UV-enhanced bioactivity and cell response of micro-arc oxidized titania coatings

Using ultraviolet (UV) irradiation of micro-arc oxidized (MAO) titania coating in distilled water for 0.5 and 2 h, we have achieved an enhanced bioactivity and cell response to titania surface. The MAO coating appears porous and predominantly consists of nanocrystallized anatase TiO₂. Compared with the MAO coating, the UV-irradiated coatings do not exhibit any obvious change in surface roughness, morphology, grain size and phase component; however, they have more abundant basic Ti-OH groups and become more hydrophilic because the water contact angle decreases significantly from 17.9 ± 0.8° to 0°. In simulated body fluid (SBF), bonelike apatite-forming ability is significantly stronger on the UV-irradiated coatings than the MAO coating. SaOS-2 human osteoblast-like cell attachment, proliferation and alkaline phosphatase of the cell are greater on the UV-irradiated coatings relative to the MAO coating. UV irradiation of titania results in the conversion of Ti⁴⁺ to Ti³⁺ and the generation of oxygen vacancies, which could react with the absorbed water to form basic Ti-OH groups. The enhanced bioactivity and cell response of the UV-irradiated coatings are proven to result from abundant Ti-OH groups on the coating surfaces. After storing the UV-irradiated coatings in the dark for two weeks, the basic Ti-OH groups on the coatings slightly decrease in amount and can induce apatite formation after a short period of SBF immersion, and show relative long-term stability.