First steps in harnessing the potential of biomineralization as a route to new high-performance composite materials

The underlying molecular mechanisms that control biomineralization have long bean thought to offer the potential for new routes to synthesis of high-performance nanocomposite materials, yet these mechanisms have until recently remained elusive. The biological mineralization of composites such as the molluscan shell generally has been thought to be directed by preformed organic arrays of proteins or other biopolymers [Venus; 1982, 41, 33.]. A less explored structure-directing factor is the role of cooperative interactions between water-soluble protein molecules and the inorganic phase during crystal nucleation and growth [Biomineralization, Chemical and Biochemical Perspectives VCH, New York, 1989, p. 133]. These interactions can control phase, morphology and growth dynamics of crystals on a time domain basis allowing the organism to rapidly introduce major structural changes in growing biomineralized composites over spatial scales ranging from angstroms to microns [Mat. Res. Sec. Symp. Pi oc., 1993, 292, 59.]. We have purified and characterized the nucleating protein sheet and polyanionic proteins from the mineralized microlaminate composites of the abalone shell and flat pearl and resolved their roles controlling biomineralization. The protein sheet directs nucleation of oriented calcite to form a "primer", while two distinct populations of soluble polyanionic proteins (at least one of which becomes occluded within the growing crystals) subsequently determine crystal phase, morphology and growth dynamics of the growing crystals. These polyanionic proteins allow us, in vitro, to abruptly and sequentially switch crystallographic phase from calcite to aragonite and vice-versa, in stereospecific directions, producing multiphase composites with micron-scale phase domains. (C) 1998 Acta Metallurgica Inc.