Kinetics of nucleation and crystal growth of hydroxyapatite and fluorapatite on titanium oxide surfaces

Although titanium has long been used as an implant material, the mechanisms of its subsequent biomineralization are not yet well understood. The nucleation and growth of hydroxyapatite (Ca-5(PO4)(3)OH, HAP) and fluorapatite (Ca-5(PO4)(3)F, FAP) on titanium oxide (TiO2) surfaces have been investigated using constant composition (CC) methods in supersaturated solutions at 37 degrees C and ionic strength 0.15 moll(-1) with relative supersaturation, 13.4-15.0 and 13.5-16.5 for HAP and FAP, respectively. The solid phases during the reaction were examined by X-ray diffraction, scanning electron microscopy (SEM) and diffuse reflectance Fourier transform infrared spectroscopy (FTIR). Both crystal phases nucleated at the TiO2 particle surfaces with supersaturation-dependent induction periods, tau(in), preceding the initial crystal growth, varying from 8 to 17 h for HAP and from 7 to 15 h for FAP. The interfacial tensions, gamma, estimated both from initial growth rate data following nucleation and from the induction times were about 32 and 39 mJ m(-2) for FAP and HAP, respectively. These values were of the same order of magnitude as those determined by a contact angle method involving thin layer techniques. The results show that TiO2 will nucleate both HAP and FAP, but only after relatively long induction periods. It is also interesting to note that the X-ray diffraction pattern of HAP on TiO2 surfaces resembles that of bone and root dentin mineral in the region 2 theta from 30 to 35 degrees. (C) 1997 Elsevier Science B.V.