Effect of Ti-OH groups on microstructure and bioactivity of TiO2 coating prepared by micro-arc oxidation

Titanium and its alloys are promising biomedical metal materials. In order to improve the bioactivity of TiO2 coatings, three processes including micro-arc oxidation (MAO) in a Ca and P ion electrolyte (Ti-MAO), MAO in a 1 M NaOH electrolyte (Ti-NaOH) which was heated to 60 °C and a two-step MAO process in which specimens were first treated by MAO in Ca and P ion solution and then further in a 1 M heated NaOH electrolyte in succession (Ti-MAO-NaOH) were carried out. Round pores were found to form on the surface of the Ti-MAO sample while homogeneously distributed hollow spherical particles were found on both the Ti-NaOH and Ti-MAO-NaOH sample surfaces. The bioactivity evaluation showed that the combination of MAO processes in different electrolytes induced more rapid formation of apatite in contrast to the simple MAO treatment in heated alkaline electrolyte. The Ti-OH groups formed in alkaline electrolyte are found to be response for the rapid formation of hydroxyapatite during a SBF soaking. The previous introduction of Ca and P can increase the opportunity to form hydroxyapatite.