Molecular Investigation of the Initial Nucleation of Calcium Phosphate on TiO2 Substrate: The Effects of Surface Nanotopographies

Nucleation and biomineralization of apatite on titanium-based material surfaces is crucial to improve surface biocompatibility, osseointegration, and rapid bone ingrowth in biomedical applications such as joint replacements. This work is designed to provide new insights on the molecular processes in the initial nucleation of calcium phosphate on TiO2 surface by means of classical molecular dynamics (MD) simulations. Aggregation of calcium and phosphate ions in pure aqueous solution was studied, and the free energies during ion adsorption was investigated by the calculation of PMFs. The MD results suggest surface hydroxylation rate and nanotopographies of TiO2 substrate contribute significantly to the initial nucleation of calcium phosphate. Our simulations suggest that surface hydroxyls on TiO2 provide active sites for the aggregation of calcium phosphate. Both calcium ions and phosphate ions could bind to the hydroxylated TiO2 surface directly or indirectly via the first water layer. Surface nanotopographies (e.g., grooves or ridges) seem to be able to restrict the diffusion of calcium ions and phosphate ions, hence offering more opportunities for adsorption by trapping ions inside channels. Therefore, it can be inferred that apatite may be formed more favorably on porous or concave surfaces.