Preparation and characterization of superhydrophilic Sr-doped hydroxyapatite coatings for medical implant surfaces

To overcome the insufficient early osteoinductivity, delayed osseointegration and potential inflammatory response of Ti-6Al-4V (TC4) implants, four Strontium (Sr)-doped calcium phosphate coatings, HA, Sr-HA5, Sr-HA10 and Sr-HA20, which the substitution ratios of Sr 2+ for Ca 2+ were 0, 5, 10, and 20 mol% were grown in situ by a one-step hydrothermal route. The influence of Sr 2+ substitution on surface wettability, bioactivity, antibacterial efficacy and early osteoconductivity was systematically evaluated. Within the investigated compositional window, the water contact angle decreased from 25.10 ± 1.00°–0° when the substitution ratios increased from 0 to 10 mol%, corresponding to a transition from hydrophilic to super-hydrophilic. Scanning electron microscopy (SEM) images showed that the surface successively presented rice-grain, rod-like, 3-D lamellar network and granular morphologies with increasing Sr 2+ content. Combined energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) confirmed that the in-situ formed phase was predominantly Sr-doped hydroxyapatite. Antibacterial assays against Escherichia coli ( E. coli ) and Staphylococcus aureus ( S. aureus ) revealed the highest bactericidal rate for the coating containing 10 mol% Sr 2+ . In vitro cell studies further identified this composition as the optimum for simultaneous cell proliferation and osteogenic differentiation. These results demonstrate that Sr-HA coatings can significantly accelerate early implant stability, thereby providing experimental and theoretical support for shortening patient recovery and improving clinical outcomes.