Many biological materials, such as natural exoskeletons (armor), are known to undergo a variety of energy dissipating mechanisms at multiple length scales due to their hierarchical, composite structures, leading to the amplification of mechanical properties beyond simple rule of mixtures. In this work, we investigate the nanoscale deformation mechanisms of the highly transparent shell of the bivalve, Placuna placenta (Linnaeus 1785). The highly mineralized shell of P. placenta (~99 wt% calcite) is composed of the foliated microstructure, in which elongated diamond-shaped calcitic laths are arranged in a mosaic-like manner. The c-axes of these calcitic laths are tilted by 24.4 ± 3.5° relative to their surface normals as measured from electron backscattered diffraction, resulting in lath surfaces closely resembling the {108} planes of calcite. Using systematic instrumented nanoindentation experiments we show that P. placenta achieves a combination of superior mechanical properties, including enhanced hardness, penetration resistance, more localized and isotropic deformation, and graceful failure, as compared to calcite, its main constitute. Most importantly, P. placenta exhibits enhanced energy absorption and damage tolerance, indicated by its much higher energy dissipation density (P. placenta: 0.290 ± 0.072 nJ/µm3, calcite: 0.034 ± 0.013 nJ/µm3). Detailed electron microscopy studies reveal two primary energy-dissipating mechanisms: deformation twinning surrounding the impact region and nanoscale plasticity through nanograin formation and reorientation, which are intimately related to the crystallographic and structural features of the nano-/micro-scopic architecture of the shell&’s foliated structure. The e-type deformation twinnings in the calcitic laths were observed in three crystallographically equivalent planes of calcite, i.e. (1-18), (018), and (-108). Deformation twinning is known to occur in geologic calcite and is hypothesized to be enhanced in P. placenta by the nanoscale thickness (~300 nm) and surface roughness of the building blocks of the foliated microstructure. The twin boundaries together with the thin (~2 nm) intercrystalline organic interfaces act as barriers to dislocation motion and catastrophic crack propagation, maintaining structural integrity and localizing nanoscopic deformation and damage to a confined region. The two mechanisms work synergistically, increasing the energy dissipation density by almost an order of magnitude relative to calcite.

While micro-FE simulations have become a standard tool in computational biomechanics, the choice of appropriate material properties is still a relevant topic, typically involving empirical grey value-to-elastic modulus relations. We here derive the voxel-specific volume fractions of mineral, collagen, and water, from tissue-independent bilinear relations between mineral and collagen content in extracellular bone tissue (J. Theor. Biol. 287: 115, 2011), and from the measured X-ray attenuation information quantified in terms of grey values. The aforementioned volume fractions enter a micromechanics representation of bone tissue, as to deliver voxel-specific stiffness tensors. In order to check the relevance of this strategy, we convert a micro Computer Tomograph of a mouse femur into a regular Finite Element mesh, apply forces related to the dead load of a standing mouse, and then compare simulation results based on voxel-specific heterogeneous elastic properties to results based on homogeneous elastic properties related to the spatial average over the solid bone matrix compartment, of the X-ray attenuation coefficients. The element-specific strain energy density illustrates that use of homogeneous elastic properties implies overestimation of the organ stiffness. Moreover, the simulation reveals large tensile normal stresses throughout the femur neck, which may explain the mouse femur neck's trabecular morphology being quite different from the human case, where the femur neck bears compressive forces and bending moments.

As a complement to the ample experimental literature on qualitative features of mineralizing bones, we here present mathematically formulated rules behind the bone mineralization process within and outside the collagen fibrils.
More precisely, we check whether the structural evolution of bone tissue during mineralization can be explained by means of fluid-solid phase transitions in the fibrils and the extrafibrillar space, considered as two thermodynamically closed systems: the mass of lost ionic fluid equals the mass of formed solid mineral in each subvolume, while the collagen mass remains unaltered. The mineralization process is accompanied by an increase in mass density, leading to shrinkage of the compartments, which can be quantified through the change in equatorial diffraction spacing at the fibrillar scale. These propositions are checked through experimental evidences, relating bone mass densities to the equatorial diffraction spacings. Therefore, the mass conservation equations are converted into mass density-diffraction relations, in three consecutive steps: First, the tissue shrinkage is evaluated based on universal bone composition rules. Secondly, this relation is downscaled to the extrafibrillar space, based on the constant mineral concentration in the extracollagenous space and the hydration swelling rule for unmineralized tissues. Thirdly, the fibrillar shrinkage is analogously derived and related to the change in diffraction spacing.
Our mathematical approach is strictly validated through comparison between experimental and predicted neutron diffraction spacings. Very low prediction errors underline the relevance of the computed evolutions of the tissue compartment volumes and volume fractions during the mineralization process in different bone tissues.
Corresponding shrinkage and composition rules are deemed beneficial for further progress in bone materials science and biomedical engineering.

Man-made nanobiocomposites offer promising mechanical properties that can surpass both natural and engineered materials by blending the best properties of biomolecules with polymers. Natural building blocks such as cellulose nanocrystals of can be effectively utilized in polymer composites due their versatility in size, shape, and surface chemistry. Cellulose molecules contain large numbers of hydroxyl side groups, allowing for a high axial modulus, greater than that of even Kevlar, while maintaining a low density and an easily functionalized surface. However, the impact of the size and surface properties of cellulose on the interfacial mechanics of polymers and all-cellulose composites remain to be fully understood.
To address this issue, here we use a molecular dynamics approach to determine the binding energy landscape between two CNC surfaces and between a CNC surface and polymer matrix, so as to predict the failure mechanism of the composite. Furthermore, we modulate the size of the CNC giving further insight into the mechanics governing the size-dependence of the interfacial interaction. We demonstrate the potential of our methodology in helping to design optimal shape and size of the CNC to achieve the desired composite behavior, and correlate our finding with composites found in biological systems.

Traumatic brain injury is a major public health issue, both in the civilian world and in the military one. In order to expand on potential injury mechanisms to the brain, a micromechanical representation of the gray matter is developed. The gray matter contains a large amount of capillaries that supply the necessary oxygen required for maintaining healthy cell and brain function. Even short duration disruption of the oxygen supply can lead to death and damage of neuronal cells present. It has been shown that shearing forces present within blood flow around obstructions can lead to activation and aggregation of the platelets, which can lead to thrombus formation. While current macro-scale models of the brain can include representation of the larger vasculature present, it is too computationally intensive to model all of the microvasculature present. However, macro-scale models can be used to determine the forces present at various points within the brain, and these forces can be used within a micro-scale model to determine the forces at the platelet level. A micromechanical computational model is developed with gray matter, capillaries, and blood, which is composed of plasma, red blood cells, and platelets. This model combines both fluids and viscoelastic solid materials, so it becomes necessary to use a combined Langrangian-Eulerian finite element approach is to determine the interaction of the platelets with the plasma present within the blood, and to determine whether damage criteria are met for potential shear-induced platelet activation. In addition, this micromechanical model can also be used to develop bottom-up constitutive models for the gray matter, based on the individual materials, in order to better model the macro-scale response of the brain.

Piezoelectricity in collagen is a fundamental phenomenon which has been linked with the ability of bone to remodel. The complex system response of bone to a wide variety of forces (compressive, shearing, axial, etc.) is poorly understood. The high degree of structural organization in collagen and the anisotropy of the piezoelectric properties of collagen make it uniquely suited as a means for cells to locally differentiate between varying stresses. Macroscopic piezoelectricity measurements made to date, however, represent a combination of numerous collagen fibrils with differing orientations. Therefore, it is difficult to obtain accurate piezoelectric coefficient values in the case of collagen. To date, the piezoelectric tensor of a single collagen fibril is not known. However, with the advent of new tools like piezoresponse force microscopy (PFM), it is now possible to probe piezoelectricity on the nanoscale. In order to investigate the dependence of piezoelectricity on the orientation of collagen, tendon, a connective tissue composed of highly aligned collagen fibrils, was studied. Rat tail tendon was embedded in epoxy and cut at angles 0, 45 and 90 degrees relative to the tendon axis. By relating the crystal and laboratory coordinate system and assuming the crystal structure of collagen to be the hexagonal C6 group, it was possible to directly measure the d33 and d15 piezoelectric coefficients via PFM and to mathematically deduce the remaining nonzero (d14 and d31) piezoelectric coefficients in order to reconstruct collagens tensor at the fibrillar scale.

The structure of human cortical bone evolves over multiple length-scales from its basic constituents of collagen and hydroxyapatite at the nanoscale to osteonal structures at near-millimeter dimensions, which all provide the basis for its mechanical properties. To resist fracture, bone&’s toughness is derived intrinsically through plasticity (e.g., fibrillar sliding) at structural-scales typically below a micron and extrinsically (i.e., during crack growth) through mechanisms (e.g., crack deflection/bridging) generated at larger structural-scales. Biological factors such as aging, irradiation and disease can lead to a markedly increased fracture risk, which is often associated with a loss in bone mass (bone quantity). However, these factors can also significantly degrade the fracture resistance (bone quality) over multiple length-scales.
Using a suite of materials science characterization techniques, including in situ small -angle x-ray scattering/wide-angle x-ray diffraction (SAXS/WAXD) to characterize sub-micron to nanoscale structural changes and synchrotron x-ray computed tomography and in situ fracture-toughness measurements in the scanning electron microscope to characterize effects at micron- to macro-scales, we show how age-related structural changes at differing size-scales degrade both the intrinsic and extrinsic toughness of bone. Specifically, we attribute the loss in toughness to increased non-enzymatic collagen cross-linking which suppresses plasticity by fibrillar sliding at nanoscale dimensions and to an increased osteonal density which limits the potency of crack-bridging mechanisms at micron-scales. The link between these processes is that the increased stiffness of the cross-linked collagen requires energy to be absorbed by “plastic” deformation at higher structural levels, which occurs by the process of microcracking. Analogous mechanisms for the embrittlement of bone due to x-ray irradiation are also presented, together with a discussion of the effect of bone diseases on bone strength and toughness, specifically due to osteogenesis imperfecta and vitamin-D deficiency.

Tropocollagen, composed of three intertwining peptide strands with characteristic sequence Gly-X-Y, where X and Y are frequently Pro or Hyp, respectively, is the principal load-bearing protein in mammals. Sequence variations and post-translational processes, such as hydroxylation of proline to hydroxyproline, or large-scale mineralization of tropocollagen fibrils into osseous structures, are known to have a profound effect on its mechanical (e.g. tensile modulus) and physical (e.g. melting point) properties and to play key roles in certain human diseases. Furthermore, due to its ubiquity in mammalian tissue, ancient collagen is of interest to archaeologists, who have found that certain degradation processes show consistent reaction rate over archaeological timescales and therefore serve as molecular clocks. In this work we explore the role of confinement on the structure and stability of collagen-mimetic systems using molecular simulations. We use a fluctuation method to compute the tensile stress-strain relationship of a (Gly-Pro-Pro)7 oligopeptide in a variety of solvent systems. Second, we calculate the partial molar volume of collagen strands in these solvents, and compare the excess volume relative to the tropocollagen triple helix to quantify the effect of solvent pressure. Finally, we study the effect of confinement by mineral phase on the structure of collagen by comparison of SAXS/WAXS data against structural changes of a compressed collagen microfibril.

Adhesion to carbohydrates presented on epithelial cell surfaces is often the first step in bacterial infection.[1] The low efficacy of monovalent inhibitors is circumvented, like in nature, by the presentation of multiple copies of the carbohydrate ligand. A great deal of attention therefore has been directed towards the synthesis of glycopolymers.[1-3] Despite this large interest, the mechanism of the ‘cluster glycoside effect&’ is not fully understood.[2]
Although multivalent presentation of carbohydrates on a polymeric scaffold can lead to inhibitors with good affinity for the target lectin or bacterial toxin, these do not discriminate between proteins that preferentially bind the same epitope. From looking at structural biology information regarding the binding pocket of different lectins and toxins, glycopolymers can be better tailored to their target and gain some specificity.[1]
Here we present a tandem post-polymerization modification strategy to systematically probe the multivalent inhibition of two distinct lectins as a function of linker length, carbohydrate density, and glycopolymer chain length.[1] This tandem post-polymerization method has been optimized to gain the optimum length spacer between the polymer backbone and the pendant saccharide.[2]

[1] Richards, S-J.; Jones, M. W.; Hunaban, M. I.; Haddleton, D. M.; Gibson, M. I., Angew. Chem. Int. Ed. 2012, 51, 7812-7816.
[2] Jones, M. W.; Richards, S-J.; Haddleton, D. M.; Gibson, M. I. Polym. Chem.
2013, 4, 717 - 723.
[3] Gou, Y.; Richards, S-J.; Haddleton, D. M.; Gibson, M. I., Polym. Chem. 2012, 3, 1634.

Cellulose nanocrystals (CNCs) are a form of pure cellulose that is exceptionally strong and stiff, yet sustainable, biodegradable and biocompatible. Hence, understanding the physical behavior of natural products, such as aerogels made of CNCs, is of utmost importance in order to make them potential candidates to replace the petroleum-based products in the 21st century. To approach a basic understanding of the properties of these aerogels, our strategy is to quantify their mechanical properties using nanoindentation, and correlate that behavior with the statistical spatial distribution of cellulose-cellulose contact points by TEM tomography. The ultimate goal is to achieve a better understanding of the cellulose-cellulose bond properties under different environmental conditions. In this initial work CNCs were prepared from pure cotton cellulose by hydrochloric acid hydrolysis. The aqueous CNC dispersion was then subjected to oxidation, converting the surface primary hydroxyls to carboxylic acids, prior to forming the dry CNC aerogels. The mechanical properties of these highly porous CNC aerogels were characterized by nanoindentation using a flat cylindrical punch of about 100 µm diameter. The flat punch was indented 5 µm into the surface to ensure a constant contact area during the measurements. The dynamic storage modulus and loss factor (tan δ) were measured over a frequency range of 1 - 150 Hz. While the storage modulus increased slightly, the loss factor showed significant change over the frequency range. There are two main deformation mechanisms that contribute to the measured mechanical properties of the aerogels - buckling of CNCs aligned parallel to the loading axis, and elastic bending of CNCs oriented at an angle to the loading axis. We postulate that buckling of CNCs is the predominant damping mechanism in these aerogels. Although at lower frequencies the buckling mechanism dominates, the aerogel behaves as a stiffer material at frequencies higher than about 90 Hz. From our results, it is reasonable to infer that the elastic bending of the CNCs plays an enhanced role during deformation at higher frequencies - resulting in an increase in storage modulus and drop in the loss factor.

Collagen constitutes one third of the human proteome, providing mechanical stability, elasticity and strength to organisms. Normal type I collagen is a heterotrimer and consists of two alpha-1 chains and one alpha-2 chain. A variation of the natural type I collagen molecule is the type I homotrimer, which consists of three alpha-1 chains, and has been found in fetal tissues, fibrotic tissues, carcinomas, and fetal and cancer cells in human. A mouse model of the genetic brittle bone disease, osteogenesis imperfect (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. Atomistic studies have revealed that the homotrimers form local kinks at molecular level. How the local kinks affect the packing of collagen molecules at the microfibril level and the relationship between the molecular content and the decrease in strength is, however, still not clear.
Here we use molecular simulations to study the mechanical differences between the heterotrimer and homotrimer collagen microfibrils. The collagen microfibril models are generated based on the in situ structure of full length collagen type I molecule with the actual amino acid sequence of real mouse collagen sequence (Collagen, type I, alpha 1 and alpha 2 chain precursor [Mus musculus]). In silico mechanical tests and structural analyses of collagen microfibril models are performed to understand the effects of the replacement of alpha-2 chain of type I collagen at the microfibril level and to address the origin of brittle bone disease.
Our results suggest that the oim microfibril is less dense compared to the normal microfibril as a result of local kink formations at molecular level. Both normal and oim microfibril models have nonlinear stress-strain relations, which are consistent with previous experiments. We find that the homotrimer microfibril has an increased lateral spacing between collagen molecules in both dehydrated and hydrated conditions, which is coincided with experiments and suggests that the increased lateral spacing is not solely due to the higher hydrophobicity of the alpha-2 chain. Mechanical tests show a higher modulus of the normal (heterotrimer) compared to the oim (homotrimer) microfibril, which suggests a molecular mechanism that the increase of lateral distance in oim fibril, due to local kink formations, causes a decrease of the modulus. Furthermore, the water distributions in both models are analyzed to reveal the structural differences at molecular level. Our studies provide fundamental insight of the effect of the loss of alpha-2 chain at the molecular level and help understanding the molecular origin of the bone brittle disease at much larger length-scales.

Nvjp-1 protein is key component of Nereis jaw which is composed of roughly 90% protein, halogens (sim;8%) and Zn ions (sim;2%). However, the hardness and stiffness of the jaws are comparable to the human dentin (~1-2 GPa of hardness and 10-20 GPa of stiffness). Nvjp-1 protein contains over 25 mol% histidine amino acid and people believe the formation of metal-coodinate crosslink between histidine-rich Nvjp-1 protein and Zn ions dominates the structural stability and marvelous mechanical properties of Nereis jaw. The metal-coordinate bond is a flexible connection due to its variable bond strength in different chemical microenvironments. The strength of metal-coordinate bond decreases in the low pH condition and the bond strength is larger as pH value is higher. Moreover, unlike most covalent bonds whose rupture is usually irreversible, metal-coordinate bonds are able to reform after rupture.
Here we report a study on building a bottom-up molecular based model of Nvjp-1 protein. A detailed analysis of geometric and mechanical properties of the Nvjp-1 at different pH values by fully atomistic simulation using pH-MD is presented. We analyze the effect of pH and ion on the structure and mechanical properties of Nvjp-1 protein and to investigate the role of metal-coordinate cross-links in Nvjp-1 proteins. Our results suggest that metal-coordinate cross-links play a significant role in achieving the characteristic mechanical properties of this protein material and compare our results with the experimental measurements. We discuss the opportunity of synthesizing peptide materials with mutable properties in the presence of metal-coordinate cross-links.

The construction or the synthesis of nanostructured materials are an expanding field of research with emphasis especially in the mechanisms involved in the formation of these structures as well as the control of shape and size. In this context, fung were used as biotemplating to obtain nanostructured systems of metal nanoparticles with potential applications in development of sensors and biosensors. This work showed the use of fungus micellar to obtain self-assembled systems of gold and silver nanoparticles on the surface of hyphae, forming stable micro tubules structures and understanding the multilayers formation mechanisms. The fungi hyphae Aspergillus aculeatus were grown in an Erlenmeyer flask containing solution of Czapeck medium. After two weeks, the medium was removed for addition of the metal colloidal dispersion. The colloidal nanoparticles were characterized by X-ray diffraction and UV-Vis spectroscopy and, the micro tubules, by scanning electron microscopy. The results showed a facile route for self-organization of colloidal metal nanoparticles on living filamentous fungi aiming to fabricate micro tubular structures with typical diameter of 2-3 mu;m and lengths exceeding a few millimeters with dimensional tube system which is mechanically stable even after drying removal of the medium liquid. Gold nanoparticles easily formed a uniform surface on the cell wall. We observed that the thickness of the micro tubule could be controlled by modifying the initial nanoparticle concentration. The strength of interaction between particles is strong enough to form a solid wall structure that allows sectioning. This allows the various layers of particles that form a wall with a thickness of 300 nm. The first layer of metal nanoparticles was formed because the hydrophobic effect and the formation of strong covalent bonds between the metal and the sulfur residues of proteins in the cell wall. However, interestingly, metal particles still forming subsequent layers because the fungus produced metabolites by consume citrate that stabilizes gold nanoparticles. The metabolites of fungi adhered nanoparticles and this results in destabilization of the double electrical layer of metal colloidal nanoparticles causing adsorption on any surface, as it was shown in an experiment in which silicon plates placed in fungus + colloid solution and its surface was coated with nanoparticle. However, the silicon in presence only of the colloid has not occurred the adsorption. In this sense, the preparation of stable nanostructures as micro tubules opens the way for different technological applications as well as in the development of sensors and biosensors.

Amyloids are an exciting class of protein fibrils that have garnered much interest due to their role in a variety of severe neurological diseases, most notably Alzheimer's disease. More recently, they have also been considered for use as functional biomaterials due to their excellent mechanical properties as well as their high propensity to self-assemble. In general, the great mechanical performance of amyloid fibrils is derived from their beta-sheet rich secondary structure, but it is still not clear how the specific arrangement and organization of the beta sheets affects the mechanical properties at both the individual fibril scale and in collections of fibrils. We use full atomistic molecular dynamics simulations to investigate the force-displacement behavior of a variety of amyloid and amyloid-like protein fibrils, all of which feature a cross-beta motif in their secondary structures. We examine fibrils with parallel and antiparallel stacked beta sheet structures, beta-helical protein fibrils, as well as structures which combine these different motifs. Our results provide key insights into the mechanisms which govern the deformation behavior and mechanical response of amyloid and amyloid-like protein fibrils and can help to guide the design of amyloid-based functional materials.

Tissue engineering is the holy grail of regenerative medicine with lofty goals of tissue and organ replacement. Specifically, many challenges exist for bone tissue engineering, a prominent one being materials design of scaffolds that have optimized mechanical and degradation properties. Natural bone is a nanocomposite of hydroxyapatite (HAP) and type I collagen. Due to its excellent bioactivity, synthetic HAP has been widely investigated for use as a bone substitute. Previous work in our group on development of the ‘Altered Phase Theory&’ of polymer clay nanocomposites has inspired the use of nanoclays in scaffold materials. Addition of montmorillonite clay to polymer scaffolds enhances the mechanical properties of the scaffolds and forms a polymer-clay nanocomposite. We have developed biomineralized HAP using amino acid modified nanoclays. Amino-acid-modified montmorillonite clay with biomineralized hydroxyapatite (in-situ HAP clay) has been shown by our group to enhance osteoconductive properties of polymer scaffolds. In this work we report Electron Energy Loss Spectroscopy (EELS) studies on the biomineralized nanoclay to determine the extent of interaction of HAP with the modified clay in in-situ HAP. The core loss K-edge of oxygen (532 eV) and the L2,3-edge for calcium (346 eV) in the HAP, modified clay and in-situ HAP clay systems were investigated and the energy-loss near-edge structures (ELNES) were compared. Differences in the calcium L2,3-edge and oxygen k-edge values for in-situ HAP clay vs. HAP and modified clay were observed. These differences are compared with the atomic interaction energy calculations conducted our group for the in-situ HAP clay system using molecular dynamics simulations. In addition, we have conducted similar studies on diseased bone (with Osteogenesis imperfecta). Electron energy loss spectroscopy is used to determine the oxygen K-edge, calcium L2,3- edge and ELNES for normal human bone and osteogenesis imperfecta bone. These experimental studies describe new insights into the molecular basis of bone diseases.

Many biomechanical imaging techniques have been developed for soft tissue characterization and tumor detection based on the differentiation of the mechanical properties in normal and pathologic tissues. Among them, acoustic radiation force imaging is an apposite imaging method using focused ultrasound to internally excite the tissue. The elastograms are obtained by measuring the responses of the excited tissue with ultrasound. Generally, the heterogeneity of soft tissue is simplified by a piecewise homogeneous model with kernel size on the order of 1mm. However, realistic breast tumors still have strong heterogeneity below the millimeter scale, and the piecewise homogeneous model cannot capture the true behavior of the underlying tissue. In order to verify the piecewise homogeneity simplification, a finite element model is built to describe the breast tumor heterogeneity in acoustic radiation force imaging. In the simulation, domains of the tumors (1 and 3mm spheres) are considered homogeneous for the piecewise homogeneous case. Within each domain, the average elasticity of the tumor is kept the same, while we study two scenarios: a) gradual changes of the tumor elasticity using a Gaussian function, and b) discrete subzones with randomly assigned tumor elasticity. For the first case, compared to the case of piecewise homogeneity, the responses inside the tumors are additionally affected by a new parameter, the standard deviation of the Gaussian function, which describes the heterogeneity below the millimeter scale. For the second case, the tumors are divided into approximately equal sized subzones with randomly assigned elasticity. For relatively large subzones, the results suggest that averaging the displacement over a larger region in the focal zone leads to more consistent results with that of the piecewise homogeneous model at the expense of reduced spatial resolution. As the size of the subzones decreases, the responses converge to the piecewise homogeneous case.

Self-healing properties have been traditionally added to resins. Typically the procedure implied the mixing of the fluid resin, resin catalyst and/or accelerator, microcapsules filled with monomer and first generation Grubbs catalyst. As the polymerization of the resin is completed a solid material that embeds within the polymeric matrix microcapsules filled with monomer (usually dicyclopentadiene) and first generation Grubbs catalysts is obtained. These "ingredients" are adding self-healing capabilities to the polymeric matrix (hardened resin). These self-healing capabilities are triggered by propagating cracks produced by the stress acting of the polymer. The cracks are generating a differential shear stress on the surface of the microcapsule that finally will rupture the microcapsule releasing the monomer, which starts its diffusion within the polymeric matrix and eventually reaches a first generation Grubbs catalyst. The interaction between the first generation Grubbs catalyst and the monomer (dicyclopentadiene) results in the polymerization of a new polymer (polydicyclopentadiene) within the polymeric matrix via a ring opening mechanism. Later, the process has been extended to polymers, by using the solution path (i.e. dissolving the polymer in an appropriate solvent and mixing it gently with microcapsules filled with monomer and first generation Grubbs catalyst. The process requires to find a solvent that does not dissolve the microcapsule and does not deactivate the catalyst.
We report on the addition of self-healing capabilities to a water soluble polymer (polyvinyl alcohol). To achieve this, the polymer has been dissolved in water. The first generation Grubbs catalyst has been added to the 10 % polymer solution. The mixture has been sonicated for 10 minutes by using a high power sonication. After that microcapsules filled with monomer (dicyclopentadiene) have been added to the mixture and gently homogenized by stirrinh. The solvent (water) was allowed to slowly evaporate for about 1 day in a vacuum oven at about 75 , and dog bone specimen were obtained from the as deposited films.
The presence of the monomer within the microcapsule has been confirmed by both Raman and FTIR measurements. The ring-opening polymerization has been supported by Raman spectroscopy. Finally, the self-healing capability has been supported by mechanical tests
(load-displacement measurements) at a very low displacement rate (0.01 mm/s) by using a TestResources equipment.

Natural active structures, such as cellular cytoskeletons, utilize dynamic networks of mechanochemical molecules and protein filaments for adaptive behavior modulation. Such components can be synthetically altered at a single molecule, and systems level to form new engineered materials. In particular, much potential exists in modifying myosin motor proteins, alpha-actinin binding proteins, and actin filaments to develop smart contractile materials that retain robust performance for varied design requirements and across environmental fluctuations.
Here, we developed an agent-based simulation platform to explain (and predict in novel variations) the emergent performance of complex mechanochemical molecular interactions. The platform consists of a virtual environment in which autonomous software agents represent synthetic molecules that interact with an actin filament traveling at a steady state. Agents were configured with varied macromolecular geometries, attachment rates, and detachment rates that influenced how agents stochastically behaved during each simulation step. System performance was measured with respect to ATPase rate, force, velocity, and stability. The emergent agents&’ behaviors were found to reliably reproduce quantitative relationships describing the hyperbolic force-velocity curve of myosins, sigmoid force-velocity curve of myosin and alpha-actinin, and the linear dependence of velocity on myosin detachment rate and step size.
We then investigated system performance with respect to varied design objectives, such as high energy efficiency, power density, and stability. Optimal systems were identified via engineering optimization methods. When trade-offs among multiple design objectives were considered, we found that rather than configuring new systems for each objective, it was possible to design a single robust system that performed well for all conditions by modulating its behavior through biochemical signaling to adapt to its environment. The discovery of such systems has potential for enabling higher performance and economical active material systems, where many potential applications exist in biomedicine and nanotechnology endeavors.

The field of computational cell mechanics encompasses different scales ranging from individual monomers, cytoskeleton constituents, full cell, up to small networks. Its focus, fueled by the development of interdisciplinary collaborative efforts between engineering, computer science, biology and medicine, until recently relatively isolated, has allowed for important breakthroughs in biomedicine, bioengineering or even neurology. Among all cells, neurons are at the heart of tremendous medical challenges (traumatic brain injuries, Alzheimer's disease, etc.). In nearly all of these challenges, the intrinsic coupling between mechanical, biochemical and electrophysiological mechanisms is of drastic relevance. This presentation aims at highlighting the role of such coupling through a set of relevant modeling efforts aimed at relating mechanical damage and cell functional alteration.

Fluids filling porosity at small length scales play an important role in the creep behavior of microheterogeneous materials. In more detail, nanoconfined fluid-filled interfaces are typically envisioned to act as a lubricant, once electrically charged solid surfaces start to glide along fluid sheets . Thereby, the fluid is typically in a liquid crystal state, referring to an “adsorbed”, “ice-like”, or “glassy” structure of fluid molecules , . Here, we aim at translating liquid crystal physics and the related viscous interface behavior into apparent creep laws at the continuum scale of materials consisting of one non-creeping solid matrix hosting almost flat fluid-filled interfaces . The liquid crystal behavior, is modeled, at the interface scale, by a linear relationship between the rate of relative displacements of neighboring fluid layers, on the one hand, and corresponding interface eigentractions, on the other hand, with an interface viscosity as the proportionality constant. Homogenization schemes for eigenstressed heterogeneous materials are used to upscale this interface behavior to the significantly larger observation scale of a matrix-inclusion composite comprising a homogeneous, isotropic, and linear elastic solid matrix, and embedded interacting, circular, and parallel interfaces. Our model describes, in an analytical fashion, how the macroscopic loading, the elastic properties of the solid, as well as the size, the density, and the viscosity of the interfaces influence the creep behavior of the composite under prescribed macroscopic stresses. The multiscale model suggests that the speed of the interfacial displacement evolution decreases exponentially with increasing time, such that viscosity-related interface eigentractions decrease exponentially down to zero. The envisioned liquid crystal-based interface behavior manifests itself at the significantly larger scale of the composite as creep of macrostrains, exhibiting again exponentially decaying deformation rates, as reported e.g. for biological tissues such as bones .

The human bone is a complex hierarchically organized nanocomposite structure made up mostly of collagen and hydroxyapatite. Genetic alterations are known to trigger specific structural nuances to the triple helical collagen structure and are often manifested as various health conditions of bone. We have also conducted experimental studies on nanomechanical testing of human cadaver bone samples. Here, we report a comprehensive study of development of multiscale models of fibrillar structures in human bone that incorporate orientational and structural details of collagen-hydroxyapatite interactions in bone. We have also described the mechanics of the full length (300 nm) collagen molecule that is structurally accurate to the collagen length in human bone. Simulations on the full length collagen reveal a third tier hierarchy in the structure of collagen that was not observed before. Collagen structure is made up of a helical arrangement (tier two) of three peptide chains that are each helical (tier one). We have discovered a third helicity in the helical arrangement of the triple helical molecule (tier three). Mechanical behavior of the three tiers indicated the resplendent nature of strengthening mechanisms that are worthy of biomimetic design, resulting from years of evolution. Molecular models of mineral-protein structures in bone have been developed. Steered molecular dynamics (SMD) is used to describe the molecular behavior and mechanics of collagen molecule in the proximity of hydroxyapatite in various different orientations. We observe that the mechanical response of collagen is significantly altered by the proximity and orientation of hydroxyapatite. A finite element (FEM) model of a collagen fibril of 50 nm diameter, and a micron length is built that is representative of the staggered arrangement of collagen molecule in human bone. The results from steered molecular dynamics are incorporated into the FEM model. The deformation characteristics and parameters of collagen and the collagen-hydroxyapatite molecular interfaces used in FEM model are directly obtained from steered molecular dynamics. We also present results of our simulation carried out in the large-displacement regime.

A combination of increased fall rates and reduced bone strength in osteoporotic patients rises the risk of hip fractures in the elderly. Osteoporotic fractures are associated with a cost of ~$20 billion per year in the United States and ~$30 billion per year in the European Union. Knowledge of bone mechanical properties and fracture strength is now required to help understand this epidemic. Computed tomography-based finite element analysis (QCT/FEA) has emerged as a very promising method used to asses bone stiffness and strength. However, QCT/FEA clinical implementation requires a unified modeling procedure, consistency in predicting bone mechanical properties, and validation with realistic test data that represent typical hip fractures.
One hundred cadaveric femora with bone densities varying from normal to osteoporotic were first CT-scanned and then fracture-tested to build, refine, and validate QCT/FEA models of femoral fractures. Image reconstructions of fractured femora were created from post-fracture CT scans to classify fracture types. Load cells were used to measure forces leading to fracture and the fracture events were recorded from both sides using high-speed video cameras.
An analysis of experimental data revealed that the final failure of the femoral cortex occurred after the peak force for 96% of the femora. Fracture forces were smaller in osteoporotic femora compared to osteopenic and normal femora and correlated moderately with bone density but weakly with age and femoral geometry. Fracture types and fracture loads were also weakly correlated. Regions of large von Mises strains obtained from digital analysis of high-speed video images correlated very well with locations of damage initiation and fracture pattern for 94% of the femora. An in-situ method developed to determine the density-elastic modulus relationship improved significantly the QCT/FEA prediction of femoral stiffness.
This study provides evidence that QCT/FEA methods hold promise for use in clinical settings. With additional improvements a robust model can be developed with high levels of confidence in its ability to predict patient specific hip fracture from CT scans.

We studied the structure and mechanical properties of DNA i-motif nanowires by means of molecular dynamics computer simulations. We built up to 230 nm-long nanowires, based on a repeated TC5 sequence from crystallographic data, fully relaxed and equilibrated in water. The unusual C#9679;C+ stacked structure, formed by four ssDNA strands arranged in an intercalated tetramer, is here fully characterized both statically and dynamically. By applying stretching, compression and bending deformations with the steered molecular dynamics and umbrella sampling methods, we extract the apparent Young&’s and bending moduli of the nanowire, as well as estimates for the tensile strength and persistence length. According to our results, i-motif nanowires share similarities with structural proteins, as far as their tensile stiffness, but are closer to nucleic acids and flexible proteins, as far as their bending rigidity is concerned. Curiously enough, their tensile strength makes such DNA fragments tough as a mild steel or a nickel alloy. Besides their yet to be clarified biological significance, i-motif nanowires may qualify as interesting candidates for nanotechnology templates, due to such outstanding mechanical properties.

Our objective is to develop a model for nanostructured biomaterials that form through the self-assembly of different amphiphilic lipid species. Individual lipid molecules are represented by a hydrophilic head group and two hydrophobic tails. The lipid species can differ in terms of specific chemical properties of the polar head groups and the hydrocarbon tail groups. The lipid molecules form a stable hybrid vesicle via the self-assembly in the presence of a hydrophilic solvent due to their amphiphilic nature. We investigate the factors that control the self-organization of the nanoscopic components of the hybrid soft biomaterials. We use a Molecular Dynamics-based mesoscopic simulation technique called Dissipative Particle Dynamics which simultaneously resolves the structure and dynamics of the nanoscopic building blocks and the hybrid aggregate. In this presentation, we characterize the morphology and dynamics of the various hybrid nanostructures. The morphological analysis of the hybrid aggregates and their material characterization can be combined to predict the structural and dynamical properties of other hybrid biomaterials.

Hydroxyapatite (HAP) (chemical formula: Ca10(PO4)6(OH)2) is the basic mineral component of mammalian bone and tooth enamel. The mechanical behavior of the bone depends on complex interplay between the hard inorganic apatite phase and the soft collagen matrix. Components in human bones are usually non-stoichiometric where Ca/P molar ratio varies from 1.5 to 1.67 for calcium deficient case, whereas the same is 1.67 for stoichiometric case. Osteoporosis is the most important underlying cause of fractures in the elderly and is a progressive metabolic disease that involves a decrease in the bone mass density with a concomitant decrease in the Ca/P ratio. For the development of effective therapeutic treatments for osteoporosis, it is imperative to understand the exact nature of the changes in mechanical behavior as a function of bone chemistry. While it is recognized that a nominal calcium deficiency in the inorganic phase leads to brittle bones, the local variation in structural and mechanical response of the inorganic constituent with varying calcium content is poorly understood.
Experimental results in the literatures show that, when Ca/P ratio changes from 1.67 to about 1.5, there will be an 80% drop in the elastic modulus and hardness and 75% reduction in toughness, which is correlated with the presence of associated defects. In this study, we have analyzed the theoretical aspects of the effect of calcium deficiency on structural and mechanical behavior of HAP using first-principles calculations, which is an ideal approach to study the precise role played by the point defects. Our calculations are based on density functional theory (DFT) as implemented in the Quantum Espresso package with a generalized gradient approximation (GGA) to exchange correlation energy of electrons, and ultrasoft pseudopotentials to represent interaction between ionic cores and valence electrons.
The calculated values of lattice constants and bulk modulus for the stoichiometric HAP are in good agreement with experimental and theoretical results reported earlier. The vacancy formation energy and five independent elastic constants are calculated for different vacancy structures of Ca-deficient HAP (Ca-dHAP). We report the existence of a metastable state, for one of the Ca-HAP defect structure, under applied pressure, which could be the reason for experimentally reported drastic reduction in the modulus and hardness for the non-stoichiometric case. The structures of stoichiometric and nonstoichiometric HAP&’s are analyzed and compared in detail using bonding characteristics and charge density plots and are correlated to their observed elastic behavior. DFT linear response is used to determine dynamical matrices on a mesh of wave vectors, which are Fourier interpolated to obtain full phonon dispersion for stoichiometric HAP and for different vacancy structures. IR active modes are analyzed, which could be useful in experimental characterization of the Ca-dHAP.

Bisphosphonates are the primary pharmacologic treatment for osteoporosis and typically reduce fracture risk by 30-50% in postmenopausal women. Atypical femoral fractures, characterized by a transverse morphology indicative of a brittle fracture, have recently emerged as a rare but serious side effect of bisphosphonate treatment. Because bisphosphonates reduce bone remodeling, they have the potential to alter bone tissue material properties. Thus, the objectives of this study were (1) to compare the nanomechanical properties of bone tissue from patients with atypical femoral fractures to that of patients with typical osteoporotic fragility fractures, and (2) to correlate the nanomechanical properties with compositional properties. Specimens of proximal femoral cortical bone were obtained from postmenopausal osteoporotic women with atypical fractures (n=2) and compared to control patients with typical osteoporotic fractures (n=2). All specimens were sectioned and polished, and nanomechanical properties were assessed with nanoindentation. The mechanical properties were then compared to compositional properties as assessed with FTIR imaging. When nanomechanical properties were examined within each specimen, less variability in reduced modulus and hardness was observed in atypical fracture specimens (mean COV= .142) than in control specimens (mean COV= .160). When all specimens were pooled, collagen maturity was the strongest predictor of reduced modulus, but did not correlate strongly with hardness. These results are consistent with prior studies showing reduced cortical bone heterogeneity with bisphosphonate treatment in postmenopausal women with fragility fractures.

Microalgae are extremely abundant and important microorganisms, which affect a variety of environmental factors. Microalgae create almost half of the oxygen in the atmosphere and also sequesters greenhouse gases, like carbon dioxide, in order to grow. Microalgae can be exposed to diverse environmental stimulations, which affect their response. Here, we investigate the environmental stimulation mode of mechanics which is directly related to their environment such as fluid flow. We mechanically stimulate single Scenedesmus dimorphus cells and understand how this affects their structural response. To accomplish this, we use atomic force microscopy (AFM) to image S. dimorphus while simultaneously capturing optical images of the cell response. This integrated approach allows us to map the AFM mechanical measurements to specific subcellular locations on the individual cells. We were then able to perform force measurements with the AFM to determine properties such as Young&’s modulus of S. dimorphus. These findings are enabling us to understand mechanical properties of a single Scenedesmus dimorphus cell, which will empower us to map these responses to environmental stimulation and optimize their environmental benefits.

The most generic representation of a virus is embodied by a protein cage encapsulating macromolecules necessary for replication of the entire complex by a host. Small icosahedral plant viruses illustrate well this basic view, but also the fact that "more with less" is the golden rule of the virus realm. Because for a few small icosahedral viruses, the protein cage can be assembled in vitro from separated proteins and could encapsulate foreign (non-genomic) cargo, it was long believed that the interaction between viral RNA and the virus coat is non-specific. This was supported by the high density of complementary charges on the interior of the capsid, and the smeared RNA distribution in X-ray and electron microscopy structural reconstructions. Nevertheless, evidence is provided here that the capsid of the Brome mosaic virus (BMV) is likely to be much more than a passive container of genomic cargo. More specifically, we present AFM indentation experiments on individual particles carrying the tripartite BMV genome with results which cannot be easily reconciled with the non-specific electrostatic interaction idea.

Wood possesses hierarchy of structure ranging from individual wood polymers to cells to growth rings. The development of new forest products is hindered by the lack of fundamental understanding of how molecular-scale modifications affect properties of bulk wood and wood composites. For instance, it is not understood how adhesive infiltration into wood cell walls relates to the performance of wood-adhesive bondlines. We are developing nanoindentation into a local probe for mechanical spectroscopy that can be used to gain insights into how molecular-scale modifications affect wood cell wall properties. Mechanical spectroscopy is the assessment of a mechanical index, such as the viscoelastic Young&’s modulus or the plastic flow stress, across a broad spectrum of time scale, deformation rate, temperature, or moisture content. In addition to providing thorough mechanical characterization, which is useful to predict material performance over a wide range of conditions, mechanical spectroscopy also provides information about the microphysical processes which are causally linked to the properties. We have invented broadband nanoindentation creep (BNC) to measure viscoplastic properties across 4-6 decades of strain rate and broadband nanoindentation viscoelasticity (BNV) to measure viscoelastic properties across greater than 4 decades of time scale. Variable-temperature BNC and BNV can also performed at temperatures between 20 and 200°C and the relative humidity during the test can be controlled between dry air and 80%. In validation studies on polymers, the data generated from nanoindentation agree with conventional viscoplastic and viscoelastic measurements and revealed the same information about the microphysical processes causally linked to the properties. Nanoindentation-based mechanical spectroscopy is performed on both unmodified wood cell walls and cell walls infiltrated with a Br-labeled phenol formaldehyde adhesive. The amount of adhesive infiltration into individual wood cell walls was quantified using synchrotron-based x-ray fluorescence microscopy. The cell wall mechanical properties increased in direct proportion to the amount of adhesive infiltration. Also, the adhesive infiltration decreased the moisture-induced softening of wood cell walls. The results indicate that adhesive infiltration into wood cell walls is an important factor in the moisture-durability of wood-adhesive bondlines.

The properties of materials are dictated not only by their atomic arrangement and molecular connectivity, but also by their nanoscale, or macromolecular, geometry. Transmission electron tomography is an advanced microscopy technique that captures the 3D structure of a specimen with nanometer resolution, and can be used to investigate the macromolecular structure of biological materials at a scale that is inaccessible to techniques such as NMR, X-Ray diffraction, and vibrational spectroscopies. We introduce a new methodology to discern and model the 3D nanoscale architecture of cellulose microfibrils within thermochemically pretreated biomass by fitting parametric space curves to tomographic datasets. This method has enabled the first quantitative measurement of the nanoscale curvature of microfibrils. Furthermore, we employ the fitted space curves to construct the first atomistic models of cellulose that reflect the macromolecular geometry of microfibrils within plant cell walls, and computational evaluation of these models has revealed relationships between the nanoscale curvature of microfibrils and their atomic configuration. Specifically, we predict the formation of kink defects in the crystal structure of cellulose I-Beta when the curvature exceeds critical threshold values. The detailed atomistic models of cellulose presented here display unprecedented semblance to real, process-relevant materials, and will benefit the biofuels and biomaterials industries by facilitating accurate, morphology-based models of the materials properties of plant of cellulose.

The ability for early detection of trace biomolecules is important for medical diagnostics and treatment of diseases. This ability requires an effective coupling of advanced materials with highly-sensitive and selective detection techniques. In this presentation, metal nanoparticles and assemblies with a series of controlled particle sizes and surface modifications in terms of surface capping structures have been investigated as novel nanoprobes coupling to surface enhanced Raman scattering (SERS) spectroscopy for monitoring biomolecular recognition of proteins in solution. One important emphasis is SERS detection in solutions, in contrast to conventional use of solid substrates for the enhancement in signal, which requires extensive sample or substrate preparation. Results of real-time SERS detection of protein modified nanoparticles in aqueous solutions will be described. New insights into the control of small aggregates of gold or core-shell magnetic nanoparticles as dimers or trimers in the solution will also be discussed in terms of SERS “hot spots”.

Scanning acoustic microscopy (SAM), when applied to biological samples has the potential to resolve the longitudinal acoustic wave speed and hence stiffness of discrete tissue components. The heterogeneity of biological materials combined with the action of cryosectioning and rehydrating can, however, create variations in section topography and hence signal attenuation. Here we set out to determine how variations in specimen thickness influence apparent acoustic wave speed measurements
Cryosections (5mu;m nominal thickness) of sheep aorta and human skin biopsies were adhered to glass slides before washing and rehydrating in water. Multiple regions (200x200 µm; n = 3) were imaged by SAM to generate an acoustic wave speed map. Subsequently a co-localised 30x30µm sub-region was imaged by atomic force microscopy (AFM) in fluid. The images were then registered using Image J. Each pixel was allocated both a height and wave speed value before their relationship was then plotted on a scattergram. The mean section thickness measured by AFM was 3.48 ± 1.12 (SD) microns in skin and 5.39 ± 1.97 (SD) microns in aorta. In both skin and aorta sections, regional height variations influenced apparent wave speed measurements; for skin a 3.5 micron height difference was associated with a 400m/s increase in wave speed. In aorta however, a 5.5 micron height difference was associated with a 400m/s increase in wave speed and the effect was only evident above a section thickness of 3.0 microns.
Local variations in specimen thickness have been shown to influence apparent wave speed. A dependency of apparent wave speed measurement and tissue type was also observed.

Dental enamel exhibits extraordinary durability being able to survive in the sometimes harsh chemical and mechanical conditions encountered in the oral cavity for over 70 years. Yet unlike other tissues in the human body it has no ability to regenerate itself, rather it&’s longevity stems from its inherently strong structure and chemical changes that occur through interchange of ions with saliva. The mechanical properties are very high for a biological material with a Young&’s modulus of 100 GPa or more and a hardness that can be in the 5-6 GPa range. Using a variety of characterization tools including nanoindentation and micro-Raman spectroscopy we have identified the relationship between enamel&’s composition and its mechanics. In particularly we have found that disease (caries) directly impacts the enamel&’s composition and this affects both hardness and Young&’s modulus. Using finite element methods (FEM) we have been able to gain an insight into the relationship between the enamel&’s mechanical properties and its structure. The hierarchical nature of its structure from nanoscale hexagonal prisms to microscale prismatic clusters and millimeter scale teeth features results in enamel having very anisotropic properties. The effects of caries can further augment this anisotropy. This is found experimentally and also shown in the FEM studies. Many oral hygiene products, including toothpastes and mouthwashes are designed to reduce the occurrence of caries by helping to maintain or in some instances restore the enamel&’s structure and chemical composition. The mechanisms involved in restoring damaged enamel&’s properties are more easily identified when the FEM is combined with experimental studies. The results suggest that in some instances reversal of the caries process is feasible, while in other circumstances it is much harder to restore the enamel to its original condition.

Increased stiffness represents a hallmark of breast cancer that is mediated by physicochemical alterations of the extracellular matrix (ECM). However, the cellular and molecular mechanisms underlying tumor-mediated ECM stiffening are poorly understood. We evaluated the effect of tumor soluble factors on the structural and mechanical properties of fibronectin (Fn) matrices deposited by 3T3-L1 pre-adipocytes at both the molecular (protein) and microscopic (cellular) scales using a combination of FRET (Fluorescence Resonance Energy Transfer)-based imaging and the Surface Forces Apparatus (SFA) technique.
Fn matrix assembly, morphology and conformation were quantified in 2-D culture systems, using traces amounts of FRET-labeled Fn exogenously added to the cultures. In parallel, the mechanical response of the 2-D matrices was recorded (in PBS at 37C) under compression using the SFA. Briefly, two sheets of mica were mounted onto half-cylindrical glass lenses and were assembled in a crossed geometry: load-indentation profiles of the mounted matrices were recorded in the quasi-static regime to extract their compressive elastic modulus E.
Our FRET data indicate that cells exposed to tumor soluble factors deposit a thicker Fn matrix than cells exposed to control media, and that this matrix is mainly composed of highly stretched and unfolded fibrils. These data also suggest higher rigidity of ECM at the single fiber level. In addition, our SFA data indicate that, at the microscopic (tissue) level, tumor-preconditioned cells deposit an overall stiffer ECM network than control cells. The combination of FRET and SFA allows for both molecular and microscopic characterization of ECM and helps gaining insight into the ECM&’s role in mechanosignaling. Collectively, these results indicate that paracrine signaling by tumor cells play a critical role in the stiffening of the tumor-associated stroma.

Researchers have been taking different approaches to address challenges in the fabrication of nanodevices such as reproducibility, reliability, and regular distribution of features. However, there are still fundamental limitations in the current fabrication methods. In an effort to resolve those challenges, investigators have been studying microorganisms in nature which have successfully created 3D biostructures such as diatoms and hexactinellid sponges. Diatoms are microalgae that feature intricate porous cell walls (frustules) made of amorphous silica (SiO₂). Recent advances in experimental and modeling techniques have expanded the field of diatom research, thus promoting the study of biostructures for applications in nanotechnology, medicine and the environment. In particular, the study of diatom biostructures can help toward better understanding the nature and properties of nanopatterned and hierarchical structures. Diatom structures have specie-specific regular shapes and well-distributed pore arrangements (ranging from nm-to-mu;m length scales), which suggest them as promising candidates as drug delivery carriers, optical devices, catalytic components, and gas sensors. In this work, we have investigated the morphology and hierarchical porous structure of selected diatom species using Scanning Electron Microscopy (SEM) imaging, and measured the size-dependent mechanical properties (e.g. Young&’s modulus and hardness) of the diatom frustules via Atomic Force Microscopy (AFM) and ambient nanoindentations. We have selected two diatom species based on well-established biological classification (Coscinodiscus sp. and Navicula sp.). These diatom structures have pore diameters ranging from 0.3 to 6 mu;m and frustule lengths from 20 to 100 mu;m. In addition, we have calculated the mechanical properties of SEM-based diatom models under compression and nanoindention conditions via Finite Element Method (FEM) simulations. FEM simulations were performed on the 3D diatom models to correlate their mechanical behavior with specific geometric variables (e.g. number of pores, pore diameter, and pore arrangement). We also calculated stress and displacement distributions to analyze the effect of loads on the diatom biostructures. Overall results from this study were found to be in good agreement with recent AFM experiments and relevant simulations reported by independent studies. The role of the hierarchical nature of the diatom structure on its overall structural strength and flexibility is analyzed. This research contributes to improving understanding of the mechanical response of these biomaterials, and it represents a step toward their future applications in drug delivery biosystems, self-repair devices, and templates for nanotechnology applications.

Electromechanical coupling, a phenomenon present in collagenous materials, may influence cell-extracellular matrix interactions. It is also postulated that collagen piezoelectricity may play a role in bone remodelling, though experimental evidence of this remains sparse. Confirmation of piezoelectricity in collagen in physiological conditions has also not been conclusively demonstrated with the literature containing conflicting results. In order to investigate fully collagens biofunctional role, it is important both to understand how piezoelectricity manifests itself in collagen and how it is affected by structure, pH and humidity. Here, electromechanical coupling has been investigated via piezoresponse force microscopy (PFM) in transparent and opaque collagen membranes consisting of helical-like arrays of aligned type I collagen fibrils self-assembled from acidic collagen solution. Using atomic force microscopy AFM, the transparent membranes were determined to contain fibrils having an average diameter of 76 ± 14 nm, while the opaque membranes comprise fibrils with an average diameter of 391 ± 99 nm. As the acidity must be neutralized before the membranes can serve as cell culture substrates, the structure and piezoelectric properties of the collagen were measured under ambient conditions before and after the neutralization process. A crimp structure (1.59 ± 0.37 µm in width) perpendicular to the fibril alignment became apparent in the transparent membranes when the pH was adjusted from acidic (pH = 2.5) to neutral (pH = 7) conditions. In addition, a 1.35 fold increase was observed in the amplitude of the shear piezoelectricity of the transparent membranes. The structure and piezoelectric properties of the opaque membranes were not significantly affected by the neutralization process. The results highlight the presence of an additional translational order in the transparent membrane in the direction perpendicular to the fibril alignment. The piezoelectric response of both membrane types was found to be an order of magnitude lower than that of collagen fibrils in rat tail tendon. This reduced response is attributed to less-ordered molecular assembly, as evidenced by the absence of D-periodicity of collagen fibrils in the membranes. Piezoelectricity in the transparent membranes was also investigated in a humidity controlled environment using band excitation PFM in order to resolve the question of collagens piezoelectricity in a wet state.

Teeth are biological composite materials with excellent mechanical properties and high fracture toughness. These qualities are the result of a complex hierarchical structure of their main constituent, i.e. crystallites of calcium-deficient carbonated hydroxyapatite in combination with small amounts of an organic matrix. Generally, teeth consist of two differently organized materials. The interior of a tooth is composed of soft and lightly mineralized bone-like dentin, whereas the outer part of a tooth is covered by very hard and mineral-rich enamel.[1] Unlike human teeth, crocodile teeth are continuously replaced during the animal&’s lifetime.
Here we present a comprehensive experimental study on teeth of the saltwater crocodile, Crocodylus porosus by using different high-end analytical techniques. Synchrotron X-ray microtomography (SRµCT) was used to investigate the internal structure of the tooth and to estimate the degree of mineralization of dentin, enamel, and cementum. Virtual CT-cuts through the tooth show that the enamel layer is rather thin (100-200 µm) compared to mammalian teeth (up to 5 mm). High-resolution scanning electron micrographs (SEM) show that the crystallites in crocodile enamel are oriented perpendicularly to the tooth surface. At the dentin-enamel-junction, the packing density of crystallites decreases and the crystallites do not display an ordered structure as in the enamel. X-ray powder diffraction in combination with elemental analysis and infrared spectroscopy shows that the biomineral phase of crocodile teeth is a carbonated calcium-deficient nanocrystalline hydroxyapatite in all three tooth-constituting tissues: dentin, enamel, and cementum. The fluoride content is very low and comparable to human teeth (<0.1 wt %). The overall mineral content of dentin, enamel, and cementum obtained by thermogravimetry is 71.3, 80.5 and 66.8 wt%, respectively. The hardness of dentin and enamel was determined by Vickers microhardness testing on embedded and polished tooth samples and amounted to 0.88±0.05 GPa and 3.23±0.14 GPa, respectively. This can be explained with the different degree of mineralization of the two tissue types and corresponds to human teeth (dentin: 0.5-0.7 GPa and enamel: 3-4 GPa).[2]
[1] S. V. Dorozhkin, M. Epple, "Biological and medical significance of calcium phosphates", Angew. Chem. Int. Ed. 41 (2002) 3130-3146.
[2] J. Enax, O. Prymak, D. Raabe, M. Epple, "Structure, composition, and mechanical properties of shark teeth", J. Struct. Biol. 178 (2012) 290-299.

In cancer therapy, antibodies plays a role as interface molecule between cancer cells and lymphocyte cells, so that activated lymphocyte cells damage cancer cells. Recent recombinant engineering enables to design smart bispecific antibodies from antibody fragments with high performance on cancer therapy, but the recombinant design has structural limitation: for instance, two fragment can be conjugated only by fusing the C-terminus of a fragment at the N-terminus of the other fragment. Here, we propose a new construction of smart multivalent/bispecific antibodies by conjugation of chemical conjugation technique and protein engineering.
IgG-type antibody is comprised of heavy chain and light chain, and the fragment of variable region (Fv), where two variable regions of heavy and light chains (VH and VL) are associated, has an antigen binding function. Here, single chain Fv (scFv), where VH was fused at the N-terminus of VL via a linker, was used as a unit of bispecific antibodies. All the lysine residues in scFv were first mutated to alternative amino acid residues by utilizing bioinformatics and molecular evolution technique to prepare the scFv with only one lysine residue at a specific position by bacterial expression. After the design of the lysine-mutated scFv fragment, the two scFv fragments are chemically conjugated via a dicarboxylic organic linker that make amide binding with the lysine in scFv; consequently, homogeneous bivalent/bispecific antibody can be generated. Here, we show the construction of smart bispecific antibody with affinity for epidermal growth factor receptor on tumor and for CD3 receptor on lymphocyte cell.

Tendon has a complex hierarchical structure composed of both a collagenous and a non-collagenous matrix. Despite several studies that have aimed to elucidate the mechanism of load transfer between matrix components, the roles of glycosaminoglycans (GAGs) remain controversial. Thus, this study investigated the elastic properties of tendon using a modified shear-lag model that accounts for the structure and non-linear mechanical response of the GAGs. Unlike prior shear-lag models that are solved either in two dimensions or in axially symmetric geometries, we present a closed-form analytical model for three-dimensional periodic lattices of fibrils linked by GAGs. Using this approach, we show that the non-linear mechanical response of the GAGs leads to a distinct toe region in the stress-strain response of the tendon. The critical strain of the toe region is shown to decrease inversely with fibril length. Furthermore, we identify a characteristic length scale, related to microstructural parameters (e.g. GAG spacing, stiffness, and geometry) over which load is transferred from the GAGs to the fibrils. We show that when the fibril lengths are significantly larger than this length scale, the mechanical properties of the tendon are relatively insensitive to deletion of GAGs. Our results provide a physical explanation for the insensitivity for the mechanical response of tendon to the deletion of GAGs in mature tendons, underscore the importance of fibril length in determining the elastic properties of the tendon, and are in excellent agreement with computationally intensive simulations.

Tooth enamel is a composite biomaterial consisting of a minor organic phase (~2%) and a hierarchically organized inorganic phase (~96-98%). Tooth enamel is a very brittle material; however it has the ability to sustain cracks without suffering catastrophic failure throughout a lifetime of mechanical function. In previous studies, we showed that the size of apatite nanocrystals in tooth enamel can affect its hardness, fracture resistance and optical properties. This important discovery raised a new question, which factors are regulating the size of enamel nanocrystals? It is well known that the crystallographic properties of synthetic apatites are influenced by the incorporation of trace elements; conversely, the role of trace elements on crystallographic properties of tooth enamel apatite is yet to be investigated. Therefore, this study was designed to investigate how trace elements can influence enamel&’s crystallographic properties and ultimately its microhardness and optical properties.
A group of 26 extracted sound teeth were collected and prepared for mechanical and physical-chemical analysis. Average crack length was calculated by measuring the propagation of cracks created by Vicker&’s indentation fracture (VIF). The enamel cell lattice parameter and crystal dimensions were calculated from X-ray Diffraction (XRD) analysis and the chemical composition was analyzed with Fourier transform infrared spectroscopy (FTIR).
The crack propagation was correlated to the enamel crystallographic dimensions and chemical composition. The statistical significance was set at P<0.05. There was variability in crack propagation among enamel samples and this variability was related to the enamel crystallographic dimensions. The average length of cracks in enamel was positively correlated with its crystal dimensions along c-axis following the Hall-Petch model for polycrystalline materials. Also, it was found that the protein content of tooth enamel had no significant effect on crack propagation.
It was shown how the dimensions of apatite nanocrystals and protein content in enamel can affect its resistance to crack propagation. It is concluded that the aspect ratio of apatite nanocrystals and the protein content in enamel determine its resistance to crack propagation. According to this finding, we proposed a new model based on Hall-Petch theory that accurately predicts crack propagation in enamel. Our new biomechanical model of enamel explains among other things why older teeth crack more and why inner layers of enamel are tougher.

Recent studies show that stem cells are multipotent and are capable of differentiating into various cell types dependent on the microenvironment provided, including chemical and mechanical factors. To study how stem cells interact with different surface mechanics without changing chemical factors, we propose using the principal, entangled elastomer surface confinement, to change the substrate mechanical response in a continuous manner without additional chemical or surface modifications. It has been shown that the viscosity of a polymer melt is a function of the film thickness. In the case of strong adsorption of the polymer chains to the surface, pinning of the first layer occurs, which in turn traps further layers, increasing the effective viscosity for distances larger than 10 radii of gyration (Rg) of the polymer. In this manner the mechanical response is varied only through physical confinement of the polymer chains without any chemical modification. In this study, we introduce the use of polybutadiene (PB), which is a biocompatible, synthesized in the monodisperse form with different molecular weights, such that Rg varies from 70 to 200Å. Since PB is a rubber under ambient condition (Tg= -95C), the effect of surface interactions on PB films of varying thickness can be probed without the high temperature conditions. Hence PB provides the ideal substrate for probing only the effects of variable substrate mechanics, without any chemical or topographical variations, on biological systems where the operational temperature range is restricted to 37C. Here we describe the use of specially processed PB films whose surface properties and thickness can be characterized with nanometer precision, to produce substrates with controlled mechanical response, in order to further investigate whether different surface mechanics can determine stem cell differentiation.

Cartilage tissue mechanics change considerably upon the onset of diseases such as osteoarthritis. In this multifactorial disease state, the tissue experiences a loss of glycosaminoglycans, the negatively charged polysaccharides that immobilize water molecules and thereby provide cartilage with significant load-bearing capacity. As glycosaminoglycans deplete, the tissue&’s hydraulic permeability increases and the compressive modulus decreases; as a result, an increased fraction of the articulating load is borne by the solid tissue components rather than the fluid component, and cartilage wear is exacerbated.
Currently, there is no osteoarthritis therapy that effectively mitigates glycosaminoglycan loss. To meet this biomedical need, we report a novel tissue supplement and technique of administration to mechanically reinforce cartilage by mimicking the hydrophilic properties of glycosaminoglycan. By in situ formation of an interpenetrating synthetic hydrogel network in the presence of articular cartilage, we demonstrate that a semi-natural and semi-synthetic double network (or interpenetrating network) may be formed and that compressive properties are augmented to restore degraded tissue to a healthy mechanical state. Specifically, a solution containing 2-methacryloyloxyethyl phosphorylcholine, ethylene glycol dimethacrylate as a crosslinking agent, eosin y and triethanolamine as photoinitiators, and N-vinyl pyrrolidone as an accelerant were incubated with 7-mm diameter ex vivo bovine osteochondral explants for twenty-four hours. The samples were then irradiated with visible light from an argon ion laser to cause polymerization of the mono- and dimethacrylate monomers, thus forming a hydrophilic hydrogel entangled with the native solid components of cartilage. When doped with a small amount of fluorescent monomer, the treatment was shown by fluorescence microscopy after histology to afford an even distribution of the hydrogel throughout the entire depth of the cartilage. Equilibrium compressive modulus, instantaneous compressive modulus, peak stress compressive modulus, and tissue relaxation times show a dose-dependent response to the treatment. Additionally, treatment of selectively glycosaminoglycan-depleted explants was able to partially recover the typically linear stress-strain behavior of cartilage under compression, whereas untreated glycosaminoglycan-depleted samples demonstrated nonlinear stress-strain behavior with lower moduli at all levels of strain. Further studies are ongoing to ascertain the tissue&’s interstitial fluid pressure directly to evaluate the hypothesis that the hydrogel serves a function mimetic of that served by glycosaminoglycan. Furthermore, the treatment&’s effect on cartilage tribology is expected to be significant, and studies to assess these properties are underway.

Melanin is a class of biopolymer-derived pigment that is composed of heterogeneous bicyclic aromatics. Melanins feature many exciting properties that are favorable to the development and enhancement of multifunctional, biocompatible electronic devices including redox active catechol groups and hybrid electronic/ionic conduction. These properties would be ideal for use in a variety of biomedical technologies such as neural interfaces and energy storage devices. Melanin films exhibit brittle mechanical properties, owing to strong pi-pi interactions and stacking of protomolecules. This property renders melanins challenging for use as materials for tissue-devices since biological structures are often soft (E < 1 MPa) and curvilinear. Here were present a class of mechanically flexible melanin thin films to address the simultaneous challenges of maintaining structural integrity and electrical functionality. Briefly, thin films are prepared by electrodeposition of 5,6-dimethoxyindole-2-carboxylic acid (DMICA) on target anodes. The electro-mechanical properties are measured by a combination of techniques including electrical impedance spectroscopy and cyclic voltammetry. The physical properties of electrodeposited synthetic and naturally-derived melanin films are compared across the composition of target substrate. The findings in this work will be pivotal for a variety of applications within flexible electronics and biomaterials.

Cellulose nanofibers have attracted much interest for the bio-reinforcement materials in polymer composites due to the high mechanical properties and biocompatible characteristics of the cellulose. It is, however, generally known to be difficult to extract cellulose nanofibers from the innate cellulose beads, as the pulverization of the cellulose had plenty of complicated processing steps, which also required harmful substances. In this study, a cellulose-nanofiber/polyvinyl-alcohol (PVA) composite was first synthesized by a newly developed water-jet nano-fabrication process called the Star Burst System (SB). The mechanical and the structural analyses of the synthesized composite were carried out. Cellulose nanofibers were extracted by the simple and harmless SB method, which produced aqueous cellulose-nanofiber solution just by running original cellulose beads under a high pressure of water in the synthetic chamber. By optimizing the process conditions, the cellulose nanofibers with high aspect ratios and small diameter of ~20 nm were obtained, which was confirmed by transmission electron microscopy (TEM). From the structural analysis of the composite by the scanning electron microscopy (SEM), it was found that the cellulose nanofibers were homogeneously dispersed in the PVA matrix. Considering the high molecular compatibility of the cellulose and PVA due to the hydrogen bonding, a good adhesive interface could be expected for the cellulose nanofibers and the PVA matrix. The effects of the number of the SB process cycles (defined as the unit “pass”) on the mechanical properties of the composites were experimentally and theoretically analysed. The Young&’s modulus rapidly increased from 3.3 GPa to 4.4 GPa up to 40 pass, whereas the modulus remained virtually constant above 40 pass. Due to the uniform dispersibility of the cellulose nanofibers, the Young&’s modulus of the 80-pass composite at the concentration of 7.7 wt% increased up to 4.7 GPa. The results corresponded well with the theory of the composite with dispersed short fibers, which clearly indicated that the potential of the cellulose nanofibers as reinforcement materials was sufficiently confirmed.

Bacterial cellulose (BC) is a promising, sustainable and biodegradable nanofibrous material that has the same chemical structure as the plant-based cellulose. However, BC fibers have diameters in the range of a few (40-70) nanometers and display many unique characteristics including high purity, high degree of polymerization, high crystallinity that results in their high strength and modulus. Because of the small fiber diameter which results in high surface area and highly porous structure, BC membranes have high water-holding capacity. BC also has high thermal stability, excellent environmental biodegradability and strong biocompatibility. As a result, BC is a promising material for many engineering applications, also in the form of BC-nanocomposites, which offer an increased mechanical performance and additional functionalities. In this contribution, we add the third dimension to BC materials, which typically have a 2D-sheet or membrane-like architecture. We report, how directionally solidifying (freeze casting) of water-based BC suspensions produces highly porous aerogel-like scaffolds with a honeycomb-like pore structure that results in remarkable, orthotropic mechanical properties. Additionally, we illustrate the excellent control over the scaffolds hierarchical structure, such as the pore morphology of the scaffolds at the micrometer scale and the nano topographic structure of the scaffold walls, both of which are of great importance in engineering fields as diverse as biomedical engineering and energy generation and storage.

Mechanics of eukaryotic cells at the nanoscale is a challenging problem due to complexity of cells. Besides general interest, such measurements are important because it has been demonstrated that the elastic modulus of cells can correlate with various diseases and even aging.
Here we describe with challenges and solutions to measure elastic modulus of cells measured by means of atomic force microscopy (AFM). Specifically, we will focus on the following problems:
1. Cellular surface brush. The majority of cells are surrounded by cellular “brush”. In the case of eukaryotic cells, the cell is typically covered with microvilli, glycocalyx proteins and polysaccharides. This brush can be of several microns in length. Using sharp probes does not eliminate the effect of brush. A simple model to separate the brush and cell deformation will be overviewed.
2. Problem of nonlinearity. After separating the contribution of the cell brush, the rigidity of the cell body can be well approximated by a homogeneous elastic medium if and only if the stress-strain relation is linear. The problem of linearity is extremely complicated and has not been solved as yet. However, there is an indirect way to test the linearity: the elastic (Young&’s) modulus should be reasonably independent of the indentation depth. We will show that it can be achieved when using relatively dull AFM probes (the radii of microns) only.
3. Heterogeneity of cell surface. Cells are not homogeneous over the surface. We will discuss the question how many points is enough to characterize the cell, and will show that it depends on the radius of curvature of the used AFM probe.
We will demonstrate that without proper consideration of the above problems, the error in defining the modulus of rigidity can easily reach an order of magnitude.

Understanding the superior mechanical responses of hierarchical materials (e.g., bone, nacre) requires a reliable extraction of local properties exhibited by these materials at the microscale and their correlation with local structure measurements. Our recently developed protocols for spherical nanoindentation offer tremendous promise in addressing this critical need. The indentation stress-strain curves obtained using this technique, when combined with complementary local structure measurements at the indentation site (e.g., obtained using Raman Spectroscopy) provide new avenues of investigation into the sub-micron level structure-property linkages in these materials. In this presentation, we demonstrate the power of this new approach through selected case studies.
In the first case study, ‘dry&’ (dehydrated-embedded) and ‘wet&’ (hydrated) femurs from two inbred mouse strains, known to differ in their whole-bone mechanical properties (A/J, and C57BL/6J [B6]) were studied. Raman spectroscopy was used to assess compositional details across the antero-medial (AM) cortex of each sample and spherical nanoindentation tests are carried out across this same cortex to obtain information about the local mechanical behavior. Indentation stress-strain (ISS) curves extracted from spherical nanoindentation, were used in combination with the mineral to matrix ratios to study property-composition relationships across the AM cortex of the two strains. Our results demonstrate inter-strain differences in nanomechanical properties consistent with those seen in macro-mechanical testing. Specifically, our ISS curves show that bone with the higher mineral-to-matrix ratio exhibits a trend towards higher elastic modulus and indentation yield strength. Consequently the less mineralized B6 mice bones show lower values of both elastic modulus and indentation yield strengths as compared to the A/J mice. Also, in both strains, newer bone exhibits lower modulus and indentation yield strength values compared to older bone regions.
In the second case study, dry attached oyster pairs were collected from the Baruch Maine Field Laboratory, South Carolina, USA. Cross sections were cut and embedded in epoxy to expose the interface between the oysters. Raman spectroscopy is used to obtain the composition of the oyster and spherical nanoindentation is performed across the width of the adhesive interface to obtain local mechanical behavior. ISS curves combined with composition maps of this bio-adhesive provide new understanding of the local structure-property relationships in these materials.
The novel approaches described in this paper represent a significant advancement in the current protocols available for extracting local structure-property relationships in hierarchical material systems.

The outer layer of shark teeth (enameloid) is a highly mineralized tissue consisting of fluoroapatite Ca5(PO4)3F[1] with an intricate hierarchical organization. Consequently, it has excellent mechanical properties and an exceptional fracture toughness, which makes it a very interesting model system for the development of new bioinspired dental materials.
We analyzed the enameloid of teeth from the shortfin mako shark, Isurus oxyrinchus, in high structural detail with high-resolution scanning electron microscopy (SEM), and elucidated the local mechanical properties using nanoindentation. The results showed that shark enameloid consists of thin fluoroapatite crystallites with diameters between 50-80 nm and a length greater than 1 µm. Clusters of parallel crystallites form bundles with different dimensions that are arranged in intriguing patterns each having an envelope consisting of organic matrix. Based on their orientation with respect to the geometry of the tooth, we distinguished different types of bundles: the majority are axial bundles, which are oriented parallel to the long axis of the tooth. They are pervaded by ribbon-shaped radial bundles initiating at the dentin-enameloid junction and proceeding perpendicular to the long axis of the tooth. The axial bundles are bound over the full length of the tooth by one layer of parallel arranged circumferential bundles with circular cross sections. The enameloid microstructure of I. oxyrinchus differs significantly from the enamel of human teeth, although their hardness is comparable.[1] Mapping of the local mechanical properties of differently oriented crystallite bundles by nanoindentation showed that both stiffness (Ered) and hardness (H) are similar, independent from the loading direction (parallel or perpendicular to the long axis of the bundled crystallites). This is in good agreement with the low elastic anisotropy measured in fluoroapatite single crystals. Additionally, two major modes of crack propagation were observed to be predominant in the enameloid: cleavage perpendicular to the long axis of individual crystallites and delamination of neighboring crystallites. In order to understand which loading direction relative to the mineral crystal axis favors which fracture mechanism, we plan to perform a series of pillar compression experiments with the goal to establish design criteria for potential synthetic tooth restoration materials inspired by the natural design principles uncovered in shark enameloid.
[1] J. Enax, O. Prymak, D. Raabe, M. Epple, "Structure, composition, and mechanical properties of shark teeth", J. Struct. Biol. 178 (2012) 290-299.

Biological composites have evolved fascinating strategies to exhibit superior structural properties. These hierarchical biomaterials utilize natural design principles to optimize static properties and fulfill their multi-functional requirements. Among this, biomineralized hard tissues are designed to achieve remarkable mechanical performance. The spearer mantis shrimp is a predatory animal known for mortal strikes using its predatory biotool. This highly mineralized and damage-tolerant biological tool is used to destroy prey&’s armour by exerting repetitive high-energy punches. It is reported that the claw is capable of exerting hundreds Newtons of forces, which is enough to destroy a crustacean&’s hard shell. Some of these hard shells are mimicked as damage-tolerant models.
Optical and electron microscopic studies revealed three distinct layers within the dactyl club. While the inner layers consist of successive fibrous layers, the outer layer is highly crystalline with crystal size decreasing 10-fold towards the free surface. Polarized microscopy showed birefringence properties of the most outer layer that can be correlated to preferred crystallite orientation. Raman spectroscopy studies of cross sectional samples highlighted a dominant amorphous calcium phases within the inner layers with increased gradient-like crystallinity of fluorapatite of the outer layer.
Nanoindentation studies on cross sectional samples showed a step-wise decrease in elastic modulus and hardness from the outer layer towards the inner most layers. Furthermore, 3D nanoindentation revealed statistically significant differences between elastic modulus in X, Y and Z planes that is correlated to preferred crystalline orientation toward the impact surface. Confocal Raman imaging and energy dispersive X-ray spectroscopy (EDS) mapping visualized the elemental and mineral phase distribution in the impact region correlating the site specific crystallo-chemistry and the relationships with the local mechanical properties.

All living organisms need sense organs to react to and interpret stimuli received from their surroundings. One such sense organ is a lyriform organ (HS-10) on the legs of the Central American wandering spider, Cupiennius salei. This organ comprises 21 strain-sensitive slits which are embedded in the stiff cuticular exoskeleton. It is used primarily to detect courtship and prey vibrations of the plant on which the spider sits. The individual slits respond to compression perpendicular to their long axis. Remarkably, the viscoelastic properties of a cuticular pad directly adjacent to the sensory organ are the main reason for the organ's pronounced high-pass characteristics. Therefore, we investigated the mechanical properties of the pad material in some detail in order to better understand its impact on the vibration sensor's function. Atomic force microscopy (AFM) measurements were taken both on the outer surface of the pad and on cross-sectional areas at different regions. Force volume measurements at different loading frequencies and varying temperatures in conjunction with viscoelastic modeling allowed mapping of the mechanical properties in different regions of the pad and revealed its complex functional morphology.

The development, growth, and maintenance of normal tissue in the body are enabled by the interactions between cells and the extracellular matrix. Recently, work has focused on the development of robust and reproducible thin films of collagen fibrils to study the effects of extracellular matrix composition and mechanics on cell behavior in physiologically relevant systems. More specifically, these model extracellular matrices were used to correlate Young&’s modulus to cell spreading and proliferation for collagen thin films subjected to dehydration and surface functionalization. In this presentation, frictional properties of native and fibronectin functionalized type I collagen thin films are studied via atomic force microscopy. The collagen lateral contact stiffness was found to be dependent only on the hydration state, indicating that bending properties were invariant with fibronectin functionalization. In contrast, the collagen coefficient of friction and shear strength varied with both functionalization and hydration state. The changes in the latter metric were found to correlate well with changes in mean cell spread areas on the same films, suggesting that shear strength may enable quantitative prediction of cell spreading on collagen and, more generally, on other extracellular matrix proteins. The results are compared with those from normal force spectroscopy.

Quantification of the mechanical stability of lipid bilayers is important in establishing the composition-structure-property relations, and shed light on understanding functions of biological membranes. A direct correlation of the self-organized structures has been demonstrated in phase-segregated supported lipid bilayers consisting of dioleoylphosphatidylcholine / egg sphingomyelin / cholesterol (DEC) in the absence and presence of ceramide (DEC-Ceramide) with their nanomechanical properties using AFM imaging and force mapping. Membrane elasticities of leukemia cancer cells before and after therapeutic treatments were also quantitatively determined by using force spectroscopy to assess the prognosis of leukemia cancer. We have designed an experiment to directly probe and quantify the nanomechanical stability and rigidity of the ceramide-enriched platforms that play a distinctive role in a variety of cellular processes. We further explored the influence of different cholesterol levels (5-40%) on the morphology and nanomechanical stability of phase-segregated DOPC/SM/Chol bilayers. The activation energies (ΔU) for generating a hole in the bilayers were calculated following the model for rupture of molecular thin films, and compared favourably with the reported values for the elementary processes involved in biomembrane and model membrane fusion using different techniques. The changes in membrane mechanics of drug treated cancer cells have demonstrated that AFM based mechanical force mapping may provide quantitative evaluation and optimization for the prognosis of leukemia cancer.

Bone is a dynamic tissue that constantly undergoes remodeling in order to maintain skeleton structural integrity. Bone remodeling is mediated by the coupling of bone cells: osteoblasts and osteoclasts which are responsible for bone formation and resorption respectively. Mechanical strain plays a critical role in the formation, proliferation and maturation of this bone cells leading to adjust the skeleton mass and architecture.
However, little is known about the mechanisms involved in the relationship between mechanical stimuli and osteoclast-mediated bone resorption in vitro. Furthermore, most of studies on resorption use bovine bone whereas its micro-structural organization is drastically different compared to human bone. It was also demonstrated that at the scale of an osteon, local Young&’s modulus and strain were heterogeneous.
Therefore, the aim of this study is first to characterize the interrelation between the mechanical properties of human cortical bone and osteoclast behavior. Then, mechanical stresses will be applied on the bone, leading to microcracks to observe the impact on osteoclast functions.
Mechanical and biochemical properties of bone were analyzed by nanoindentation, high resolution computed tomography, raman microscopy. Osteoclast precursors were isolated from murine bone marrow cells and differentiated in the presence of receptor activator of NF-kB ligand (RANKL) and macrophage colony-stimulated factor (MCSF). Mature osteoclasts were seeded on human cortical bone slices with transversal or longitudinal orientation. Resorption was quantified by TRAP assay and blue toluidine staining. Interferometry approach determined bone resorbed volume. Mechanical stimuli will be performed using compression testing inside a bioreactor.
Preliminary results showed that osteoclast activity was different on human bone compared to bovine bone. Human bone resorption appeared in specific areas that will be characterized by young&’s modulus and bone mineral content. In order to determine this in vitro model, the impact on osteoclast activity of slice orientation and bone porosity will be analyzed. Finally, mechanical testing will be carried out with precursor and mature osteoclast to analyze effects of mechanical stress on migration, fusion, differentiation and activity of osteoclasts.
This ongoing invitro study will provide a deeper understanding of mechanical strain implication on osteoclast activity during human bone remodeling.

Tooth enamel is a mineralized tissue made almost entirely of inorganic carbonated apatite nanocrystals arranged in a highly organized architecture that contributes to their physical properties. During the aging process, the average size of hydroxyapatite crystals in enamel increases and the enamel becomes softer, darker and less resistant to fracture. Recently we have shown that the crystallographic ultrastructure of enamel has a major role in determining the physical and optical properties of teeth. Microhardness and fracture resistance were found to have an inverse correlation with the size enamel nanocrystals along the c-axis that follows the Hall-Petch model and smaller enamel crystals imparted a lighter shade to the tooth than larger crystals. Therefore, we hypothesized that inducing a reduction in enamel crystal size could rejuvenate the physical and optical properties of mature teeth.
This study was designed to assess the reactivity of magnesium (Mg) ions with mature tooth enamel and assess whether these ions can induce changes in its crystallographic structure, thus modifying the mechanical and optical properties of teeth. A group of one hundred and six sound anterior teeth were treated with either saturated solutions of Mg ions or deionized-distilled water (as a control). Teeth were immersed for a period of 14 hours in each specific treatment. We assessed the elemental and crystallographic composition and the mechanical and optical properties before and after each specific treatment using the following techniques: x-ray diffraction, Raman spectroscopy, scanning electron, inductively coupled plasma mass spectrometry, ion exchange chromatography, pycnometry, BET analysis, TGA analysis, Vickers microhardness and tooth shade measurements.
Here we show that Mg ions reacted with tooth enamel and by doing so modified its crystallographic structure. Mg ions decreased significantly the apatite crystallinity and crystal size in enamel (p<0.05), while it increased the specific surface area of enamel (p<0.05). This change in crystallographic structure affected the mechanical and optical properties of the enamel making it harder and whiter (p<0.05).
Crystallographic ultrastructure plays a key role in defining the properties of the tooth enamel, which can be tailored through ionic substitution for improvement of mechanical and optical properties. For over a century, tooth whitening has been focused on the use of oxidizing agents such as hydrogen peroxide and carbamide peroxide. However, these substances can be hazardous and may have a negative effect on enamel hardness. This is the first study ever to use a new approach for whitening teeth through modifying the crystallographic structure without using oxidizing agents, thus avoiding the potential harmful effects of oxidizing agents while providing additional benefits such as increased enamel hardness.

Caddisflies are aquatic insects that share a common silk-spinning ancestor with terrestrial moths and butterflies, including the domesticated silk moth. Similar to terrestrial moths and butterflies, aquatic caddisfly larvae spin silk from a pair of modified salivary glands. Caddisflies use their silk to assemble a variety of structures underwater, ranging from composite body armor to food harvesting nets. The H-fibroins of both orders have blocky primary structures of repeating motifs, although the specific motifs notably vary across orders. In terrestrial moth silk, large blocks of poly(GAGAGS) motifs are arranged in ordered β-sheet domains forming strong inter-chain crosslinks interspersed in extensible amorphous regions. In contrast, caddisfly H-fibroins contain less than 5 mol% alanine, although X-ray spectroscopy suggests they contain crystalline components with periodicities consistent with β-sheet structures. Decomposition of the amide I region of the casemaker Hysperophylas sp. silk was consistent with ~35% of the residues occupying an anti-parallel β-conformation. Instead of alanine-rich motifs, the sub-repeats of caddisfly H-fibroins contain (SX)_n motifs, where X is usually an aliphatic amino acid and n = 2-6. As demonstrated by several methods, the serines in the (SX)_n motifs are extensively phosphorylated. Elemental analysis of native caddisfly silk fibers revealed a nearly stoichiometric ratio of Ca to P, with much smaller amounts of Mg and other trace metals. Together, these experimental results suggested that the (SX)_n motifs may form Ca²⁺-stabilized β-sheet domains analogous to the alanine β-domains of terrestrial silks. Exchange of divalent metal ions with Na⁺ EDTA decreased the content of β-sheet conformation, as determined by amide I decomposition. Removal of EDTA and re-addition of Ca²⁺ returned the β-sheet content back to near native proportions. From decomposition of the phosphate region of FTIR spectra, ~75% of phosphates are complexed with Ca²⁺ and ~25% are in an uncomplexed dianion form. Metal ion exchange with Na⁺ EDTA converted ~81% of the Ca²⁺ phosphate to an uncomplexed dianion form, which was reverted back to ~66% Ca²⁺ complex by re-addition of Ca²⁺. The results suggest there may be two populations of phosphate groups in native silk fibers. Single fiber mechanical testing revealed that native fibers are highly extensible, display a yield point, force plateau, and load cycle hysteresis. Metal ion exchange decreased the modulus and eliminated the yield point, force plateau, and hysteresis. In the absence of Ca²⁺ the fibers behaved as weak elastomers. Re-addition of Ca²⁺ qualitatively restored the modulus, strength, and mechanics of the fibers. Based on these results, a structural model will be presented in which the (SX)_n motifs assemble into two- and three-stranded, Ca²⁺-stabilized β-sheets that stack to form inter-chain crystalline crosslinks analogous to the poly(GAGAS) β-sheet networks of terrestrial moth silks.

Long fiber networks form the microstructure of electrospun scaffolds, which are widely used for mechanical support of growing artificial tissues. Quantitative characterization of such networks is essential for developing a better understanding of their behavior, and for design of optimal tissues. In this work, we establish relationships between certain macroscopic and microscopic characteristics of scaffolds that reveal the important geometric parameters. We also present a simple numerical procedure for capturing essential geometrical and topological features of 3-D scaffolds by creating contacting layers of long, curved fibers, using a self-intersecting random walk inside a periodic box. Sets of electron micrographs (SEM) of electrospun poly (ester urethane) urea (PEUU) scaffolds were analyzed. Mechanical anisotropy and fiber alignment were controlled, and a combination of thresholding and morphological procedures enabled so that fibers-overlaps positions, connectivity and fiber angle distributions were extracted from the generated network to fully describe the network topology The RVE size was determined studying the stabilization of the material extracted features over areas of increasing sizes. The scaffold simulations reveal the representative volume element needed to provide an adequate statistical description of the scaffold and aid in microscopic data acquisition by supplementing the image measurements. The generated geometry was converted into a finite element mesh and used to investigate important classes of problems and quantities of interest, such as general 3-D loading and the evolving 3-D microstructure, including the formation of emergent structures at larger length scales. The result of the parametric studies of the effects and their relationships to network geometry will allow one to understand scaffold mechanical behavior and identify optimal network geometry. The proposed modeling strategy was able to elucidate how local morphology shaped the mechanical response across a wide range of size scales, and offers a tool for engineering bio-inspired elastomeric electrospun scaffolds.

Tissue engineering offers a paradigm shift in the treatment of injury and disease. Hydrogels, highly hydrated polymer networks, have frequently been considered as substitutes for the extracellular matrix of soft tissue. However, they lack much of the structural complexity of native soft tissue, and in turn exhibit poor mechanical properties. Here, random and aligned electrospun nanofiber mats are infiltrated with hydrogels for use in mechanically critical tissue engineering applications, forming novel composite materials that mimic the fiber-reinforced structure of the native soft tissue. Structure-properties relationships in the composites are explored with mechanical testing techniques across a range of length-scales, from nanometer—comparable to the fiber diameter—to macroscopic, and using both experimental and computational techniques. The approach allows for the targeted design of composite systems with mechanical properties, including stiffness, toughness, and time-dependent behavior, optimized for their functional tissue engineering application.

Applications of nanoparticle-based strategies have gained recognition in biomedical research fields for their promising therapeutic potential in targeting various cell types. Hybrid gold nanoparticles are long sought after as prospective targeting contrast candidates for practical bioimaging and diagnostic applications due to their unique optical properties. Clinical success of hybrid nanoparticle systems depends on many parameters, including packing functional biological probes onto material interface with orientation control. Conventional fabrication approaches have so far only limited success due to low biocompability, restricted modularity and poor bio/nano interface control as a result of complex, multistep synthetic and chemical cross-linking bioconjugation procedures usually involving biology-unfriendly stringent conditions and harsh reducing agents. However in Nature, molecular recognition is the key element in highly ordered and organization of hierarchical complex structures at both nano- and micro-scale. Building upon the lessons from nature, we demonstrate a single-step biofabrication route to produce hybrid gold nanoprobes via self-organized modular multifunctional peptides. Mimicking molecular recognition in biological systems, our group developed multifunctional modular peptide probes that can self-organize on nanoscale particles and simultaneously display biologically-active motifs. Our extensive research on molecular biomimetics leaded to development of specific peptide linkers and assemblers enabling the formation of highly ordered and robust supramolecular monolayers as well as synthesizers with morphological control over micro- and nano-scale. Specifically, we fused engineered gold-binding binding peptides (AuBP) with other biologically-active peptide probe molecules, i.e. cell-targeting (integrin- and glioma-binding) or antibody-binding (Fc-binding) motifs producing novel chimeric peptides. AuBPs provide a feasible basis for self-assembling hetero-functional monolayers on gold nanoprobes that are biocompatible and capable of targeted cell bioimaging. When correctly designed, such peptide-functionalized nanoprobes retain exceptional stability in high-salt content buffer solutions as tested in stability assays, dynamic light scattering, and gel agarose electrophoresis. Moreover, the produced peptide-functionalized nanoprobes successfully target various cell receptors, as demonstrated in fluorescence and scanning electron microscopy bioimaging experiments. The produced biocompatible, self-assembled gold nanoprobes have a great potential for development of clinically feasible nanoprobe tools for cell-targeted diagnostics application.

The outermost layer of human skin, the stratum corneum (SC), is a natural nanocomposite that is exposed daily to variable and potentially harsh environmental conditions. The hydration level of the SC is crucial for the barrier function and physical appearance of skin. Water loss causes the development of drying stresses, which provide a driving force for damage in the form of chapping and cracking. Raman spectroscopy was used to investigate the structure of the tissue&’s protein and lipid components during water desorption and to characterize the water loss profile of SC for both free and bound water. Very good correlations of water loss data to mechanical stress of stratum corneum were obtained. We then developed a diffusion model to explain the observed water loss profiles and found that the model accurately captured the free water loss, which is also closely correlated with the development of drying stresses. We demonstrate how the diffusion model can be used to predict the development of drying stresses and hence the onset of dry skin damage. The research has important implications for the treatment of skin disorders such as chronic xerosis.

Structure and composition analysis are crucial for understanding biological materials and the development of new approaches in life science, medicine and bio-mimetics. This contribution demonstrates available analysis instrumentation which has substantially evolved.
Energy dispersive X-ray spectroscopy (EDS) in the electron microscope (SEM/FIB/TEM) allows high efficiency element distribution mapping in 2d and 3D with a spatial resolution reaching from the mm to the atomic scale. Multiple detectors ensure large collection angles and avoid shadowing effects. Thus, samples can be investigated fast and sometimes in a close to natural state. One bio-mineralization example is the embedding of skin irritating calcium oxalate crystals into the agave leaf for its protection investigated using a multiple EDS-detector system in SEM.
Micro X-ray fluorescence (mu;XRF) using an X-ray source (instead of electrons as in case of SEM) is the tool for the composition analysis of larger scale objects, up to several cm in size, with higher sensitivity, particularly for heavier elements. The incorporation of Si into plant leaves for stabilization was studied using micro-XRF.
Crystal structure and orientation is characterized using electron backscatter diffraction (EBSD) in SEM. An EBSD study of the egg shell structure for different species reveals how evolution fulfills the task to provide a stable egg shell easy to crack from the inside though.
Furthermore, non-destructive 3D analysis of the inner structure of different objects by X-ray micro-tomography (CT) will be explained. Various insects were dissected non-invasively showing for instance internal heat deflection plates which help some beetle species to control temperature. A micro-CT attachment for SEM allows the further increase of spatial resolution and study structural details below one micrometer.
These and further examples demonstrate new powerful tools for biomaterial characterization in 2D and 3D on the millimeter, micrometer and nanoscale.

Microenvironments of biological cells are dominated in vivo by macromolecular crowding and resultant excluded volume effects. This feature is absent in dilute in vitro cell culture. Here, we induced macromolecular crowding in vitro through the incorporation of synthetic macromolecular globules of nm-scale radius at physiological levels of fractional volume occupancy in basal media. We quantified the impact of induced crowding on the extracellular and intracellular protein organization of human mesenchymal stem cells (MSCs) via immunocytochemistry, atomic force microscopy (AFM), and AFM-enabled nanoindentation. Macromolecular crowding in extracellular culture media directly induced supramolecular assembly and alignment of extracellular matrix proteins deposited by cells, which in turn increased alignment of the intracellular actin cytoskeleton. Furthermore, macromolecular crowding of collagen type-I gels induced similar levels of alignment, even in the absense of cells. The resulting cell-matrix reciprocity further affected adhesion, proliferation, and migration behavior of MSCs. Macromolecular crowding can thus aid the design of more physiologically relevant in vitro studies and devices for MSCs and other cells, by increasing the fidelity between materials synthesized by cells in vivo and in vitro.

Hydrogels are attractive materials for biological applications due to their ability to absorb quantities of water on the same order as soft tissues and to act as scaffolds and substrates for cell culture. However, their use is limited by unsatisfactory mechanical performance involving poor elastic modulus, toughness, and ductility. Mechanical characterization of hydrogels provides another challenge. Spherical indentation, specifically colloidal probe atomic force microscopy (AFM), is non-destructive and allows for testing a local region of the sample with contact areas small compared with the sample size, and at scales relevant to biological systems. Poly(ethylene glycol) dimethacrylate (PEGDMA) hydrogel systems with 10 % to 20 % polymer content were tested. The polymer is bifunctional, so crosslinking density depends on polymer concentration only. Spherical indentation load-relaxation data were obtained using an AFM with a fluid cell environment. Gold colloidal probe tips with a nominal radius of 3 mu;m were used for indentation. The probes were calibrated for their spring constant using the thermal fluctuation method and confirmed via laser Doppler vibrometry; spring constants were on the order of 2 N/m. Indentation data were obtained as a function of maximum load (10 nN to 200 nN), contact rise time, and attachment-detachment rate. Load-relaxation data were fit to extract elastic modulus and viscoelastic and poroelastic parameters. Preliminary results indicate polymer concentration relates directly to elastic modulus, and inversely to hydraulic permeability. A complete understanding of how to tailor hydrogel mechanical properties through controlled fabrication will aid in the design and use of these materials for biological applications.

Naturally occurring composite structures like antler bone and albone nacre have a highly ordered structural design at the nanoscale. Natures&’ successful architecture has attracted a widespread interest in mimicking such systems artificially, the goal being to design tough composite materials with adaptable mechanical properties.
Here we report results on the synthesis in fabricating these nanocomposites via chemical infiltration where nanoparticles nucleate and grow within precursor organic multilayers via a layer-by-layer route. A highly ordered array of hollow chambers are fabricated by nanoimprinting on sacrificial templates, these methods are incorporated with the layer-by-layer assembly of polyelectrolytes resulting in an array of three-dimensional nanocomposite pillars which are then mineralised. The structural and mechanical similarities with naturally occurring biomineralised systems are discussed.
The thickness and microstructure of the polyelectrolyte multilayers are measured using SEM, a considerable change in morphology of micro-pillars is evident, with the mineralised composites looking stiffer and thicker and the non-mineralised pillars more flaccid. TGA was used to calculate the rate of mineralization within multilayer films, infiltrated multilayers have shown up to 65% presence of calcium carbonate which is comparable to structures like bone. X-ray diffraction to characterise the structure and micromechanical testing involving nanoindentation have also been conducted. The mechanical performance for PSS/PAH mineralised composite pillars, significantly increased up to 10 GPa after infiltration in comparison to the non-mineralised pillars with a strength of only 3 GPa. The synthetic composites can thus be compared with natural biomineralised tissues like nacre.

Gelatin is prepared through partial thermal denaturation and chemical degradation of collagen.[1] The latter is the most abundant structural protein in the extracellular matrix of multicellular animals.[2] In vertebrates, it accounts for about 30 wt% of the total body protein. Although gelatin is a low-cost biopolymer with great potential, the mechanical properties of untreated gelatin are not on par with synthetic polymers. Studies have shown that the physical properties of gelatin films and fibers can be improved by adapting the production process.[3, 4] Up to now, gelatin spinning is mostly done in labor-intensive, multi-step processes, which rely on harmful solvents. We report a robust spinning method for the fabrication of stable protein fibers from a ternary system consisting of protein, solvent and non-solvent.[5] The developed process is continuous, can easily be scaled up, requiring only minimal energy input and harmless solvents. Through continuous stretching of the gelatin fiber, orientation of the fibrous protein is achieved. Independent measurements by XRD and SEM proved this observation. Tensile tests revealed that the fibers&’ mechanical properties are significantly improved by stretching and attain the level of similar biopolymers (e.g. wool, tendon collagen) and synthetic polymers (e.g. PTFE, PA 6). These results may pave the way for the use of gelatin fibers in diverse applications such as fiber-reinforced bioplastics and textiles.
[1] W. F. Harrington, P. H. Von Hippel, in Advances in Protein Chemistry, Vol. 16 (Ed.: C. B. Anfinsen, M. L. Anson, K. Bailey, J. T. Edsall), Academic Press, 1962, pp. 1-138.
[2] M. Djabourov, Contemporary Physics, 1988, 29, 273-297.
[3] R. Fukae, T. Midorikawa, J. Appl. Polym. Sci., 2008, 110, 4011-4015.
[4] A. Bigi, B. Bracci, G. Cojazzi, S. Panzavolta, N. Roveri, Biomaterials, 1998, 19, 2335-2340.
[5] P. R. Stoessel, R. N. Grass, W. J. Stark, European patent application, 2013, nr. 13 002 820.2.

Protein-solid interactions and assembly of proteins on surfaces is utilized in many fields to integrate biomolecules and their diverse, genetically-controlled functions with engineered solid substrates, including nanoparticles, quantum dots, nanorods, thin films and bulk materials. Examples include bioelectronics, biosensors, and bioimplants. In biology, proteins are the major biopolymers that enable dynamism of bio systems but they also catalyze mineralization, growth, and intricate hard tissue formation with complex multifunctional properties. These are all desirable merits in engineered systems, currently impossible to achieve. Controlling proteins at bio/solid interfaces relies on establishing key correlations between primary sequences and resulting molecular interactions that follow spatial organizations on substrates. Using combinatorial mutagenesis, similarity analysis in bioinformatics and rational design principles, our Center engineers short peptides (7-25 amino acids long) by controlling their folding patterns and, hence, tailoring the molecular interactions that leads to a variety of addressable self-assembled peptides (SAP) nanostructures. The peptides are tailored via simple point and domain mutations to control fundamental interfacial processes, including initial binding and molecular recognition, surface aggregation and growth kinetics, and intermolecular interactions leading to self and targeted assembly. Tailoring short peptides and their molecular interactions offers versatile control over molecular well-defined surface properties essential in building engineered, chemically and electronically rich, bio-solid interfaces. As demonstrated, peptides alone, or in chimeric forms, as bifunctional constructs can bridge nanosolids to form molecularly hybrid systems for a variety of biophotonics and bioelectronics implementations as well as solution and surface biofunctionalization for implants (to enhance osteointegration and nano-fouling surfaces), nanoparticle and thin film mineralization, for tissue regeneration) and bifunctional chimeric enzyme-peptide conjugates. This presentation will summarize recent advances and provide future prospects in proof-of-principle demonstrations at the confluence of biology and solid materials.

The leaves of the common sundew (Drosera) serve as active traps that lure, capture and eventually digest their prey. Some of the species exhibit slower secondary movements of the leaf blades that engulf the immobilized prey and hasten the nutrient uptake. Auxin-mediated differential growth is implicated in the localized response yet the underlying mechanics is unknown. Our experiments on slender leaves of Cape sundews (D. capensis) show that the differential expansion on the ventral surface is along the long axis and localized around the prey. Surprisingly, it is graded laterally with a maximum (sim; 30%) along the leaf midvein and the effectively ellipsoidal growth region scales with the size of the prey. We use a combination of numerical simulations and macroscale experiments on model bilayered laminae to elucidate the mechanisms that lead to efficient and localized shape changes. Our results highlight mechanical principles in play during resource-limited adaptations that lead to complex shape changes in a range of plant tropisms and have implications for reversible morphing of synthetic laminae.

The silk from the Loxosceles spider features a unique, ribbon-like morphology unlike any other spider silk or synthetically spun polymer fiber. We report extraction of this fiber from animals under controlled conditions and a first structural and mechanical analysis. These protein ribbons represent free-standing polymer films with a thickness of about 50 nm, corresponding to only a few molecular layers of protein, thus approaching a 2D polymer that is dominated by surface rather than bulk properties. Their thinness allows these fibers to bend and wrinkle easily, which promises interesting adhesive properties due to facilitated surface conformation to contacted objects. The surface of these ribbons is covered with dot-like protrusions with diameters of a few nanometers that are not found on other silk fibers, and which may contribute to the adhesive properties of the material.
Due to its relative structural simplicity, this material is an ideal model system to study silk. Using an atomic force microscope we carried out stress-strain analysis of individual fibers with a corresponding Young&’s modulus as high as 19±5 GPa; the strain at break was determined to be 27±3%. Thus, despite effectively featuring the morphology of a nanoscopically thin protein film, the mechanical performance of these ribbons rivals the strongest and toughest silk fibers known.

Understanding the structure-property relations and thus the design principles of biological materials is a valuable source of inspiration for the development and improvement of synthetic structural materials with tailored properties. Our model system, the crustacean cuticle, is a hierarchically organized composite material that covers the whole animal and forms the continuous exoskeleton. The organic phase of the cuticle is organized in at least seven hierarchical levels. It consists of chitin molecules forming crystalline fibrils enveloped by proteins. These fibrils form planes that are stacked in a twisted plywood structure constituting three of the cuticle&’s four principal layers. At least two of them, the exo- and endocuticle, contain a mineral phase of mainly Mg-calcite, amorphous calcium carbonate (ACC) and phosphate (ACP). In addition, the cuticle is pervaded by an elaborate pore canal system. The high number of hierarchical levels and the structural and compositional diversity provide a high degree of freedom for varying the mechanical properties of the material. We investigated the structure, composition and mechanical properties of cuticle from different crustacean species and used the experimental data as input for a hierarchical model [1,2] for cuticle that uses ab initio calculations for the properties of basic components like chitin and calcite, hierarchical homogenization performed in a bottom-up order for the meso-level, and a Fast-Fourier-Transforms approach that is able to describe the local development of stress and strain fields within the bulk material on the macro-level. By using realistic, experimentally determined model geometries, our model enables us to investigate the structure-property relationships in terms of volume fractions, shapes and arrangement of selected constituents, their dimensions as well as mechanical properties to evaluate the “ideal” parameters necessary to fulfil a certain function. Since the properties used as input can be varied, the model allows for knowledge based design of biomimetic hierarchical composite materials by virtual prototyping.
[1] S. Nikolov, M. Petrov, L. Lymperakis, M. Friák, C. Sachs, H. Fabritius, D. Raabe, J. Neugebauer, Adv. Mater. 2010, 22, 519-526.
[2] S. Nikolov, H. Fabritius, M. Petrov, M. Friák, L. Lymperakis, C. Sachs, D. Raabe, J. Neugebauer, J. Mech. Behav. Biomed. Mater. 2011, 4, 129.

Learning lessons from nature is the key element in the design of tough and light composites. Nacre shows the nature&’s ability to combine strong and brittle mineral with tough organic material into a multilayered composite. In this study, mechanical behavior and toughening mechanisms of nacre-inspired multilayered materials are explored using a physics-based mechanics model. In nacre&’s structure tensile pillars, shear pillars and the roughness of the tablets play an important role in its overall mechanical performance. A micromechanical physics-based model of nacre is proposed to simulate the mechanical deformation and toughening mechanisms in the multilayered materials. Moreover,a freeze casting method are used to create multilayered polymer/polymer and ceramic/polymer composites, respectively. The modeling results are compared with the experimental results for a range of multilayered materials.

Nacre (abalone shell) is a naturally occuring composite material with remarkable material properties. Over the past decade, advanced processing techniques (e.g., freeze casting, layer-by-layer self assembly, etc.) have significantly increased the potential for cost effective and efficient routes to processing nacre-like biomimetic structures in bulk form. While the strength and stiffness of these materials have been modeled extensively in terms of their microstructure, their response to non-uniform stress and deformation is largely uncharacterized. In this work, we present a discrete element energy-based computational model to capture the complex fracture properties of these highly anisotropic, heterogeneous composites. Stable Monte Carlo numerical schemes implemented and parallelized on modern GPU architecture have enabled large fracture simulations containing millions of degrees of freedom. When coupled with basic fracture mechanics based scaling laws, the simulation results clearly show the effect of microstructure on the macroscopic composite toughness along with the dominant failure mechanisms. Prior to crack initiation, the results closely resemble the scaling relationships found in a continuum material.

Fascinating properties of many surfaces occurring in nature originate from nano- and micro-hairs covering them. Prominent examples include air-retaining leaves of plants like the floating water fern Salvinia [1], as well as water beetles [2]. Inspired by these and other examples, we introduce a highly scalable low-cost fabrication technique for producing a dense fur of high aspect ratio nanohairs out of polymer surfaces. This hot pulling technique is a modified hot embossing process in which softened polymer is locally elongated with a heated sandblasted steel plate, creating densely packed nanohairs on the polymer surface.The resulting nanofur is suitable for various biomimetic applications. The wettability of polycarbonate, for example, changes from hydrophilic on unmodified samples to superhydrophobic (> 170°) on nanofur samples. Liquid traps and channels result from changing the local wettability of the nanofur by mechanical structuring. By locking an intermediary liquid on the nanofur surface self-healing surfaces are fabricated.
[1] Barthlott, Schimmel, Wiersch, Koch, Brede, Barczewski, Walheim, Weis, Kaltenmaier, Leder, Bohn, Adv. Mat. 22, 2325 (2010); [2] Ditsche-Kuru, Schneider, Melskotte, Brede, Leder, Barthlott, Beil. J. Nanotechnol. 2, 137 (2011).

Aim
Calcium phosphate (CaP) ceramics are widely used as bone graft substitutes (BGS) due to their excellent biocompatibility and osteotransduction [1]. However, their inherent brittleness limits their application to non-load bearing locations or requires additional metallic fixations [2]. Unfortunately, stronger and tougher materials usually exhibit inferior biological properties (e.g. metals) [3].
The aim of this project is to combine the good biological properties of CaP ceramics with the strength and toughness characteristic of highly ordered composite structures [4]. Specifically, strong CaP platelets [5] were mixed with a tough biopolymer to form structured composites to be tested for bioresorbable load-bearing BGS.
Materials and Methods
We developped a synthesis route to produce prismatic dicalcium phosphate (DCP) platelets a few µm long and with an aspect ratio of about 20 [5]. The platelets were mixed in varied volume ratios with a 3wt% chitosan solution. Slurries were dried over 5 days at 30°C in silicone molds. The drying polymer films reached a final thickness of 0.1-0.3mm and forced the platelets to lie horizontally. XRD rocking curves were measured to determine the platelets&’ orientation. Dogbone-shape samples were cut in the composite films and tensile tested. Finally, scanning electron microscopy (SEM) was performed on ruptured surfaces. Significance of differences was evaluated by ANOVA (α=0.05).
Results and discussion
Yield (σ_y) and tensile strength (σ_t) linearly increased with increasing ceramic content up to 15vol%, exhibiting maximum σ_y and σ_t of 110 and 151MPa. Such strength are similar to values found for cortical bone in the literature [6], highlighting the relevance of this system in load-bearing biological applications. Interestingly, strength values declined at 20vol%. SEM pictures revealed high orientation of platelets up to 15vol% of platelets, and much more disordering and defects at 20vol%. Rocking curves measurements quantitatively confirmed these observations with an increase of FWMH (“mean angle” of the platelets) of 7°. Though not statistically significant, the Young&’s modulus continuously increased up to 4GPa and the strain at rupture decreased to 6%.
Conclusions
The present results show the feasibility and potential of using resorbable CaP platelets to reinforce biopolymers (increase of 40% of σ_y) and obtain synthetic composites with similar mechanical properties to natural ones. This study offers a step forward in the synthesis of biodegradable, strong and tough materials.
References
[1] LeGeros RZ. Clin Orthop. 2002
[2] Wagoner Johnson AJ. Acta Biomater.2011
[3] Karageorgiou V. Biomat. 2005
[4] Bonderer LJ. Science. 2008
[5] Galea L. Biomat.2013
[6] Landis WJ. J. Bone Miner. Res. 1995

Recent studies have shown that fibrillar structure is one of the key factors for many insects and lizards to cling and maneuver on various surfaces. Van der Waals force has been proven to be the primary interaction mechanism between gecko&’s feet and target surfaces during attachment, and easy detachment is control by making the angle of the stalk of setae larger than a critical value to peel the feet off. During animal locomotion with repeatedly attachment and detachment, self-cleaning is the key to keep their feet working. While strong attachment, easy detachment, and self-cleaning occurs, which is equally crucial for the survival of the animals like gecko. Advanced functional materials mimicking gecko feet are highly desired in the fields of military, semiconductor industry, medical care, and outer space explorations. However, how the gecko clean their sticky feet in dusty environment still remains unclear. Here, we provide experimental evidence that self-cleaning occurs at the microscale by dynamically induced change in adhesion forces. Simulation shows that the unique seta-spatula structure and dynamic effects are key to this dynamically induced self-cleaning at the microscale. Under hyperextension, dynamic effect pushes setal array into effective self-cleaning regimen, making a live gecko easily remove dirt particles during locomotion.

A key goal for nanotechnology is to develop a universal strategy by which prescribed structures can be rational-designed and self-assembled from small components. Structural nucleic acid nanotechnology has provided an effective approach for constructing sophisticated synthetic molecular structures and devices. An effective method for assembling megadalton nanoscale 2D and 3D shapes is DNA origami, in which a long “scaffold” strand is folded to a predesigned shape via interactions with short “staple” strands. Recently, we developed a modular design strategy for making complex discrete 2D shapes (Nature 485:623, 2012), using only short synthetic DNA strands. The modularity of this approach allowed the authors to build more than a hundred distinct shapes from a common two-dimensional molecular “canvas”. Here we further extended this modular approach to three dimensions: 3D discrete DNA structures (Science 338:1177, 2012) and crystals were assembled from short single-stranded “DNA bricks”.
We showed that large DNA structures with prescribed shapes can be assembled from hundreds of DNA bricks in an one-pot reaction. We demonstrated the generality of this method by successfully designing and characterizing 123 structures, including two 8 MDa cuboids. The modularity of this approach allowed us to experimentally constructed 100 distinct shapes from a “3D canvas” with 10 by 10 by 10 voxels, simply by repipetting various subsets of a master strand collection. These structures include solid shapes with sophisticated geometries and surface patterns, and hollow shapes with intricate tunnels and enclosed cavities.
In addition, we demonstrated complex DNA crystals using the DNA-brick method. The new approach allows us to design complex repeating units (from 1,024nt to 11,648nt, or 0.34 MDa to 3.84 MDa) by implementing distinct sequences for each DNA brick within the repeating unit. A total of 32 DNA crystals were constructed using same design rules, including (1) 9 1D-growth DNA-bundle crystals that show different intersectional shapes, chiral feature, tunnels, and porous features. (2) 8 2D-growth DNA-multilayer crystals that exhibit different height (up to 20 layers, or 50nm), channel feature, and cross tunnel features. (3) 8 2D-growth DNA-forest crystals that are up to 256 bp (84 nm) in height, and contain features like channels, pores, and tubes. These crystals successfully realized many three-dimensional features that were unattainable using previous approaches.

Designing in vitro cellular environments with topographical features and material stiffness that can be dynamically altered on-demand would allow researchers to probe the cellular response to changing surface environments, providing insights into cell motility and differentiation in real-time. Here, we report on the development of novel biomaterials created using micro-3D printing (µ3DP) technology with feature sizes of less than 1 µm that have both tunable elastic properties and environmental responsiveness (temperature, pH, ionic strength, light, etc.). When irradiated with a focused, pulsed laser beam, the pre-fabricated biomaterials undergo a rapid volumetric reduction. The magnitude of this responsive behavior is influenced by various factors, including the crosslinking density of the material (lower density results in greater response), post-fabrication laser fluence, and molecular additives that may modulate contraction via photogeneration of singlet oxygen, peroxide radicals, etc. Due to the nature of the focused laser beam, the material response is highly localized with submicron resolution and can be initiated in the presence of cells. This technology provides a new paradigm in materials development, providing the capacity to dynamically manipulate the topographical features and material stiffness presented to a cell. With high spatiotemporal control and real-time monitoring, this technology substantially extends abilities of existing methods for studying cellular responses to changing topographical features.

Pancreatic ductal carcinoma is one of the most lethal forms of cancer with 5 year survival rate less than 5%. Liposomes are representative lipid nanoparticles and are used for delivering anticancer drugs, DNA fragments or siRNA in pancreatic cancer cells. However, upon targeting, the contents are passively released from the liposomes and this process is often slow. As a potential solution, we are preparing liposomes encapsulating a precursor which will generate gas bubbles in situ in response to acidic pH of cancer cells. By incorporating a folic acid conjugated lipid, we have successfully targeted these liposomes to the pancreatic ductal carcinoma cells overexpressing the folate receptors. The disturbance created by the escaping gas bubbles leads to the rapid release of the encapsulated anticancer drugs from the liposomes. Atomic force microscopic studies indicate that the liposomal structure is destroyed in reduced pH. The escaping gas bubbles render the liposomes echogenic - allowing successful imaging using a medical ultrasound transducer. Contents release from these liposomes is further enhanced by mechanical stimulation of the gas bubbles with diagnostic frequency ultrasound, resulting in substantially reduced viability for the pancreatic cancer cells (14%). Hence, these liposomes have the potential for ultrasound image-guided, controlled cytosolic delivery of anticancer drugs at tumor site.

In this paper we report a fundamental morphological instability of constrained 3D microtissues induced by a positive chemomechanical feedback between actomyosin-driven contraction and the mechanical stresses arising from the constraints. Using a 3D model for mechanotransduction we find that perturbations in the shape of contractile tissues grow in an unstable manner leading to formation of "necks" that lead to the failure of the tissue by narrowing and subsequent elongation. Unlike necking in passive materials, here the instability is caused by the active contraction (extension) of the regions of the tissue where the mechanical stresses are smaller (greater) than the characteristic actomyosin stall stress of the tissue. The magnitude of the instability is shown to be determined by the level of active contractile strain, the stiffness of the extra cellular matrix (ECM) and the stiffness of the boundaries that constrain the tissue. A phase diagram that demarcates stable and unstable behavior of 3D tissues as a function of these material parameters is derived. The predictions of our model are verified by analyzing the necking and failure of normal human fibroblast (NHF) tissue constrained in a loopended dogbone geometry and cardiac microtissues constrained between microcantilevers. In the former case, the tissue fails first by necking of the connecting rod of the dogbone followed by failure of the toroidal loops. In the latter case we find that cardiac tissue is stable against necking when the density of the ECM is increased and when the stiffness of the supporting cantilevers is decreased, in excellent agreement with the predictions of our model. By analyzing the time evolution of the morphology of the constrained tissues we have quantitatively determined the chemomechanical coupling parameters that characterize the generation of active stresses in these tissues. More generally, the analytical and numerical methods we have developed provide a quantitative framework to study how contractility can influence tissue morphology in complex 3D environments such as morphogenesis and organogenesis.

Currently, proliferation and differentiation of stem cell is usually accomplished either in vivo, or on chemical coated tissue culture petri dish with the presence of feeder cells. Here we investigated whether they can be directly cultured on polymeric substrates, in the absence of additional factors.
We found that mouse embryonic stem cells did not require gelatin and could remain in the undifferentiated state without feeder cells at least for four passages on partially sulfonated polystyrene. The modulii of cells was measured and found to be higher for cells plated directly on the polymer surface than for those on the same surface covered with gelatin and feeder cells. When plated with feeder cells, the modulii was not sensitive to gelatin. We also found that cell growth was much better on electrospun PMMA and P4VP fibers other than on spin coated thin film, which showed that topography could also influence stem cell behavior.
Whereas the differentiation properties of human bone marrow stem cells, which are not adherent, are less dependent on either chemical or mechanical properties of spin-casted substrate. However, they behave differently on different toughness hydrogels as oppose to on polymer coated thin films. The bone marrow stem cells preferred soft hydrogels and formed a “cluster” at the center of the gel. The AFM tests showed that bone marrow stem cells could adjust their modulii to different surroundings.

Cell migration and growth in multicellular tissues play important roles in morphogenesis, wound healing, and tumor metastasis. Many studies have focused on single cell mechanics and the response of motile cells to topological cues but little is known about the responses of migratory multicellular tissues to complex topological cues. Understanding cell and tissue migratory responses to well defined 3D substrates and the factors that regulate those dynamic responses are key to understanding the principles of cell sheet migration during embryonic development, which can contribute to fields including the development of biomaterials for tissue engineering and regenerative medicine. Our approach to investigating this biomaterial based response of living system is to present micropost arrays fabricated via MicroElectroMechanical Systems techniques, which will provide advantages for studying multicellular system response to substrate-based biophysical stimuli. We implement micropost arrays with different diameters (e.g., different spacing gaps) and Xenopus laevis tissues cultured in a well-controlled microenvironment. Our topographical controlled approach for cellular application enables us to achieve a high degree of control over micropost positioning and geometry via simple, accurate, and repeatable microfabrication processes. These microfabricated post arrays provide an opportunity to study the influence of topology on cell sheet migration. Through our work, we found that cells in ectodermal Xenopus tissues migrated outward as cohesive sheets from their initial morphology. Furthermore, post arrays slowed tissue movements over the course of 20 hours and through sequential time-lapse imaging, the migration of ectodermal tissues on flat substrates was faster on flat substrates than on post arrays. We then investigated how the multicellular systems would respond when presented with both flat substrates and 3D micropost arrays simultaneously. The tissues placed on flat-substrates at the edge of the post array, at the interface between flat-substrate and posts, and completely on the post array suggest topological cues are essential and can be used to shape tissue form. Using microfabricated post arrays, we found that the multicellular tissue system was directly correlated to topology differences and controlling 3D microenvironments could impact the shape, formation, and the rate of tissue cells migration.

Mechanical characteristics of single cells are used to identify and possibly leverage interesting differences among cells or cell populations. Fluidity---hysteresivity normalized to the extremes of an elastic solid or a viscous liquid---can be extracted from multiple rheological measurements of cells, including creep compliance and oscillatory phase lag. With multiple strategies available for acquisition of this nondimensional property, fluidity may serve as a useful and robust parameter for distinguishing cell populations, and for understanding the physical origins of deformability in soft matter. Here, for three disparate eukaryotic cell types deformed via optical stretching, we examine the dependence of fluidity on chemical and environmental influences around a time scale of 1 s. We find that fluidity estimates are consistent in the time and the frequency domains under a structural damping (power-law or fractional derivative) model, but not under an equivalent-complexity lumped-component (spring-dashpot) model; the latter predicts spurious time constants.These results emphasize the utility of collecting whole-cell viscoelastic data from multiple rheological domains, particularly when aiming to extract and compare mechanical parameters among cells or populations thereof.

Interfaces in biomaterials are not as smooth as in an inorganic material. The variety lies in structural configuration, non-planarity, composition gradients and structural gradients getting mixed and influencing the overall material behavior. Most accurate simulations approaches cannot model such behavior due to limitations in size and time scales. Most simulation based approaches that are appropriate to model such interfaces in terms of length scales are not accurate enough. Experiments to model interfacial biomechanics are limited in their ability to resolve structural features simultaneously with the mechanical behavior of interfaces at the length scale of interfaces. In the present work, an experiment-simulation based approach is presented to quantify mechanical strength and molecular structure of interfaces simultaneously using a combination of nanomechanical Raman spectroscopy with classical molecular simulations. Analyses are interpreted to present a combined outlook regarding how a combined experiment-simulation approach can be used to decipher multiphysical interfacial interactions in complex biomaterials and bioinspired materials.

Drug delivery issues that pertain to low loading and bioavailability can be addressed by incorporating the drug within complexing molecules such as cyclodextrin. The use of cyclodextrin-drug complexes in nanofibrous form offers a powerful method to enhance poorly water soluble drugs solubility while protecting the active material from degrading. However, cyclodextrin nanofibers dissolve instantly, limiting their use to sublingual delivery thus modification are needed extend the mat dissolution duration. Our approach entails blending cyclodextrin with different polymers and chemical crosslinking to expand the mat dissolution duration allowing for the release of poorly water soluble drugs over an extended time period. In this case mat morphology and dissolution duration was altered by blending cyclodextrin with poly(vinyl alcohol) or chitosan and chemical crosslinking with glutaraldehyde. Both blending and crosslinking were found to alter the mat dissolution duration and affect the drug crystallinity and release profiles.

Lipid bilayers are one of the most important structures found in nature. They provide a protective layer for cells as well as hosting the machinery for transport across the cell membrane. Solid supported lipid bilayers provide an excellent model system for studying the surface chemistry of cells and make it possible to investigate processes such as the interactions of polyelectrolytes with lipid bilayers. Polyelectrolytes interact strongly with lipid bilayers composed of zwitterionic lipids or with lipids having the opposite charge as the polymer. Here we present a study that uses single particle tracking to reveal the kinetic and thermodynamic properties of branched polyethylenimine (PEI, 25kD) as it attaches to, and diffuses in, a number of solid supported lipid bilayers such as DMPC (dimyristoylphosphocholine) and POPC (palmitoyloleoylphosphorcholine). These kinetic and thermodynamic properties (as well as mobility) were studied as a function of lipid composition, pH, temperature, and phase. The experiments were performed at two different temperatures of 15 and 25°C and for each temperature the observations were repeated at pHs of 6.5, 7.4 and 8.5.
The results indicate that at 15 °C the mobility of PEI inside DMPC (gel phase at 15 °C) is quite slow and insertion into the membrane is greatly reduced. However, PEI easily inserts into membranes composed of POPC (liquid phase) and less transient surface attachment is observed. At 25 °C PEI easily inserts into the both DMPC and POPC membranes with high mobility and little surface attachment. The pH of the buffer also has a dramatic effect on the mobility of the PEI in lipid bilayers. As the pH decreases more amines are protonated increasing the cationic nature of the polymer, thus increasing the PEI interaction with the zwitterionic membrane and facilitating its insertion into the bilayers. When the lipid composition includes a negatively charged lipid such as brain-PS, it will associate strongly with PEI causing the creation of PS rich domains surrounding the polyethylenimine. The pH dependence on mixed lipid composed of 80% DMPC, or POPC, with 20% Brain-PS were also measured. While PEI inserted much less freely in membranes containing brain-PS at pH of 6.5, little change was observed at the pH 7.4 and 8.5.

Nature has evolved efficient strategies to synthesize complex mineralized structures that exhibit exceptional damage tolerance. One such example is found in the hyper-mineralized hammer-like dactyl clubs of the stomatopods, a group of highly aggressive marine crustaceans. The dactyl clubs from one species, Odontodactylus scyllarus, exhibit an impressive set of characteristics adapted for surviving high velocity impacts on the heavily mineralized prey on which they feed. Our analysis has revealed that the dactyl club consists of a multi-phase composite of oriented crystalline hydroxyapatite and amorphous calcium phosphate and carbonate, in conjunction with a highly expanded helicoidal organization of the fibrillar chitinous organic matrix. We identified three primary regions within the club with varied material composition and ultrastructures. The outermost region of the club that makes contact with its prey during an impact consists of an extremely hard outer region characterized by highly oriented calcium phosphate (hydroxyapatite, HAP) crystals. This region also demonstrates a slight gradient in phosphorus concentration. In addition, there is an energy absorbing region rich in α-chitin that has a rotated plywood structure mineralized with amorphous calcium carbonate and calcium phosphate and pores, which may nucleate cracks intentionally to locally dissipate energy. Finally, the sides of the club consist of alpha-chitin fibrils that are oriented perpendicular to the cross-section of the club and are also mineralized.
By investigating the structure-property relationships of these unique damage-tolerant crystallized structures using modern chemical, morphological, and mechanical characterization techniques, we are now developing the necessary tools for the design and fabrication of cost-effective and environmentally friendly engineering composites with impact resistance.

The stomatopods are a group of highly aggressive marine crustaceans that use their 5-mm wide light-weight appendages, made of an ultra strong organic-inorganic composite structure, as a hammer to smash their heavily shelled preys with accelerations of a .22-caliber bullet producing forces of 0.5 to 1.0 kilonewtons. This hammer, so called dactyl club, is capable of enduring the incredibly high speeds with tremendous forces from its smashing blows creating excessively high internal stresses without inducing damage. However, what makes this material stand out is that the stomatopod can actually smash and defeat their armored preys (mostly armored mollusks and other crustaceans) which have been an important focus of research during the last two decades for their damage-tolerance and excellent mechanical properties. Additionally, this material has evolved an ultrastructure designed for high-velocity strikes, which is a unique feature in biological materials rarely explored before. Thus, understanding the structure-property relationships in these extremely strong biological structures may provide critical insight into the development of the high-performance and multifunctional biomimetic materials. We have identified two main hierarchical structures: one that resemble a helicoidal fiber-reinforced composite and a second one based on unidirectional fiber reinforced composite wrapped around the dactyl club. This talk will focus on the modeling and mechanical analysis of these two important components, and how they contribute to this hammer&’s high impact and damage tolerant materials behavior. We successfully combined computational modeling, 3D printing and mechanical testing to evaluate some important hypotheses about the key morphological features of the microstructure and most important toughening mechanisms that are unique in these hierarchical materials.

Deep fascia is a type of connective tissue found throughout the body that encloses many muscles and connects them to bones. There is growing evidence that fascia can influence limb stability, force transmission, and elastic energy storage during locomotion. Studying the role of fascia during active movement presents challenges: (1) its sheet-like structure contains multiple layers of well-organized collagen fibers making the tissue anisotropic and (2) the multiple connections to muscles and bones generate complex nonhomogeneous states of strain. The complex structure and loading environment of fascia is one reason for which current musculoskeletal models ignore its role in simulations of muscle function [1, 2]. This study restricts attention on the deep fascia of the thigh, specifically on goat fascia lata.
In the present study we first characterize the structure of fascia using histology and light microscopy. The images provide detailed information of the layered structure, the waviness and directions of collagen fibers, and the volume ratios between individual layers. To determine the stress-deformation response in vitro we perform a series of uniaxial and planar biaxial tension tests using applied strain as the independent parameter. Using the data, we propose a constitutive description of the tissue that uses structurally-driven assumptions to capture and reproduce the anisotropic response.
References:
1. Tang, C. Y., Zhang, G., Tsui, C. P., 2009. A 3D skeletal muscle model coupled with active contraction of muscle fibres and hyperelastic behaviour. J. Biomech. 42, 865-872.
2. Lee, D., Glueck, M., Khan, A., Fiume, E., Jackson, K., 2010. A survey of modeling and simulation of skeletal muscle. Acm T. Graphic. 28, 106.

Tissue engineering for skin cancer and burn patients often requires removing and grafting of the keratin tissues which is an extremely painful process. An ideal process would therefore be using modified keratin tissues from other sources for skin grafting. Hence, recent research interests have focused on the mechanical properties of some keratin-based biological materials (e.g., wool, horn, hooves and avian quills). However, data on the properties of porcupine quill are uncommon. For this purpose, we report the mechanical behavior of porcupine quills and the structural evolution of the keratin intermediate filaments under tension.
Porcupine quill is a keratin-based biological material composed of a cylindrical outer shell with a reinforcing inner foam core. The mechanical properties and fracture behavior of North American porcupine quills conditioned at relative humidities of 65% and 100% showed that increasing the water content decreased the tensile stiffness and strength and increased the ductility of the porcupine quills. The present tests also indicated that the shell of the porcupine quill, in contrast to the inner foam structure, carried the majority of the axial tensile loads. In addition, the quill shells&’ axial properties and resistance to nanoindentation were generally higher than similar mechanical properties measured in the circumferential direction of shells, in agreement with previous reports of an axial orientation of the keratin intermediate filaments in the shells. In-situ measurement of ATR-FTIR under tensile test of quill shells in the amide I band showed that the content of α-type keratin decreased while β-type keratin increased as the strain increased to 15% - consistent with strain accommodation in wool via unfolding of α-helices to β-sheets as reported in other studies. Scanning electron microscopy of the fracture surfaces of porcupine quills revealed that the outer shells of quills are composed of 2 - 3 distinct layers with different fracture characteristics, especially when the samples contain 100% RH. The outer layer of the porcupine shell appears to resist the plasticizing effects of moisture due to the presence of hydrophobic lipids in the outer layer. Microfibrils and intermediate filaments were also observed aligned parallel to the quill growth direction. Finally, nanoindentation revealed that the hardness and elastic modulus of the inner most layer of the quill shell, which consists of highly ordered α-type keratin, are about 20% higher than the middle and outer layer of the quill shell.

Keratin fibres are an important structural component of cells, such as found in skin and nails, that form a cytoskeleton network which gives these cells an elastic response and flexibility [1]. Synthetic keratin films could give inspiration for novel biomaterials where flexibility or shock absorbance could be advantageous. The keratin fibre consists of a hierarchical assembly, starting with the lowest sub-unit; the keratin dimer [2]. While some macro-scale measurements are possible on keratin films and some in vivo fibres, it is very challenging experimentally to isolate and measure the physical properties of an intact dimer, without these denaturing or recombining into the full filament structure.
We have recently obtained full atomistic structures of the K1/K10 keratin dimer [3] and are using these as a platform for gaining new insights into the mechanical properties of the fibre [4]. We perform molecular simulations of the keratin dimer in its native solvent environment as found in skin - i.e. in the presence of natural moisturizing factors. By changing the environment we investigate the impact on dimer mechanical properties. The aim of these simulations is to optimize mechanical parameters to be incorporated into a mesoscale model of the keratin fibre in which properties can be efficiently predicted.
[1] P Strnad, V Usachov, C Debes, F Gräter, F. DAD Parry, MB Omary, Unique amino acid signatures that are evolutionarily conserved distinguish simple-type, epidermal and hair keratins, Journal of Cell Science (2011) 124: 4221-4232.
[2] CH Lee, MS Kim, BM Chung, DJ Leahy, PA Coulombe,
Structural basis for heteromeric assembly and perinuclear organization of keratin filaments,
Nature Structural and Molecular Biology (2012) 19:707-715.
[3] DJ Bray, TR Walsh, M Noro, R Notman, Simulated full atomistic dimer structure of the epithelial keratin K1/K10 Intermediate filament, including the elusive head and tail domains, in preparation (2013).
[4] CC Chou, MJ Buehler, Structure and mechanical properties of human trichocyte keratin intermediate filament protein, Biomacromolecules (2012) 13: 3522 - 3532.

Bone is a natural protein (collagen)-mineral (hydroxyapatite) nanocomposite with hierarchically organized structure. Our previous work has demonstrated orientational differences in stoichiometry of hydroxyapatite resulting from orientationally dependent collagen-mineral interactions. The nature of these interactions have been investigated both through molecular dynamics simulations as well as nanomechanical and infrared spectroscopic experiments. In this study, we report experimental studies on human cortical bone with osteogenesis imperfecta (OI), a disease characterized by fragility of bones and other tissues rich in type I collagen. About 90% of OI cases are due to a causative variant in one of the two structural genes (COL1A1 or COL1A2) for type I procollagens. OI provides an interesting platform for investigating how alterations of collagen at the molecular level cause changes in structure and mechanics of bone. Fourier transform spectroscopy, electron microscopy, scanning probe microscopy, and nanomechanical tests describe the structural and molecular differences in bone ultrastructure due to presence of diseases. Photoacoustic-Fourier transform infrared spectroscopy (PA-FTIR) experiments have been conducted to investigate the orientational differences in molecular structure of OI bone, which is also compared with that of healthy human cortical bone. Further, comprehensive dynamic and static nanomechanical testing is conducted in the transverse and longitudinal directions in the OI bone. Microstructural defects and abnormities of OI bone were ascertained using scanning probe and scanning electron microscopies. These results provide an insight into molecular bases of deformation and mechanical behavior of healthy human bone and OI bone.

Soft biomaterials that act as scaffolds for bone and tissue growth have a multitude of applications in tissue engineering and regenerative mechanisms. The mechanical properties of such materials are particularly important. Human cells have themselves been shown to be sensitive - in terms of their response - to the mechanical rigidity of the underlying scaffold on the nanometer scale. Biomaterials are traditionally difficult to characterize in terms of micro and nanoscale response. These are complex, inhomogeneous networks with multiple structural length scales, as well as being of low modulus and viscoelastic in nature giving rise to a time-dependency in the mechanical response.
We demonstrate the use of novel and unique nanoindentation techniques and methodologies to explore the true stress-strain response of such biomaterials on the micro to nanoscale. We use a diamond, cylindrical flat-punch in conformal contact with a collagen scaffold layer of known thickness supported on a flat, high stiffness solid substrate. By the use of a flat punch geometry we are able to increase the window of sensitivity for such materials, commonly with elastic moduli in the kPa range, while maintaining an invariant contact geometry. Using a load-controlled instrumented nanoindenter, we create a unique state of confinement and directly access the stress-strain response. We gain access to structural length scales from tens of nanometers up to tens of microns, and sensitivity of mean contact stiffness in a range typically from 1000 to 150,000 N/m for a 30 micron diameter punch. Viscoelasticity is explored by both standard creep measurements and also small-amplitude (one to tens of nanometers) harmonic oscillations superimposed on the load signal. Such small-amplitude harmonic perturbations prove to be a powerful tool for reproducibly studying the local response.
We compare dehydrated collagen membranes of a few microns thick, deposited onto polished silicon wafers of sub nanometer surface roughness. The diameter of the diamond punch is around ten times that of the film thickness, such that we maintain an optimal aspect ratio that has been previously determined by experiment and simulation. Thermal drift is determined to be of the order of 0.05 nm/s and is systematically corrected for, together with instrument and substrate compliance. We study the effect of both collagen network cross-linking and profusion of the matrix with hyaluronic acid, which tends to be naturally present in many tissues of the human body. We show that at small strain (typically less than 10 percent) we are able to acquire both comparative viscoelastic stress-strain and harmonic stiffness data, with a single measurement duration of a few minutes. We show quantitatively how the viscoelastic response and average stiffness of the collagen matrix is modified by the introduction of cross links and approximately ten percent hyaluronic acid by mass.

Random fiber networks are micromechanical models representing many soft biological and engineering materials. Their mechanical behavior depends on system parameters such as the fiber density, orientation, type of cross-linking, fiber constitutive response, etc. In this work we study the relationship between these parameters and the system-scale response, and the size effects resulting from the intrinsic heterogeneity of the network. The presentation will review the main results of this work, such as the dependence of the size effect on system parameters, differences between 2D and 3D networks, and the effect of the presence of an embedding matrix on the overall elastic moduli and their size effect. Implications for the selection of representative volume elements in multiscale models of fibrous systems will be discussed.

Loss of myelin from axons impairs neuronal signal transduction and promotes axon death leading to permanent neurodegeneration, and is a hallmark of demyelinating diseases such as multiple sclerosis (MS). The natural process of myelin regeneration is carried out in the central nervous system by myelin producing oligodendrocytes that differentiate from oligodendrocyte precursor cells (OPCs). However, in MS remyelination efficiency declines with progression of the disease. It remains unclear how the changes in the biomechanical and biochemical microenvironment of demyelinating lesion contribute to decreased remyelination. Here, we show that the mechanical changes of the extracellular materials and the extracellular acidic pH, both characteristic features of demyelinating lesions, affect key biological processes involved in the response of OPCs to myelin loss. Specifically, we show that OPC survival, proliferation, migration, and differentiation in vitro depend on the mechanical stiffness of polymer hydrogels representing the range of brain tissue stiffness, and that these processes decrease on more compliant gels. Separately, we show that these processes decrease in acidic extracellular pH, and that OPCs migrate toward acidic pH in pH gradients representative of the interface between healthy tissue and acidic brain lesions. As these processes are integral to OPC response to axon demyelination, our results suggest that changed mechanical properties and acidity of demyelinating lesion could contribute to decreased remyelination.

Quantitative characterization of mechanical properties for soft materials, such as gels and biological tissues, remains a significant challenge for the field of materials science. Difficulties are due to the following: large strain and low force responses; nonlinear constitutive behaviors; and the impact of surface energy on deformation responses. This talk describes a simple, though not yet broadly adopted, procedure called Cavitation Rheology, which triggers responses that quantitatively correspond to Young&’s modulus and strain energy release rate in gels. Most importantly, this technique provides direct measurement of mechanical properties on local length scales, ranging from sub-micron to millimeter, and allows for 3-D characterization of property gradients, critical for biomaterials. While quantitative correlation has been demonstrated, open questions remain with regards to underlying deformation mechanisms in Cavitation Rheology. The principal concern of this talk is the observed deformation response, comprised of a triggered instability whose partially understood physical mechanisms and limitations are explained in detail and verified by experiment.
Cavitation rheology consists of the insertion, pressurization, and monitoring of a fluid filled needle. Pressurization is accomplished using an attached syringe. At the tip of the embedded needle, optical microscopy reveals that first, a cavity grows in direct response to plunger displacement (corresponding to increased pressure). At a critical point, the cavity expands rapidly, independently of plunger movement, resulting in a marked pressure drop as monitored by a pressure sensor. It is this pressure peak value, or critical pressure, that has been shown to correspond to both fracture toughness and elastic moduli in several hydrogel materials depending on the size of needle used.
The rapidity of the bubble expansion indicates that the deformation is associated with the onset of instability. This talk describes the nature of this instability, either elastic (reversible) or corresponding to fracture. It demarcates the limited parameter space for which rapid void growth may correlate to an elastic instability. It also leverages existing mechanisms for spherical void fracture to understand fracture in the embedded needle geometry using a combination of analytical theory and finite element analysis. These deformation mechanisms are translated into predictions for critical pressure as a function of needle size, void surface energy, and, interestingly, the ability of the pressurizing fluid to store energy. Using this new understanding of the relation between critical pressure and material properties, a transition between cavitation and fracture is illustrated for a triblock co-polymer gel system. These results not only provide a basis of understanding for a promising new experimental technique but also hold general implications for void-governed failure mechanisms in soft materials.

The aragonite-protein composite material out of which the shells of most molluscs are made has a fracture toughness of about 8 MParadic;m. This is surprisingly high considering that the aragonite phase has a bulk fracture toughness of only 0.9 MParadic;m. One reason for the improved performance of the shell relative to pure aragonite is its microstructure which consists of a strongly textured array of aragonite plates in a conchiolin matrix in a roughly 95:5 ratio by volume. Cracks that initiate in an aragonite platelet are deflected, blunted or arrested when they reach the more ductile conchiolin. We speculate that a compressive residual stress at strategic locations in the shell may further improve its resistance to crack propagation. A compressive residual stress in a composite material can cause bridging or closure of the crack tip and hence can slow crack growth.
To investigate this hypothesis we used neutron diffraction in an attempt to identify whether a detectable stress distribution exists in a large mollusc shell and, if so, whether this stress state can provide enhanced fracture toughness. The crystallographic texture of mollusc shells in general is relatively well understood and was not directly an objective of the study. Freshly collected shells of the gastropod Ninella torquata (family Turbinidae), which has a diameter of about 10 cm, were used. Samples were mounted in the diffractometer with the axis of coiling and surface patch of a whorl having a defined orientation with respect to the beam. The texture of the samples was readily extracted. The possible existence of a non-uniform stress distribution through the shell is analysed and discussed.