The Organic-Mineral Interface in Biominerals

Abstract

Numerous living organisms form minerals. These biogenic minerals, or biominerals, are composite materials containing an organic matrix and nano- or micro-scale amorphous or crystalline minerals. In this chapter we will review the molecular aspects of biomineralization and describe as completely as is currently possible the organic-mineral interface, the location in which organic-mineral interactions occur. Biomineral composite materials include bone, dentine, enamel, statoliths, otoliths, mollusk and crustacean shells, coccolith scales, eggshells, sponge silica skeletons, algal, radiolarian and diatom silica micro-shells, and a variety of transition metal minerals produced by different bacteria (Lowenstam and Weiner 1989; Addadi and Weiner 1997; Banfi eld and Nealson 1997; Fortin et al. 1997; Fitts et al. 1999; Templeton et al. 1999; Lower et al. 2001a; Mann 2001; Glasauer et al. 2002; De Yoreo and Vekilov 2003; Weiner and Dove 2003; De Yoreo and Dove 2004).From a materials science perspective, organic molecules are soft, compliant and fracture resistant while inorganic crystals are hard and brittle. Biomineral composites combine the best of these properties and minimize the weaknesses: they are both hard and fracture resistant (tough) (Currey 1977; Jackson et al. 1988; Schaffer et al. 1997; Kamat et al. 2000; Gao et al 2003). This is due to several factors: structure, nano-size and chemical composition. Only recently materials scientists have begun to learn how to build a synthetic composite material that outperforms each component taken separately, and have done so inspired by shell nacre (Tang et al. 2003).The mechanisms of biomineral formation are not fully understood (Mount et al. 2004) and while they are of interest in their own right, they may also provide models for new materials concepts, inspire design solutions and give new insights into the genetic control of biological structure (e.g., Schaffer et al. 1997).Lowenstam (1981) introduced the distinction between the biologically induced mineralization, which is enacted extracellularly or on the cell surface by many algae and bacterial species, and the organic-matrix mediated mineralization performed by many animals (later termed biologically controlled mineralization) (Bazylinski and Frankel 2003; Frankel and Bazylinski 2003; Veis 2003).Eukaryotic biominerals often show complex hierarchical structure from the nanometer to the macroscopic scale. This structure confers mechanical strength and toughness: despite being highly mineralized, with the organic component constituting not more than a few percent of the composite material, the fracture toughness exceeds that of single crystals of the