Silver Ion Implantation into Orthodontic Stainless-Steel Materials: Physicochemical Characterization and In Vitro Microbiological Evaluation

Fixed orthodontic appliances create retention sites for bacterial biofilm, increasing the risk of white spot lesions and periodontal complications. Silver ion (Ag) implantation offers a potential solution by conferring antimicrobial properties while preserving mechanical integrity. This study evaluated the physicochemical properties, Ag release, biocompatibility, and antibacterial efficacy of AISI 301 stainless-steel orthodontic brackets (Ormco Bios) modified by silver ion implantation using a Pulsed Filtered Cathodic Arc Vacuum System. Physical and micro-structural surface characterizations—including SEM/EDS, interferometric roughness analysis, contact angle measurements, nanoindentation hardness testing, and friction coefficient determination (under dry and saliva-lubricated conditions)—were conducted directly on the modified brackets. In compliance with international testing standards requiring defined geometric areas, AISI 301 flat samples of same chemical composition and microstructure were used to quantify Ag release in artificial saliva (cumulative silver ion release in artificial saliva was quantified via ICP-MS over time, with measured concentrations corrected for the volume extracted at each sampling interval), as well as to perform in vitro biological (human fibroblast cytocompatibility) and microbiological assays against seven bacterial strains (Streptococcus mutans, Streptococcus gordonii, Lactobacillus salivarius, Lactobacillus acidophilus, Porphyromonas gingivalis, Enterococcus faecalis, and Staphylococcus aureus). SEM/EDS confirmed successful Ag incorporation (0.93 wt.%) on the brackets. Surface roughness remained unchanged (0.16–0.17 mm), whereas hydrophilicity, surface energy, and nano-hardness increased significantly. Friction coefficients decreased under both dry and saliva-lubricated conditions. ICP-MS analysis revealed Ag release stabilizing at 119 ± 5.0 ppb after one week. Biocompatibility testing demonstrated no cytotoxic effects on fibroblasts. Furthermore, modified surfaces exhibited marked antibacterial activity against Lactobacillus spp., P. gingivalis, E. faecalis, and S. aureus, though no significant inhibition was observed against S. mutans or S. gordonii. Ag ion implantation directly improves the mechanical and tribological properties of stainless-steel orthodontic brackets. The use of standardized specimens confirmed steady Ag release, cytocompatibility, and selective antibacterial efficacy, supporting Ag implantation as a viable surface modification strategy for clinical orthodontic applications.