Mechanical, Biological, and Antibacterial Characteristics of Plasma-Sprayed (Sr,Zn) Substituted Hydroxyapatite Coating

Implant-related infections are a major concern in total joint prostheses, occurring up to 3% in operations. In this work, 5% Zn²⁺ was added in HA to offset bacterial activity and 5% Sr²⁺ was also incorporated as a binary dopant to reduce the cytotoxic effect of Zn²⁺. The nanosized HA powder was synthesized by the hydrothermal method and then heat-treated at 600 °C for 4 h. The heat-treated powder was plasma-sprayed on a titanium alloy Ti-6Al-4V substrate. The addition of the dopant did not significantly influence the physical and mechanical properties of the coating. However, the cytocompatibility, antimicrobial, and contact-angle properties statistically enhanced. Moreover, the (Sr,Zn)-HA coating was post-heat treated at 500 and 600 °C for 3 h. X-ray diffraction confirmed that after heat treatment phase purity and crystallinity increased and residual stress decreased. Mechanical stability was evaluated by adhesive bond strength, and the results showed that after heat-treatment bonding strength increased from 26.81 ± 2.93 to 29.84 ± 3.62 and 34.66 ± 2.57 MPa, at 500 and 600 °C, respectively. Similar to the mechanical property, antibacterial activities and biological functions are also significantly improved. More interestingly, it was also observed that the Zn²⁺ ions released from the coating depend on Ca²⁺, P, and Sr²⁺ ions while Ca²⁺, P, and Sr²⁺ ions relied on heat treatment temperatures. However, (Sr,Zn)-HA coating at 600 °C demonstrates cytotoxic effects on MC3T3-E1 cells, characterized by poor cellular morphology on the coating surface and ultimately, cell death. The doping of Sr²⁺ with Zn²⁺, therefore, can offset the cytotoxic effects and enhanced biological performance. All of the outcomes of this study signify that (Sr,Zn)-HA coating heat-treated at 500 °C showed not only excellent mechanical and biological performance but also enhanced the antibacterial properties.