Humans perform fascinating science experiments at home on a daily basis when they undertake the modification of natural and naturally-derived materials by a cooking process prior to consumption. The material properties of such foods are of interest to food scientists (texture is often fundamental to food acceptability), oral biologists (foods modulate feeding behavior), anthropologists (cooking is probably as old as the genus Homo and distinguishes us from all other creatures) and dentists (foods interact with tooth and tooth replacement materials). Material scientists may be interested in the drastic changes in food properties observed over relatively short cooking times. In the current study, the mechanical properties of one of the most common (and oldest at 4,000+ years) foods on earth, the noodle, were examined as a function of cooking time. Two types of noodles, each made from natural materials (wheat flour, salt, alkali and water) by kneading dough and passing them through a pasta-making machine. These were boiled for between 2–14 min and tested at regular intervals from raw to an overcooked state. Cyclic tensile tests at small strain levels were used to examine energy dissipation characteristics. Energy dissipation was >50% per cycle in uncooked noodles, but decreased by an order of magnitude with cooking. Fractional dissipation values remained approximately constant at cooking times greater than 7 min. Overall, a greater effect of cooking was on viscoplastic dissipation characteristics rather than fracture resistance. The results of the current study plot the evolution of a viscoplastic mixture into an essentially elastic material in the space of 7 minutes and have broad implications for understanding what cooking does to food materials. In particular, they suggest that textural assessments by consumers of the optimally cooked state of food has a definite physical definition.

Nacre, the shiny inner layer of seashells is a model biomimetic system composed of 95% of inorganic (aragonite) phase and 5% of organic phase (mainly proteins and polysaccharides). Nacre exhibits an interlocked layered “brick and mortar” structure where the bricks are made up of aragonite and mortar is the organic phase. We have performed nanoindentation and dynamic nanoindentation tests to study the nanomechanical and dynamic nanomechanical response of nacre. The indentation experiments performed at low loads indicate an elastic modulus of about 15 GPa for the organic phase. The low load, low penetration experiments appear to be better indicators of nanomechanical behavior. Dynamic nanomechanical response of nacre was studied using dynamic nanoindentation (nano-DMA). Significant increase in the values of tan δ was observed with increase in frequency. Also the dynamic nanoindentation experiments indicate that nacre exhibits viscoelastic behavior. Further, fourier transform spectroscopy experiments of nacre in innate and undisturbed state indicate the presence of water in nacre. The nanograin structure of nacre platelets, as well as the entrapped and adsorbed water, is two important contributors to the viscoelastic response of nacre. Atomic force microscopy experiments also indicate a very high force to remove organic from the aragonite in nacre. These experiments provide important insight into nanomechanical response of nacre, its constituents and also interfaces.

In the current study, the effects of polar solvents on tissue volume and mechanical properties are considered. Area shrinkage measurements are conducted for mineralized, bone tissue samples soaked in polar solvents. Area shrinkage is used to calculate approximate linear and volume shrinkage. Results are compared with viscoelastic mechanical parameters for bone in the same solvents (as measured previously) and with both shrinkage measurements and mechanical data for nonmineralized tissues, as taken from the existing literature. As expected, the shrinkage of mineralized tissues is minimal when compared with shrinkage of nonmineralized tissues immersed in the same polar solvents. The mechanical changes in bone are also substantially less than in nonmineralized tissues. The largest stiffness values are found in shrunken bone samples (immersed in acetone and ethanol). The mineral phase in bone thus resists tissue shrinkage that would otherwise occur in the pure soft tissue phase.

Of all the internal organs, mechanical behavior of the lung is arguably most closely related to physiologic function. In inflation and deflation, lung parenchyma may be treated as an elastic material with some viscous damping. However, quasi-plasticity is observed, in the form of atelectasis, which is the localized collapse of alveoli under different conditions. General anesthesia in lung is known to increase tendency for pulmonary atelectasis, and this condition is typically removed by mechanical inflation to high pressures, which can be hazardous. The specific mechanisms of atelectasis are not fully known, one reason for this being the difficulty in developing a direct characterization method to perform causal investigations. In a previous abstract, we described the potential for controlled indentation tests to probe atelectatic tendency in lung. In this talk, we present the first results of ‘hardness testing’ on dog and rabbit lung, using different inflation schemes. Specifically, we indented excised lungs at physiologic pressures, inflated with air, pure oxygen, and 0.2% isoflurane in oxygen. Between these three conditions, we found marked differences in ‘hardness’ of the lung, when indented with tip radii comparable to that of ribs. That is to say, large contrasts in residual impressions, as well as re-inflation behavior, were observed. In addition, effects of different inflation gases occurred within a much shorter time than previously reported in other surgical experiments, indicating perhaps different, faster mechanisms controlling atelectasis than previously considered.

A nanostructural hydroxyapatite (HA) coating was deposited to a porous Ti6Al4V femoral stem implant using electrophoretic deposition (EPD) to improve the bond strength, healing speed, and durable service life. The nanostructure of HA and micro-pores created in the coating can enhance the bioactivity and increase the adhesion between the implant and hard tissue. In order to achieve homogenous HA coating on the stem implant with a porous surface profile, a compensation electrode was designed and prepared. Rotation of the implant was conducted during the coating process so that a uniform coating could be achieved. The EPD combined with a dip coating was used to compensate for the complex profile and led to a homogenous coating throughout both peak and valley areas.

This developed coating process is able to deposit homogeneous HA coating on a complicated implant surface. Compared to the coating made by plasma thermal spray, the EPD HA coating can cover inner cores and exhibit a better adhesion and bioactivity. The EPD HA coating possesses significantly improved corrosion resistance compared to thermal spray HA coating. Most importantly, the biomechanical property was significantly promoted based on an in vivo animal test.

Hydroxyapatite (HA) is a calcium phosphate mineral analogous to the mineral part of bone. This similarity makes this material bioactive and suitable to coat medical implants. However, at present, there is not a coating technique which gives the coated implant the desired properties and long life required for medical implants.

In an effort to produce HA coatings with improved properties, calcium phosphate films were prepared using magnetron sputtering deposition on a silicon substrate at 600°C. Initial efforts resulted in the deposition of amorphous films with a distinctive grain-like surface morphology. The morphological grain size was studied using SEM and found that it was possible to control the average diameter value of the round shaped grains by adjusting the deposition time. Increasing the deposition time increases the mean grain diameter. EDS spectra showed the unintentional addition of carbon, iron and nickel to the samples during deposition. After eliminating the impurities, it was possible to prepare calcium phosphate films in the HA phase but without the grain-like surface morphology. These results suggested that the impurities prevented the formation of the calcium phosphate HA phase while acting as nuclei for the heterogeneous nucleation of the grains. This is an important result where the deposition process parameters can be controlled to functionalize the films in order to produce distinctive nanoscale features in the surface morphology.

A type of static friction between two polymer brush coated surfaces, resulting from fluctuations from mean field theory is found, but with creep-like motion for forces below the force of static friction, which is much more rapid than the usual creep between solid surfaces in contact.

The seed shell of Mezzettia parviflora (Annonaceae), an understory tree from the lowland tropical rain forest of Malaysia, has an extraordinarily sophisticated structure. The shell is entirely made of sclerenchyma (mechanical support tissue) that is in all parts equally dense (about 95% of volume occupied by cell wall). Yet, differences in cell shape and orientation have a strong influence on fracture resistance. The structure appears designed to allow the seed to open for germination, but to limit predation by large animals.

A bio-inspired approach is employed to deposit the oxide films on the substrates coated with self-assembled organic layers. Particularly, titania and zirconia films are grown in aqueous precursor solutions at near room temperatures. This process, directed by the nanoscale organic template, mimics the controlled nucleation and growth of the biominerals such as bones and teeth. Multiscale structural evolution resulting from initial bulk nucleation, nanoparticle aggregation, and ultimate film formation are systematically studied by adjusting the precursor solution conditions. Dynamic light scattering (DLS) is utilized to characterize initial nanoparticles and their associated aggregates/clusters formed in situ in solution. Corresponding nano- and microstructure developments of the oxide films are investigated through high-resolution transmission electron microscope (TEM) and scanning electron microscope (SEM). In addition, mechanical performance is evaluated with the aid of a dynamic nanoindentation testing to establish the structure-property relationships of the bio-inspired oxide films. The goal of this study is to have a capability to tailor microstructures and mechanical behaviors by identifying the controlling mechanisms responsible for nucleation and growth of such oxide films.

Bone materials are characterized by an astonishing variability and diversity. Still, because of ‘architectural constraints’, their fundamental hierarchical organization or basic building plans remain largely unchanged during biological evolution. These building plans govern the mechanical interaction of the elementary components of bone (hydroxyapatite, collagen, water; with directly measurable tissue-independent elastic properties), which are here quantified through a multiscale homogenization scheme delivering effective elastic properties of bone materials: At a scale of 10 nm, long cylindrical collagen molecules, attached to each other at their ends by ∼1.5 nm long crosslinks and hosting intermolecular water inbetween, form a contiguous matrix called wet collagen. At a scale of several hundred nanometers, wet collagen and mineral crystal agglomerations interpenetrate each other, forming the mineralized fibril. At a scale of 5 microns, the solid bone matrix is represented as collagen fibril inclusions embedded in a foam of largely disordered (extrafibrillar) mineral crystals. Remarkably, needle and sphere type representations of disordered minerals deliver quasi-identical mechanical behavior of such extrafibrillar porous polycrystals. At a scale above the ultrastructure lacunae are embedded in extracellular bone matrix, forming the extravascular bone material. Model estimates predicted from tissue-specific composition data agree remarkably well with corresponding stiffness experiments across cortical and trabecular materials, which opens new possibilities in the exploitation of computer tomographic data for nano-to-macro mechanics of bone organs, especially in combination with currently investigated extensions towards damage and failure.

Indentation techniques are employed for the measurement of mechanical properties of a wide range of materials. In particular, techniques focused at small length-scales, such as nanoindentation and AFM indentation, allow for local characterization of material properties in heterogeneous materials including natural tissues and biomimetic materials. Typical elastic analysis for spherical indentation is applicable in the absence of time-dependent deformation, but is inappropriate for materials with time-dependent responses. Recent analyses for the viscoelastic indentation problem, based on elastic-viscoelastic correspondence, have begun to address the issue of time-dependent deformation during an indentation test. The viscoelastic analysis has been shown to fit experimental indentation data well, and has been demonstrated as useful for characterization of viscoelasticity in polymeric materials and in hydrated mineralized tissues. However, a viscoelastic analysis is not necessarily sufficient for multi-phase materials with fluid flow. In the current work, a poroelastic analysis—based on fluid motion through a porous elastic network—is used to examine spherical indentation creep responses of hydrated biological materials. Both analytical and finite element approaches are considered for the poroelastic Hertzian indentation problem. Modeling results are compared with experimental data from nanoindentation of hydrated bone immersed in water and polar solvents (ethanol, methanol, acetone). Baseline (water-immersed) bone responses are characterized using the poroelastic model and numerical results are compared with altered hydration states due to polar solvents.

This paper reports mechanical properties evolution of polydimethylsiloxane (PDMS) during the crosslinking process. In this work, PDMS crosslinking was induced by mixing base prepolymer and curing agent at certain ratios. The liquid prepolymer was spun coated on a silicon wafer, and the curvature change of the wafer was measured continuously using a curvature measurement system. The relationship between the curvature change and typical mechanical properties was investigated using a bilayer model; and the evolution of the properties was derived, as a function of operational parameters. This work is expected to help better understanding of the crosslinking process and provide practical strategies for controlling the mechanical behavior of the resulting polymer structures, especially those for mechanical sensing applications.

The specific adhesion mediated by receptor-ligand binding in the nanoindentation of cells was studied by using a continuum-kinetics approach. It was found that the adhesion force only causes a shift to the indentation loading curve whereas the unloading curve is tilted downward by the adhesion force, resulting in a hysteresis. Parametric studies were conducted to investigate how experimental conditions influence the adhesion force. It was found that the maximum adhesion force is sensitive to the geometry of the indenter but insensitive to the indentation depth and the mechanical properties of cells. It was also shown that the dependence of the adhesion force on the loading and unloading rates is controlled by the association rate of the receptor-ligand binding, leading to three well-defined regimes.

Polyacrylamide-based hydrogels are popular materials that have been extensively studied for their applications in the field of biomaterials due to their permeability and biological compatibility. A major limitation of this polymeric material, however, in biological applications stems from their limited mechanical stiffness. The current study examines the mechanical properties of polyacrylamide-polyacrylic acid-based hydrogels at varying volume fractions and compositions using oscillatory rheology. For a fixed chemical cross-linker concentration, an increase in the volume fraction of hydrogel resulted in an increase in the shear elastic modulus of the hydrogel. Additionally, varying the relative ratio of polyacrylamide to polyacrylic acid resulted in modest changes in the shear elastic modulus.

The excellent biocompatibility, biofunctionality, and non-antigenic property make chitosan an ideal material for tissue regeneration. In addition to that its hydrophilic surface promotes cell adhesion, proliferation, and differentiation, and evokes a minimal foreign body reaction on implantation. In spite of these favorable properties, the inadequate mechanical strength and loosening of structural integrity under wet conditions, limit its application for bone tissue engineering. To improve the suitability of chitosan for bone tissue engineering, we have biomimetically synthesized composites of chitosan, polygalacturonic acid and hydroxyapatite. Polygalacturonic acid (PgA) is biocompatible, biodegradable and electrostatically complementary to chitosan. The strong interactions between negatively charged carboxylate groups of PgA and positively charged amino groups of chitosan lead to complex formation. This biopolymer complex provides improved mechanical strength and better structural integrity under wet condition. In this study, we have investigated the applicability of chitosan-PgA-hydroxyapatite composites for bone tissue engineering.

This paper reports typical failure modes of polymer microstructures. Polydimethylsiloxane (PDMS) microstructures were fabricated using replication-based methods. The mechanical collapses of these structures were observed using scanning electron microscopy. In this work, we classified the mechanical collapses based on their individual geometries and discussed the mechanism of each failure mode. We also worked on solutions to recover these failed PDMS structures. This work helps to better understand the collapsing mechanism and to facilitate the development of control strategies to enhance the mechanical reliability of polymer microdevices.

The wasp petiole (waist) is a self-assembled, multifunctional, hierarchically structured tube, which in some species has a simple, near-cylindrical shape that simplifies mechanical property characterization. We describe studies performed by scanning electron microscopy and transmission electron microscopy that reveal several details about the hierarchical structure of mud dauber wasp petiole, including: the number and thickness of the concentric layers of cuticle comprising the wall; the similarity (and differences) between the wall structure and the structure of multi-layer corrugated cardboard; and the different anisotropies of the dorsal and ventral internal surfaces. We also describe a simple experiment in which a petiole is used as a cantilever to determine its stiffness in bending; the result (1.5 GPa) demonstrates a material efficiency in bending similar to that of GFRP and aluminum.

Bone is a highly ordered nanocomposite system consisting primarily of organic (collagen) and inorganic (hydroxyapatite(HAP)) phases. The nanocrystals of hydroxyapatite (HAP) mineralize at specific locations in bone. The collagen molecule in bone has C and N terminals which are known as C and N telopeptides. We have performed Molecular Dynamics (MD) and Steered MD (SMD) simulations to study the interface between HAP (mineral) and collagen with N-terminal telopeptides. The force field parameters for HAP were taken from our previous study and the force field parameters for collagen with telopeptides were taken from CHARMm force field. We have investigated the detailed conformation of telopeptides interacting with specific surfaces of mineral and the interface between HAP and collagen. The load-deformation behavior of collagen is obtained with SMD simulations. It has been found that the load deformation behavior of collagen is different when in close proximity of and interacting with HAP.

We report molecular modeling of stretching single molecules of tropocollagen, the building block of collagen fibrils and fibers that provide mechanical support in connective tissues. For small deformation, we observe a dominance of entropic elasticity. At larger deformation, we find a transition to energetic elasticity, which is characterized by first stretching and breaking of hydrogen bonds, followed by deformation of covalent bonds in the protein backbone, eventually leading to molecular fracture. Our force-displacement curves show excellent quantitative agreement with optical tweezer experiments, suggesting a persistence length of approximately 16 nm. We demonstrate that assembly of single TC molecules into fibrils significantly decreases their flexibility, leading to decreased contributions of entropic effects during deformation. We develop a simple continuum model capable of describing entire deformation range of TC molecules.

In living eukaryotic cells a crosslinked network of polymer fibers, the cytoskeleton, endows the cells with structural integrity and mechanical stability and flexibility. To understand the mechanisms that are at the base of these functions, it is important to know in what way the microstructure and the mechanical behavior of the cytoskeleton change as a function of the type and the density of crosslinking molecules. To address this issue, we have developed a new modeling approach based on the discretization of polymeric fibers that are modeled as homogeneous straight beams in a constant volume. Crosslinks between adjacent fibers are taken into account by creating additional beams between the fibers if their spacing is smaller than a meaningful upper bound. By varying their geometrical and mechanical properties, the influence of the crosslinks on the shear modulus of the network can be studied systematically. Our simulations predict interesting new scaling behaviors that depend on the degree of crosslinking.