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 human population today. In the year 2000, excessive UV exposure caused the loss of approximately 1.5 million disability-adjusted life years (DALYs) (0.1% of the total global burden of disease) and 60,000 premature deaths globally. Climate change is expected to exacerbate this problem due to the effect of ozone-depleting gases, altered cloud distribution, and modified clothing choices and time spent outdoors, all of which may increase UV exposure. The biophysical and biochemical aspects of UV damage on skin has been studied, but little is understood regarding its effects on mechanical properties of skin, particularly the SC. Using a thin-film biomechanics approach, we demonstrate the effect of UV exposure on the cell cohesion and mechanical integrity of SC, as a function of UV wavelength, source (narrowband vs. broadband UVB and UVC), dosage and tissue depth. Thin-film testing methodologies including micro-tension, bulge testing and double cantilever beam testing were used to measure the stiffness, distensibility and intercellular delamination energy of SC following UV exposure. Attenuated total reflectance Fourier transform infrared spectroscopy was used to determine the effects of UV on SC intercellular lipids and keratin. Both SC fracture stress and strain, values chiefly determined by the intercellular lipids, were observed to decrease with UV exposure, while the tissue stiffness, largely dependent on the corneocyte keratin filaments, remained constant. Additionally, the intercellular delamination energy of SC significantly decreased with UV exposure suggesting decreased cohesion of the intercellular lipids with UV. ATR-FTIR on UV treated tissue showed oxidation of the lipids and keratin and confirmed both structural and content changes in the SC intercellular lipid network, which likely results in the decreased cohesion of the intercellular lipids with UV. UV exposure has dramatic effects on SC cell cohesion and mechanical integrity that are related to its effects on SC intercellular lipids and keratin. Clinical implications of this work include prevention and treatment of sunburn and long term skin damage such as skin cancer.
3:45 PM - OO5.5
Tough, Strong Hydrogels with Elastomeric Fiber Reinforcement.
Paul Calvert 1 , Animesh Agrawal 1 , Vijaya Chalivendra 1 , Nima Rahbar 1 , Dapeng Li 1 Show Abstract
1 , University of Massachusetts Dartmouth, N Dartmouth, Massachusetts, United States
Until recently synthetic hydrogels were notably weak when compared to their biological equivalents. As a result they have been mainly used for food and cosmetics with contact lenses as the only real engineering application. Recent advances, especially in double network gels, have shown that synthetic hydrogels can be made with strength and toughness comparable to biological tissues. Since then, several other routes to the preparation of strong gel have been developed. Just as in other types of materials, it appears that many different mechanisms may be used to develop toughness in gels.One obvious route is to reinforce gels with fibers to make a composite just as a polymer matrix can be reinforced with carbon or glass fibers. Since part of the essence of a gel is a large extension to break, the reinforcing fibers should be elastic. The combination of highly elastic fibers with a soft but brittle gel puts us into a materials regime that is essentially unexplored. We have succeeded in making very controlled examples of these composites using freeform fabrication techniques and report here on the properties of the resulting composites, including toughness up to 10 kJ/m2, about 10 times that of cartilage. This paper will discuss the applicability of conventional composite theory to this special case with large extensions, substantial non-linearity and the possibility of poroelastic flows in the matrix.
OO6: Poster Session: Hierarchical Materials
Tia Benson Tolle
Wednesday AM, November 30, 2011
Exhibition Hall C (Hynes)
9:00 PM - OO6.1
Topology and Geometrical Perspectives in Hierarchical Bio-Molecular Systems.
Sanju Gupta 1 , A. Saxena 2 Show Abstract
1 Chemistry & Biophysics, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 2 Theoretical Div and Center for Non-linear Science, LANL, Los Alamos, New Mexico, United States
Biological systems are naturally occurring self-assembled and hierarchical, and involve a multitude of geometrical shapes and topological variations. Unlike their inorganic counterparts, they are elastically soft and highly deformable. Despite extensive structuraland physical property characterization, they have not been viewed aslow-dimensional topologically distinct bio-molecular systems although their geometrical aspects led to a new research paradigm. An additional interest in them stems from the fact that the exotic geometries are accompanied by (local) topological defects. Motivated by our recent work within the framework of "global" topology and geometry for a range of nanocarbons having exotic structural diversity [1, 2], here we attempt to invoke a similar approach to understanding the bio-macromolecular systems. In this contribution bio-molecular systems including bio-membranes (planar or curved), microtubules (cylindrical),vesicles (ring-shaped or toroid), rods and cone (conical) cells, amphipathic (helical screw), globular proteins and DNA (double-helix and complex) systems are analyzed within the framework of differential geometry in order to obtain important information such as Gaussian (K) and mean (H) curvatures, bending elasticity (Canham-Helfrich) Hamiltonian and especially their topological features. Through this analysis we hope to derive an overarching bio-molecular shape/topology --> functionality paradigm. Specifically, we (i) examine the topology (e.g. genus) and statistical properties of lipid bilayer membranes and vesicles and (ii) use differential geometric methods to analyze the surface structure of proteins. We also study the transition states from one geometric (and/or topological) form to another and emphasize that curvature leads to nonlinearity that manifests itself in some form of symmetry breaking or statistical property variation with direct implications for synthetic biofunctionality. S. Gupta and A. Saxena JAP (2011).Selected for Virtual Journal of Nanoscale Science & Technology April Issue (2011)];  ibid. JRS (2009).
9:00 PM - OO6.10
Bioinspired Colorimetric Sensors for Explosive TNT Detection Using Hierarchical Self-Assembled Structures.
Jin-Woo Oh 1 2 , Woo-Jae Chung 1 2 , Byung Yang Lee 1 2 , Joel Meyer 1 2 , Seung-Wuk Lee 1 2 Show Abstract
1 Bioengineering, UC Berkeley, Berkekey, California, United States, 2 Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Skin color changes are widely adapted strategies utilized in animal kingdom for expression of a mood of its temper, communication between species, and camouflage from the predators. Although emulation of biological colorization strategies and functions is very useful, however biomimetic approaches to emulating natural structures is still limited and tailoring its functions are challenging. Here, we report novel bioinspired colorimetric sensors which can be tailored with their structures and functions through directed evolution and self-assembly process of M13 phage. We first fabricated multicolor hierarchical phage films (phage-litmus) through meniscus-controlled self-assembly process. Upon exposure to various organic solvents and water, the phage-litmus showed different color change responses due to structural changes depending on the polarity of solvents. Through directed evolutionary screening, we imparted trinitrotoluene (TNT) recognition motifs on the phage and incorporated the specific function of detecting explosive molecules to the phage-litmus. Utilizing commonly used handheld device (iPhone), we could distinguish target molecule over similar chemical structures at very low concentration in a selective manner. Our sensitive and selective bioinspired colorimetric phage-litmus sensors promise to be useful in detecting various target chemicals, preventing their harms to human health and the environment in the future.
9:00 PM - OO6.11
Characterization of Incipient Carious Lesions in Human Dental Enamel Using Raman Spectroscopy and Nanoindentation.
Bedabibhas Mohanty 1 2 , Elayne Schneebacher 3 , Adrian Mann 1 2 3 Show Abstract
1 Department of Materials Science & Engineering, Rutgers The State University of New Jersey, Piscataway, New Jersey, United States, 2 Institute for Advanced Materials, Devices and Nanotechnology, Rutgers The State University of New Jersey, Piscataway, New Jersey, United States, 3 Department of Biomedical Engineering, Rutgers The State University of New Jersey, Piscataway, New Jersey, United States
Dental caries is known to be the most common oral disease among various population groups across the globe. Caries lesions are the precursor to dental cavities and form due to demineralization of the enamel by a complex process of chemical reactions involving the ever changing environment of the oral cavity. The process is one of slow demineralization over several months, even years. Demineralization is to a degree reversible with ions (i.e. Ca2+, F-) able to diffuse in to help preserve the tooth’s structural integrity. The lesion can also become “arrested” and unable to develop further. Early detection of caries lesions can significantly improve patient care as the formation process lesions can be reversed to avoid cavity formation. In the present work, we have investigated the growth of incipient carious lesions in vitro using Raman spectroscopy and nanoindentation. Incipient carious lesions were grown on the buccal faces of human premolars by exposure to lactic acid solution for 14 days. Lesions were cross-sectioned and examined using Raman spectroscopy and nanoindentation. Our results from Raman spectroscopy showed that the calcium and phosphate content were significantly lower in the lesions when compared to the sound enamel. The intensity and area of the phosphate bands were observed to be lower in the region where lesions have formed compared to the higher values seen in healthy enamel closer to the dentin-enamel junction. The total depth of demineralization was observed to be 100 mm below the enamel surface towards the interior. Nanoindentation tests were performed across the lesions from the outer enamel surface towards the dentin-enamel junction. Nanomechanical results showed the lesion had significantly lower elastic modulus and hardness when compared to sound enamel. Although it remains difficult to detect the developmental stages of carious lesions and to identify the border between sound enamel and enamel with lesions, our results do suggest that Raman spectroscopy has the potential to be of utility in detecting early dental caries.
9:00 PM - OO6.12
Young's Modulus of Individual Osteon Lamellae by Nanoindentation of Orthogonal Facets in a Cubic Bone Specimen.
Anna Faingold 1 , Sidney Cohen 2 , H. Daniel Wagner 1 Show Abstract
1 Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot Israel, 2 Chemical Research Support, Weizmann Institute of Science, Rehovot Israel
A correlation between the mechanical properties and microstructure of the fundamental building blocks of bone, such as the osteonal lamellae, would shed light on how it fulfills a variety of mechanical functions through its complex hierarchically organized structure. In an attempt to move closer to the determination of such correlation, we systematically measured the Young’s modulus of individual lamellae within three perpendicular planes of a single osteon by nanoindentation. We find that in the plane perpendicular to the osteon axis (OA) the lamella closest to the canal exhibits 30 percent higher Young's modulus compared to other lamellae, whereas at the plane parallel the OA this difference is statistically insignificant. Moreover, in planes parallel to the OA an unexpected asymmetry of the lamellae shows up on opposing sides of the canal, potentially supporting the validity of the rotated plywood structure model of bone lamellae.
9:00 PM - OO6.13
Self-Assembly of Cellulose Nano-Fibrils from Bacterial and Animal Sources.
Mudrika Khandelwal 1 , Alan Windle 1 Show Abstract
1 Dept. of Metallurgy and Materials Science, University of Cambridge, Cambridge United Kingdom
Cellulose is the most common naturally occurring structural bio-polymer. Its sources vary from higher living organisms like plants and trees, to primitive organisms like bacteria. Cellulose occurs as a unique supra-molecular organisation of the polymer chains into microfibrils. The crystallinity, degree of polymerization, dimensions of the microfibrils varies with the cellulose source. In this work, cellulose from bacterial and tunicate (sea-squirt) origin has been treated with sulphuric acid to obtain rigid rod-like nano-whiskers. The various aspects of the whiskers,such as crystallite size, crystallinity, width, thickness and aspect ratio, from the two different origin have been analysed by XRD, SAXS, AFM and SEM. Owing to the rigid rod-like morphology, these nano-whiskers self assemble to form microscopically oriented domain and thus a liquid crystalline phase. Visually, the phase separation, with the onset of formation of an anisotropic phase, is observed at a very low concentration (below 1 wt%) for both the materials. The complete transition to liquid crystalline region occurs via a broad bi-phasic region. This phase transition behaviour has been compared for the two sources. An attempt has been made to relate the phase transition behaviour to various attributes (aspect ratio, poly-dispersity) of the cellulose nano-whiskers from the two sources. A clear advantage in terms of lower concentration for the onset of liquid crystalline phase formation can be seen for the sources of cellulose chosen in this work, due to large aspect ratios of these nano-whiskers as compared to conventional plant cellulose. This work is a step forward to transfer of mechanical properties of individual microfibrils (such as Young's modulus of the order of 170 GPa ) at nano-scale to macroscopic products like fibres. K. Tashiro and M. Kobayashi, Calculation of crystallite modulus of native cellulose. Polymer Bulletin, 1985. 14(3): p. 213-218
9:00 PM - OO6.14
Contrast-Transfer Efficiency Study of Harmonic Motion Imaging for Viscoelastic Soft Tissues.
Assimina Pelegri 1 , Xiaodong Zhao 1 Show Abstract
1 Mechanical and Aerospace Enigneering, Rutgers University, Piscataway, New Jersey, United States
Numerous biomechanical imaging techniques have been proposed for tissue discrimination and detection of breast tumors. In general, they are developed based on differences in responses of biological tissues with distinct mechanical properties under external loading or stimulus. Harmonic motion imaging (HMI) is one of these techniques, which uses focused ultrasound (FUS) force to generate sinusoidal excitation in a localized region of soft tissues. The corresponding induced displacement amplitude is used to estimate the soft tissue stiffness, or stiffness contrast of the inclusion to the background. In this study, we investigate the contrast-transfer efficiency (CTE) of harmonic motion imaging, which is defined as the ratio of the measured stiffness contrast to the real stiffness contrast. A finite element model (FEM) is built to model the soft tissue consisting of an inclusion and the surrounding normal tissue, both of which are assumed to be isotropic, homogenous, linear viscoelastic, and near-incompressible media. The steady-state response of the soft tissues is computed via finite element simulation. The finite element model and steady-state analysis procedure are validated via experimental data. Then, the CTE of harmonic motion imaging is investigated for different inclusion to background contrast ratios, from soft inclusion/hard background to hard inclusion/soft background. The effects of viscous coefficient and inclusion size on CTE are also studied. The results illustrate that a better CTE of harmonic motion imaging is obtained for hard inclusion in a soft background than soft inclusion in a hard background, which is also observed in static elastography and constitutes the fundamental limitation of elastography. The CTE of harmonic motion imaging reduced as the inclusion size decreased. It was also observed that a large viscous coefficient would highly decrease the CTE for hard inclusion with high stiffness contrast.
9:00 PM - OO6.15
Hierarchical Microstructure and Elastic Properties of the Leaf Petiole in the Philodendron Melinonii Species.
Md. Tanvir Faisal 1 , Alejandro Rey 2 , Damiano Pasini 1 Show Abstract
1 Mechanical Engineering, McGill University, Montreal, Quebec, Canada, 2 Chemical Engineering, McGill University, Montreal, Quebec, Canada
The leaf petiole is a plant organ that connects the leaf blade to the stem of the plant. From a structural viewpoint, the petiole resembles a cantilever beam that withstands torsion loadings due to the wind action and bending deformation due to gravity forces acting on the leaf blade. During growth, the petiole develops defined structural features at multiple length scales which synergistically determine its mechanical response to external stimuli. In this work, we focus on five levels of the petiole structural hierarchy, from the constituent elements of the primary cell wall to the cross-sectional shape of the petiole. At the first level, the cell wall is generally comprised of cellulose microfibrils cross-linked with hemicellulose and embedded in pectin matrix. From micro to mm, the walls of the cellular tissue are made of parenchymatous pith with large pore-like aerenchyma. At the last level of the hierarchy, the petiole can be considered as the integration of three tissues, namely epidermis, collenchyma and parenchyma, with gradients of relative density and spatial distribution that define the organ morphology.The focus of this paper is on the multiscale analysis of the flexural stiffness of the Philodendron Melinonii petiole. The goal is to capture the impact that the structural features defined at each level of the hierarchy has on the flexural response of the petiole. Since the microstructure of the plant tissues resembles a non-periodic cellular pattern, we resort to a 2-D Centroidal Voronoi tessellation (CVT) to generate a realistic network of polygonal cells. The Voronoi network is coded in MATLAB (R2008b, The MathWorks) and the stress-strain regime of the resulting geometric model is analyzed through a finite element (FE) model. The boundary of each cell of the CVT represents the cell wall of the cellular tissue; each cell wall is thus modeled with a beam element. The FE analysis of the Voronoi model explores the relation between the microstructure and the effective properties of the cellular tissue. As a result, the effective homogenized elastic property of the cellular tissue is obtained and used to calculate the overall flexural stiffness of the P. Melinonii petiole. The results are used to plot domains of flexural stiffness as a function of the volume fraction of the cell wall. The impact of cross-sectional geometry, layer-architecture and the material properties of the cell wall components and tissues are visualized into efficiency maps, displaying property boundary curves at each level of the structural hierarchy. Within each domain fall all the possible combinations of materials that are shaped into a certain geometry during the petiole growth, offering a comparative overview of the hierarchical development of the mechanical properties of the petiole. The results help to gain insight into the contribution that a scale dependent structural feature brings to the overall mechanical efficiency of the leaf petiole.
9:00 PM - OO6.18
Parametric Study of Nanofiber Formation by Rotary Jet-Spinning.
Holly McIlwee 1 , Paula Mellado 1 , Mohammad Badrossamay 1 , Josue Goss 1 , L. Mahadevan 1 , Kevin Kit Parker 1 Show Abstract
1 Wyss Institute for Biologically Inspired Engineering, School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States
Nanofibers are useful materials for a wide range of applications. Despite the effort devoted to understanding the parameters affecting fiber quality in common techniques such as electrospinning, a simple method to control fiber diameter and morphological characteristics remains elusive [1, 2]. The centripetal force driven assembly of nanofibers, termed, Rotary Jet-Spinning (RJS), has been demonstrated as an efficient, reproducible technique for nanofiber production . We hypothesized that by identifying parameters that allow for control of fiber diameter and morphology, a straightforward method to design and manufacture nanofibers could be realized. During RJS, polymer solutions are fed into a perforated reservoir rotating at high speeds. As the polymer is ejected from the reservoir orifice, the balance of viscous and centripetal forces stretch the jet to form nanofiber structures. Solvent evaporation produces solid fibers at the collector, while it was observed that complete solvent evaporation occurs over longer timescales with solvent continuing to evaporate hours after spinning. When parameters such as solution viscosity, angular speed, distance to the collector, and orifice diameter were varied, fiber diameter could be tuned in a controlled way. A phase diagram was constructed experimentally, describing the parameter space in which continuous fibers are formed as a function of angular speed and solution viscosity. Based on the results of this parametric study, we were able to design experiments to achieve fibers with diameter as small as 20 nm from low viscosity solutions composed of poly(lactic acid) dissolved in chloroform with tunable bead content. References1. Fridrikh, S.V., et al., Controlling the fiber diameter during during electrospinning. Physical Review Letters, 2003. 90(14).2. Theron, S.A., E. Zussman, and A.L. Yarin, Experimental investigation of the governing parameters in the electrospinning of polymer solutions. Polymer, 2004. 45(6): p. 2017-2030.3. Badrossamay, M.R., et al., Nanofiber Assembly by Rotary Jet-Spinning. Nano Letters, 2010. 10(6): p. 2257-2261.4. Senthilram, T., et al., Self Crimped and Aligned Fibers. MaterialsToday, 2011. 14(5):p. 226-229.5. Huttunen, M., et al., A simple and high production rate manufacturing method for submicron polymer fibres. Journal of Tissue Engineering and Regenerative Medicine, 2011, 5: n/a. doi: 10.1002/term.421
9:00 PM - OO6.3
Structural and Mechanical Properties of Human Intermediate Filament Keratin Proteins.
ChiaChing Chou 1 , Markus Buehler 1 Show Abstract
1 Laboratory for Atomistic and Molecular Mechanics, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Keratin, a type of intermediate filament protein, is the key component of hair, nail and skin in vertebrates, including mammals. Inside the cell, keratin plays the role which maintains the stability of the cell and contributes to the stability of adjacent cells. The hard alpha-keratin intermediate filaments which are rich in disulfide bonds found in materials such as hair and nail features a hierarchical structure, ranging from alpha-helical protein, a dimer composed of alpha-helical coiled coils and two globular C- and N-terminal domains, to full-length intermediate filaments embedded in a sulfur-rich protein matrix. At the molecular level, the alpha-helical coiled coils are stabilized by clusters of hydrogen bonds. At the molecular level, bundles of the keratin filaments in the sulfur-rich protein matrix are stabilized by disulfide bonds that play a defining role in their mechanical and physical stability. Here we report a study on building a bottom-up molecular based model of keratin, starting at the atomistic scale and by using the full human keratin type K35 and K85 amino acid sequence. A detailed analysis of geometric and mechanical properties of the keratin dimer and tetramer by fully atomistic simulation using reactive (ReaxFF) and nonreactive (CHARMM) forcefield is presented. In order to understand its outstanding mechanical properties at larger scales of filaments and filament networks, we upscale our simulation with coarse-grained multi-scale model. We analyze the deformation behavior of the keratin filaments while applying force. Our results suggest that keratin filaments contribute to the cell stiffness and disulfide bonds play a significant role in achieving the characteristic mechanical properties of this protein material. We discuss the opportunity of studying disease states associated with genetic mutations and other structural defects in keratin.
9:00 PM - OO6.5
Probing of Single Cells Using Carbon Nanotube Pipettes in near Field Scanning Optical Microscope.
Yang Gao 1 , Riju Singhal 2 , Zulfiya Orynbayeva 2 , Michael Schrlau 2 , Gary Friedman 1 , Yury Gogotsi 2 , Adam Fontecchio 1 Show Abstract
1 Electrical and Computer Engineering, Drexel University, Philadelphia, Pennsylvania, United States, 2 Material Science and Engineering, Drexel University, Philadelphia, Pennsylvania, United States
Measuring time or population average cellular responses often provided insufficient information about cell function. Single cell analysis is a rapidly developing area aimed at characterizing cell-to-cell variations in a population as well as variation of individual cell behavior over longer time periods following multiple stimuli. Only few reports exist on prolonged tracking of a cell’s long term reaction to stimuli because most conventional single cell probes are intrusive and often kill the cell during probing or injection. In earlier work [refs], we have proposed single cell multi-functional probes which caused minimum perturbation of the cell behavior. Two different types of probes were fabricated: carbon nanopipettes prepared by chemical vapor deposition (CVD), and assembled carbon nanotube cellular endoscopes [refs]. Each type of probe has its own merits. The CVD probes are electrically conducting along their entire lengths and can be easier to fabricate, while cylindrical carbon nanotube endoscopes were demonstrated to cause less cell damage in piercing the cell membrane. In both cases, the hollow structure of the probes allows transport of material.In this work we demonstrate simultaneous acquisition of force-displacement data obtained by probing MDCK IIG cells with the aforementioned probes, while optically recording the probe positioning and cell behavior. The probe was mounted on a Near-Field Scanning Optical Microscope (NSOM) system, by which its position can be precisely controlled and manipulated, while the NSOM system also offers an uninterrupted optical path to combine an inverted microscope with the system. Therefore, this approach allows performing multiple tasks (due to multifunctionality of the probe) during a single operation which could record the reaction of a single cell to various stimulations. In future, the influence of varying the parameters of the probe (such as the diameter, and the direction of approach, etc) would be studied. Additionally, more cell cultures would be used for experiments on this system to determine the response of the system for different cell types.
9:00 PM - OO6.6
Ultrastructural Origins of Optical Properties in the Exoskeletons of Beetles.
Xia Wu 1 , Andreas Erbe 2 , Helge Fabritius 1 , Dierk Raabe 1 Show Abstract
1 Microstructure Physics and Metal Forming, Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf Germany, 2 Interface Chemistry and Surface Engineering , Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf Germany
The cuticle of Arthropoda does not only have structural functions like serving as an exoskeleton that forms and supports the body, but also plays an important role for the ecophysiology of the organisms by generating colors used for signaling, camouflage or warning. Many insects, which represent the largest group inside the Arthropoda, display a vivid iridescent coloration. In beetles, this coloration is generated by modifications in structure and chemical composition of the outermost layers of the cuticle, mostly the epi- and exocuticle. These modifications can be found either in the load-bearing parts themselves or in specialized structural elements like scales formed on their surface. The resulting microstructures can act as organic photonic crystals. Understanding the building principles of these structures and their physical interaction with light can help to improve and develop new, optically active materials. We investigated and compared the ultrastructure, chemical composition and optical properties of colored cuticle parts from different beetle species. In beetles from the families Carabidae and Scarabeidae, the metallic green color is generated by constructive interference of light reflected by different ultrastructures in different regions of the elytral cuticle. In Carabidae, the color originates from a multilayer structure located in the epicuticle, in Scarabaeidae by the helicoidal structure of the exocuticle. Scales of beetles belonging to the family Curculionidae contain three-dimensional photonic crystals with a diamond-based lattice structure that can generate nearly all colors from blue to red by local changes in orientation of the photonic crystal within different domains and even become transparent by substitution of the air phase with inorganic material.
9:00 PM - OO6.7
Mechanical Switching at the Molecular Scale: The Actin-α-Actinin Interaction.
Guillaume Copie 1 , Fabrizio Cleri 1 Show Abstract
1 IEMN, University of Lille I, Lille France
Actinin is a microfilament protein, and α-actinin is a cross-linking protein necessary for the attachment of actin filaments to the Z-lines in skeletal muscle cells, and to the dense bodies in smooth muscle cells. The functional protein is an anti-parallel dimer, which cross-links the thin filaments in adjacent sarcomeres, and therefore coordinated contractions between sarcomeres in the horizontal axis. Both ends of the α-Actinin terminals are composed of two calmodulin (CH1 CH2) attached to one monomer, and an EF-domain attached to the antiparallel monomer.We investigated, by means of combined rigid-docking and molecular mechanics, the complex structure of α-actinin attached to actin filaments. For the first model we obtained rigidly docked structures of the closed CH1-CH2 conformation to an actin monomer, which was then relaxed in physiologic water. The second and third models consists of an actin trimer or pentamer, representing a fragment of an actin filament in the Holmes configuration. We performed rigid docking of the α-actinin CH1-CH2 terminal on the actin fragments, and studied the differential adhesion free energy.Subsequently, we selected the best candidate configurations from rigid docking, to be equilibrated in physiological water by very-large-scale MD simulations (comprising as many as 50,000 atoms and about 100,000 water molecules). We obtained interacting complete structures of the actin-α-actinin complex. Such structures were then subject to mechanical stresses (both traction and shear), to study of conformational changes in the actin-α-actinin binding upon mechanical deformation. This is supposed to be the initial step in the signalization process occurring at the contact region, when the actin fibers are strained and sheared during muscle action.
9:00 PM - OO6.8
Lysozyme Pattern Formation in Evaporating Drops.
Heather Meloy Gorr 1 , John Barnard 1 Show Abstract
1 Materials Science and Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
A water droplet containing suspended particles deposited on a solid surface will form a ring-like structure due to the redistribution of solute during evaporation, a phenomenon sometimes referred to as the "coffee ring effect." The complex patterns left on the substrate after evaporation are characteristic of the nature of the solute and its interaction with the substrate. This has attracted recent interest in the biological sciences due to potential applications in medical screening and diagnosis. However, due to the complexity of ‘real’ biological fluids, a comprehensive understanding of the solute dynamics during droplet evaporation of these fluids is lacking. In this study, we examine by scanning probe and optical techniques, the morphological evolution and conditions for coffee ring formation for simplified model biological solutions of DI water and lysozyme, a globular protein found in high concentration, for example, in human tears and saliva. Unlike a number of recent studies which model evaporation dynamics using latex particles, the lysozyme molecules are much smaller with dimensions 3 nm × 3 nm × 4.5 nm and are expected to carry a net positive charge at physiological pH. In addition, the drop diameters in this study are very small, ranging from 1 µm – 50 µm, whereas many experimental studies of these systems are at the millimeter scale. For the size range of the drops in this study, the protein movement and the resulting dried residue morphology are highly influenced by the decreased evaporation time of the drop. Here, we consider the effect of concentration, liquid evaporation rate, and particle diffusive motion on the morphology of the deposited drop as well as the minimal conditions for coffee ring formation.
9:00 PM - OO6.9
Multi-Scale Analysis and Modeling of Robocasted Scaffolds Mechanical Properties.
Martin Genet 1 , Manuel Houmard 1 , Salvador Eslava 2 , Eduardo Saiz 2 , Antoni Tomsia 1 Show Abstract
1 Materials Science Department, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 Materials Department, Imperial Colledge London, London United Kingdom
Robocasting is considered one of the top candidates to fabricate highly structured porous ceramics for numerous applications including bone regeneration implants.While the process-microstructure link is completely straightforward, precise relations between microstructure and mechanical properties, which are critical for most applications, remains to be elucidated.Indeed, qualitative understanding of the physics underlying the response of robocasted scaffolds under mechanical loading has been established, but robust quantitative tools are still missing.Here, we present a multi-scale theoretical and experimental analysis of the scaffolds mechanics, as well as associated numerical tools.Even though we focus on hydroxyapatite inks, most of the study could be transposable to other inks.The final aim is to quantitatively optimize their morphology with regards to fundamental properties such as stiffness or strength.On the rod's scale (200-800 μm), three point bending tests were performed on single rods with different diameters.On the scaffold's scale (5-20 mm), compression tests were performed on several samples.Sanchez-Palencia homogenization and Weibull analysis of periodic representative volume elements were used to bridge those two scales.In terms of stiffness, it is established that the Young modulus is an intrinsic property of the rods.The measured modulus is quite scattered, from 2 GPa to 10 GPa, with a mean value of 5.5 GPa.This rather classical observation validates the delicate experimental analysis on single rods.It is also established that the link between rods and scaffolds stiffness can be achieved through numerical homogenization.It is important to point out that once this bridge is validated, it can also be used in an inverse manner to derive rods stiffness from scaffolds data.In terms of strength, we have observed a surprising yet highly interesting fact: Weibull moduli are not intrinsic, and depend on rods diameter.Increasing the diameter, while changing the length to keep the same volume, increases the mean strength and decreases the dispersion.It is most probably due to a process-induced change in defects distribution, and ongoing morphological analysis of rods sections will help us conclude on that.Using these data to compute scaffold compressive strength distributions thanks to Weibull theory leads to encouraging yet not satisfactory enough predictions when compared to experimental data.This is certainly due to the fact that the scaffolds do not actually break following the weakest link rule, and possibilities to extend our model will be discussed.In conclusion, we present a multi-scale analysis of the mechanical properties of robocasted scaffolds, with powerful numerical tools for the quantitative optimization of the scaffolds microstructure, which will allow us to build tougher and stronger materials, e.g., for the repair of load-bearing bone defects.
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
OO7: Carbon I
Wednesday AM, November 30, 2011
Room 208 (Hynes)
9:30 AM - **OO7.1
Biscrolling Nanofiber Sheets and Functional Guests into Multifunctional Yarns for Energy Applications.
M. Lima 1 , S. Fang 1 , X. Lepro 1 , C. Lewis 1 , R. Ovalle-Robles 1 , J. Carretero-Gonzalez 1 , E. Castillo-Martinez 1 , M. Kozlov 1 , J. Oh 1 , N. Rawat 1 , C. Haines 1 , M. Haque 1 , V. Aare 1 , S. Stoughton 1 , A. Zakhidov 1 , S. Kim 2 , Ray Baughman 1 Show Abstract
1 Alan G. MacDiarmid NanoTech Department, University of Texas at Dallas, Richardson , Texas, United States, 2 National Creative Research Initiative Center for Bio-Artificial Muscle, Hanyang University, Seongdong-gu, Seoul, Korea (the Republic of)
Though yarn spinning has prehistoric origins and remains vital today, a host of important materials cannot be made into yarns by previously known methods. Generically applicable methods are demonstrated for producing continuous yarns comprising up to 99 wt % of otherwise unspinnable nanopowders or nanofibers that remain highly functional. These methods utilize the strength and electronic connectivity of sometimes minute amounts of carbon nanotube sheets that are helically scrolled in the yarns. Scrolled 50 nm thick nanotube sheet or sheet stacks can confine nanopowders, micropowders, or nanofibers in the corridors of often irregular scroll sacks, whose observed complex structures are related to twist-dependent extension of Archimedean or Fermat spirals or spiral pairs into scrolls. This new technology is used to make yarns of graphene ribbons, superconductors, high performance battery materials, catalytic oxygen electrodes for fuel cells, TiO2 for release of active oxygen, and strong sutures containing biomedical agents. The observed mechanical properties enable yarn knotting and the weaving and sewing of biscrolled multifunctional yarns into textiles.
10:00 AM - **OO7.2
Structure and Properties of Carbon Nanotube Fibre: A Hierarchical Model.
Alan Windle 1 , James Elliott 1 , Nagore Ibarra-Gonzalez 1 , Matthew James 1 , Krzysztof Koziol 1 , Agnieska Lekawa-Raus 1 , Jing Qui 1 , Jermonimo Terrones-Portas 1 , Juan Vilatela 1 Show Abstract
1 , Cambridge University, Cambridge United Kingdom
Carbon nanotube fibres are typically ~10 microns in diameter. They are however yarn-like, in that the basic fibrous elements are the nanotubes, or more probably, bundles of mutually aligned nanotubes. Structural observations, using a combination of scattering methods, spectroscopy and electron microscopny, demonstrate that although the vast majority of the nanotubes are very well aligned indeed with the fibre axis, the fibres contain a variety of defects. These can be catagorised as included particles - which are frequently associated with catalyst residues, voids, low molecular weight carbonaceous deposit and topological defects arising from the entanglements present in the original 'elastic smoke' produced in the synthesis zone. A hierarchical model is proposed with is consistent with the structural observations, but at the same times provides a rational basis for understanding a considerable range of mechanical and electrical properties, and some combinations of these properties. The model is then tested against observations of the consequences of post spinning treatments, such a thermal annealing and other physical and chemical approaches to fibre modification. The results are discussed in the context of strategies to optimise the fibres for different applications.
10:30 AM - OO7.3
Crosslinking Enhancements in Hierarchical Carbon Nanotube Bundles.
Tobin Filleter 1 , Rodrigo Bernal 1 , Shuyou Li 2 , Horacio Espinosa 1 Show Abstract
1 Mechanical Engineering, Northwestern University, Evanston, Illinois, United States, 2 NUANCE Center, Northwestern University, Evanston, Illinois, United States
Carbon nanotubes (CNTs) are envisioned to be ideal building blocks in hierarchical macroscopic composite fibers due to their extraordinary strength and stiffness. The study of the mechanics of natural materials, such as tendon and spider silk, has provided great insight into the design of structures that can simultaneously achieve a remarkable marriage of strength and toughness. Macroscopic materials based on CNTs, however, have been limited by weak shear interfaces between adjacent CNT shells and composite matrix elements. In previous multiscale experimental studies of hierarchical DWNT yarns, we identified the need to introduce crosslinks at the individual DWNT bundle level to enhance the mechanical performance . In addition, a better fundamental understanding of tailoring load transfer at nanometer length-scales through cross-linking is needed to identify how the exceptional mechanical properties of individual CNTs can be effectively utilized in bioinspired composite fibers. To address this issue we have applied a MEMS based in-situ transmission electron microscopy (TEM) tensile testing method to elucidate load transfer mechanisms within double-walled nanotube (DWNT) bundles, crosslinked via high energy electron irradiation. We have demonstrated that load can be effectively transferred to inner DWNTs and shells within the bundles by applying a controlled dose (9-11 x 1020e/cm2) of (200 keV) electron irradiation to induce covalent crosslinks at the interfaces of adjacent DWNTs and shells . In particular, crosslinking is induced on two lengths scales within the hierarchical bundle structures, both between the outer shells of adjacent DWNTs as well as between outer and inner shells within each DWNT. By tailoring this irradiation induced cross-linking we have observed order of magnitude increases in both the effective strength and modulus of the individual DWNT bundles to ~17 GPa and ~700 GPa respectively. In-situ TEM imaging during tensile testing of crosslinked DWNT bundles reveled distinct failure mechanisms for low and highly crosslinked bundles, confirming the beneficial effects of bridging adjacent shells and tubes. In future the dramatic enhancements in mechanical properties as a result of irradiation induced covalent cross-linking has the potential to lead to high-performance bioinspired hierarchical fibers and composites with tunable mechanical properties made of DWNT precursors. M. Naraghi, T. Filleter, A. Moravsky, M. Locasio, R. O. Loufty, and H. D. Espinosa, "A multi-scale study of high performance DWNT-polymer fibers," ACS Nano, vol. 4, pp. 6463-6476, 2010.T. Filleter, R. A. Berrnal, S. Li, and H. D. Espinosa, "Ultrahigh Strength and Stiffness in Cross-Linked Hierarchical Carbon Nanotube Bundles," Adv. Mater., (in press), 2011.
10:45 AM - OO7.4
Effect of Aligned CNT Length on the Fracture Toughness of Hierarchical Composite Materials.
Soraya Kalamoun 2 , Sunny Wicks 1 , Brian Wardle 1 Show Abstract
2 , ENSICAEN, Caen France, 1 Department of Aeronautics and Astronautics, MIT, Cambridge, Massachusetts, United States
Aerospace composite materials are being increasingly employed due to their high specific strength and toughness, as well as tailorability of directional properties that can be controlled by designing the direction and proportion of advanced fibers. One disadvantage of composites are the interlaminar properties, between the layers of fibers, that can fail and lead to delamination. Numerous approaches to reinforcing the through-thickness direction of the fibers have been developed like z-pinning, stitching, and the incorporation of nano-additives and particles such as carbon nanotubes (CNTs). All of these approaches come with accompanied drawbacks like fiber damage and resin pockets in z-pinning and stitching, as well as difficulty in homogenous dispersion and increase resin viscosity for the addition of nanoparticles. In this paper, a nanoengineered hierarchical architecture is employed, in which aligned CNTs are grown in situ on advanced fibers, providing mechanical reinforcement between layers of the laminate. This system is called a fuzzy fiber reinforced plastic (FFRP), and overcomes typical hurdles including control over alignment and dispersion. Prior work on mechanical characterization of the FFRP made by hand lay-up with a viscous boat-building epoxy yielded an increase of 76% over baseline of the steady state fracture toughness. Vacuum assisted resin infusion (VARI) is a common manufacturing method for the production of large and complex fiber reinforced composite materials, that depends on the use of a low-viscosity polymer that can flow into a laminate. In this abstract, we are focused on laminate infusion of FFRP laminates by an aerospace grade epoxy, and the interlaminar mechanical properties of the laminates that result. We studythe effect of CNT length on the Mode I fracture toughness, and varied the length by controlling the length of the CVD process to yield laminates with 6 and 19 microns in length. The Mode I fracture toughness was studied by Double Cantilever Beam, or DCB (ASTM D5528), and notched translaminar fracture (ASTM E1922). In Mode I fracture toughness testing by DCB to isolate GIc, or the critical strain energy release rate, testing of 20 laminates revealed interesting trends based on the length of CNT. Baseline laminates firstly exhibited no toughening during crack propagation, as is evidenced by comparing initiation and steady state toughnesses. Because of this, our focus was placed only on the steady state toughnesses of the laminates. 19 micron long CNTs lead to a 23% increase of the GIc_steadystate, while 6 micron long CNTs lead to a 50% decrease for the same property. The fracture surface will be imaged and characterized to understand the source of this opposite behavior. Ongoing work includes manufacturing and testing of notched translaminar fracture laminates to further study the fracture toughness of FFRP laminates made by infusion.
11:30 AM - **OO7.5
Mechanical Behavior of Nanodiamond-Containing Polymers.
Yury Gogotsi 1 , Ioannis Neitzel 1 , Vadym Mochalin 1 Show Abstract
1 , Drexel University, Philadlephia, Pennsylvania, United States
Due to their favorable strength to weight ratio, polymer-matrix composite materials find numerous applications in construction, transportation, biomedical, sports industry and other areas. A nanocomposite is a composite material in which at least one of the components has at least one spatial dimension smaller than 100 nm. At these small length scales, the specific surface area becomes large and polymer-filler interactions become increasingly important, since polymer properties change in the vicinity of the surface. The resulting third phase, called interphase, plays an important role in the overall properties of the nanocomposite since its volume fraction becomes significant at the nanoscale. Current nanocomposite research largely focuses on fillers such as CNTs, graphene, clays, silica and titania. However, all nanofillers have certain limitations. CNTs suffer from poor dispersibility, while most oxides have primary particles that are larger than 20 nm. Thus, new nanofillers that can improve mechanical properties and add other useful functionalities to polymers are being researched.Nanodiamond (ND), which was discovered decades ago in the former USSR, is a unique nanomaterial with excellent mechanical, thermal, and optical properties that is manufactured in ton quantities and is very attractive as a filler. The small 5-nm diameter of detonation ND particles in combination with their rich surface chemistry  makes ND an optimal candidate for polymer reinforcement. Furthermore, ND can be used to engineer multifunctional composites and build hierarchical structures, e.g. porous and fibrous materials with up to 60 wt.% diamond. In the case of an epoxy matrix , ultimate mechanical reinforcement has been achieved by using high loadings of ND powder. Also, the effect of functionalized (aminated) ND on the epoxy stoichiometry and the resulting mechanical properties has been investigated and resulted in new insights in engineering nanocomposites. Several complementary mechanical characterization techniques have been used to understand the reinforcing mechanisms of ND and deformation mechanisms of the nanocomposites. Results of compression and fracture toughness measurements have been compared with depth-sensing indentation data. This research gives new insights in the reinforcing mechanisms of ND in polymer matrices.S. Osswald, G. Yushin, V. Mochalin, S. Kucheyev, Y. Gogotsi, JACS, 128, 11635 (2006). I. Neitzel, V. Mochalin, I. Knoke, G. R. Palmese, Y. Gogotsi, Composites Science and Technology, 71 (5) 710–716 (2011).
12:00 PM - **OO7.6
The Assembly of Functionalized Graphene and Carbon Nanotubes into Macroscopic Materials: Tuning Mechanical Properties through Surface Chemistry.
Zhi An 1 , Sourangsu Sarkar 1 , Owen Compton 1 , SonBinh Nguyen 1 Show Abstract
1 Chemistry, Northwestern University, Evanston, Illinois, United States
As allotropes of carbon, graphene and carbon nanotubes can be easily functionalized using a plethora of chemical strategies. The resulting functionalized materials can be made to compatible with both aqueous and organic media as well as in polymer matrices. This presentation will focus on the syntheses of functionalized graphene and carbon nanotubes and their assembly into macroscopic materials whose properties can be tuned through the modification of surface chemistry.
12:30 PM - OO7.7
Graphene Slows down Creep Deformation of Epoxy.
Ardavan Zandiatashbar 1 , Catalin Picu 1 , Nikhil Koratkar 1 Show Abstract
1 Mechanical, Aerospace & Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
The monotonic, cyclic, and creep mechanical behavior of nanocomposites of epoxy and graphene platelets (GPL) is studied. It is observed that the addition of nanofillers does not improve the monotonic material properties, but significantly slows down creep and to some extent, crack growth rate under cyclic loading. The discrepancy between the creep of unfilled and filled epoxy increases with increasing stress and temperature. Similar conclusions are obtained from macroscopic and nanoscale (nanoindentation) tests. The filler weight fraction leading to optimal creep and fatigue behavior is identified to be 0.1 wt%. Potential mechanisms controlling this behavior will be discussed.
12:45 PM - OO7.8
Hierarchical Graphene Materials: From Monolayers to Papers and Nanocomposites.
Zhiping Xu 1 Show Abstract
1 Engineering Mechanics, Tsinghua University, Beijing, Beijing, China
Graphene, the ultimately monatomic layer material, features outstanding mechanical properties including ultrahigh tensile stiffness, strength and resilience. Recently there are continuous efforts in understanding the underlying structure-properties relationship. However, many of these features point to macroscopic applications, but an efficient way to transfer the unique properties of graphene monolayers to macroscopic materials is lacked, which prohibits large-scale applications.Macroscopic graphene materials such as papers and nanocomposites consist of chemically modified graphene sheets and intersheet crosslinks of various types, which hold great promises in high-performance, multifunctional and light-weighted applications. In this talk, we will present a multiscale approach (density functional theory, molecular dynamics and continuum mechanics) to understand simultaneously atomistic mechanisms, microscale structures and microscopic performance of these hierarchical graphene materials, and provide general principles for materials design that are supported by experimental evidence.Hierarchical structures of graphene-based materials will be also discussed comparably with biological materials that are widely studied recently, such as bones, nacre and collagen fibrils, which possess similar materials hierarchies and inspire many novel concepts in new materials development.References Y. Liu, B. Xie and Z. Xu, J. Mater. Chem. 21, 6707 (2011) Y. Liu, B. Xie, Zhong Zhang, Quanshui Zheng and Z. Xu, http://arxiv.org/abs/1105.0138 (2011) Z. Xu, J. Comp. Theor. Nanosci. 6, 625 (2009)
OO8: Carbon II
Wednesday PM, November 30, 2011
Room 208 (Hynes)
2:30 PM - **OO8.1
Mechanics of Multilayer Carbon Nanotube Structures.
Chiara Daraio 1 Show Abstract
1 Graduate Aerospace Laboratories (GALCIT), California Institute of Technology, Pasadena, California, United States
Carbon nanotube (CNT) foams are characterized by a complex hierarchical microstructure, with naturally graded functional properties and excellent recovery of large compressive deformations. We exploit these and other properties of CNT foams to fabricate lightweight energy absorbing materials with tunable properties. We assemble multilayer structures by alternating CNT foams with polymer or metal layers and characterize their mechanical response under static and dynamic loading. These materials present large energy absorption under quasistatic loading, strain localization, and dynamic “softening” effects under impact loading. We develop a numerical model directly inspired by the micromechanical response for these materials, where CNTs are represented by collections of uniform bi-stable springs.
3:00 PM - OO8.2
In Situ SEM Shear and Peeling Tests - Chemo-Mechanics of Crosslinking Molecules.
Horacio Espinosa 1 , Mohammad Naraghi 1 , Michael Roenbeck 1 , Zhi An 2 , SonBinh Nguyen 2 Show Abstract
1 Mechanical Engineering, Northwestern University, Evanston, Illinois, United States, 2 Chemistry, Northwestern University, Evanston, Illinois, United States
Recently, CNT yarns with high CNT content, has received considerable attention. In these emerging materials, interactions between neighboring CNTs control the composite’s mechanical performance. However, so far, the mechanical properties of CNT composites, such as their strengths, are only a small fraction of the strength of CNTs, primarily due to weak interactions between CNTs. To achieve a fundamental understanding of the load transfer mechanisms between CNTs and elucidate methods to tailor them, we have developed a nanomechanics-based experimental approach to explore the interactions between CNTs in peeling (mode I and mixed mode) and in shearing mode (mode II). These modes describe the dominant interactions between CNTs in CNT yarns. The experimental approaches consisted of mounting a MWNT to the tip of a cantilever (load sensor). To study CNT-substrate interactions, in mode I and mixed mode, the MWNT is brought in contact with a substrate and peeled off at different angles in situ a SEM. Given the sp2 structure of both CNT and substrate material, the experiment is assumed to mimic the interactions between two CNTs pulled apart during the stretching of a yarn. Peeling experiments conducted with this configuration resulted in stable debonding. The cohesive energy between MWNTs was estimated to be ~0.4 nJ/m, comparable to theoretical estimates of the vdW interactions between graphitic surfaces. When the CNT is pulled away normal to the surface, debonding happens in mode I, while at lower contact angles mixed mode fracture is observed. In the limit, when the contact angle reaches zero, fracture occurs in mode II (Shear). In view that small angles (less than ~ 40 degrees) are not practical within the current setup, shear experiments between two MWNTs were carried out. The overlap length between the two MWNTs was varied to investigate its effect on lateral interactions. In general, the load capacity of the junctions increased with overlap length, with no sign of saturation. In nearly all cases, the fracture of the junction between the two tubes was abrupt with no pre-failure sliding. At sufficiently large overlap length, however, instead of junction failure, MWNTs fracture. Tube functionalization with carboxylic groups marginally enhanced the interactions between CNTs, pointing to the effect of hydrogen bonds in improving CNT interactions. The study suggests several approaches to enhance the mechanical behavior of CNT yarns. For instance, alignment of CNTs along the yarn axis results in mode II (shear failure), which maximizes the junction process zone. The method is particularly suited to investigate the role of chemistries in the hierarchical design of novel composites.
3:15 PM - OO8.3
Measuring and Visualizing Stress Transfer in Polymer–Graphene Oxide Nanocomposites at the Single Sheet Level.
Minzhen Cai 1 , Hannes Schniepp 1 Show Abstract
1 Applied Science, The College of William & Mary, Williamsburg, Virginia, United States
Graphene oxide (GO) sheets have outstanding mechanical and processing properties and can be manufactured economically and at a large scale. They are, thus, excellent candidates to be applied in high-performance polymer nanocomposites. However, in order to make full use of the outstanding strength of the GO sheets, it will be necessary to achieve very efficient stress transfer between the polymer and the sheets, ideally up to fracture strain of GO. This is especially challenging, as the GO sheets (E=200 GPa) are about two orders of magnitude stiffer than the surrounding polymer matrix. We developed an experimental technique to quickly assess the relative strength of the GO–polymer interface for a series of different polymers and found that polyvinyl alcohol (PVA) features a particularly strong interface with GO. In order to assess the load transfer across the PVA–GO interface more quantitatively, we monitored the strain in individual GO sheets inside the polymer via force modulation microscopy, as a function of the external strain of the composite. We found that, more than 80% of the matrix strain is directly transferred so some of the GO sheets in this system for strains of up to 8%. This directly shows that, in principle, it is possible to make such nanocomposites work. We complemented this atomic force microscopy-based study by in-situ Raman spectroscopy on relaxed and strained composites. We found that there is a significant variation of the achieved load transfer for different GO sheets. We attribute this to significant property variations within in the ensemble of GO particles. Our findings suggest that techniques as ours — with the capability of revealing the mechanical properties of nanocomposites at the single sheet level — are needed in order to fully understand and optimize the performance of nanocomposites.
3:30 PM - OO8.4
Microstructural Modeling of Electro-Mechanical Sensing and Damage Modes in Carbon Nanotube Polymer Composites.
S. Xu 1 , O. Rezvanian 1 , K. Peters , Mohammed Zikry 1 Show Abstract
1 , North Carolina State University, Raleigh, North Carolina, United States
New specialized large-scale validated microstructurally-based finite-element techniques have been developed to predict how crystalline-amorphous interfaces affect coupled electrical and stress fields in polymer composites with different distributions and orientations of carbon nanotubes. The computational formulation accounts for the interrelated effects of chirality, interfacial mismatches, current densities, volume fractions, and tube aspect ratios at scales ranging from the nano to the micro scales. New methodologies are developed to optimize behavior for desired electro-mechanical sensing, failure prevention, and functional device design.
3:45 PM - OO8.5
Static and Active Interplay between Geometry, Mechanics and Electronic Transport in Assembled Carbon Nanotube Networks.
Moneesh Upmanyu 1 , Hailong Wang 1 , Yung Jung 1 , Myung Hahm 2 1 Show Abstract
1 Mechanical and Industrial Engineering, Northeastern University, Boston, Massachusetts, United States, 2 Mechanical Engineering and Materials Science, Rice University, Houston, Texas, United States
Nanoelectronic devices that rely on the superior electronic transport in filamentous networks of carbon nanotubes are typically beleaguered by reliability and scalability issues, the primary reason for the ongoing paradigm shift towards thin films consisting of CNT networks as active elements. The extent of the hierarchical structure in these films, consisting of bundled and branched CNT bundles, is modulated by the conformations of the individual nanotubes and the extent of their self-assembly. In this talk, we present a combination of multi-scale coarse-grained computations and experiments on understanding the role CNT-scale mechanics and assembled network topology on the form of electronic transport across the thin films. We first present results the static response of the networks, wherein we explore the effect of nanoscale confinement that arises during template-assisted directed assembly of CNT solutions. Our results show that the width and thickness of the nanoscopic trenches result in a topology that acts to alleviate the effect of the electrical heterogeneity in the networks. The active response is critical in almost all envisioned applications, and to this end, we present results on the effect of extrinsic deformation, via densification or stretching strains, on the electronic transport. Our results are compared with experimental characterization of i) thin films formed by fluidic assembly of ultra-thin CNNs on micropatterned silica substrates, and ii) mechanically deformed sandwich structures consisting of vertically-aligned and fully percolated CNNs. The qualitative aspects of the results presented here have broad implications nanostructure-property relations in for several classes of nanotube and/or nanowire networks.
4:30 PM - **OO8.6
Theory and Computational Studies of Carbon Nanotube Bundles.
George Schatz 1 Show Abstract
1 Chemistry, Northwestern University, Evanston, Illinois, United States
This talk will describe theory and calculations aimed at understanding the structural and mechanical properties of double-wall carbon nanotube bundles. The studies include both electronic structure studies and empirical potential molecular mechanics studies, and much of the focus is on modeling carbon-nanotube pullout experiments. Related calculations with graphene structures are also presented.
5:00 PM - OO8.7
Controlling the Mechanical Properties of Hierarchical Structures of Aligned Carbon Nanotubes.
Jordan Raney 1 , Chiara Daraio 1 Show Abstract
1 Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California, United States
We have synthesized and assembled hierarchical multilayer structures based on aligned carbon nanotube (CNT) arrays, using either polymer or metal interlayers. Under quasistatic compression these structures present two orders of magnitude improved energy dissipation compared to commercial materials of similar density. We further tune their mechanical response by manipulating synthesis parameters during CNT growth, and by reinforcing the nanotubes afterward. These procedures can modify the structure of the CNT arrays from the nano- to the micro-scale, allowing significant control over the mechanical properties of the final materials. Mechanical characterization of these systems shows controllable strain localization and increased energy dissipation under compression and impact. These materials hold promise as tunable low-density energy dissipative systems.
5:15 PM - OO8.8
Mechanical Properties of Bio-Inspired DWNT Yarns.
Mohammad Naraghi 1 , Michael Roenbeck 1 , Owen Compton 2 , Zhi An 2 , SonBinh Nguyen 2 , Horacio Espinosa 1 Show Abstract
1 Mechanical Engineering, Northwestern University, Evanston, Illinois, United States, 2 Chemistry, Northwestern University, Evanston, Illinois, United States
The superior mechanical properties of many natural materials, compared to engineering materials such as ceramics and metals, have motivated researchers to study nature’s design principles. These principles have allowed for the marriage of seemingly desperate properties such as high strength and toughness at low weights. For instance, at equal weights, spider silk can carry several times more load and dissipate much more energy before failure as compared to steel. Studies into the structure of natural materials have pinpointed some of these principles, including the hierarchy in structure and bonds, the abundance of hydrogen bonding which can reform upon their deformation induced rupture (e.g. spider silk), and correspondence between geometrical parameters such as overlap length between constituents and their mechanical properties (e.g. abalone shell) [1, 2]. In this study we investigated two approaches to exploit the design principles in nature to develop composite yarns with high mechanical performance. Given their remarkable mechanical properties, such as their high strength, we chose CNTs as main constituents of the yarns. In the first approach, we developed a spinning-stretching technique to fabricate CNT composite yarns from CNT mats. CNTs were fabricated in a CVD reactor and functionalized in situ CVD with substituted acrylic acid esters. To improve the mechanical behavior of CNT yarns the interactions between CNTs were enhanced by infiltrating yarns with polyvinyl alcohol (PVA). The polymer molecules induce a network of hydrogen bonds to generate chemo-mechanical interactions between functionalized CNTs and PVA chains, thus, mimicking the load transfer mechanisms of natural materials. Moreover, modifying the functional groups of the CNTs from only hydrogen bond acceptors to hydrogen bond acceptors-donors enhanced the mechanical properties of the yarns, pointing to the significance of hydrogen bonds in improving their mechanical properties. By incorporating these design principles we were able to achieve strengths and toughness as high as ~0.5 GPa and ~90 J/g, respectively. H. J. Gao, B. H. Ji, I. L. Jager, E. Arzt, and P. Fratzl, "Materials become insensitive to flaws at nanoscale: Lessons from nature," Proceedings of the National Academy of Sciences of the United States of America, vol. 100, pp. 5597-5600, May 13 2003.M. J. Buehler and T. Ackbarow, "Fracture Mechanics of protein materials," Materials Today, vol. 10, pp. 46-58, 2007.
5:30 PM - OO8.9
A Bottom-up Approach to Thermo-Mechanical Properties of Graphite-Metal Nanocomposites.
Shu-Wei Chang 1 2 , Arun Nair 1 2 , Markus Buehler 1 2 Show Abstract
1 Laboratory for Atomistic and Molecular Mechanics, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Center for Computational Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Graphite has excellent in-plane mechanical properties (stiffness, strength), thermal and electrical conductivity. The weak interactions along the cross-plane direction of graphite and the challenges in fabricating large-scale single crystal graphite, however has become the bottleneck for macroscopic applications. Low-cost metal–graphite nanoplatelet composites are promising solutions for functional materials in digital and high-power electronics applications, which feature reliable mechanical properties and low electrical/thermal resistance. Strongly adhesive metals can glue together isolated graphite platelets and thus graphite platelet in metal matrix composites has the great potential for high stiffness, strength, and conductivity in materials. The performance of the metal-graphite materials depends on the choice of metals (interactions at graphite-metal interface), geometries, working temperature, and sizes. Therefore, comprehensive understanding on the size and temperature effects is crucial for the design of graphite-metal nanoplatelet composites.In this talk, we will present how density functional theory (DFT) is used to predict the interfacial structure between graphite and metal and how the material parameters from DFT are used to perform molecular dynamics (MD) simulations to study the thermal and mechanical properties of graphite-metal nanocomposites. Two different metals that are studied in this research are copper and nickel. Atomistic simulations using MD are used to study the size and temperature dependency on the thermal and mechanical properties of the layered graphite-metal materials. To calculate the thermal conductivity we use the Muller-Plathe approach to systematically explore the effect of layer thickness of metal versus graphite on thermal properties. We also use atomistic simulations to investigate the mechanical reliability of graphite-metal composites. The lattice mismatch that is computed for different temperature regimes will be discussed in light of mechanical performance of the composites. Our results show that the increase in volume fraction of graphite would increase the mechanical properties along the in-plane direction of graphite, but decreases it along the cross-plane direction. The effect of volume fraction of graphite on thermal properties of graphite-metal system will be presented. These studies help in understanding the structure, electronic and phononic coupling properties at the interfaces between graphite and metals, which determine their mechanical properties and transport performance and provide guidance for optimum design of graphite-metal nanoplatelet composites.
5:45 PM - OO8.10
Viscoelasticity in Vertically Aligned Carbon Nanotube Foams and Polymer-Carbon Nanotube Sandwich Structures.
Ludovica Lattanzi 1 , Luigi De Nardo 1 , Jordan R. Raney 2 , Chiara Daraio 3 4 Show Abstract
1 Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”, Politecnico di Milano, Milano Italy, 2 Materials Science, California Institute of Technology, Pasadena, California, United States, 3 Graduate Aerospace Laboratories, California Institute of Technology, Pasadena, California, United States, 4 Applied Physics, California Institute of Technology, Pasadena, California, United States
We study the macroscale viscoelastic response of vertically aligned carbon nanotube (VACNT) foams both as freestanding materials and partially embedded between two layers of Poly(dimethylsiloxane) (PDMS). These materials present a multiscale fibrous structure, low density, and remarkable mechanical properties in compression. We test their creep and stress-relaxation responses, and model their behavior with nonlinear power-laws. Both freestanding and sandwich structures exhibit a time-dependent behavior under compression that is well captured by a power-law function of time, where the rate of creep is dependent on the stress level, and the rate of relaxation is dependent on the strain level. Results show a marginal effect of the PDMS layers on the overall viscoelastic response. The presence of the polymer, however, affects the buckling mode and stability of the structures. At high strain levels, the VACNT foams partially embedded in polymer reach higher peak stresses than the freestanding VACNT foams.
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
OO9: Modelling I
Thursday AM, December 01, 2011
Room 208 (Hynes)
9:30 AM - **OO9.1
Stacking Bonds: Programming Molecular Recognition Using the Geometry of DNA Nanostructures.
Sungwook Woo 2 , Paul W. Rothemund 1 Show Abstract
2 Bioengineering, California Institute of Technology, Pasadena, California, United States, 1 Bioengineering, C.S. & C.N.S., California Institute of Technology, Pasadena, California, United States
From ligand–receptor binding to DNA hybridization, molecular recognition plays a central role in biology. Over the past several decades, chemists have successfully reproduced the exquisite specificity of biomolecular interactions. However, engineering multiple specific interactions in synthetic systems remains difficult. DNA retains its position as the best medium with which to create orthogonal, isoenergetic interactions, based on the complementarity of Watson–Crick binding. Here we show that DNA can be used to create diverse bonds using an entirely different principle: the geometric arrangement of blunt-end stacking interactions. We show that both binary codes and shape complementarity can serve as a basis for such stacking bonds, and explore their specificity, thermodynamics and binding rules. Orthogonal stacking bonds were used to connect five distinct DNA origami. This work, which demonstrates how a single attractive interaction can be developed to create diverse bonds, may guide strategies for molecular recognition in systems beyond DNA nanostructures.
10:00 AM - OO9.2
Proton Switched Cytidine Interactions and the Self-Assembly of i-motif DNA Nanowires.
Raghvendra Singh 1 2 , Dominique Collard 3 , Ralf Blossey 2 , Fabrizio Cleri 1 Show Abstract
1 IEMN, University of Lille I, Lille France, 2 , Institute for Interdisciplinary Research, Villeneuve d'Ascq France, 3 LIMMS/Cnrs, University of Tokyo, Tokyo Japan
The DNA "i-motif" is formed by stretches of two or more cytidines (C). Two duplexes containing a sequence of Cs’ can associate head-to-tail, in an intercalated tetramer structure. Tetramer formation is driven by a slight acidification of the solution below pH 7, each CC+ pairing being mediated by a proton, resonantly exchanged between the two bases. It has been experimentally shown that intercalated structures can organize into long nanowires, up to a length of several microns, representing templates for the self-assembly of nanoobjects. We study of the molecular mechanisms leading to the intercalation of poly-C fragments, by means of empirical Molecular Dynamics simulations. Notably, the key mechanisms of proton-mediated base-pairing are still unknown. For a deeper understanding of such phenomenology, which also serves to understand the biological functions of non-Watson-Crick DNA and RNA assembly, we characterized the protonation interactions of poly-C strands in physiological water. We initially used standard motifs for intercalation, such as single A,C,T,G bases, or the TGT insulin spacer. In a further step of the Project, we also use DNA aptamers for intercalation. Such condition should be directly comparable to experiments carried out with the Cnrs-LIMMS.
10:15 AM - OO9.3
Bond Energy Effects on Strength, Cooperativity and Robustness of Hierarchical Molecular Assemblies.
ChiaChing Chou 1 , Markus Buehler 1 Show Abstract
1 Laboratory for Atomistic and Molecular Mechanics, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
A fundamental challenge in engineering biologically inspired materials and systems is the identification of molecular structures that define fundamental building blocks. Here we report a systematic study of the effect of the energy of chemical bonds on the mechanical properties of molecular structures, specifically their strength and robustness. By considering a simple model system of an assembly of bonds in a cluster we demonstrate that weak bonding, as found for example in H-bonds, results in a highly cooperative behavior where clusters of bonds operate synergistically to form relatively strong molecular clusters. The cooperative effect of bonding results in an enhanced robustness since the drop of strength due to the loss of a bond in a larger cluster only results in a marginal reduction of the strength. Strong bonding, as found in covalent interactions such as disulfide bonds or in the backbone of proteins, results in a larger mechanical strength. However, the ability for such bonds to interact cooperatively is lost, and as a result, the overall robustness is lower since the mechanical strength hinges on individual bonds rather than a cluster of bonds. A series of molecular dynamics simulations with a simple coarse-grained multi-scale model of alpha-helical protein domains with bonds of varied strength (from weak H-bonds to strong covalent bonds) are also carried out, aimed at probing the interplay of protein domains and cross-links during mechanical deformation. We specifically consider the mechanics of disulfide bonded structures under reducing or oxidizing chemical microenvironments, an issue of great relevance for the understanding of physiological and disease conditions. We find that in the presence of reducing agents, the system with weaker disulfide bonds shows a lower strength but higher robustness. Conversely, the presence of oxidizing agents leads to a greater stabilization of disulphide bonds so the multi-scale model with more stable disulfide bond has higher strength but lose robustness. The fracture mechanism of the disulfide bond based system is consistent with the predictions from our theoretical model. The results presented here provide general insight into the interplay of bond energy, robustness and other geometric parameters such as bond spacing and hierarchical architecture. We conclude our analysis with a correlation of structural data of natural protein structures, which confirms the conclusions derived from our study and links our findings to evolutionary mechanisms of selection of universal protein architectures. C.C. Chou, M.J. Buehler, “Bond energy effects on strength and robustness of molecular structures,” Interface Focus, 2011
10:30 AM - OO9.4
Orchestrating Population Shifts along the Landscape of Biological Macromolecules.
Ali Rana Atilgan 1 , Burak Okan 2 , Canan Atilgan 1 Show Abstract
1 , Sabanci University, Istanbul Turkey, 2 , Rensselaer Polytechnic Institute, Troy, New York, United States
Conformational multiplicity of proteins under different environmental conditions is satisfactorily described herein by simple analytical methods. To efficiently monitor the equilibrium landscape and to orchestrate the population shifts, we derived the following set of equations originating from continuum mechanical considerations: i) the equilibrium equation of each repeating unit, ii) the constitutive relation for each (non)bonded short and/or long-range contact, and iii) the compatibility equation between the fluctuation of an element and fluctuations of its neighboring bonds. Together with the updated incremental Lagrange formalism, we put forward an effective single-molecule manipulation methodology. After each incremental move due to the perturbation induced by a ligand, the response kernel is updated and the new position on the free-energy surface is calculated.In this study, we additionally demonstrate that cooperatively inserted intra-residue fluctuations, resembling different ligand insertions into the protein, moderate the positional motion of residues that are responsible for desired activities of the protein. We identify a feedback mechanism between sensory regions and adaptively distributed actuating parts. We construct a template that is a subset of the native structure containing the controller and we show that the template is conserved within the families of evolved sequences. These templates are shown to be the key elements for protein-protein interactions.We study the relationships between the statistical and spectral properties of networks derived from the protein conformations located at the different minima of the landscape. We determine how the shortest path betweenness distribution of the edges is altered from one minimum (apo form) to another one (holo form). The results indicate that the changes in the redundancy index, which is the ratio of the number of alternative two-step paths a given residue i generates to its non-bonded contacts and its overall reachability, is significant. Furthermore, we relate the redundancy of each residue to its bond orientational order and demonstrate the symmetry group shifts when the folded structure is altered due to ligand perturbations or changes in pH levels.
11:15 AM - **OO9.5
A New Approach to Molecular Simulation.
Vijay Pande 1 Show Abstract
1 Vijay Pande, Vijay Pande, Stanford, California, United States
In order to rationally design molecular systems for dynamical properties, an important step is the ability to quantitatively predict dynamics of molecules at the atomic scale. The traditional approach to molecular simulation, eg. running Molecular Dynamics simulations and analyzing the resulting trajectories, has numerous short comings. In particular, such methods typically yield only anecdotal result and reach timescales thousands to millions of times too short to connect directly with experiment. I will present a new approach to molecular simulation. Through a combination of Molecular Dynamics simulation, Bayesian inference, and worldwide distributed computing, one can directly simulate in chemical detail, many complex, experimentally systems on length scales and timescales previously unimaginable. I will give examples of how these new methods can lead to significant breakthroughs in classically challenging problems, such as protein folding, as well as biological and biomedical applications, including Abeta protein misfolding in Alzheimer’s Disease.
11:45 AM - OO9.6
Maximum Stability of Alpha-Helical Protein Domains is Reached at a Critical Length Scale.
Zhao Qin 1 2 , Andrea Fabre 1 4 , Markus Buehler 1 2 3 Show Abstract
1 Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 4 Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Center for Computational Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
The alpha-helix secondary structure protein motif, characterized by a right-handed coil structure with 3.6 residues per convolution, is a universal architectural feature of proteins and its formation, stability and breakdown is crucial for many biological processes. Here we show that a favored alpha-helix length exists at 1.4 nm/9 residues, caused by a competition between alpha-helix folding, unfolding and formation of higher-order tertiary structures. We use a combination of thermodynamical modeling, well-tempered metadynamics and statistical analysis of experimental results to reveal the mechanism by which the different possible conformations of alpha-helices compete and coexist. Our thermodynamics model reveals the probability for a polypeptide to form each of the three typical structures, including an intact alpha helix, partly unfolded structures and self-folded higher-order tertiary structure, as functions of the polypeptide length. We survey tens of thousands of experimentally obtained protein structures for alpha-helix lengths preferences and find an agreement with the probability distribution obtained from our model, showing that alpha-helices with 0.8 to 2 nm/5 to 14 residues length are most common, presumably because they are most stable. This theoretical model is further directly substantiated by direct metadynamics simulations using a full-atomistic explicit solvent model. Our results represent general insight into the size dependent structural stability of alpha helices. The stabilities and mechanical properties of alpha-helix motifs profoundly influence the material properties and hence biological functions of associated protein materials. The results advance our understanding of size effects in protein stability with relevance to diseases associated with protein misfolding, and may further enable the design of synthetic alpha-helical protein materials.
12:00 PM - OO9.7
Mechanics of Materials with Fractal Microstructure.
Catalin Picu 1 Show Abstract
1 , Rensselaer Polytechnic Institute, Troy, New York, United States
Fractal structures are the epitome of hierarchical structures. Many materials, mostly biological, have multiscale structures with fractal characteristics over certain range of scales. These fractals are stochastic in nature. A brief review of such structures will be presented in the introduction, followed by an analysis of the nature of the deformation of ideal fractals, and of structures with fractal characteristics over a range of scales. Differences in the overall mechanical behavior between a material with fractal microstructure and a material with the same phase composition but without fractal structure will be discussed. The effect of the width of the range of scales of fractality on the homogenized behavior (equivalent to a size effect) is also discussed. This analysis is possible due to a new formulation of continuum mechanics for hierarchical structures with self-similarity.
12:15 PM - OO9.8
A Micromechanics Homogenization Model on Kinematics of Brain White Matter.
Assimina Pelegri 1 , Yi Pan 1 , David Shreiber 2 , Sagar Singh 2 , Vivak Patel 2 Show Abstract
1 Mechanical and Aerospace Enigneering, Rutgers University, Piscataway, New Jersey, United States, 2 Biomedical Engineering, Rutgers University, Piscataway, New Jersey, United States
A novel microstructural kinematics model is presented that captures the kinematics of brain tissue at axonal level, and possibly enables evaluation of the injury state (e.g., partial recovery of the neurological functions or irreversible damage) of the patient while proving a reliable insight into the injury location. In this paper, homogenization on the central nervous system’s white matter using representative volume elements (RVEs) is performed based on the local micro-structure, in which a forward boundary value problem is solved using finite element analysis to obtain homogenized mechanical behavior of the composite material. The brain tissue white matter and random fiber composites share common features, from the mechanics perspective. The former is treated as a “composite material”, in which the undulated, reinforcing axon fibers are embedded within a supportive tissue matrix comprised primarily of glia. Recent study showed that axon kinematics during simple elongation follows neither pure affine nor non-affine models, and that their behavior changes according to the level of macroscopic, tissue stretch. At low levels of macroscopic strain, axon’s behavior was predominantly non-affine. As strain increases, the behavior becomes increasingly affine. It was suggested that the unusual microstructure of white matter, where there is little to no structural extracellular matrix but instead axons are connected to a cellular matrix of asctrocytes and myelinating oligodendrocytes at Nodes of Ranvier, was responsible for the atypical behavior, and that the transition from non-affine to affine behavior was caused by the recruitment of cellular cross-links.For modeling the atypical white matter behavior and to determine the mechanical properties of the brain tissue, we have developed a composite mechanics and micromechanics analysis coupled with finite elements into a biomechanical and microstructure interacting model. In this paper, a representation of the brain white matter is generated using a random walk approach. The geometric description of the axon and the surrounding matrix is obtained from the neurofilament immunohistochemistry images. The geometric model reported here is used for finite element analysis. Hyperelastic material constitutive models are applied to describe the behaviors of the axon and surrounding matrix, respectively. The axon’s kinematics describing the interaction of the axon and the surrounding matter under different stretch is embedded into this model to accurately capture the mechanical model at macroscopic level.
OO10: Modelling II
Thursday PM, December 01, 2011
Room 208 (Hynes)
2:30 PM - OO10.1
Design of de Novo Hierarchical Materials: Application of Category Theory to Protein Materials.
Tristan Giesa 1 2 , Markus Buehler 1 3 4 Show Abstract
1 Laboratory for Atomistic and Molecular Mechanics, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Department of Mechanical Engineering, RWTH Aachen University, Aachen Germany, 3 Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 4 Center for Computational Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Biological hierarchical materials are currently largely understood through top-down approaches by dissecting their macroscopic structure. In order to make advances in our understanding of multiscale mechanisms obtained from experimental and computational studies available for engineering, novel descriptive methods based on mathematical tools must be introduced.Using a category theoretical system representation, hierarchical structures are not only described, but the origin of overall mechanisms by which functionality emerges can be revealed. This includes for example the role of H-bond clustering in biological materials (such as spider silk) for superior strength and robustness. Moreover, an abstraction of the material’s building blocks and their interactions allows a mathematically rigorous transfer of insights gained from various fields to materials science and vice versa. This novel concept overcomes current major limitations of graph theoretical approaches in materials science. In summary, category theory combined with extensive multiscale simulations provide appropriate means to gather and share knowledge of material structure-function relationships which enables the engineering of advanced biological and biomimetic fibers, composites, and other de novo materials for applications that span from regenerative medicine and energy applications to civil engineering.We present several case studies to outline the approach used here and to make an analogy between protein filaments and social networks which exhibit remarkably comparable hierarchical buildups. Furthermore, we show that depending on the composition and structural organization of basic building blocks on multiple scales, a wide variety of functions is achieved, despite the reliance on a limited number of building blocks. Category theory enables us for instance to relate spider silk, where polypeptides assemble into a cross-linked fibrillar network connected by weak interactions to organize into a macroscopic orb web, to music, where basic waveforms assemble into complex hierarchies to form intricate compositions, e.g. fugues or symphonies. We demonstrate how these hierarchies can be identified in a stringent mathematical framework and how the structure preserving transformations from seemingly disparate field can teach us about the design of de novo hierarchical materials.
2:45 PM - OO10.2
Multiscale Model of Collagen Fibril Incorporating the Effect of Mineral Collagen Interactions.
Shashindra Pradhan 1 , Kalpana Katti 1 , Dinesh Katti 1 Show Abstract
1 Civil Engineering, North Dakota State University, Fargo, North Dakota, United States
The hierarchical structure of bone spans from collagen and mineral nano-crystals at the molecular scale to osteons at the macroscopic scale. Understanding the deformation mechanism of bone requires a robust multiscale model that bridges various hierarchical levels and which capture the interactions at each of the scales. In our study, we have conducted hierarchical multiscale modeling of fibril to investigate the deformation characteristics of the collagen fibril using a combination of steered molecular dynamics and finite element (FEM) methods. Fibrils are cylindrical structures composed of staggered array of collagen molecules and mineral in the hole-zone. At the molecular scale, we have conducted steered molecular dynamics (SMD) simulations to evaluate the deformation mechanisms and investigate the mechanics of collagen in the proximity of hydroxyapatite. We have also conducted SMD simulations to investigate the mechanics of full-length collagen molecule. Our simulations show that the mechanical response of collagen is highly influenced by the mineral proximity. Moreover, the type of mineral surface in the vicinity and the orientation of collagen were also found to have a significant impact on the deformation behavior of collagen. The nano-mechanical parameters for collagen evaluated from the SMD simulations were incorporated into the finite element (FEM) model of fibril. The FEM model of fibril, 50 nm in diameter is constructed and replicates the staggered arrangement of collagen and the influence of mineral present in the hole-zone on the adjacent collagen. The micro-architecture of this fibril model also mimics the banded pattern of fibril along the length due to alternating overlapping and gap regions. From this FEM model we have investigated the mechanics of fibril under tensile load and report the impact of mineral-collagen interaction in the deformation behavior of fibril.
3:00 PM - OO10.3
Phase-Field Model of Fracture in Disordered Composites.
Qian Xiao Li 1 , Mark Jhon 1 Show Abstract
1 Materials Science and Engineering, Institute of High Performance Computing, Singapore Singapore
Nacre is a model biological composite, exhibiting a very high resistance to fracture relative to its constitutent materials. Because of the many levels of structural hierarchy in nacre, it is challenging to accurately simulate fracture in such a material. The phase-field model of fracture is a computational technique that can naturally incorporate structural hierarchy. In the present study, we consider the role of disorder in the elastic constant on the phase-field model of fracture. We calculate the toughness of a composite with varying levels of structural disorder, and find that strongly disordered composites show increased toughness. We also determine the effect of spatially correlated disorder on toughness. We observe crack deflection at sufficiently long spatial correlations, further increasing the toughness of the composite.
3:15 PM - OO10.4
Structural and Mechanical Differences between Collagen Homo- and Heterotrimers: From Molecular to Microfibril Scales.
Shu-Wei Chang 1 , Sandra Shefelbine 2 , Markus Buehler 1 Show Abstract
1 Laboratory for Atomistic and Molecular Mechanics, Department of Civil and Environmental Engineering, Massachusetts institute of technology, Cambridge, Massachusetts, United States, 2 Department of Bioengineering, Imperial College, Cambridge, London, United Kingdom
Collagen is a crucial structural protein, formed through a hierarchical assembly of tropocollagen molecules, arranged in collagen fibrils, which constitutes the basis for larger-scale fibers. Collagen constitutes one third of the human proteome, providing mechanical stability, elasticity and strength to organisms. Normal type I collagen is a heterotrimer consisting of two alpha-1 chains and one alpha-2 chain. The homotrimeric isoform of type I collagen, which consists of three alpha-1 chains, is only found in fetal tissues, fibrosis, and cancer in humans. A mouse model of the genetic brittle bone disease, osteogenesis imperfecta (oim), is characterized by a replacement of the alpha-2 chain by an alpha-1 chain, resulting in a homotrimer collagen molecule. Experimental studies of oim mice tendon and bone have shown reduced mechanical strength compared to normal mice. The relationship between the molecular content and the decrease in strength is, however, still unknown. In this research we present a comprehensive study of the structural differences between the heterotrimer and homotrimer from the atomistic simulation at the single molecular level to the microfibril level. We use a molecular simulation approach to study the structural differences between type I heterotrimer and homotrimer of real sequence mouse collagen molecules. We calculate the persistence length and analyze the structure to determine difference in mechanical behavior of hetero- and homotrimers. The persistence length quantifies the stiffness of long polymer chains, making it an appropriate parameter for estimating the stiffness of collagen molecules. The structural analyses allow us to assess the differences in behavior along the collagen molecules. Furthermore, we use atomistic collagen microfibril models that include full-length molecules with the actual sequence defined by the mouse heterotrimer and homotrimer collagen gene, in order to describe the mechanical behavior at the microfibril level to reveal the origin of brittle bone disease. The results show that homotrimer persistence length is half that of the heterotrimer, indicating that it is more flexible. Structural analyses reveal that the homotrimer kinks and freely rotates with angles much larger than heterotrimer. The local kinks found at the single molecule level may result in a larger lateral distance between collagen molecules in the fibril and have implications for reducing the intermolecular cross-linking which is known to reduce the mechanical strength. Atomistic modeling allows us to examine the behavior from a single collagen molecule to collagen microfibril, helping to define the basic structural and mechanical differences responsible for the changes throughout the hierarchy. Our studies provide fundamental insight of the effect of the lost of alpha-2 chain at the single molecular and help understanding the molecular origin of the bone brittle disease at much larger length-scales.
4:00 PM - OO10.5
The Molecular Mechanism of Rapid Aging Disease: Insights into the Structural and Mechanical Stability of the Normal Lamin A and Its Mutant.
Zhao Qin 1 2 , Agnieszka Kalinowski 4 , Kris Dahl 4 5 , Markus Buehler 1 2 3 Show Abstract
1 Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 4 Department of Biomedical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States, 5 Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States, 3 Center for Computational Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Hutchinson–Gilford progeria syndrome is a premature rapid aging syndrome caused by the expression and accumulation of a mutant form of lamin A, progerin. As a component of the cell’s nucleoskeleton, lamin A plays an important role in the mechanical stabilization of the nuclear envelope and in other nuclear functions. It is largely unknown how the characteristic 50 amino acid deletion affects the conformation of the tail domain of lamin A, which is a protein segment with mostly intrinsically disordered secondary structure. Here we use replica exchange method to simulate the folding process of the segment and obtain an ensemble of semi-stable structures. We confirm our simulated structures by comparing the geometric and thermodynamic properties of the ensemble structures to in vitro stability measurements preformed by tryptophan fluorescence and circular dichroism. Using this combination of experimental and computational techniques, we compare the size, heterogeneity of size, thermodynamic stability of the normal tail and its mutant, as well as the difference in mechanisms of force-induced denaturation. Our experimental and computational results quantitatively and consistently show that the mutant is a more stable structure (by approximately 37–70 kcal/mol or 62–117 kBT), as well as a more compact molecule (by 78%). Altogether these results suggest that the altered structure and stability of the tail domain can explain changed protein–protein and protein–DNA interactions and may represent an etiology of the disease. Our results show that by combining the power of molecular simulation and experiment, we manage to investigate t