Formation and in vitro/in vivo performance of "cortex-like" micro/nano-structured TiO2 coatings on titanium by micro-arc oxidation.

A novel "cortex-like" micro/nano dual-scale structured TiO2 coating was prepared on a titanium surface by micro-arc oxidation (MAO) with tetraborate electrolytes. This structure, which combines microslots with nanopores, exhibits super hydrophilicity. This coating modified the surface structure, chemistry, and hydrophilicity in a one-step treatment. Evolution of the coating together with its surface and structure properties was studied. We propose a forming mechanism of the dual-scale structure in which the oxides formed during the MAO discharge dissolve in the tetraborate electrolytes, leaving little or no deposition outside the discharge channels. We performed in vitro tests using human bone marrow-derived mesenchymal stem cells (hBMSCs) that compared this coating with a "volcano-like" MAO coating and a commercial sandblasted, large-grit and acid-etched (SLA) coating. The adhesion, morphology, proliferation, and differentiation of hBMSCs, together with the matrix mineralization, were investigated. Results suggest that the "cortex-like" structure significantly promotes the adhesion, spreading, and differentiation of cells and increases the matrix mineralization. In vivo tests with mongrel dogs showed an excellent osseointegration of the "cortex-like" coating. The combination of the dual-scale structure and the hydrophilicity of the "cortex-like" TiO2 coating synergistically resulted in an outstanding cytocompatibility and osseointegration, which may facilitate a higher level of implant success.