“Bio-metals”: Ancient biological materials with nanoindentation size effects: Experiments and elements of manifold micromechanics

Abstract

The Nix-Gao nanoindentation size effect, where squared hardness scales with the inverse of the indentation depth, qualifies as a major hallmark of crystalline metals. It is generally accepted that this effect arises from the accumulation of dislocations, i.e., irregularities in the atomic lattice in the form of line defects. A recent study has strongly suggested that the very same effect holds for a totally different type of material, namely, an unusual class of metal-coordinated proteins making up the jaws of different bristle worm species. In order to gain further evidence, we here complement this study by a systematic investigation on the midplane (rather than the surface) of a single jaw from one species, Perinereis cultrifera. Six different indentation depths were repeatedly realized at eleven specific testing areas, covering the central and tip regions of the jaw's midplane. Both in the tip and the central region, the hardness-to-depth relations clearly obey the Nix-Gao size effect law, which is classically linked to strain gradient plasticity. Interestingly, our experiments also reveal an elastic size effect. The latter is rationalized via a review of elements of “manifold micromechanics”: Concentrated microscopic volume forces of the Peach–Koehler format, associated with dislocation-type “folds” in the ion-coordinated structural protein matrix, induce non-negligible macroscopic strain gradients at the level of a representative volume element of the ion-spiked structural protein material matrix. Given the remarkable blend of polymeric and metallic properties in the investigated ancient biomaterials, we suggest to call the latter “bio-metals,” and we regard them as emerging area of interest in biophysics and bioengineering.