Influence of frequency and duty cycle on the properties of antibacterial borate-based PEO coatings on titanium for bone-contact applications

In this work, copper and zinc co-doped coatings were successfully produced on commercially pure titanium samples by plasma electrolytic oxidation (PEO) using a sodium borate-based electrolyte. The effect of different duty cycles and frequencies on the treatment process was assessed, showing a significant influence of these parameters on the morphology and physical-chemical properties of the produced coatings. In particular, the coatings formed by applying duty cycles above 50% and 100 Hz frequency showed the highest thickness, surface homogeneity and porosity among all the treatment conditions. Moreover, the treatment significantly increased the wettability of all the samples, leading to a superhydrophilic behavior. The undoped PEO surface showed a clear bacteriostatic capability, given by its tuned morphology (high homogeneity and controlled pore size) and chemical-physical properties (high wettability), while the doping ions conferred antibacterial properties to the specimens, significantly reducing the number and the metabolic activity of the typical pathogens responsible for bone infections upon surgery (S. aureus, S. epidermidis, E. coli). Furthermore, the same ions did not induce any cytotoxic effect on Saos-2 osteoblasts, while the release of the early osteogenic marker alkaline phosphatase (ALP) suggested a pro-osteogenic activity of the surfaces.