A Kinetic Approach to Synergize Bactericidal Efficacy and Biocompatibility in Silver-Based Sol-Gel Coatings

Silver ions are antimicrobial agents with powerful action against bacteria. Applications in surface treatments, as Ag⁺-functionalized sol-gel coatings, are expected in the biomedical field to prevent contaminations and infections. The potential cytotoxicity of Ag⁺ cations toward human cells is well known though. However, few studies consider both the bactericidal activity and the biocompatibility of the Ag⁺-functionalized sol-gels. Here, we demonstrate that the cytotoxicity of Ag⁺ cations is circumvented, thanks to the ability of Ag⁺ cations to kill Escherichia coli (E. coli) much faster than normal human dermal fibroblasts (NHDFs). This phenomenon was investigated in the case of two silver nitrate-loaded sol-gel coatings: one with 0.5 w/w% Ag⁺ cations and the second with 2.5 w/w%. The maximal amount of released Ag⁺ ions over time (0.25 mg/L) was ten times lower than the minimal inhibition (MIC) and minimal bactericidal (MBC) concentrations (respectively, 2.5 and 16 mg/L) for E. coli and twice lower to the minimal cytotoxic concentration (0.5 mg/L) observed in NHDFs. E. coli were killed 8-18 times, respectively, faster than NHDFs by silver-loaded sol-gel coatings. This original approach, based on the kinetic control of the biological activity of Ag⁺ cations instead of a concentration effect, ensures the bactericidal protection while maintaining the biocompatibility of the Ag⁺ cation-functionalized sol-gels. This opens promising applications of silver-loaded sol-gel coatings for biomedical tools in short-term or indirect contacts with the skin.