Induction of calcium phosphate precipitation by titanium dioxide

Titanium powder and various titanium dioxides were tested for their capacity to reduce the induction time for calcium phosphate precipitation from supersaturated solutions. Only after a pretreatment aimed at increasing its oxide surface layer was titanium powder found to accelerate the precipitation from solutions containing 2 mmol/L CaCl2, 2 mmol/L KH2PO4, 50 mmol/L Hepes, pH 7.2, and to induce precipitation from metastable solutions containing 1.2 mmol/L CaCl2, 1.2 mmol/L KH2PO4, 50 mmol/L Hepes, pH 7.2, at 37 degrees C. Even stronger effects were found when suspensions of the titanium dioxides anatase or rutile (10-50 micrograms/mL) were added to these solutions. TiO2 appeared to serve as a reactive substrate for secondary nucleation at a wide range of calcium-to-phosphate ratios and concentrations, even in the presence of 40 mg/mL bovine serum albumin, which completely inhibited precipitation in control incubations. These results suggest that the oxide surface layer of titanium implants may induce calcium phosphate precipitation in the metal-to-bone interface, which may play a role in the integration of such implants in bone.