In vitro study: Bond strength, electrochemical and biocompatibility evaluations of TiO2/Al2O3 reinforced hydroxyapatite sol–gel coatings on 316L SS

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To improve the biocompatibility and corrosion resistance of 316L SS (Stainless Steel) metal implants, HAp(hydroxyapatite)/TiO2/Al2O3 nanocomposite coating was created using dip sol-gel method sintered at 550 °C. The weight percentage of TiO2+ Al2O3 in these coatings was 20, 30, and 40% wt. In this study, characterization was performed using FTIR (Fourier-Transform Infrared Spectroscopy), XRD, FE-SEM (Field Emission Scanning Electron Microscope), EDS (Energy Dispersive Spectroscopy), DLS (Dynamic Light Scattering), Pull-off, and AFM (Atomic Force Microscopy) analysis. The potentiodynamic polarization and EIS (Electrochemical Impedance Spectroscopy) were performed in the simulated body fluid (SBF) solution. The samples were immersed in SBF solution for 28 days to investigate the ability to form a bone-like layer on the coatings. The amount of calcium and phosphate in the SBF solution and on the coating surface was measured. The MTT assay was performed to examine human MG63 cells viability for various coatings (24 and 72 h). The results showed that the HAp/TiO2/Al2O3 reinforced coatings have the lowest cracks and porosity and have the highest bond strength to the 316L SS substrate (26.5 MPa). Also, the lowest corrosion current density is related to nanocomposite coatings (HAp + 40%wt Al2O3 + TiO2:0.091 μA/cm2), which is pretty low compared to the uncoated substrate (40.35 μA/cm2) or HAp coating (25.26 μA/cm2). The EIS test results showed that in Nyquist curves, the semicircular diameter is more extensive for nanocomposite coatings, which indicates the higher corrosion resistance of these coatings. Cell culture studies showed that the reinforced coatings were non-cytotoxic to human MG63 cell line (HAp + 30%wt Al2O3 + TiO2:99.50% MG63 cell viability), and these coatings have a higher ability to form a bone-like layer than other coatings. However, adding 40% wt TiO2 + Al2O3 negatively affects biocompatibility (HAp + 40%wt Al2O3 + TiO2:87.7% MG63 cell viability). Therefore, HAp + 30%wt(TiO2 + Al2O3) coatings have biocompatibility, adhesion strength, corrosion resistance higher than other coatings, which means that this improved coating has better properties than hydroxyapatite coatings, and this is a new turning point for biocomposite coatings for medical applications.