Bioactivity of novel self-assembled crystalline Nb2O5 microstructures in simulated and human salivas

Crystalline, self-assembled niobium oxide microstructures formed in situ via potentiostatic anodization of niobium foil in an HF(aq) electrolyte solution are proposed as exceptional nucleators of Ca-P minerals, including hydroxyapatite. This material was tested for bioactivity through immersion in simulated and pooled human salivas. The simulated saliva formulation was based on mineral content found in stimulated human saliva and has a molar Ca/P ratio of 1:3.7. Oxide microstructures and mineral morphologies were examined using scanning electron microscopy. Differences in the mineral phase and morphology were attributed to the contrasting complexities of the two supersaturated solutions, with proteins and enzymes in human saliva most likely imparting a significant role. Dimensions of the niobium oxide microstructures and mineral deposits were characterized using profilometry. Energy dispersive spectroscopy, x-ray diffraction, Raman spectroscopy and electron microprobe analysis were utilized in identifying the nucleated mineral phases. Nucleation from human saliva resulted in mixed-phase mineral formations including amorphous calcium phosphate and poorly crystalline apatites. On the other hand, mineral nucleation from simulated saliva was more specific to hydroxyapatite. Based on these results, we demonstrate that a crystalline, self-assembled metal oxide is a unique and efficient nucleator of hydroxyapatite and other Ca-P minerals in supersaturated salivary solutions.