Nacre, the shiny inner layer of mollusk shells is a model biomimetic nanocomposite system. Its exceptional mechanical properties have been the inspiration for materials scientists for several decades. Nacre exhibits a layered brick and mortar structure. It is composed of 95% inorganic (aragonitic CaCO3) phase and 5% organic (mainly proteins and polysaccharides) phase that are arranged in interlocked brick and mortar architecture with the mineral as bricks and organics as the mortar. In the current work, we describe the dynamic nanomechanical behavior of nacre using dynamic nanoindentation (nano-DMA) experiments. Two sets of loads were applied to obtain the dynamic response from varying depths in nacre. These tests were performed at three different frequencies (25, 50, and 100 Hz) to study the effect of frequency on the dynamic properties of nacre. The loss modulus (E″) and the loss factor (tan δ) were measured. Both of these parameters were observed to increase with increase in depth. Significant increase in tan δ was observed with the increase in frequency. Photoacoustic Fourier transform infrared spectroscopic studies on nacre indicate the presence of water in nacre. This water may be present at nanograin interfaces in nacre platelets, at organic–inorganic interfaces, and also in the organic phase in nacre. We believe that water is one of the significant contributors to the viscoelasticity of nacre. Our results indicate that the aragonite platelets in nacre may also contribute to viscoelasticity.

White spot lesions are clinically detectable areas of demineralized enamel that often form during orthodontic treatment. Fluoride has been shown to prevent demineralization from occurring. Mechanical properties of white spot lesions are not well characterized. Bovine enamel slabs, with and without fluoride treatment, were placed under demineralization conditions. Through a series of microindentations at incremental loads, mechanical strength was measured using a novel method, specific volume absorbing energy (SVAE). SVAE is equal to work energy divided by the indentation volume. The supra-surface area (outermost 5 μm) of enamel slabs with fluoride demonstrated decreased mechanical strength compared to enamel slabs without fluoride. Fluoride may not impart protection over long periods of demineralization.

Nastic structures are synthetic constructs capable of controllable deformation and shape change similar to plant motility, designed to imitate the biological process of nastic movement found in plants. This paper considers the mechanics and bioenergetics of a prototype nastic structure system consisting of an array of cylindrical microhydraulic actuators embedded in a polymeric plate. Non-uniform expansion/contraction of the actuators in the array may yield an overall shape change resulting in structural morphing. Actuator expansion/contraction is achieved through pressure changes produced by active transport across a bilayer membrane. The active transport process relies on ion-channel proteins that pump sucrose and water molecules across a plasma membrane against the pressure gradient. The energy required by this process is supplied by the hydrolysis of adenosine triphosphate. After reviewing the biochemistry and bioenergetics of the active transport process, the paper presents an analysis of the microhydraulic actuator mechanics predicting the resulting displacement and output energy. Experimental demonstration of fluid transport through a protein transporter follows this discussion. The bilayer membrane is formed from 1-Palmitoyl-2-Oleoyl-sn-Glycero-3-[Phospho-L-Serine] (Sodium Salt), 1-Palmitoyl-2-Oleoyl-sn-Glycero- 3-Phosphoethanolamine lipids to support the AtSUT4 H+-sucrose cotransporter.

The silica skeleton of the deep-sea sponge Euplectella aspergillum was recently shown to be structured over at least six levels of hierarchy with a clear mechanical functionality. In particular, the skeleton is built of laminated spicules that consist of alternating layers of silica and organic material. In the present work, we investigated the micromechanical properties of the composite material in spicules of Euplectella aspergillum and the giant anchor spicule of Monorhaphis chuni. Organic layers were visualized by backscattered electron imaging in the environmental scanning electron microscope. Raman spectroscopic imaging showed that the organic layers are protein-rich and that there is an OH-enrichment in silica near the central organic filament of the spicule. Small-angle x-ray scattering revealed the presence of nanospheres with a diameter of only 2.8 nm as the basic units of silica. Nanoindentation showed a considerably reduced stiffness of the spicule silica compared to technical quartz glass with different degrees of hydration. Moreover, stiffness and hardness were shown to oscillate as a result of the laminate structure of the spicules. In summary, biogenic silica from deep-sea sponges has reduced stiffness but an architecture providing substantial toughening over that of technical glass, both by structuring at the nanometer and at the micrometer level.

Tendons may be classified as intrasynovial or extrasynovial based upon their anatomic location. We studied the physical properties of extrasynovial and intrasynovial tendons. Extrasynovial tendon has a rougher surface and higher friction than intrasynovial tendon. A number of carbodiimide derivatized biomaterials were developed to modify the irregular surface of an extrasynovial tendon to mimic the smooth surface of an intrasynovial tendon. We found that carbodiimide derivatized hyaluronic acid-gelatin (cd-HA-gelatin) chemically bonded to the extrasynovial tendon surface and optimally reduced gliding resistance in vitro. This biosynthetic, biomimetic modification of the extrasynovial tendon surface may prove to be a useful clinical product.

The mechanical response of living tissue is important to understanding the injury-risk associated with impact events. Often, ballistic gelatin or synthetic materials are developed to serve as tissue surrogates in mechanical testing. Unfortunately, current materials are not optimal and present several experimental challenges. Bulk measurement techniques, such as compression and shear testing geometries, do not fully represent the stress states and rate of loading experienced in an actual impact event. Indentation testing induces deviatoric stress states as well as strain rates not typically available to bulk measurement equipment. In this work, a ballistic gelatin and two styrene-isoprene triblock copolymer gels are tested and compared using both macroscale and microscale measurements. A methodology is presented to conduct instrumented indentation experiments on materials with a modulus far below 1 MPa. The synthetic triblock copolymer gels were much easier to test than the ballistic gelatin. Compared to ballistic gelatin, both copolymer gels were found to have a greater degree of thermal stability. All of the materials exhibit strain-rate dependence, although the magnitude of dependence was a function of the loading rate and testing method.

Investigations of bone mechanical properties are of major importance for bone pathology research, biomaterials, and development of in vivo bone characterization devices. Because of its complex multiscale structure, assessment of bone microstructure is an important step for understanding its mechanical behavior. In this study, we have investigated the strain rate influence on the mechanical properties of interstitial lamellae on two human femur bone samples. Nanoindentation tests were performed with the continuous stiffness measurement technique. Young's modulus and hardness were calculated using the Oliver and Pharr method. A statistical significant influence of strain rate on hardness was found (p < 0.05) showing a viscoplastic behavior of interstitial bone at the micrometer scale. This phenomenon may reflect the role of the organic component in the bone matrix mechanical behavior.

A variety of biological materials composed of hierarchical phase composites can be found. These biological materials successfully combine impressive toughness with high stiffness and strength. Deposition techniques combined with high-power laser beams can imitate biological structures in technical systems. Interference phenomena, such as surface scattering, Lloyd's mirror arrangements, or interference of coherent beams, can be used to create these biomimetic long-range ordered structures on the scale of nanometers to micrometers. These structures are not limited to topographic texturing, as in the case of holographic gratings; rather, they can also create composite structures and phase transformations. This article presents a brief overview of interference techniques, their possibilities, and their limits.

Cancellous bone plays a crucial structural role in the skeleton, yet little is known about the microstructure-mechanical property relationships of the tissue at the microscale. Cancellous tissue is characterized by a microstructure consisting of layers interspaced with transition zones with different proportions of collagen and mineral. In this study, the quasistatic and dynamic mechanical properties of lamellar and interlamellar tissue in human vertebrae were assessed with nanoindentation, and the collagen content and organization were characterized with second harmonic generation microscopy. Lamellar tissue was 35% stiffer, 25% harder, and had a 13% lower loss tangent relative to interlamellar tissue. The stiff, hard lamellae corresponded to areas of highly ordered, collagen-rich material, with a relatively low loss tangent, whereas the compliant, soft interlamellar regions corresponded to areas of disordered or collagen-poor material. These data suggest an important role for collagen in the tissue-level mechanical properties of bone.

Control over mechanical properties of hydrogels is of primary importance for the use of these materials in drug delivery and tissue engineering applications. We demonstrate here that crystallinity and block length of poly(lactide) (PLA) can be used to tune the elastic modulus of associative network gels of poly(lactide)–poly(ethylene oxide)–poly(lactide) over several orders of magnitude. Polymers made with crystalline L lactic acid blocks formed very stiff hydrogels at 25 wt% concentration with an elastic modulus that was almost an order of magnitude higher than hydrogels of polymers with a similar molecular weight but containing amorphous D/L-lactic acid blocks. The relaxation behavior and crosslink density of gels are also significantly influenced by crystallinity of PLA and are again a function of PLA block length. Using these variables we can design new tailor-made materials for biomedical applications with precise control over their structure and mechanical properties.

Computational models of the cell nucleus, along with experimental observations, can help in understanding the biomechanics of force-induced nuclear deformation and mechanisms of stress transition throughout the nucleus. Here, we develop a computational model for an isolated nucleus undergoing indentation, which includes separate components representing the nucleoplasm and the nuclear envelope. The nuclear envelope itself is composed of three separate layers: two thin elastic layers representing the inner and outer nuclear membranes and one thicker layer representing the nuclear lamina. The proposed model is capable of separating the structural role of major nuclear components in the force-induced biological response of the nucleus (and ultimately the cell). A systematic analysis is carried out to explore the role of major individual nuclear elements, namely inner and outer membranes, nuclear lamina, and nucleoplasm, as well as the loading and experimental factors such as indentation rate and probe angle, on the biomechanical response of an isolated nucleus in atomic force microscopy indentation experiment.

A micromechanical model was developed to predict the elastic moduli of hydroxyapatite (HA) whisker reinforced polymer biocomposites based upon the elastic properties of each phase and the reinforcement volume fraction, morphology, and preferred orientation. The effects of the HA whisker volume fraction, morphology, and orientation distribution were investigated by comparing model predictions with experimentally measured elastic moduli for HA whisker reinforced high-density polyethylene composites. Predictions using experimental measurements of the HA whisker aspect ratio distribution and orientation distribution were also compared to common idealized assumptions. The best model predictions were obtained using the experimentally measured HA whisker aspect ratio distribution and orientation distribution.

The cementum–enamel junction (CEJ) is a unique interface that plays a critical role in effectively transferring force from the top of the tooth to the jawbone. A comparative study of bovine and equine teeth has been carried out experimentally to establish the relationship among the interfacial structure, fracture resistance, and mechanical functions. The CEJs were analyzed by optical microscopy and scanning electron microscopy (SEM). Microhardness and elastic modulus were measured with a Vickers microhardness tester and a nanoindentation system. A modified cantilever beam technique was used to study CEJ fracture process and the toughness (Gc,). It was observed that CEJ is a distinct interface connecting the hard enamel (200–350 kg/mm2) with the soft cementum (∼20–30 kg/mm2). Bovine CEJ was macroscopically flat and smooth. However, under SEM, a close integration of cementum bumps and enamel pits at the 5 μm level and dimples at the 200 nm level were clearly seen. In equine CEJ, the similar flat cementum–enamel interface was interrupted by hemispherical cementum protrusions (average size of 28.3 μm) at high density (340/mm2). Despite the microscopic roughness, bovine CEJ was easy to fracture at relatively low toughness (128 J/m2). The measured toughness for equine CEJ was about seven times that of bovine (1140 J/m2). The high toughness has been attributed to the mechanical interlocking provided by the cementum protrusions. The underlying crack bridging mechanism and the applications to the design of orthopaedic implants are briefly discussed.