Apatite-forming ability of CaO-containing titania

It was recently shown that titanium metal and its alloys spontaneously form a bonelike apatite layer on their surfaces in the living body and bond to the bone through the apatite layer, when the sodium ions are incorporated into titanium oxide layer of their surfaces by chemical and heat treatments. It is expected that their apatite-forming ability, and hence their bone-bonding ability, could be enhanced, if the calcium ions are incorporated into their surface titanium oxide layers instead of the sodium ions, because the calcium ions released from their surface layers can increase the ionic activity product of the apatite of the surrounding fluid more effectively than the sodium ions. In the present study, in order to investigate the effect of incorporation of the calcium ions into the titanium oxide layer on its apatite-forming ability, apatite-forming abilities of titania gels which have different CaO contents and subjected to different heat treatments were examined in a simulated body fluid with ion concentrations nearly equal to those of the human blood plasma. It was found that CaO-containing gels do not form the apatite on their surfaces as far as they take an amorphous phase in spite of the fact that they release larger amounts of the calcium ions with increasing CaO contents of the gels. They form the apatite when they take an anatase-like structure even though they do not contain CaO. These results indicate that a specific structure of the titanium oxide is more important for the apatite nucleation than the magnitude of the ionic activity products of the apatite in the surrounding fluid.