quantities of energy. Periodic lattices are characterized by high peak strength but a marked post-peak softening, which makes them inefficient at absorbing energy. Many applications, such as protective equipment and packaging, require materials that are both efficient at absorbing energy and have high strength. In this work we study a composite designed to meet these needs by combining a stochastic foam with a periodic lattice. The system under study has two interpenetrating constituents, a thiol-ene periodic lattice whose mm-scale pores are filled with polyurethane foam with μm-scale porosity. The resulting composite is co-continuous and combines both types of cellular architectures. In this work we present the results of quasi-static compression tests on both the composite and its constituents. We use both synchrotron x-ray microtomography and 3D digital image correlation to study the deformation mechanisms of the constituents within the composite. We show that there is synergy between the constituent materials--the composite has both high strength and a high plateau stress, making it ideal for the applications mentioned.
OO2: Living Systems II
Monday PM, November 28, 2011
Room 208 (Hynes)
3:00 PM - OO2.2
Water-Activated, Shape Memory Twist Effect in Wood.
Nayomi Plaza 1 , Sam Zelinka 2 , Don Stone 3 , Joseph Jakes 1 Show Abstract
1 Performance Enhanced Biopolymers, USDA Forest Products Laboratory, Madison, Wisconsin, United States, 2 Durability and Wood Protection Research, USDA Forest Products Laboratory, Madison, Wisconsin, United States, 3 Department of Materials Science and Engineering, University of Wisconsin - Madison, Madison, Wisconsin, United States
Passive plant actuation has received increasing attention from scientists, especially in context of biomimetic and smart material applications. Recently, we discovered a water-activated, shape memory twist effect in wood slivers. This effect is of interest not only because of its novel behavior, but also because it holds clues about the thermodynamics of water in wood and the roles played by water in altering the mechanical and ionic relaxations of the polymeric components of wood. Even though scientists have known generally about the critical importance of water in wood for over a century, where exactly the water resides in wood and what it does to the polymers in wood remains – remarkably – poorly understood. By carefully examining this shape memory effect, and by dissecting it into its mechanical, thermodynamic, and transport (kinetic) components, we provide new insight into the interactions between water and wood. We study the twist effect by constructing simple torsional pendulums from wood slivers. Upon wetting the slivers twist. The amount and rate of twist depend on sliver diameter, cellulose microfibril angle with respect to the longitudinal cell axis, and equilibrium moisture content of the sliver before wetting. In some pendulums, more than a full revolution is possible within seconds after wetting. If the specimen is allowed to dry unrestrained, the sliver eventually returns to its pre-wetting configuration. However, if the sliver is restrained in the twisted state during drying, the majority of the twist remains ‘locked in’ once the restraint has been removed. The sliver can then be ‘unlocked’ by rewetting and allowing it to dry without restraint. The effects of sliver size, cellulose microfibril angle, and initial equilibrium moisture content were all studied. In addition, nanoindentation experiments are performed. Nanoindents placed in the S2 layer of the dry cell wall or compound middle lamella (lignin-rich layer that holds the cell walls together) are essentially permanent if the humidity is held fixed. However, the nanoindents can be made to nearly disappear (S2 layer) or completely disappear (middle lamella) if the wood is moistened. The nanoindentation water-activated shape memory effect is used along with water vapor sorption isotherm data to help better understand the shape memory twist effect in the wood slivers.
3:15 PM - OO2.3
Hierarchically-Structured Nanocellulose Composite Scaffolds.
Amalie Donius 1 , Andong Lui 2 , Philipp Hunger 1 , Lars Berglund 2 , Ulrike G. Wegst 1 Show Abstract
1 Materials Science & Engineering, Drexel University, Philadelphia, Pennsylvania, United States, 2 Wallenberg Wood Science Center, Royal Institute of Technology (KTH), Stockholm Sweden
The use of a highly controlled, water-based, ice-templating technique enables the creation of materials with hierarchical structures. The resulting scaffolds are anisotropic and possess a honeycomb-like structure with properties which, in the strong direction, are one to two magnitudes higher than those of equiaxed foams of identical overall porosity. Important for the performance of these materials is the overall porosity, or relative density, and the pore morphology at the first level of the hierarchy and the solid from which the cell walls are made at the second. Presented in this contribution will be correlations found between the honeycombs’ structure, cell wall composition and structure, and overall performance in compression for freeze-cast scaffolds made from pure chitosan (deacetylated chitin), pure nanocellulose, and composites that have been reinforced with nanocellulose and particles. They show that the scaffolds’ stiffness, strength, and toughness can significantly be increased by a change in freezing velocity and the addition of nanofibers and particles. The results illustrate how the scaffold properties can be controlled independently through both composition and processing parameters, and that those of anisotropic honeycombs are significantly higher than those of equiaxed foams of identical composition. With their unusual structure-property profile, freeze-cast scaffolds are ideal candidates for a vast range of applications that range from filtration and catalysis to tissue scaffolds.
3:30 PM - OO2.4
A Multiscale Approach to Assess the Effect of Multilevel Structuring on the Properties of Hierarchical Lattice Materials.
Andrea Vigliotti 1 , Damiano Pasini 1 Show Abstract
1 Mechanical Engineering, McGill University, Montreal, Quebec, Canada
Natural materials have often a defined multilevel hierarchy which governs their macroscopic properties and mechanical efficiency. The structural features at a given length scale impart a specific mechanical contribution. One of these features is embodied by cellular patterns, which can be revealed at one level of the structural hierarchy, or can appear, in certain instances, repeated times at multiple levels. Cellular arrangements, either random, or periodic, or a mix of the two, yield a major contribution to the overall performance of the material. When they are nested at multiple length scale, however, they confer a remarkable cumulative effect on the mechanical properties. Recent advances in additive manufacturing techniques enable the artificial replica of nested cellular substructures across length scale, opening up new opportunities for the next generation of hierarchical materials. To capture the contribution and interaction of the substructures at different length scale, accurate constitutive models of hierarchical materials are necessary. This paper presents a multiscale approach to the analysis of a hierarchical lattice component which exhibits nested levels of lattice. The material at each layer is idealized with a lattice structure. Starting from the shape and topological properties of the unit cell at a given layer, the method allows the estimation of the mechanical properties of the overall hierarchical component. The structural properties of a layer at a given scale are first modeled independently by defining proper boundary conditions and are then used to solve the resulting equilibrium problem to obtain the homogenized properties of the layer. Followed at each layer, the method enables to capture the effect of nesting layers with a lattice structure. The procedure allows at a given length scale to assess yield and buckling strength of the material allowing a thorough estimation of the macroscopic strength.The results of the analysis are plotted onto material property charts, which can visualize the contribution of each structural layer to the overall macroscopic performance. A number of three-dimensional, open and closed cell, topologies as well as the effect of cell geometric parameters of the lattice have been investigated. The maps show that at every multiscale step the mechanical properties of a given layer merge into those of the layer that follows in the hierarchy. The maps permit to follow the properties development as they evolve across length scale. The method can be a resource to explain the behavior of natural hierarchical materials as well as to guide the design of next generation of hierarchical structures, such as for the design of bone-replacement prostheses made out of nested layers of lattice material.
3:45 PM - OO2.5
Structure-Property-Processing Correlations in Freeze-Cast Titanium Foams.
Jordan Weaver 2 , Surya Kalidindi 2 , Ulrike G. Wegst 1 Show Abstract
2 Mechanical Engineering and Mechanics, Drexel University, Philadelphia, Pennsylvania, United States, 1 Material Science and Engineering, Drexel University, Philadelphia, Pennsylvania, United States
The performance of cellular metals is determined by the metal from which the cell walls are made, by the overall porosity, and by the pore morphology. Freeze casting, the directional solidification of a water-based metal slurry, has been shown to produce complex metal foams. Freeze casting also offers great control of the overall porosity and pore morphology and hence the performance of the cellular material. The above mentioned tunability is demonstrated in the construction of honeycomb-like Ti-6Al-4V scaffolds with correlations between slurry composition, additives, pore size distribution and mechanical properties. The results show that cellular metals can be freeze cast that have unique structure-property combinations at porosities that may exceed 60%.
OO3/SS4: Joint Session: Living Systems III: Nano and Submicron Mechanical Testing
Monday PM, November 28, 2011
Constitution A (Sheraton)
4:30 PM - **OO3.1/SS4.1
New Techniques Using Nanoindentation of Biological Tissues in Fluid Environments.
Michelle Dickinson 1 Show Abstract
1 Chemical and Materials Engineering, Univeristy of Auckland, Auckland New Zealand
Nanoindentation has become a common and useful technique for measuring the mechanical properties of heterogeneous and microscale materials. Recently there has been a trend to use this technique for testing biological tissues, however the standard preparation methods of dehydrating, mounting and polishing specimens combined with the Oliver and Pharr technique for data analysis are not best suited for such delicate and visco-elastic materials. This study will introduce some of the key challenges for obtaining realistic mechanical properties of biological tissues using nanoindentation including the immersion of samples in fluid, mounting and preparation techniques, contact area error, substrate effects and appropriate data analysis models. Results showing a new nanoindentation test technique for measuring the elastic properties of elastin modified epithelial tissue layers ranging from 7-20μm (3 to 5 cell layers) in thickness will be given. Although previous measurement attempts on these films using tensile testing have failed due to their compliant nature leading to gripping and aligning difficulties, this study will introduce a new drumhead indentation technique. Using this technique, the membrane can be grown and floated over the drum mounting device without removal from the incubation fluid thus removing the need for sample fixation or storage. The membrane is gently held down along the periphery to create a drum-like skin upon which indentation tests are carried out. The indentation loads ranging from 1-5μN result in load-displacement curves where both the linear and non-linear deflection responses can be analysed. This study will show the first results measured for this type of thin biological film displaying modulus values ranging from 200-1000kPa. To highlight the issues related to hard biological tissues, SEM images showing indentation crack propagation and deflection paths in dehydrated mineralized tissues from E. chloroticus sea urchins will be compared to those tested without sample preservation emphasizing the importance of appropriate fluid immersion and testing for biological composites.
5:00 PM - OO3.2/SS4.2
Determination of Mechanical Properties in Escherichia Coli by Nanoindentation.
Cody Wright 1 Show Abstract
1 Department of Mechanical Engineering, Old Dominion University, Suffolk, Virginia, United States
Escherichia coli, like other gram-negative bacteria, is protected from the surrounding harsh environment by a cell wall consisting of the peptidoglycan and outer membrane. Whereas the cytoplasmic membrane is the selective barrier, the cell wall provides mechanical strength for the cell. As bacteria navigate various environments, osmotic pressure can change dramatically. The peptidoglycan together with cellular proteins mitigate the osmotic stress that would otherwise cause lysis. The mechanical properties of E. coli cells and its individual layers have been largely indeterminable until the recent development of probe-based measurement tools. Since their invention, scientists have reported significant data measuring elasticity, modulus, and stiffness using atomic force microscopy (AFM). Fundamentally, in order to determine these mechanical properties through probe-based techniques the contact area and load should be well defined. The load can be precisely calculated through the AFM cantilever spring constant. However, the silicon tip contact area can only be estimated, potentially leading to compounding uncertainties. Therefore, we propose a methodology to determine nanomechanical properties of E. coli using a nanoindenter. The mechanical properties of the live bacteria will be tested in liquid.
5:15 PM - OO3.3/SS4.3
Linking Nano- and Micromechanical Measurements of the Bone-Cartilage Interface.
Sara Campbell 1 , Virginia Ferguson 2 , Donna Hurley 1 Show Abstract
1 , NIST, Boulder, Colorado, United States, 2 2.Department of Mechanical Engineering, University of Colorado, Boulder, Colorado, United States
A thin (~10 to 100 µm) region of articular calcified cartilage (ACC) anchors stiff (~20 GPa) bone to the significantly more compliant (~100’s of MPa) hyaline articular cartilage (HAC). Although this bone-cartilage, or osteochrondal, interface resists remarkably high shear stresses and rarely fails, its mechanical properties are largely unknown. In this hierarchical study, we combine nanoindentation and atomic force microsopy (AFM) methods to elucidate the mechanisms that facilitate load transmission across the ostetochondral interface. A rabbit femoral head embedded in PMMA was sectioned in the coronal plane and ultramicrotomed to provide a flat surface for testing. Nanoindentation tests (maximum load of 2 mN) were placed in an array traversing the interface region from the ACC into the HAC with 3 µm spacing. Nanoindentation measurements revealed a transition zone in mechanical properties between the calcified and uncalcified region approximately 9 µm wide. Contact resonance force microscopy (CR-FM) measures the frequency and quality factor of the AFM cantilever’s vibrational resonance in contact mode. With this technique, measurements of relative storage modulus E' and loss modulus E'' with 300 nm spacing were possible. CR-FM measurements indicated a substantially narrower (~3 µm) interface, demonstrating the importance of multiscale testing. The inherent nanoscale heterogeneity of ACC was evidenced by significantly higher coefficients of variation than those for HAC for E' values measured by both nanoindentation and CR-FM. This nanoscale heterogeneity is likely to contribute to energy dissipation and the functionality of the bone-cartilage interface at the macroscale. Complimentary measurements with quantitative backscatter electron imaging indicated a decrease in mineral content across the transition zone that corresponded with the increase in E' values. Understanding the functionality of the osteochondral interface will further aid in the development of biomimic interfaces.
5:30 PM - OO3.4/SS4.4
Spherical Nanoindentation Applied to Biomimetic Composites.
Mohammed Abba 1 2 , Surya Kalidindi 1 , Ulrike Wegst 2 Show Abstract
1 Mechanical Engineering, Drexel University, Philadelphia, Pennsylvania, United States, 2 Materials Science & Engineering, Drexel University, Philadelphia, Pennsylvania, United States
Over time, scientists and engineers all over the world have used nature as a template to design and fabricate materials with varying functionality. While there have been considerable advances made in replicating and improving on naturally occurring materials, man has yet to fully understand the science behind some of them. One of the most widely studied fields is that of biological composites and their multifunctionality. Usually made of biopolymers and some minerals these composites have been found to have a resistance to fracture that is orders of magnitude greater than their constituent materials. With the increasing demand for “green” materials, these composites can serve as a useful template as they are made from safe, biodegradable and readily available materials. This study will focus on how hierarchical structures, such as those found in nacre, can be replicated using a fast and repeatable method. Using chitosan and alumina platelets, we will design films by casting and by smearing the solution. The smeared solution will show more aligned layers and higher mechanical properties. Mechanical properties will be measured in tension as well as using spherical nanoindentation to better understand properties and interfaces at the nanoscale level.
5:45 PM - OO3.5/SS4.5
Failure of Bone at the Sub-Lamellar Level Using In Situ AFM-SEM Investigations.
Ines Jimenez-Palomar 1 , Asa Barber 1 Show Abstract
1 Department of Materials, School of Engineering and Materials Science, Queen Mary University of London, London, London, United Kingdom
Bone is a fibrous biological nanocomposite material, which is optimized to avoid catastrophic failure [1, 2]. The fracture behavior of bone is expected to be controlled by the various structural features present across the many existing hierarchical length scales . However, micron sized lamellae in bone present the simplest composite unit in bone consisting of mineralized collagen fibrils within a protein matrix, with some work suggesting that this length scale dominates the fracture of whole bone . In this paper we examine the mechanical properties of individual lamellae using novel atomic force microscopy (AFM)-scanning electron microscopy (SEM) techniques . Individual lamellar beams are selected from bone using focussed ion beam (FIB) microscopy and mechanically deformed with the AFM while observing failure modes using SEM. Both the elastic and fracture behavior of the bone lamellae are determined using these techniques. Composite analysis is used to evaluate the mechanical behavior of lamellae and results at micron and sub-micron length scales related to the overall toughness of bone material. Thus, the contribution of micron and sub-micron toughening mechanisms to the fracture of whole bone is considered. References1.Fratzl, P. and R. Weinkamer. Nature's hierarchical materials. Progress in Materials Science. 528 (2007) p. 1263-1334.2.Peterlik, H., et al. From brittle to ductile fracture of bone. Nature Materials. 5 (2006) p. 52-55.3.Gupta, H. S. and P. Zioupos. Fracture of bone tissue: The ‘hows’ and the ‘whys’. Medical Engineering and Physics. 30 (2008) p. 1209-1226.4.Hang, F. and A.H. Barber. Nano-mechanical properties of individual mineralized collagen fibrils from bone tissue. J. R. Soc. Interface. 857 (2011) p. 500-505.
Markus J. Buehler Massachusetts Institute of Technology
Horacio D. Espinosa Northwestern University
Tia B. Tolle U. S. Air Force Research Laboratory
Mark R. Van Landingham U. S. Army Research Laboratory
Ulrike Wegst Drexel University
OO6: Poster Session: Hierarchical Materials
Tia Benson Tolle
Tuesday PM, November 29, 2011
Exhibition Hall C (Hynes)
OO4: Living Systems IV
Tuesday PM, November 29, 2011
Room 208 (Hynes)
9:30 AM - **OO4.1
Interactions between Myofibroblasts and Cardiomyocytes in a Tissue Model of Cardiac Fibrosis.
Teresa Abney 1 , Pavi Anand 2 , Ali Nekouzadeh 2 , Tetsuro Wakatsuki 3 , Elliot Elson 4 5 , Guy Genin 1 5 Show Abstract
1 Mechanical Engineering and Materials Science, Washington University at St. Louis, St. Louis, Missouri, United States, 2 Biomedical Engineering, Washinton University in St. Louis, St Louis, Missouri, United States, 3 Department of Physiology, Medical College of Wisconsin, Milwaukee, Wisconsin, United States, 4 Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, Missouri, United States, 5 Center for Materials Innovation, Washington University, St. Louis, Missouri, United States
Interactions between myofibroblasts and cardiomyocytes are important to understanding the long-term consequences of cardiac fibrosis and myocardial infarction, but are difficult to quantify in natural tissue. We study how myofibroblasts perturb electromechanical function of cardiomyocytes in a 3D culture environment using computational and experimental models designed specifically for this purpose.
The idealized model system, which we call engineered heart tissues (EHTs), is assembled from embryonic cardiomyocytes and containing defined fractions of myofibroblasts distributed randomly throughout the tissue. EHTs are assembled by suspending ~10e6 cells obtained from 10-12 day chicken embryos in 1 ml of ~1mg/ml type I rat tail collagen. Over several days of incubation the primary fibroblasts convert to myofibroblasts that compress and stiffen the collagen. Within 4-7 days the cardiomyocytes, which begin contracting independently, establish gap junctions and begin to beat coherently. Then the EHT twitch force is readily measurable with an isometric force transducer, and the spread of electrical excitation can be measured using optical mapping techniques. The fraction of cardiomyocytes can be varied from ~5% to ~95%.
The computational model is formulated at the cellular level taking into account individual cardiomyocyte and myofibroblasts to yield the pattern of impulse spread as modulated by the presence of myofibroblasts acting either as insulators or resistors. The excitatory impulse activates the contraction of individual viscoelastic cells that are mechanically linked to other cells and the extracellular matrix (ECM). Three classes of models are linked in these simulations: electrophysiologic models, models of the contractile response of individual cardiomyocytes as a function of their internal non-bound calcium levels, and models linking these cellular responses to the overall mechanics of an EHT.
We predict measure the effects of myofibroblasts on electrical and mechanical functioning of EHT specimens at the hierarchical levels of ion channel, single cell, and EHT. Central questions are how myofibroblasts and cardiomocytes are coupled electrically in EHTs, and how overgrowth of tissues by proliferative myofibroblasts affects mechanical function. We will present results that shed light on how myofibroblasts can both improve and attenuate the active mechanical function of EHTs.
10:00 AM - OO4.2
Nanomechanics of Murine Articular Cartilage Reveals the Effects of Chondroadherin Knockouts.
Michael Batista 1 2 , Alan Grodzinsky 2 3 4 , Christine Ortiz 1 , Dick Heinegard 5 , Lin Han 1 Show Abstract
1 Department of Material Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 4 Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 5 Department of Rheumatology, University of Lund, Lund Sweden
Previous biomechanical studies of articular cartilage have focused primarily on its major molecular constituents, collagen and aggrecan. Despite their important roles in cartilage matrix formation, integrity, and function, other cartilage biomolecules have received less attention. Direct quantification of cartilage biomechanical properties using murine knockout models can provide important insights into how these molecules affect the function and pathology of cartilage. This study focuses on chondroadherin (CHAD), a leucine-rich repeat protein from the territorial matrix, which binds to type II collagen and the integrin α2β1, and is hypothesized to function in the communication between cells and their surrounding matrix as well as in the regulation of collagen fibril assembly. Femoral condyles from murine knee joints were dissected from the hind legs of wild type and CHAD knockout specimens with ages of 1 year, 4 month, and 11 weeks (n≥4 joints for each group). AFM-based nanoindentation was performed on the femoral medial condyle of each specimen using a gold-coated SiO2 spherical colloidal probe tip (spring constant ~ 4.5 N/m, end radius ~ 2.5 μm), functionalized with a neutral, hydrophilic self-assembled monolayer (11-mercaptoundecanol). All the tests were conducted in PBS with 0.1-10 μm/s z-piezo displacement rates. The effective indentation modulus, Eind, was calculated in the loading portion of each force-indentation depth curve using the Hertz model to account for tip geometry. For all the tested age groups, the CHAD-knockout cartilage had significantly lower Eind at all indentation rates (p<0.05, 2-way ANOVA). For example, Eind was 0.77±0.1 MPa (mean±SEM) for 1 year wild type specimens and 0.25±0.07 MPa for CHAD knockout specimens at 1 μm/s. The same effect was observed among the 4 month and 11 week groups, in which Eind was significantly lower for CHAD knockout specimens. In addition, significant increase in Eind with indentation rate for all specimens (pFriedman<0.05) suggested the presence of poroviscoelastivity in both wild type and CHAD knockout joints. The results of this study demonstrated a significant reduction in compression resistance of the cartilage tissue in the absence of CHAD macromolecules. This weakening of cartilage tissue appears to be consistent with the hypothesis that CHAD has a biomechanically important function as a connecting linkage in the formation of an appropriately assembled fibrillar collagen network. Lack of CHAD appears to weaken development of the load bearing extracellular matrix. This could consequently affect the local concentration and compressive resistance of aggrecan, and influence osmotic swelling pressure and permeability. Ongoing studies are investigating the biochemical properties and nanostructure of CHAD-knockout murine joints, to provide further evidence on the important role of CHAD in cartilage tissue formation and function.
10:15 AM - OO4.3
Molecular Interactions between Aggrecan and Collagen from the Cartilage Extracellular Matrix.
Fredrick Rojas 1 , Alan Grodzinsky 2 3 4 , Christine Ortiz 1 , Lin Han 1 Show Abstract
1 Materials Science and Engineering , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Biological Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 4 Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
The functional behavior of articular cartilage is dependent on the integrated interactions of its extracellular matrix molecular constituents including the fibrillar collagen network and negatively charged proteoglycans, such as aggrecan. Understanding interactions between these molecules will provide important insights into the mechanistic origins of cartilage biomechanical behavior. In this study, atomic force microscopy (AFM) was utilized to quantify the interactions between a spherical probe tip (R ~ 2.5 μm) end-functionalized with aggrecan at physiological-like packing densities, and the surface of trypsin-treated, proteoglycan-depleted cartilage samples, consisting primarily of type II collagen fibrils, in physiological-like aqueous conditions. The aggrecan-collagen interactions were measured by: 1) indenting into the trypsin-digested cartilage disk up to a depth of ~500 nm at 0.5 μm/s indentation rate; 2) holding at this constant indentation depth for a given surface dwell time (t = 0 – 60 s); and 3) retracting from the sample at the same indentation rate as loading. This experiment was repeated for cartilage disks obtained from a minimum of three calf knee joints, in various aqueous solutions (NaCl, ionic strength (IS) = 0.01 – 1.0M; phosphate buffered saline without Ca2+, IS = 0.15M; NaCl, IS = 0.15M with [Ca2+] = 0 – 20mM). The maximum adhesion force and energy were extracted from each of the retraction force-indentation depth curves. For the aggrecan-tip, heterogeneous long-range adhesion was observed up to ~2.5 μm upon retraction from the surface after indentation for all dwell times. The adhesion force, Fad, showed an asymptotic, nonlinear increase with t, reaching a maximum of 3.1±0.2 nN at t = 60 s. Aggrecan-collagen interactions displayed a significant dependence on IS (p < 0.001), where Fad varied from 2.5±0.2 nN at 0.01M NaCl to 4.3±0.3 nN at 1.0M NaCl (t = 30 s); as well as a significant [Ca2+]-dependence (p < 0.001), where Fad increased from 2.95±0.2 nN with a [Ca2+] = 0mM to 7.4±0.3 nN with a [Ca2+] = 20 mM (IS = 0.15M NaCl, t = 30 s). Aggrecan-collagen interactions displayed ~50% higher magnitude in adhesion force and ~3× longer adhesion distance than previously studied aggrecan-aggrecan interactions. Our results provide a quantitative assessment of the magnitude and environmental dependence of the molecular interactions between aggrecan and collagen. These molecular interactions are important in determining the overall integrity of the cartilage extracellular matrix and the relative mobility of matrix molecules, such as aggrecan, within the fibril network during tissue deformation. Understanding aggrecan-collagen interactions can provide additional insights into normal cartilage tissue function (e.g., osmotic swelling, hydraulic permeability and energy dissipation) and altered function in disease states, as well as a molecular perspective for approaches to tissue engineering for cartilage repair.
10:30 AM - OO4.4
Specific Biomechanical and Biochemical Interactions between Neuronal Cells and Guidance Factors Measured by Atomic Force Microscopy.
Elise Spedden 1 , Ross Beighley 1 , James White 2 , Marie Tupaj 2 , David Kaplan 2 3 , Cristian Staii 1 Show Abstract
1 Physics and Astronomy, Tufts University, Medford, Massachusetts, United States, 2 Biomedical Engineering, Tufts University, Medford, Massachusetts, United States, 3 Chemical Engineering, Tufts University, Medford, Massachusetts, United States
Studying how neuronal cells grow and interact with each other as well as their biomechanical and biochemical interactions with surrounding guidance factors (extracellular matrix proteins, mechanical and topographical cues etc.) is of fundamental importance for understanding the nervous system. However, the mechanism of axonal navigation to their target region and their specific interactions with guidance factors are largely unknown. Here we describe the use of Atomic Force Microscope (AFM) based nanolithography and AFM force spectroscopy to characterize and explore the interplay between biochemical and biomechanical cues on length scales ranging from nanometers to microns. Specifically, we use AFM nanolithography techniques, fluorescence spectroscopy and AFM force spectroscopy measurements to systematically investigate the adhesion and real time growth of neuronal processes in the presence of different types of extracellular matrix proteins and adhesion factors: poly-lysine, laminin, and fibronectin. We therefore create a series of systems, containing identical number of neurons growing on well-defined geometrical patterns, while the type of the underlying growth promoting protein is different from sample to sample. For each system we measure several key biomechanical/biochemical parameters related to neuronal growth such as adhesion strength and rupture forces between the axons/dendrites and the protein patterns, axon/dendrite stiffness and elastic modulus, and recovery time of the axon/dendrite in response to cell membrane damage induced by AFM tip. We show that systematic measurements of these parameters yield fundamental quantitative information about the role played by various biomechanical and biochemical growth factors in determining axon/dendrite outgrowth and connectivity.
11:15 AM - **OO4.5
The Role of Bone’s Hierarchical Structure in Fracture Toughness.
Sandra Shefelbine 1 Show Abstract
1 Department of Bioengineering, Imperial College London, London United Kingdom
Bone is a natural composite material consisting of protein (mainly collagen), mineral (hydroxyapatite), and water. Like many other natural composites bone exhibits a unique combination of mechanical properties, in particular high strength and toughness. Bone’s fracture resistance (toughness) derives from toughening mechanisms that act both at the molecular level and at much larger dimensions (the micron level and up). We still do not know how changes in the bone building blocks at the molecular level influence its hierarchical architecture and how structural features interact across a wide range of length scales. Osteogenesis imperfect (OI or brittle bone disease) is caused by a mutation of the collagen type 1 gene that results in brittle bones and many fractures. Because OI is caused by a molecular defect and results in altered whole bone mechanics, particularly a massive decrease in toughness, OI bone provides an ideal platform for studying how molecular changes disrupt the structure and toughness of bone throughout the structural hierarchy. In a mouse model of OI (oim) we have recently employed a multi-scale approach, analyzing bone structure and mechanics using a variety of imaging and mechanical testing modalities of mouse bone from wild type (WT) and OI mice.Whole bone level: Three point bending tests were performed on femur specimens from 8 week old WT and OI mice. OI bones exhibited classic brittle fracture with little plastic deformation.Tissue level: Decalcified histology showed a woven tissue fabric in the OI bones compared to the organized lamellar bone of WT. There was a significant decrease in Young’s modulus (E) in the OI bones compared to WT (nanoindentation). Interestingly the decrease in modulus was accompanied by an increase in mineral density (qbSEM). In situ and ex situ fracture toughness measurements demonstrated the lack of toughening mechanisms in OI bones; WT bones exhibited classic toughening, particular crack deflection and microcracking. Cell level: Synchrotron CT demonstrated a larger number of osteocyte lacunae as well as more vascular canals in OI bones compared to WT bones, which is in accordance with its higher metabolic demands. Mineral/Protein level: Raman spectroscopy indicated OI bones had less carbonate substitution and less crystallinity. Transmission electron microscopy images of the mineral illustrated the smaller, tightly packed, and disorganized structure of the OI apatite crystals compared to WT. By studying pathologic bone, we can probe the ramifications of molecular defects throughout the bone hierarchical structure and in order to develop an understanding of how this structure is created from the molecular level up. Multi-scale analysis that links cellular expression to whole organ pathology is critical in a wide array of disease and treatment paradigms.
11:45 AM - OO4.6
Age-Related Changes in the Plasticity and Toughness of Human Cortical Bone at Multiple Length-Scales.
Elizabeth Zimmermann 1 2 , Eric Schaible 2 , Hrishikesh Bale 2 , Holly Barth 1 2 , Simon Tang 3 , Peter Reichert 2 , Bjorn Busse 2 , Tamara Alliston 3 , Joel Ager 2 , Robert Ritchie 1 2 Show Abstract
1 , University of California Berkeley, Berkeley, California, United States, 2 , Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 , University of California San Francisco, San Francisco, California, United States
The complex structure of human cortical bone evolves at multiple length-scales from its basic constituents of collagen molecules and hydroxyapatite crystals at the nanoscale to the osteonal (Haversian) structures at near-millimeter dimensions. Such a characteristic multi-scale structure provides the basis for its mechanical properties. With respect to resistance to fracture, toughness in bone is derived intrinsically, i.e., from plastic deformation, at structural scales typically below a micrometer by such mechanisms as fibrillar sliding, whereas bone’s extrinsic (crack-growth) toughness results from mechanisms such as crack deflection and bridging that are generated at much larger structural levels in the tens to hundreds of micrometers. Biological factors can degrade fracture resistance, such that the risk of bone fracture is markedly increased with aging. Although an important reason for the decreased fracture resistance is the loss of bone mass (or bone-mineral density) with age (bone quantity), we find here that significant age-related changes in the bone-matrix structure can occur at multiple length-scales, which are detrimental to fracture resistance (bone quality). Specifically, non-enzymatic advanced glycation end-product measurements and (synchrotron) small-/wide-angle x-ray scattering (SAXS/WAXS) were used to characterize changes in the bone structure at sub-micrometer dimensions, while computed synchrotron x-ray tomography and in situ fracture toughness measurements in the environmental scanning electron microscope (ESEM) were used to characterize corresponding changes at micron-scale dimensions and above. Our results show how age-related structural changes at differing size-scales can degrade the intrinsic toughness by increased cross-linking at nanoscale dimensions suppressing plasticity, as well as the extrinsic toughness of bone by an increased osteonal density at the scale of tens to hundreds of micrometers, which acts to limit the potency of crack bridging mechanisms. The link between these processes is that the increased stiffness of the collagen fibrils from cross-linking requires energy to be absorbed by “plastic” deformation at higher structural levels; this is primarily achieved by the process of microcracking, which in turn has a profound effect on the mechanisms of extrinsic toughening in bone.
12:00 PM - OO4.7
Consideration of Bone and Teeth as Fiber Reinforced Polymer Composites Using In Situ Nanomechanical Testing.
Fei Hang 1 , Dun Lu 1 , Asa Barber 1 Show Abstract
1 Department of Materials, Queen Mary University of London, London United Kingdom
Bone and teeth are examples of biological materials that have structures adapted to a mechanical function. Importantly, the structural composition of these biological composites show striking similarities to fiber reinforced polymer composites. The bones of mammals and the teeth of limpets are studied in this work as nanofibers of collagen incorporating hydroxyapatite and the mineral goethite respectively reinforce biological polymers. The mechanical properties of individual constituents in these biological composites are measured using advanced atomic force microscopy (AFM) techniques . Composite theory is applied in order to understand how nanomechanical behavior in biological composites relates to overall mechanical performance. References1. F. Hang, A. H. Barber. J. R. Soc. Interface 8 (2011) 500.
12:15 PM - OO4.8
Magnetically Responsive Bi-Stable Hierarchical Assemblies.
Stoyan Smoukov 1 , Elena Blanco 1 , Stephanie Lam 1 , Sumit Gangwal 1 , Krassimir Velikov 2 , Orlin Velev 1 Show Abstract
1 Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Debye Institute for Nanomaterials, Utrecht University, Utrecht Netherlands
Hierarchical responsive systems have a number of existing applications including actuators, sensors, displays and tunable viscosity liquids. Fields and environmental gradients are often needed not only to actuate a change, but to maintain the system in a particular state. We describe two systems where bi-stable assemblies are a potential solution to this problem. In the first we form staggered linear chain assemblies from magnetic Janus particle building blocks. The structures are well-defined and stable, and can remain assembled even in the absence of external fields. The strength and directions of the interactions between particles in a chain can be controlled by the type and thickness of the metal shells in the Janus particles. The chains can also be switched between multistable straight and curved configurations by the magnetic field. Most importantly, on command, the chains can be disassembled remotely and broken up into constituent particles. This is accomplished by the use of a demagnetizing coil and is a convenient way to recycle the particles into new assemblies. In the second system we also demonstrate a new class of Ramsden/Pickering foams, which can be stable for weeks in the absence of fields, but can be destroyed on-demand using a magnetic field. We have previously used hypromellose phthalate (HP-55) particles to create superstable foams. The addition of HP55-magnetic particle composites above a certain concentration allows the remote manipulation and rapid destruction of the foams. Such foam systems can be applied in a range of industrial and environmental applications that require controlled defoaming on-demand. We analyze foam stability in the absence of a field by measuring the rate of water drainage from the foam as a function of time. We also correlate the collapse behavior of the foam to the liquid volume fraction as well as the concentration of magnetic particles in the system. Aging effects associated with film wetness lead to different collapse mechanisms. Particles and bubbles in wetter foams are able to move around and rearrange and are more resistant to collapse, whereas in aged foams they have decreased mobility and the drained films break more easily. In summary, we demonstrate hierarchical materials with novel functionality which can be manipulated remotely or destroyed/disassembled on-demand using magnetic fields.
OO5: Synthesis and Characterization I
Tuesday PM, November 29, 2011
Room 208 (Hynes)
2:30 PM - **OO5.1
High Speed X-Ray Imaging.
Wah-Keat Lee 1 Show Abstract
1 Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois, United States
The high flux, wide energy spectrum (5-150 keV), coupled with high-repetition rates and a flexible timing structure makes synchrotrons ideal places for high-speed x-ray imaging. At the APS, each x-ray pulse is <150 ps, and the inter-pulse time can be varied, up to a maximum of 3.68 us, in 2.8 ns steps. With the addition of high speed beam choppers to mitigate the heat load on the sample and detector, and the use of detectors with fast electronic shutters ( < 1 ms), high quality time resolved images with sub-microsecond temporal and micrometer spatial resolutions are feasible. In addition, due to the small electron source sizes and the large source-sample distances, phase-contrast is easily achieved, thereby greatly enhancing the image contrast and is particularly useful in cases where absorption contrast is weak. Application examples include: fatigue crack propagation in superalloys, automobile fuel injector and spray dynamics and three-dimensional fluid flow through a porous media.
3:00 PM - OO5.2
Mechanical Characterization of Skin Using Indentometry and Viscoelastic Model.
Sean Morrisroe 1 2 , Wei Tian 1 , Chung-yi Chiang 1 Show Abstract
1 Advanced Measurement and Data Modeling, Unilever, Trumbull, Connecticut, United States, 2 Mechanical Engineering, University of Connecticut, Storrs, Connecticut, United States
Skin mechanical properties are highly relevant to skin hydration, skin aging, and various skin diseases (such as pseudoxanthoma elasticum and cutis laxa). A change or a disorder of skin mechanics could affect not only the tactile perceptions but also the appearance of the skin. In any clinical practice, a non-invasive characterization to effectively assess skin mechanical properties is highly desired by the dermatologists and clinicians. Indentometry is one of the non-invasive methods measuring skin mechanical properties but is rarely used in clinical skin research due to the lack of understanding and model development to correlate indentation measurements with skin viscoelastic behaviors. A handheld device was modified and developed to perform indentation measurements in both in-vitro experiments and in-vivo clinical research. A standard linear solid model was used to interpret the viscoelastic properties of skin measured from the indentation test. Using the porcine skin model, the mechanical properties at various skin depths were examined to understand the mechanical attributes in the hierarchical layers of skin. Incorporating with other measurements such hydration, skin indentometry with viscoelastic model potentially can be used to investigate the correlations between skin physiology and skin mechanics. It is believed that indentometry will be a useful technique to probe skin mechanics, allowing dermatologists to precisely assess skin conditions and clinicians to effectively perform skin research in the skin care industry.
3:15 PM - OO5.3
Suspended Cell Rheology in the Frequency Domain as Characterized by Optical Stretching.
John Maloney 1 , Eric Lehnhardt 3 , Krystyn Van Vliet 1 2 Show Abstract
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States, 3 Biomedical Engineering, Arizona State University, Phoenix, Arizona, United States, 2 Biological Engineering, MIT, Cambridge, Massachusetts, United States
Tissue cells are known to exhibit power-law rheology at physiologically relevant probe frequencies, along with a broad distribution of stiffness values when probed in the attached state by contact techniques. However, previous results have been equivocal whether stress fibers -- prominently featured by attached cells -- and the resulting internal stresses are a necessary enabler of power-law rheology. Do suspended cells deform similarly to lipid vesicles, which behave as simple spring-dashpot assemblies, or similarly to attached cells, which exhibit a power-law rheological response similar to soft glassy materials? Furthermore, can the wide distribution of stiffness measurements be attributed to the vagaries of probe-cell contact, or is this distribution an inherent feature of cells? We address both questions with the first measurements of fully suspended cells in the frequency domain via the non-contact technique of optical stretching. We find that suspended cells exhibit power-law rheology despite lacking stress fibers. Moreover, single cells examined by a non-contact technique are found to exhibit considerable stiffness heterogeneity, indicating that such broad distribution of cell stiffness is an inherent feature of cell populations. These results confirm the benefit of using optical stretching to eliminate probe-cell and cell-substratum contact and thus to avoid active adhesion response, including focal adhesion and stress fiber formation, during single-cell rheological measurements. Additionally, the results prompt further study of the dominant origin of mechanical variation among cells.
3:30 PM - OO5.4
Effect of UV Damage on Biomechanics of Human Stratum Corneum.
Krysta Biniek 1 , Kemal Levi 1 , Reinhold Dauskardt 1 Show Abstract
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
The outermost layer of skin, the stratum corneum (SC), is a hierarchical structure composed of corneocytes, made up largely of aligned keratin filaments, intercellular lipids, and cellular protein junctions, or corneodesmosomes, and provides mechanical protection and a controlled permeable barrier to the external environment while subject to highly variable conditions including changing temperature, humidity, solar radiation, and mechanical and abrasive contact. One of the most common and hazardous environmental conditions the SC encounters on a daily basis is solar radiation. UV is a potent and ubiquitous carcinogen responsible for much of the skin cancer in the hum