Formation of octacalcium phosphate by heterogeneous nucleation on a titania surface

Biocompatibility of the surfaces of titanium dental implants can be improved by covering them with calcium phosphate crystals, which makes the surface bioreactive. Possibly the most effective bioreactive foreign material that improves osteointegration and adsorption/binding of extracellular proteins and structural proteins is crystalline octacalcium phosphate {2×[Ca₄H(PO₄)₃·2.5H₂O] or Ca₈(HPO₄)₂(PO₄)₄·5H₂O, OCP}. In this work the building up of OCP crystals on the surface of TiO₂ anatase is examined in the process of heterogeneous nucleation from constant-composition solutions of CaCl₂ and KH₂PO₄ at constant pH (pH 6.8) and ionic strength (I=0.05 M), in dense titania suspensions. Constant relative supersaturation with regard to the calcium phosphate formation was maintained by the controlled addition of the reagent solutions, according to the desired speed of crystallization. The surface saturation value of calcium ion adsorption was measured by detecting the pH decrease during CaCl₂ addition in a separate experiment. The OCP crystallization was also conducted on the surface of an evaporated titanium layer, and on titanium metal disks. The surface of the disks was modified by the laser ablation method in order to increase the oxide layer thickness. Calcium phosphate crystals formed on the surface of the modified titanium disks, but not in an appreciable amount on the surface of the evaporated titanium layer.