Multifunctional hybrid Nylon/Chitosan/Hydroxyapatite/TiO₂ biocoatings for enhanced corrosion protection and cellular response of 316L stainless steel implants

Although numerous polymer and ceramic coatings have been developed for metal implants, achieving long-term corrosion protection while maintaining favorable immune responses remains a significant challenge. This study developed and evaluated multifunctional hybrid nylon (PA)/chitosan (CS)/hydroxyapatite (HAp)/TiO₂ biocoatings deposited on 316 L stainless steel (316 L SS) by dip-coating to improve corrosion resistance and biological performance for implant applications. Two formulations, PA84%/CS10%/HAp5%/TiO₂1% and PA89%/CS5%/HAp5%/TiO₂1%, were characterized by scanning electron microscopy (SEM), mechanical testing, cell viability assays with RAW-Blue macrophages, NF-κB activation analysis, and electrochemical evaluation in Hank's solution for 120 days. The coatings exhibited homogeneous porous morphologies, indicating integration between the polymeric matrix and ceramic phases. This microstructure was accompanied by cell viability approaching 98%, while NF-κB activation remained low compared with the inflammatory control, demonstrating a cellular response compatible with biomedical applications. Although CS, HAp, and TiO₂ incorporation reduced the mechanical performance relative to neat PA, the hybrid biocoatings retained ductility and resilience suitable for flexible coating applications, with PA84%/CS10%/HAp5%/TiO₂1% showing the best balance among the composites. Electrochemical tests demonstrated enhanced corrosion protection compared with uncoated 316 L SS. Among the evaluated systems, PA84%/CS10%/HAp5%/TiO₂1% showed the best performance, exhibiting the lowest corrosion current density (3.00E-07 mA/cm 2 ) and the highest impedance modulus (90372 Ω.cm 2 ) after 120 days of immersion. This behavior was attributed to the multifunctional protective layer, which combines barrier effect, interfacial stability, and diffusion-controlled ionic transport, due to the coating-passive layer synergy. These results indicate the potential of PA/CS/HAp/TiO₂ biocoatings to enhance long-term corrosion protection and biologically compatible surfaces for metallic implants.