Knowledge of the structure-property-function relationships of dermal scales of armoured fish could enable pathways to improved bioinspired human body armour, and may provide clues to the evolutionary origins of mineralized tissues. Here, we present a multiscale experimental and computational approach that reveals the materials design principles present within individual ganoid scales from the 'living fossil' Polypterus senegalus. This fish belongs to the ancient family Polypteridae, which first appeared 96 million years ago during the Cretaceous period and still retains many of their characteristics. The mechanistic origins of penetration resistance (approximating a biting attack) were investigated and found to include the juxtaposition of multiple distinct reinforcing composite layers that each undergo their own unique deformation mechanisms, a unique spatial functional form of mechanical properties with regions of differing levels of gradation within and between material layers, and layers with an undetectable gradation, load-dependent effective material properties, circumferential surface cracking, orthogonal microcracking in laminated sublayers and geometrically corrugated junctions between layers.

Nanomechanical heterogeneity is expected to influence elasticity, damage, fracture and remodelling of bone. Here, the spatial distribution of nanomechanical properties of bone is quantified at the length scale of individual collagen fibrils. Our results show elaborate patterns of stiffness ranging from approximately 2 to 30 GPa, which do not correlate directly with topographical features and hence are attributed to underlying local structural and compositional variations. We propose a new energy-dissipation mechanism arising from nanomechanical heterogeneity, which offers a means for ductility enhancement, damage evolution and toughening. This hypothesis is supported by computational simulations that incorporate the nanoscale experimental results. These simulations predict that non-uniform inelastic deformation over larger areas and increased energy dissipation arising from nanoscale heterogeneity lead to markedly different biomechanical properties compared with a uniform material. The fundamental concepts discovered here are applicable to a broad class of biological materials and may serve as a design consideration for biologically inspired materials technologies.

Concrete, bone and shale have one thing in common: their loadbearing mineral phase is a hydrated nanocomposite. Yet the link between material genesis, microstructure, and mechanical performance for these materials is still an enigma that has deceived many decoding attempts. In this article, we advance statistical indentation analysis techniques that make it possible to assess, in situ, the nanomechanical properties, packing density distributions, and morphology of hydrated nanocomposites. These techniques are applied to identify intrinsic and structural sources of anisotropy of hydrated nanoparticles: calcium-silicatehydrate (C-S-H), apatite, and clay. It is shown that C-S-H and apatite, the binding phase in, respectively, cement-based materials and bone, are intrinsically isotropic; this is most probably due to a random precipitation and growth process of particles in calcium oversaturated pore solutions, which can also explain the nonnegligible internanoparticle friction. In contrast, the load-bearing clay phase in shale, the sealing formation of most hydrocarbon reservoirs, is found to be intrinsically anisotropic and frictionless. This is indicative of a 'smooth' deposition and compaction history, which, in contrast to mineral growth in confined spaces, minimizes nanoparticle interlocking. In all cases, the nanomechanical behavior is governed by packing density distributions of elementary particles delimitating macroscopic diversity.

We have directly measured the entropic elasticity due to the uncoiling of individual polymer chains of poly(methacrylic acid) (PMAA) using the atomic force microscope (AFM). Covalent attachment of one chain end to a substrate and sufficiently low chain grafting densities were achieved by using a mixed monolayer technique that involved the co-chemisorption of (mono)thiol-functionalized PMAA and self-assembling alkanethiols on gold. Single molecule force spectroscopy experiments were carried out in good solvent conditions where the chains were tethered to a Si3N4 probe tip via nonspecific physisorption interactions. Upon retraction of the probe tip from the surface, single, continuous, attractive peaks in the force versus distance profiles were frequently observed. These peaks could be fit, for all chain bridging lengths, to entropic-based, statistical mechanical, random-walk formulations, i.e., the free ly j ointed chain (FJC) model and wormlike chain (WLC) model. The fits to both models yielded a statistical segment length or persistence length of ≈0.3 nm (approximately the length of a single PMAA monomer unit), thus suggesting that locally the chains are quite flexible. In addition to measuring entropic elasticity, we have also shown that single molecule force spectroscopy experiments are able to provide quantitative information on the statistical nature of adsorption of single polymer chains.

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