Functional bioactive glass topcoats on hydroxyapatite coatings: Analysis of microstructure and in-vitro bioactivity

A bioactive glass topcoat was introduced to modify the surface morphology and in-vitro reactivity of hydroxyapatite (HA) coatings for biomedical applications. With this aim, a CaO-rich bioactive glass, termed BG_Ca (wt.%: 4.7 Na2O, 42.3 CaO, 6.1 P2O5, and 46.9 SiO2), was selected due to its good bioactivity and low tendency to crystallize at high temperature. The standard HA coatings were sprayed through atmospheric plasma spray (APS) on steel substrates starting from commercial powders (“APS-HA” samples). The HA coatings, in turn, were subsequently coated with a thin layer of bioactive glass by suspension plasma spray (SPS), thus obtaining the duplex systems (“APS-HA/SPS-BG_Ca” samples). The samples with and without the BG_Ca layer were analysed by microstructural characterization and by in vitro tests in simulated body fluid (SBF). The analysis revealed an increased reactivity of the APS-HA/SPS-BG_Ca samples compared to the glass-free APS-HA coatings.