Mechanical property measurement protocols have their origins in metallurgy as well as in mechanical and civil engineering—metals being the first materials used on a broad industrial scale. Recent decades have evidenced growing interest in applying these protocols to biological materials or materials mimicking or replacing biological tissue. However, the mechanical properties of biological materials have been found to be highly variable and seemingly hard to capture by traditional protocols. A slow, emerging thought is that perhaps the mechanical theories underlying the testing protocols emanating from the metals field might not be fully applicable to the highly complex, hierarchically organized biological materials and might need further development. The articles in this issue highlight different and complementary, yet also interdependent, approaches to the challenge of extending theoretical and applied mechanics to the level needed for satisfying and reliably capturing the properties of biological materials. This issue also encompasses corresponding, far-reaching consequences for measurement techniques and evaluation protocols aimed at the determination of mechanical protocols.

This article presents an overview of various material models used to represent the mechanical behavior of arteries and the inverse problems posed by the identification of their constitutive parameters. After briefly defining inverse problems and describing the general features of arteries, this article addresses three main queries involving inverse problems and arterial wall characterization: (1) macroscopic identification of the parameters of sophisticated constitutive models from traditional uniaxial and biaxial experiments; (2) mesoscopic identification of regional variations in the material parameters of arteries, tracking the effects of functional adaptation or lesions; and (3) how constitutive models and inverse problems allow information to be obtained on the arterial microstructure and how the structural constituents interact in the mechanical response. Finally, the article shows that while significant effort has been made to relate the complex mechanical behavior of arteries to their microstructure, a new class of inverse problems has recently appeared related to the identification of mechanobiological parameters, which are involved in the numerical models of growth and remodeling.

Lumbar intervertebral disc (IVD) degeneration is the leading cause of lower back pain. While lumbar IVDs have low cellularity and limited capacity of regeneration, they bear high mechanical loads. Accelerated disc degeneration may happen because of undue cell stimulation, and cell nutrition also seems to be particularly influent in the disc. Cell nutrition depends on exchange of oxygen, glucose, and lactate between the periphery and the various regions of the IVD, which in turn depends on mechanical deformations. The mechanical regulation of disc cell nutrition, known as indirect mechanotransduction, is difficult to study in vivo or in vitro. This review reports on an important alternative in the form of numerical methods, which are based on the ability of poromechanical models to be coupled to solute transport-reaction models. Models need to address intricate nonlinear and time-dependent phenomena, but have allowed for important mechanisms to be proposed, complementing current knowledge gained from in vivo or in vitro observations.

Certain biocomposites exploit the combination of soft and hard elements to achieve high strength and toughness. In nacre, found inside certain seashells or on the surface of pearls, hard layers of micron-scale thickness are glued together by thin layers of soft proteins to realize remarkable strength and toughness. In spider webs, stiffer radial threads are connected by softer spiral threads to produce a light and resistant structure. In the exoskeleton of lobsters, organic fibers form a chiral structure in an inorganic matrix. This article reviews progress in the understanding of the mechanical superiority of such soft-hard biocomposites. In particular, simple physical views are presented that allow an intuitive understanding of how their remarkable structures contribute to enhancing their fracture resistance in the presence of cracks, and how such structures are physically optimized in terms of mechanical properties. Such fundamental insights could be useful as guiding principles for developing artificial, reinforced materials.

Cellulose nanocrystals (CNCs) are naturally occurring, structural material building blocks, which exhibit great potential for future multifunctional nanocomposites due to their high bioavailability, low cost, and impressive mechanical properties. Recent research on CNCs has focused on isolation techniques, crystal morphology, mechanical property characterization, and development of hierarchical materials, including CNC thin films and CNC-based nanocomposites. These studies have revealed that the unique conformation, structure, and surface chemistry of CNCs contribute directly to their outstanding mechanical performance and anisotropic features. To better facilitate their applications in hierarchical, bioinspired materials and exploit the inherent benefits of these biological building blocks, interfacial interactions and mechanics of CNCs with various materials, including other nanocrystals, polymer matrices, and small-molecule solvents, must be explored. This review highlights recent work focusing on the interfacial mechanics of CNCs. We discuss the current progress that has advanced our understanding of their behavior, and future challenges that must be addressed in order to fully exploit the potential of CNCs in engineered materials.

From the biological/chemical perspective, interface concepts related to the cell surface/synthetic biomaterial interface and the extracellular matrix/biomolecule interface have wide applications in medical and biological technologies. Interfaces also play a significant role in determining structural integrity and mechanical creep and strength properties of biomaterials. Structural arrangement of interfaces combined with interfacial interactions between organic and inorganic phases significantly affects the mechanical properties of biological materials, allowing for a unique combination of seemingly inconsistent properties, such as fracture strength and tensile strength being both high—as opposed to traditional engineering materials, which have high fracture strength linked to low tensile strength and vice versa. While there has been a tremendous amount of work focused on the effects of structural arrangements on biomaterial properties, both experimental and computational studies of the strength, deformation, and viscosity of the interface itself are limited to just a few systems. Even in such studies, the actual interface stress is rarely analyzed, and correlated to the overall material strength or creep properties. This article provides a focused overview of such studies in hard biological materials, followed by a new vision of how the results of interfacial molecular studies could be consistently linked to multiscale, micromechanics-based perceptions of hierarchical biological materials.

This article describes a new microscope, based on angle-resolved cathodoluminescence (CL) imaging spectroscopy, which enables optical imaging and spectroscopy at deep-subwavelength spatial resolution. We used a free electron beam in a scanning electron microscope as a direct excitation source for polarizable materials, and we collected the emitted coherent visible/infrared CL radiation using a specially designed optical collection system that is integrated in the electron microscope. We have demonstrated the use of this new technique for the excitation of plasmons in single metal nanoparticles, surface plasmon polaritons at metal surfaces, resonant Mie modes in dielectric nanostructures, and cavity modes and Bloch modes in photonic crystals. Using angle-resolved detection, we are able to derive the nature of localized modes and the dispersion of propagation modes in dielectric and plasmonic geometries. An outlook about new directions and applications of CL imaging spectroscopy is also provided.