Hierarchical structure of marine shell biomaterials: biomechanical functionalization of calcite by brachiopods

Abstract Biologic structural materials for skeletons or teeth show a hierarchical architecture, where organic macromolecules and mineral substance form a hybrid composite material with its components inter-weaved on many length scales. On the nanostructure level brachiopods form hybrid composite mesocrystals of calcite with occluded organic molecules. On the microstructure level several types of materials are produced, on which the electron back-scatter diffraction (EBSD) technique gives insight in texture and architecture. We describe the calcite single-crystal fiber composite architecture of the secondary layer involving organic matrix membranes, the competitive-growth texture of the columnar layer and the nano-structuring of the primary layer. In the overall skeleton the organic biopolymers provide flexibility and tensile strength while the mineral provides a high elastic modulus, compressive strength, hardness and resistance to abrasion. The hierarchical composite architecture, from the nanostructure to the macroscopic level provides fracture toughness. The morphogenesis of the biomaterial as a whole and of the mineral crystals is guided by the organic matrix and most probably involves amorphous calcium carbonate (ACC) precursors. In this paper we review the hierarchical architecture of rhynchonelliform brachiopod shells, which is very distinct from mollusk nacre.