Biomineralization: bioinorganic solid-state chemistry
Biominerals, such as sea urchin spines and mollusk shells, are biogenically formed composite ceramics which show superb adaptation for given tasks. The 542 million years of evolution since the Cambrian explosion has rendered biominerals an immense source of inspiration for both material design and for the creation of ceramic materials at ambient temperature. The key step during genesis of these biogenic ceramics is the controlled solid-amorphous to crystalline transformation from a transient precursor to a highly co-oriented mosaic crystal. This solid-to-solid phase transformation is pseudomorphic, i.e. it proceeds with conservation of the macroscopic morphology of the mineral body which eventually leads to non-equilibrium shaped crystalline forms. Therefore, biominerals and biomineralization itself represents a huge source of inspiration for the synthesis of functional materials under morphological control. As of yet, the formation of ceramics at ambient conditions for anthropogenic ceramic materials is still in its infancy and Nature may guide us on this venture.
Fundamentals of Nucleation and Crystal Growth in Real Systems
Our research in biomimetic crystallization taught us that crystallization processes are not well understood for real systems; the classical models as given in 1920's work explicitly well for simple model systems (i.e. melts, NaCl, etc.). But already simple additives, such as polyacrylic acid, can strongly intervene in the crystallization process and trigger so-called Noncassical Crystallization processes. These processes i.e. oriented attachment, prenucleation cluster, and liquid-condensed mineral precursors (PILP) question our current understanding of crystal birth and growth.
Bioinspired Materials Chemistry
The third member of the nonclassical triumvirate is the so-called Polymer-Induced Liquid-Precursor Process (PILP), a process which has pronounced morphosynthetical potential. It proceeds in a colloid-mediated fashion via a liquid-phase amorphous intermediate. By addition of tiny amounts of poly-ionic polymers like poly-aspartate, poly-amines or selected biomineralization proteins, classical nucleation of a solid crystalline phase is suppressed which, in turn, promotes the formation of a liquid-condensed phase of mineral precursor. This unusual ion-enriched liquid-amorphous phase becomes the crucial agent of the precipitation reaction; the process of mineralization is converted from a solution crystallization process to a pseudomorphic solidification process. This change of pathway provides an efficient means to synthesize an impressive multitude of mineral morphologies, many of which mimic the features long considered enigmatic in biominerals.