Titania nanotube interface to increase adhesion strength of hydroxyapatite sol-gel coatings on Ti-6Al-4V for orthopedic applications

Abstract

Sol-gel coating hydroxyapatite (HA) allows for non-line-of-sight coatings on metallic implants that enhance biocompatibility and osseointegration, but coatings are on the order of a few microns thick and have poor adhesion, especially at low sintering temperatures; Both issues can limit implant longevity. In order to improve coating adhesion strength, a titania nanotube interface was used to reduce thermal mismatch and provide a nano-scale surface morphology for mechanical interlocking, while multiple dip-coatings achieved coating thicknesses >70 μm. In this study, Ti-6Al-4V disks were anodized to produce self-assembled titania (TiO2) nanotubes (NTs) on the surface of the substrates, while HA sol-gel was used to dip-coat the samples. The titania layers measured 800–900 nm thick, with nanotube pore widths of 90 ± 12 nm. Pure HA coating thicknesses were measured at 73.3 ± 10.5 μm and 84.97 ± 18.1 μm for polished Ti-6Al-4 V and anodized Ti-6Al-4V, respectively. The adhesion strengths of pure HA coatings on anodized (NT) surfaces were significantly higher (P < 0.05, N = 7) than polished surfaces at 19.02 ± 3.36 MPa and 13.8 ± 3.28, respectively. Strontium was doped into sol-gel coatings (Sr-Sol) to enhance bioactivity and showed a significant increase in preosteoblast MTT optical density after 3 days of culture compared to pure HA sol-gel, and plasma sprayed pure HA and strontium-HA (Sr-HA). SEM imaging showed well attached osteoblast cells with a similar morphology of multiple, long filopodia across all samples. This study shows that forming titania nanotubes through anodization is a viable approach to increase the adhesion strength of sol-gel coatings sintered at low temperatures, and demonstrates a facile method to incorporate dopants that enhance the osteoconductivity of HA sol-gels.