FLAT PEARLS FROM BIOFABRICATION OF ORGANIZED COMPOSITES ON INORGANIC SUBSTRATES

THE study of biomineralization is inspiring new approaches to the cont rolled fabrication of synthetic materials such as nanoparticles, polym er-mineral composites and templated crystals(1-3). Although this biomi metic approach is gaining momentum, the biological mechanisms involved in biomineralization remain relatively unexplored. One major reason f or this is the difficulty of analysing biomineralization processes in their native dynamic state. Here we demonstrate that a highly organize d composite material-a 'flat pearl'-can be biofabricated on disks of g lass, mica and MoS2 inserted between the mantle and shell of Haliotis rufescens (red abalone). We shove that the construction of this materi al is spatially and temporally regulated and proceeds through a develo pmental sequence that closely resembles that at the growth front of th e natural shell. Recognition of the implanted inorganic surfaces by ma ntle cells apparently governs a switch, perhaps genetically controlled , from aragonite to calcite biomineralization. Once a partially orient ed calcite-protein primer layer has been deposited, there is a switch back to the nucleation and assembly of columnar stacks of highly order ed aragonitic nacre. Thus the presence of an inorganic surface between the mantle and shell of the organism triggers a change in the nature of the mineral phase deposited.