Fabrication of silver- and strontium-doped hydroxyapatite/TiO2 nanotube bilayer coatings for enhancing bactericidal effect and osteoinductivity

Hydroxyapatite (HA)-coated implants are more susceptible to bacterial infection because their bioactive surface, which is favourable for osseointegration, could also become a reservoir for bacterial colonisation. To solve this problem, an electrodeposition method for preparing silver- and strontium-modified antibacterial HA layers onto TiO2 nanotubes (TNs) was developed. Ag was incorporated into the HA coating to improve its antimicrobial properties. Sr was added as a second binary element to offset the potential cytotoxicity of Ag. Results showed that Sr2+ and Ag+ could be evenly incorporated into the HA lattice to form SrAgHA coatings. The TN layer with a diameter of 100 nm strengthens the adhesion via the anchoring effect. In vitro electrochemical corrosion studies demonstrated that the SrAgHA/TN coating sustains the stimulated body-fluid (SBF), thus indicating excellent corrosion resistance with a lower corrosion penetration rate than the bare commercially pure (CP)-Ti substrate. The composite coatings were found to be bioactive, based on the promotion of additional apatite onto the SrAgHA coating surface from SBF. Staphylococcus aureus growth was inhibited by SrAgHA/TN coatings, whereas the coatings without Ag had no effect on bacterial growth. MC3T3-E1 cell culture revealed that SrAgHA/TN demonstrated better cytocompatibility, and permitted stimulated cell proliferation, attachment and differentiation capacities than uncoated CP-Ti substrate. The addition of Sr to the AgHA coatings effectively counteracted the potentially negative effects and improved the performance compared with CP-Ti. The improved antibacterial effects, correlated with superior cytocompatibility and mechanical behaviour, suggested that SrAgHA/TN can be an alternative to pure HA for the preparation of reliable implant coatings for orthopaedic applications.