Thermal, Biological, and Bioactive Characterization of Sol–Gel Coating Materials for Biomedical Stainless Steel

The development of bioactive hybrid coatings for biomedical implants requires materials exhibiting suitable thermal stability, bioactivity, and biocompatibility. In this study, hybrid organic–inorganic sol–gel coatings based on silica (SiO₂) and polyethylene glycol (PEG, 24 wt%) were functionalized with different concentrations of caffeic acid (CafA 5, 10, and 15 wt%) and deposited onto AISI 304 and AISI 316 stainless steel substrates by dip-coating without surface pre-treatment. The proposed approach enabled the formation of homogeneous hybrid coatings on untreated stainless steel substrates through a simple and scalable deposition process. A thermal analysis demonstrated the stability of the hybrid network and the effective integration of the organic and inorganic phases. Bioactivity was evaluated by in vitro immersion in simulated body fluid (SBF), while SEM observations revealed the formation of mineral deposits on the coating surface, and an EDX analysis confirmed the presence of calcium and phosphorus within the deposited layer. The formation of crystalline hydroxyapatite (HA) was subsequently confirmed by X-ray diffraction (XRD), confirming that all investigated formulations retained their ability to induce apatite formation after SBF immersion. In addition, in vitro biocompatibility assays confirmed that the developed materials exhibited concentration-dependent cytocompatibility, with the cellular response being influenced by the amount of incorporated CafA. Overall, the results demonstrate that the proposed hybrid materials combine thermal stability, bioactivity, and cytocompatibility, highlighting their potential as bioactive coatings for biomedical applications.