Nanomechanics of Biological Single Crystals

Abstract

Examples of ceramic materials in which the organic framework is stiffened by inorganic particles, are well known in nature and are produced synthetically by man. Organisms also form a different type of composite in which the host is a inorganic single crystal and the guests are proteins deliberately occluded into the crystal. The best-studied examples, to date, are biogenic calcites, and in particular those formed by the echinoderms. In vitro experiments with calcite crystals grown in the presence of echinoderm intracrystalline proteins and mollusk shell proteins show that these macromolecules are occluded inside the crystal on specific planes that are oblique to the cleavage planes, and their presence significantly improves the mechanical properties of the crystal host. Furthermore, the proteins also influence the crystal textural properties: the coherence length is reduced in directions perpendicular to the planes on which the proteins adsorb. This textural anisotropy is generally consistent with the gross morphology of the single crystal elements, suggesting that these proteins may also function in determining the shape of the crystal during growth. These novel single crystal-protein composites may be just one example of strategies used in nature for producing materials with special properties.