Studies of early growth mechanisms of hydroxyapatite on single crystalline rutile: a model system for bioactive surfaces

Previous studies have shown that crystalline titanium oxide is in vitro bioactive and that there are differences in the HA formation mechanism depending on the crystalline direction of the titanium oxide surface. In the present study, the early adsorption of calcium and phosphate ions on three different surface directions of the single-crystal rutile TiO₂ substrate has been investigated. A crucial step in the nucleation of HA is believed to be the adsorption of Ca²⁺ and PO₄ ³⁻ from phosphate buffer solutions. The (001), (100) and (110) single crystalline rutile surfaces were soaked in phosphate buffer saline solution for 10 min, 1 h and 24 h at 37°C. The surfaces were then analyzed using time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS). The results show that the adsorption of Ca²⁺ and PO₄ ³⁻ is faster on the (001) and (100) surfaces than on the (110) surface. This study also shows that TOF-SIMS can be used as a tool to better understand the adsorption of calcium and phosphate ions and the growth mechanism of HA. This knowledge could be used to tailor new bioactive surfaces for better biological reaction.