Effect of heat treatment on apatite-forming ability of Ti metal induced by alkali treatment

The present authors previously showed that titanium metal forms a bone-like apatite layer on its surface in a simulated body fluid (SBF), when it has been treated with a NaOH solution to form a sodium titanate hydrogel layer on its surface. This indicates that the NaOH-treated Ti metal bonds to living bone. The gel layer as-formed is, however, mechanically unstable. In the present study, the NaOH-treated Ti metal was heat treated at various temperatures in order to convert the gel layer into a more mechanically stable layer. The gel layer was dehydrated and transformed into an amorphous sodium titanate layer at 400–500°C, fairly densified at 600°C and converted into crystalline sodium titanate and rutile above 700°C. The induction period for the apatite formation on the NaOH-treated Ti metal in SBF increased with the transformation of the surface gel layer by the heat treatment. Ti metal heat treated at 600°C, however, showed a fairly short induction period as well as high mechanical stability, since it was covered with a fairly densified amorphous layer.