Biological performance of ZnO-doped hydroxyapatite coatings on PEEK: <i>in vitro</i> antibacterial, cytotoxic, and osteogenic assessment for orthopedic implants

ZnO-doped hydroxyapatite (HAp) coatings were developed on thermally sensitive polyetheretherketone (PEEK) substrates using a hybrid plasma spraying approach that combines powder and solution precursor feedstocks. Three coating architectures with different ZnO contents were designed to assess the influence of zinc incorporation on antibacterial and osteogenic performance. All coatings were deposited at a low plasma power (5.7 kW), enabling successful deposition without thermal degradation of the PEEK substrate, and achieving bond strengths up to 17 MPa. ZnO-doped coatings exhibited antibacterial activity against<i>E. coli</i>and<i>S. aureus</i>, with significantly higher efficacy against<i>E. coli. In vitro</i>tests using MC3T3-E1 pre-osteoblasts showed enhanced cytocompatibility and osteogenic differentiation at low ZnO concentrations, as indicated by increased alkaline phosphatase (ALP) activity and calcium deposition exceeding those of undoped HAp coatings by over 50% after 21 d. The combination of antimicrobial and osteoinductive properties suggests that ZnO-doped HAp coatings are promising candidates for PEEK-based orthopedic implants.