Formation and Bioactivity of SrTiO₃ Nanotubes on Titanium by Modified Anodization and Hydrothermal Treatment

SrTiO₃ nanotube films with good adhesion strengths to Ti substrates were fabricated by using a hybrid approach with a modified anodization and a hydrothermal treatment (HT). The effect of Sr²⁺ concentration in HT solutions on the morphologies and phase components of the nanotubes were investigated, the SrTiO₃ nanotubes formation mechanism was explored, and the adhesion strengths, hydrophilicity and apatite-forming ability of the SrTiO₃ nanotubes were also evaluated. The results demonstrated that with increasing the incorporation of Sr²⁺ into the nanotubes, no obvious changes of the lengths and outer diameters of the nanotubes were observed, but the wall thickness and the crystallinity of SrTiO₃ in the nanotubes increased. The accumulation of Sr at the inner tube wall indicated that the reaction of Sr²⁺ with TiO₂ mainly occurred in the vicinity of internal surfaces of the closely arranged nanotubes. The formation of the SrTiO₃ nanotubes could be attributed to an in situ dissolution–recrystallization process. The SrTiO₃ nanotubes exhibited good hydrophilicity and bioactivity, and the induced apatite preferred to nucleate on the nanotubes with higher crystallinity and Sr content, indicating a good bio-adaptability of the SrTiO₃ nanotubes for orthopedic application.