Formation of Hydroxyapatite Coating on Anodic Titanium Dioxide Nanotubes via an Efficient Dipping Treatment

Hydroxyapatite (HA) depositions on metallic biomedical implants have been widely applied to generate bioactive surfaces in simulated biological environments. Meanwhile, highly ordered TiO₂ nanotubes obtained via anodization have attracted increasing interest for biomedical applications. However, the capability to grow HA coating on TiO₂ nanotubes at room temperature remains problematic. In this study, we applied a dipping treatment for biomimetic formation of an adhesive HA coating on titanium dioxide nanotubes. The coatings formed using this procedure did not require high-temperature annealing or high supersaturation of the simulated biological condition. The as-formed TiO₂ nanotubes on titanium were treated using several dip-and-dry steps, through which the TiO₂ nanotubes were filled and covered with calcium phosphate nucleation sites. The specimens readily grew HA once immersed in the original simulated biological fluid (SBF) after little more than 12 hours. The carbonated HA coating was formed with 10-μm thickness after 4 days of immersion, while only a few calcium phosphate particles were observed on annealing TiO₂ nanotubes immersed in the same solution for the same duration. Tensile testing showed that the bonding strength between HA coating and substrate was 27.2 ± 1.6 MPa. This treatment dramatically improved efficiency for promoting HA formation on anodic TiO₂ nanotubes at room temperature.