PMMA-TiO₂ and PMMA-ZrO₂ coatings modified with hydroxyapatite and β-TCP for Ti6Al4V metallic implants

Full text: Novel PMMA-TiO₂ and PMMA-ZrO₂ organic-inorganic hybrid coatings modified with hydroxyapatite and β-tricalcium phosphate (β-TCP) have been developed to improve the corrosion resistance and osseointegration of Ti6Al4V titanium alloys, which are widely used as metallic orthopedic implants. The organic-inorganic hybrids were synthesized using the sol-gel process, by reacting titanium isopropoxide, or zirconium propoxide, with 2-hydroxyethyl methacrylate (2- HEMA) and methyl methacrylate (MMA). Both hybrid structures, studied by small-angle X-ray scattering and by X-ray photoelectron spectroscopy, are composed of PMMA matrix covalently bonded through HEMA molecules to TiO₂ or ZrO₂ nanoparticles, having fractal geometry and an average distance of 2 nm. Optimized synthesis parameters resulted in transparent, homogeneous and crack free coatings of less than 10 μm thickness, with a corrosion resistance of up to 10¹⁰ Ω cm², 4 orders of magnitude higher than that of bare Ti6Al4V, as determined by electrochemical impedance spectroscopy in simulated body fluid (SBF) solution. Results provided by MTS assay (cell metabolic activity), Ca⁺² deposition assay and alkaline phosphatase assay using osteoblasts (bone-forming cells) revealed a significant improvement of the biocompatibility and osseointegration for hydroxyapatite and β-TCP modified coatings, compared to the uncoated alloy. This was explained by the increase of nanoscale roughness, measured by atomic force microscopy, and the higher surface free energy, quantified using contact angle measurements, which lead to an enhanced protein adsorption on the coatings. The PMMA-TiO₂ hybrids presented higher corrosion resistance and biocompatibility, compared to the PMMA-ZrO₂ hybrids. (author)