Effect of NaOH etching and oxygen plasma treatments on surface characteristics and their potential to activate micro-arc oxidized TiO2 coatings

• Anodic porous TiO 2 coatings were activated by NaOH etching or O 2 plasma. • NaOH etching produced titanate structures on the surface and inside the coatings. • O 2 plasma has only interfered with surface chemistry state of the coatings. • O 2 plasma provided a more stable interface for molecules conjugation. Activation treatments such as NaOH etching or O 2 plasma can play an essential role in surface conjugation of titanium with biomolecules, providing a better interaction at the bone-implant interface. However, their application on complex titanium dioxide (TiO 2 ) surfaces is still not explored. In this contribution, bioactive and porous TiO 2 coatings produced by micro-arc oxidation (MAO) were treated with NaOH etching or O 2 plasma and then placed in contact with a reactive isocyanate test compound to evaluate the potential of molecule conjugation. Results suggested that O 2 plasma treatment has only changed the surface chemistry of the coating through carbon contaminants removal, plasma-driven oxidation and generation of functional OH species, including reactive carboxyl groups. This chemical modification by plasma has made the surface superhydrophilic. After NaOH etching, the coating became rougher and also superhydrophilic, containing titanate structures doped with sodium and calcium on its surface and inside the inner pores. Upon reaction with butyl isocyanate, the O 2 plasma-treated surfaces seem to better provide molecule conjugation, introducing characteristic conjugation bonds, and also making MAO coatings more hydrophobic due to the surface-terminated methyl chains from isocyanate. This proof-of-concept study has demonstrated the promising grafting potential given by O 2 plasma on complex TiO 2 surfaces.