Thermal Oxide Layer Enhances Crystallinity and Mechanical Properties for Plasma-Sprayed Hydroxyapatite Biomedical Coatings.

The stability of plasma-sprayed hydroxyapatite (HA) coatings on metallic implants in vivo remains a significant challenge for load-bearing orthopedic implants despite their excellent mechanical and osteoconductive properties. This study focuses on oxide layer formation on the surface of Ti6Al4V samples through furnace heating at 600, 700, and 800 °C for 10 min for optimization of the most effective oxide layer to increase plasma coating crystallinity and improve plasma coating bond strength with the metal surface. The 800 °C heat treatment shows an effective oxide layer which increases coating crystallinity from 64 to 75% and coating adhesive bond strength from 25.9 ± 2.3 to 30.7 ± 1.1 MPa, while simultaneously reducing the dissolution rate of HA coatings. The addition of biologically relevant dopants, MgO and SiO2, show negligible effects on crystallinity and adhesive bond strength on plasma-sprayed HA coatings and additionally show an enhancement effect on osteoblast proliferation and differentiation. Moreover, the inclusion of these additivess shows an increase in osteogenesis in a rat distal femur model after 6 and 10 weeks of implantation. Overall, this study provides a direct solution to improve the crystallinity, adhesive bond strength, and osteogenic properties of plasma-sprayed HA coatings on orthopedic implants that is more manufacturable and translational from research to an industrial scale.