Multi-scale structure and growth of nacre: a new model for bioceramics

Nacre is the internal lustrous 'mother of pearl' layer of many molluscan shells. The structure is a brick and mortar arrangement: the bricks are flat polygonal crystals of aragonite and the mortar consists of organic compounds. The biological mineralization of composites such as the molluscan shell has generally been thought to be directed by preformed organic arrays of proteins or others biopolymers. The possibility that the organic matrix behaves as a template for crystal formation by heteroepitaxial growth, is examined. The structure and growth of nacre of Pinctada margaritifera and Pinctada maxima are studied at different scales. We propose a new model in nacre growth and maturation. Understanding the molecular mechanisms that regulate biomineralization may thus provide practical routes to the synthesis of new high-performance composite materials. Nacreous shell growth was studied by electron microscopy after fixation of immature nacre. It shows that the first step in crystal formation takes place in a biofilm, as evidenced by Laser Raman Spectroscopy. This biofilm is composed of organic matrix and calcium carbonate in the aragonite crystal form. Crystalin nuclei grow in the biofilm as an aligned sequence, starting with a row of intitial nucleation centres, gradually increasing concentrically in size and fusing together to form polygons in the next highest nacreous level. In Pinctada biomineralization takes place in a biopolymer that has the ability to initiate nucleation and to control orientation in the aragonite crystal form and growth. Also is stressed the existence of several levels or steps of mineralization, simultaneously, as a key mechanism for the thickening of the shell. This natural bioceramic is studied with the aim of using it as a biomaterial in bone repair.