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 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 ℃ for 10 min for optimization of the most effective oxide layer to increase plasma coating crystallinity and improve coating bond strength. The 800 ℃ 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 MPa 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, however, in vitro evaluations show an enhancement effect on osteoblast proliferation and differentiation. Moreover, the inclusion of these dopants 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.