Biomimetic Morphogenesis of Fluorapatite-Gelatin Composites: Fractal Growth, the Question of Intrinsic Electric Fields, Core/Shell Assemblies, Hollow Spheres and Reorganization of Denatured Collagen

Abstract

The biomimetic growth of fluorapatite in gelatin matrices at ambient temperature (double-diffusion technique) starts with elongated hexagonal-prismatic seeds followed by self-similar branching (fractal growth) and ends up with anisotropic spherical aggregates. The chemical system fluorapatite/gelatin is closely related to in vivo conditions for bone or tooth formation and is well suited to a detailed investigation of the formation of an inorganic solid with complex morphology (morphogenesis). The fractal stage of the morphogenesis leads to the formation of closed spheres with diameters of up to 150 μm. The self-assembled hierarchical growth thereby shows immediate parallels to the topological branching criteria of the macromolecular starburst dendrimers. A second growth stage around the closed spheres of the first stage is characterized by the formation of concentric shells consisting of elongated prismatic fluorapatite units with nearly parallel orientation (maximum diameter of the complete core/shell spheres of 1 mm). The specific structure of the core/shell assembly is similar to the dentin/enamel structure in teeth. Together with the idea of the biological significance of electric fields (pyro-, piezoelectricity) during apatite formation under in vivo or biomimetic conditions the present paper considers the composite character of the material and the mechanisms of fractal growth (branching criteria and architecture, the influence of intrinsic electric fields etc.).