Structural and biodegradable properties of hydroxyapatite-SiO₂ reinforced MgO bioceramics by dual plasma electrolytic oxidation

In the present work, hydroxyapatite (HA) and silicon dioxide (SiO₂) were in-situ synthesized in a MgO bioceramic on AZ31 via sequential dual plasma electrolytic oxidation (PEO). This process integrates bioactive compounds into the coating, which results in a dense and functional surface with significantly enhanced corrosion resistance and biological response. The structural analysis of the resulting films indicated that the first PEO in silicate electrolyte yielded MgO-SiO₂ hierarchical porous coating with relatively big pores. The second PEO in calcium glycerol phosphate (CaGP) can be resulted in synthesized HA in the pancake-like areas and inner pores of MgO-SiO₂ film, reducing porosity to 8.63% and yielding an average pore size of 3.5 μm². In vitro bioactivity assessment clarified that the simultaneous presence of SiO₂ and HA in the modified PEO coating, with an optimum morphology and an average thickness of 37 µm, leads to the significantly improved biological sustainability of magnesium. The improvement is due to development of non-stoichiometric hydroxyapatite-rich regions over 3 to 10 days of exposure in simulated body fluid (SBF). Moreover, the biodegradation results revealed that HA-SiO₂ reinforced PEO coating as a result of the second PEO had retained a remarkable 16-fold increase in its protective performance after a 10-day cycle.