Composite coatings of lanthanum-doped fluor-hydroxyapatite and a layer of strontium titanate nanotubes: fabrication, bio-corrosion resistance, cytocompatibility and osteogenic differentiation

Poor bio-corrosion resistance and undesirable incomplete osseointegration restrict the application of hydroxyapatite (HA) as an implant coating material. In this study, a novel F-and-La co-substituted hydroxyapatite (FLaHA) coating, which was reinforced with strontium titanate nanotubes (STNTs), was applied on Ti substrates using a combination method of anodization, electrochemical deposition and hydrothermal treatment. To the best of our knowledge, this is the first report on the development of FLaHA/STNT coatings for improving the chemical stability and the mechanical and biological properties of Ti substrates. The STNT exhibits an evenly-distributed porous and latticed structure on Ti substrates that favours the infiltration of FLaHA crystals. Different characterisation techniques, such as x-ray photoemission spectroscopy, x-ray diffraction, field-emission scanning electron microscopy and energy-dispersive spectroscopy, have clearly confirmed the successful synthesis of STNT-FLaHA coatings that constitute oriented nanorod arrays. Isolated hexagonal nanorod grains, with diameters of 200-300 nm, that stand on a substrate provide a uniform morphology to the surface of electrodeposited thin films at micro-scales. The survival of the coatings was prolonged because of their good degradation resistance. Owing to the anchoring effect of the STNT layer, the adhesion strength of the FLaHA/STNT coating was 15.9 ± 5.4 MPa, which was two times higher than that of STNT-free HA coatings. The potentiodynamic polarisation curves and the Nyquist plot confirmed that the conversion coating significantly improved the bio-corrosion resistance of the Ti substrates in the SBF solution. Roughness and hydrophilicity of the control HA layer were even greater than those of the FLaHA/STNT coating. However, it provided better cell adhesion, spreading, proliferation and osteogenic differentiation for mouse pre-osteoblasts cells. That is, the FLaHA/STNT coating could enhance osteoconductivity by improving the cell-adhesion, proliferation and differentiation of osteoblast. Therefore, FLaHA/STNT nanocomposite coatings can be used as implant materials with multi-functional properties, such as good biocompatibility and high mechanical and corrosion-inhibiting properties.