Aspects of mechanical deformability and biorheology of the human red blood cell are known to play a pivotal role in influencing organ function as well as states of overall health and disease. In this article, consequences of alterations to the membrane and cytoskeletal molecular structure of the human red blood cell are considered in the context of an infectious disease, Plasmodium falciparum malaria, and several hereditary hemolytic disorders: spherocytosis, elliptocytosis, and sickle cell anemia. In each of these cases, the effects of altered cell shape or molecular structure on cell elasticity, motility, and biorheology are examined. These examples are used to gain broad perspectives on the connections among cell and subcellular structure, properties, and disease at the intersections of engineering, biology, and medicine.

From a public health perspective, developing a detailed mechanistic understanding of the well-known increase with age in fracture risk of human bone is essential. This also represents a challenge from materials science and fracture mechanics viewpoints. Bone has a complex, hierarchical structure with characteristic features ranging from nanometer to macroscopic dimensions; it is therefore significantly more complex than most engineering materials. Nevertheless, by examining the micro-/nanostructural changes accompanying the process of aging using appropriate multiscale experimental methods and relating them to fracture mechanics data, it is possible to obtain a quantitative picture of how bone resists fracture. As human cortical bone exhibits rising ex vivo crack-growth resistance with crack extension, its fracture toughness must be evaluated in terms of resistance-curve (R-curve) behavior. While the crack initiation toughness declines with age, the more striking finding is that the crack-growth toughness declines even more significantly and is essentially absent in bone from donors exceeding 85 years in age. To explain such an age-induced deterioration in the toughness of bone, we evaluate its fracture properties at multiple length scales, specifically at the molecular and nano dimensions using vibrational spectroscopies, at the microscale using electron microscopy and hard/soft x-ray computed tomography, and at the macroscale using R-curve measurements. We show that the reduction in crack-growth toughness is associated primarily with a degradation in the degree of extrinsic toughening, in particular involving crack bridging, and that this occurs at relatively coarse size scales in the range of tens to hundreds of micrometers. Finally, we briefly describe how specific clinical treatments, e.g., with steroid hormones to treat various inflammatory conditions, can prematurely damage bone, thereby reducing its fracture resistance, whereas regulating the level of the cytokine Transforming Growth Factor-β can offer significant improvements in the stiffness, strength, and toughness of bone and as such may be considered a therapeutic target to treat increased bone fragility induced by aging, drugs, and disease.

In the last decade, most publications on the mechanical properties of dental calcified tissues were based on nanoindentation investigation. This technique has allowed a better understanding of the mechanical behavior of enamel, dentin, and cementum at a nanoscale. The indentations are normally carried out using pointed or spherical indenters. Hardness and elastic modulus are measured as a function of indenter penetration depth and from the elastic recovery upon unloading. The unique microstructure of each calcified tissue significantly contributes to the variations in the mechanical properties measured. As complex hydrated biological composites, the relative proportions of the composite components, namely, inorganic material (hydroxyapatite), organic material, and water, determines the mechanical properties of the dental hard tissues. Many pathological conditions affecting dental hard tissues cause changes in mineral levels, crystalline structures, and mechanical properties that may be probed by nanoindentation. This review focuses on relevant nanoindentation techniques and their applications to enamel, dentin, and cementum investigations.

A single elastic modulus is not sufficient for describing the mechanical behavior of a living cell due to its viscoelastic nature and heterogeneity beneath the membrane. In this paper, the nanoscale elastic and viscoelastic behavior of individual living Chinese hamster ovary (CHO-K1) cells in a physiological environment were probed by atomic force microscopy (AFM) indentations at various loading rates. Based on Hertzian fits of the force–distance curves, the apparent elastic modulus of the cells was determined and found to be a function of the loading rate as well as the indentation depth. Notably, contributions from the substrate were negligible up to 50% of the cell thickness. For increased indentation rates and depths, healthy spindle-shaped CHO-K1 cells were found to exhibit an increased change of stiffness, whereas for unhealthy oval- shaped CHO-K1 cells there was little stiffening at equivalent loading rates and depths. Furthermore, a larger hysteresis between the loading and unloading curves was observed with increasing loading rates, which was related to the viscoelastic behavior of CHO-K1 cells. This work demonstrates differences in the rate- and depth-dependent elastic behavior at the nanoscale level between healthy and unhealthy mammalian cells.

The secondary osteon is the fundamental building block of compact cortical bone at the tissue level. Light and scanning electron microscopy have shown that the osteon consists of a laminated cylindrical composite of mineralized collagen fibril lamellae ∼5–7 μm thick. Using scanning nanoindentation and quantitative backscattered electron imaging on secondary osteons from the human femoral midshaft, we found that the indentation modulus shows a periodic variation between ∼24 GPa and ∼27 GPa within a single lamella. The average lamellar value remains nearly constant across the osteon and increases abruptly to more than 30 GPa at the interstitial bone interface. The local mineral content, determined from quantitative backscattered electron imaging at the indented locations, shows also a lamellar level modulation and is positively correlated with the indentation modulus at the same tissue position. We propose that such a mechanically and compositionally modulated structure may be an effective crack-stopping mechanism in bone.

This paper presents a shear assay method for the determination of the viscoelastic properties of biological cells. The method was applied to the measurement of the viscoelastic properties of human osteosarcoma (HOS) cells. It involves a combination of shear assay experiments and digital image correlation techniques. Following in situ observations of cell deformation during shear assay experiments, a digital image correlation (DIC) technique was used to determine the local displacement and strain fields. The creep curves were also extracted from multiple digital images that were used to extract the time dependence of local strain under constant stress conditions. The measured creep curves were well described by a generalized viscoelastic Maxwell model. The extracted elastic and viscous parameters were in good agreement with results obtained from prior studies with other techniques. The results also suggested that the nucleus is stiffer than the surrounding cytoplasm of HOS cells.

Spider silk fibers were obtained by the monitored forced silking method. This procedure allows measurement of the silking force during the process and retrieving the fibers so their tensile behavior can be characterized. Silking conditions, including the reeling speed and the use of an anaesthetising gas, were varied to ascertain their influence on the tensile properties of the silk. In all cases, it was found that the tensile properties are determined by the silking stress, obtained by dividing the silking force by the diameter of the fiber. This suggests that the sophisticated spinning system of the spider can be characterized essentially by a single parameter, which controls the properties of spider silk almost independently of the reeling conditions.

We present a structural feature of nacre in the red abalone shell: micrometer-scale screw dislocations in the aragonite layers and resultant growth via spiral motion. Compared to typical ionic or covalent crystals, nacre contains 106 screw dislocations per square centimeter, a difference of three orders of magnitude. Using electron microscopy, ion microscopy, and an in situ nano-manipulator, we studied the structure of screw dislocation cores in detail. We considered that these screw dislocations contribute significantly to the strengthening mechanisms that lead to nacre's extraordinary work of fracture, which is three orders of magnitude greater than that of aragonite and other monolithic crystals. This work suggests that the lamellar layers of aragonite propagate via a large number of continuous spiral growth domains as the “stacks of coins” become confluent. This model may provide a basis for creating new comparable micro/nanocomposites through synthetic or biomineralization means.

We report studies of the mechanical properties of tropocollagen molecules under different types of mechanical loading including tension, compression, shear, and bending. Our modeling yields predictions of the fracture strength of single tropocollagen molecules and polypeptides, and also allows for quantification of the interactions between tropocollagen molecules. Atomistic modeling predicts a persistence length of tropocollagen molecules ξ ≈ 23.4 nm, close to experimental measurements. Our studies suggest that to describe large-strain or hyperelastic properties, it is critical to include a correct description of the bond behavior and breaking processes at large bond stretch, information that stems from the quantum chemical details of bonding. We use full atomistic calculations to derive parameters for a mesoscopic bead-spring model of tropocollagen molecules. We demonstrate that the mesoscopic model enables one to study the finite temperature, long-time scale behavior of tropocollagen fibers, illustrating the dynamics of solvated tropocollagen molecules for different molecular lengths.

Bone is a composite material with hydroxyapatite mineral, collagen, and water as primary components. Water plays an important role in maintaining the mechanical integrity of the composite, but the manner in which water interacts within the collagen and mineral at ultrastructural length-scales is poorly understood. The current study examined changes in the mechanical properties of bone as a function of hydration state. Tissues were soaked in different solvents and solutions, with different polarities, to manipulate tissue hydration. Mineralized bone was characterized using nanoindentation creep tests for quantification of both the elastic and viscoelastic mechanical responses, which varied dramatically with tissue bathing solution. The results were considered within the context of solution physical chemistry. Selectively removing and then replacing water provided insights into the ultrastructure of the tissues via the corresponding changes in the experimentally determined mechanical responses.

The structure, composition, and mechanical response of Australian bamboo were investigated. The graded structure, composition, and mechanical properties were confirmed by depth profiles obtained using synchrotron radiation diffraction and Vickers indentation. The mechanical performance of bamboo was strongly dependent on age. Results indicated that young bamboo has a higher strength, elastic stiffness, and fracture toughness than its older counterpart does. In addition, the hardness of bamboo is both load dependent and time dependent as a result of an expanding interfacial damage zone and indentation creep, respectively. In addition to fiber debonding, crack deflection and crack-bridging are the major energy dissipative processes for imparting a high toughness in bamboo.

The mechanical properties of nacre constituents from red abalone were investigated. Electron microscopy studies revealed that the tablets are composed of single-crystal aragonite with nanograin inclusions. Both nanoasperities and aragonite bridges are present within the interfaces between the tablets. By means of nanoindentation and axial compression tests, we identified single tablet elastic and inelastic properties. The elastic properties are very similar to those of single-crystal aragonite. However, their strength is higher than previously reported values for aragonite. A finite element model of the interface accounting for nanoasperities and the identified properties revealed that the nanoasperities are strong enough to withstand climbing and resist tablet sliding, at least over the initial stages of deformation. Furthermore, it was observed that the model over-predicts strength and under-predicts ductility. Therefore, we conclude that other interface features must be responsible for the enhanced performance of nacre over its constituents.

The mechanical properties of biological materials are well adjusted to their function. An excellent example for such materials is the cuticle or exoskeleton of arthropods. In this study, dehydrated cuticle of the American lobster Homarus americanus was examined as a model for a mineralized biological composite material. Nanoindentation testing is a powerful method for revealing gradients and anisotropy in the hardness and the elastic properties of such materials. The air-dried test specimens stem from different parts of the crusher claw with different biological functions. Both the exocuticle and the endocuticle were probed in normal and in the transverse direction to the cuticle surface. For estimating variations in the grade of mineralization, the samples which were tested as cross-sections of the cuticle were analyzed by the use of energy dispersive x-ray mapping. The microstructure of fracture surfaces of the test specimens was investigated using scanning electron microscopy. Due to the use of dehydrated samples, our results do not reflect the exact properties of lobster cuticle in the natural hydrated state, but they can be regarded as a fairly good approximation to the in vivo state.

Acquired salivary pellicle is a thin protein-rich film formed by the adsorption of saliva onto teeth. It plays important roles in lubrication during mastication and protecting the teeth from chemical attack. Pellicle can become colonized by bacteria to form dental plaque which can lead to dental caries if the bacteria are acidogenic. Abrasive polishing with a dentrifice is used periodically to remove the pellicle from teeth. Pellicle can interact with dietary polyphenolic compounds (tannins) to create extrinsic stains on the tooth surface. The staining can modify the pellicle's mechanical properties and change its morphology resulting in a “squeaky” feeling when the tongue is rubbed over the teeth. Atomic force microscopy imaging and nanoscale mechanical measurements show that unstained pellicle has a dense undulating morphology and is a surprisingly stiff, viscoelastic solid. In contrast, tannin-stained pellicle has fewer but larger surface undulations and exhibits substantial viscous creep.

Elastic-viscoelastic correspondence was used to generate displacement–time solutions for spherical indentation testing of soft biological materials with time-dependent mechanical behavior. Boltzmann hereditary integral operators were used to determine solutions for indentation load-relaxation following a constant displacement rate ramp. A “ramp correction factor” approach was used for routine analysis of experimental load-relaxation data. Experimental load-relaxation tests were performed on rubber, as well as kidney tissue and costal cartilage, two hydrated soft biological tissues with vastly different mechanical responses. The experimental data were fit to the spherical indentation ramp-relaxation solutions to obtain values of short- and long-time shear modulus and of material time constants. The method is used to demonstrate linearly viscoelastic responses in rubber, level-independent indentation results for costal cartilage, and age-independent indentation results for kidney parenchymal tissue.

Over millions of years, natural constructions have been developed by evolutionary processes in nature with the general aim to generate high-performance structures with minimum material consumption. It is well known that one of the major strategies is to reach a largely homogeneous material load in the biological structure. In this paper, we demonstrate that this natural design principal applies not only to the macroscopic level but also to microstructural optimization, and in particular, to adhesive joints in nature. Furthermore, we introduce the computer aided adhesive and assembly optimization method (CA3O), which transfers the natural mechanically stimulated growth processes to technical adhesive joints by means of the finite element method.

Ionic polymer metal composite (IPMC) is a promising candidate for artificial muscles and other bio-related applications. Compared with the widely reported Nafion, Flemion-based IPMC has some more attractive advantages. To improve its mechanical stability in dry environments, the organic solvent glycerol was applied. The mechanical characteristics such as tip displacement, tip force and durability were measured and analyzed. The results showed that Flemion-based IPMC with glycerol as solvent has optimum mechanical stability in dry conditions and will be a good candidate for artificial muscles working in air.

The underlying relationships between the microstructure and time-dependent mechanical properties of hydrated fibrin, collagen, and fibrin/collagen composite materials have been explored using an adaptation of the classical rigid, cylindrical, flat punch loaded normally to a planar specimen surface. A suite of quasi-static elastic and viscoelastic indentation experiments have been conducted with uniformly mixed fibrin, collagen, and fibrin/collagen composites, in addition to macrolayered collagen materials. Coupled with insights obtained from optical and confocal fluorescence microscopy, a simple micromechanics model has been developed for the effect of local microstructural variables on the macroscopic mechanical stiffness. These results demonstrate the efficacy of this technique to efficiently and reproducibly probe hydrated engineered tissue replacement materials for local variations in viscoelastic material behavior without the need for extensive specimen preparation or grips, as well as being suitable for performing directly comparable measurements with explants of human skin.

Spider silk is a material with unique mechanical properties under tension. In this study, we explore the anisotropic mechanical properties of spider silk using instrumented indentation. Both quasistatic indentation and dynamic stiffness imaging techniques were used to measure the mechanical properties in transverse and longitudinal sections of silk fibers. Quasistatic indentation yielded moduli of 10 ± 2 GPa in transverse sections and moduli of 6.4 ± 0.5 GPa in longitudinal sections, demonstrating mechanical anisotropy in the fiber. This result was supported by dynamic stiffness imaging, which also showed the average reduced modulus measured in the transverse section to be slightly higher than that of the longitudinal section. Stiffness imaging further revealed an oriented microstructure in the fiber, showing microfibrils aligned with the drawing axis of the fiber. No spatial distribution of modulus across the silk sections was observed by either quasistatic or stiffness imaging mechanics.