We can grow a...tooth?

Nature provides us a wide variety of beautiful structures and functionalities via incredibly high level of self-assembly, which has been inspiring scientists and engineers for decades. In particular, the hard tissues of organisms (shells, bones, teeth, etc.), often composite materials of organic and inorganic components, resemble a heated subject in both biological and material research. These hard tissues, or biominerals, contain crystals whose shapes are very different from the crystal habits produced inorganically. Large-scale and defined crystal structures are hardly found in biominerals; instead, the crystals may be 'molded' into elaborate structures which have non-faceted crystal surfaces, wherein the 'mold' is a vesicular compartment within which the crystal is formed. The ability to 'mold', or grow such energetically unfavorable structures inorganic components that cannot be melted at physiological temperatures has deeply intrigued the materials engineering community. Understanding the process of biomineralization may find potential application in low-temperature processing techniques in materials industry.

Biomineralizaion is controlled at multiple levels, including the regulation of particle size, shape, crystal orgientation, polymorphic structure, defect texture, and particle assembly. Formation of complex composites with hierarchical composite structures involves all level of biological elements, from bio-macromolecules to cells. Insights into the mechanisms of biomineralization, thanks to biology, enable materials scientists to develope similar techniques that either mimic (biomimetic) or are inspired by (bio-inspired) biological processes. Although it would be difficult for scientist to mimic complex cellular processes, however, the materials chemistry aspects of biomineralization can be studied by model systems, and utilized for biomimetic engineering.

One particular aspect of interest to the materials chemist is the means by which these organisms use organic constituents (macromolecular templates, vesicular compartments, solubilized peptides or proteins, etc.) to mediate the growth of the mineral phase. In particular, with the development of the controlled polymerization techniques such as atom transfer radical polymerization (ATRP) or reversible addition-fragmentation transfer (RAFT) polymerization and sophisticated understanding of the properties of polymers, synthetic macromolecules have been recognized as a promising route to direct the growth of defined crystal structures of inorganic materials.

In a recent report by Dirk Volkmer and co-workers (Angew. Chem. Int. Ed. 2006 45 7458-7461. DOI: 10.1002/anie.200602382) described how transient amorphous calcium carbonate (ACC) can be stabilized by ionotropic polymethacrylic acid (PMAA) brushes with defined thickness on glass surfaces, and transformed into polycrystalline calcite films with similar thickness of the brush by a simple thermal treatment. Then a glass substrate covered with patterned PMAA brushes fabricated via a TEM grids as photomasks under Hg-Xe irradiation was tried (see the scheme on the right^†). Again, polycrystalline calcite with similar thickness and patterns to the precursor polymer brush was obtained.

The calcite thin film is an exact 3D replica of the ionotropic PMMA matrix, which means the thickness of the film can be adjusted by varying the length polymer brush, which can be easily controlled owing to the 'living' nature of ATRP. Furthermore, in addition to photolithography, a 'top-down' approach, 'bottom-up' approaches such as SAMs or self-assembly of copolymers provide more patterning possibilities. The transient ACC-process of biomineralization of calcium carbonate has been regarded as the most possible mechanism of the formation of various complex biogenic calcite structures such as sea urchin spines. It has also been exploited as a transient precursor in vitro to produce CaCO₃ crystals in constrained geometries, to fabricate microstructured calcite single crystals, and to synthesize nacre-type laminated CaCO₃ coatings. This approach could lead to potential fabrication of novel inorganic or composite materials with interesting optical, catalytic, or biological properties; and presumably, we will be able to grow a tooth with 'real' quality and pre-designed shape. Which do you like, vampire or rabbit style?

† The figure was adapted from the original paper. Copyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.