Calcium phosphate interactions with titanium oxide and alumina substrates: an XPS study

Besides the excellent mechanical properties of titanium and alumina (Al2O3) in the case of load bearing applications, their bone-bonding properties are very different. In osseous environment, Al2O3 ceramic is encapsulated by fibrous tissues, whereas bone can bind directly to titanium, via its natural titanium dioxide (TiO2) passivation layer. So far, this calcification dissimilarity between TiO2 and Al2O3 was attributed to respectively their negative and positive surface charge under physiological conditions. The present study aims at studying the chemical interactions between TiO2 and Al2O3 (phase α) with the diverse ions contained in simulated body fluids (SBFs) buffered with trishydroxymethyl aminomethane (TRIS) at pH=6.0 and pH=7.4. After 1 h of immersion, TiO2 and α-Al2O3 powders were analyzed by X-ray photoelectron spectroscopy (XPS). The results indicated that Ca and HPO4 groups were present on TiO2 surface. In addition, HPO4 groups were found to be in a higher amount than Ca on TiO2, which does not comply with the surface charge theory. With regard to Al2O3, little HPO4 but no Ca was detected on its surface, and TRIS bound to Al2O3 substrate in all of the immersion experiments. The fact that both Ca and HPO4 were present at the vicinity of TiO2 might be at the origin of its calcification ability. On the other hand, Al2O3 did not show any affinity towards Ca and HPO4 ions. This might explain the inability of Al2O3 substrate to calcify.