Nucleation and growth of apatite by a self-assembled polycrystalline bioceramic.

The formation aspects of a polycrystalline self-assembled bioceramic leading to the nucleation of hard-tissue mineral from a supersaturated solution are discussed. Scanning electron imaging and surface-sensitive interrogations of the nucleated mineral indicated the presence of an intermediate amorphous layer encompassing a rather crystalline phase that formed on niobium oxide (Nb(2)O(5)) microstructures. The crystalline phase was identified from Raman spectroscopy as hydroxyapatite (HAP), while the phosphorous-rich amorphous layer is suggested to have the chemical form CaO-P(2)O(5). In addition, the mechanism favoring HAP nucleation is discussed in terms of the (0 0 2) and (0 0 1) diffraction planes of HAP and Nb(2)O(5), respectively. The small mismatch along several lattice dimensions strongly suggests epitaxy as a dominant mode in the heterogeneous nucleation of HAP. Furthermore, the effectiveness of this mode was shown to critically depend on the self-organization of the Nb(2)O(5) microstructures. Because nucleation does not appear to depend solely on the integrity of Nb(2)O(5) crystals, the self-organization of Nb(2)O(5) crystals also contributes significantly to HAP nucleation. Based on our results, we propose the organized arrangement of bioceramic crystals as a new mode for the bioinspiration of hydroxyapatite and other hard-tissue mineral.