Wear performance and osteogenic differentiation behavior of plasma electrolytic oxidation coatings on Ti-6Al-4V alloys: Potential application for bone tissue repairs

Plasma electrolytic oxidation (PEO) can be used to obtain oxide coatings able to improve the biocompatibility, wear and corrosion properties of metal implants, due to complex morphology and the formation of oxides with the possibility of incorporating bioactive elements. This study aims at producing coatings on Ti-6Al-4V alloys with potential application for bone tissue repairs, evaluating cytocompatibility behavior and the potential for osteogenic differentiation, by using mesenchymal stem cells. Coatings have been produced on Ti6Al4V alloys by PEO with a calcium and phosphate-based electrolyte, by applying the potentials of 200 V (Ti-PEO200V) and 300 V (Ti-PEO300V), with a pulsed electrical regime. The SEM/FEG characterization shown that the samples obtained are typically porous, have different thicknesses and roughness, which were found to be dependent on the maximum applied voltage. The bioactive elements (Ca and P) were incorporated into the coatings and their concentrations grew with the maximum applied voltage, as indicated by RBS analysis. In addition, the same analysis indicated that the coatings were mainly composed of Ti oxides. XRD patterns confirmed that the coatings consist mainly of titanium oxides in the rutile and anatase phases. The tribological properties were evaluated by a ball-on-plate tribometer. Both samples show very similar tribological behavior. The wear resistance of Ti-PEO300V was more impaired by the microcracks present in its morphology and its great surface roughness than the Ti-PEO200V sample. Biological properties were assessed by studying the cytocompatibility with MSC cells. The coating obtained for the Ti-PEO200V sample showed a great cytocompatibility level and initial adhesion, as well as an innate ability for osteo-differentiation of MSCs, displaying considerable potential for usage as bone-repair applications.