Crystallinity of anodic TiO ₂ nanotubes and bioactivity

Anodic TiO ₂ nanotubes were produced on titanium at 20 V using 1 M Na ₂ SO ₄ and 0.5 wt% NaF. Oxidation for 3 hours produced amorphous tubes of diameter 100 nm and thicknesses 2 μm. Heat-treatments were done for 3 hours at different temperatures. 300 °C treatment converted the amorphous coatings to anatase. 550 and 700 °C treatments formed dual anatase and rutile; 850 °C treatment crystallized to rutile. The treatment at 700 °C produced an oxide surface with higher roughness, lower wetting angle and higher coating adhesion. Bioactivity of the as-oxidized and heated coatings were evaluated by treating them in a simulated body fluid (SBF) to form hydroxyapatite (HA) and the rates of HA formation were compared. Deposits of HA could be seen on the dual oxide structure within 3 days. HA was detected after 7 days in the anatase structure and only after 21 days in the amorphous and rutile structures. In vitro cell culture tests done using mouse osteoblasts indicated that, the 700 °C-heated surface showed higher levels of cell activity than the other surfaces. It is concluded that the dual rutile and anatase structure formed by heating the oxide at 700 °C is the best of the five surfaces tested.