Comparison of cytotoxicity, antibacterial activity and surface roughness, among Ag-, Ag/ZnO-, and Ag/TiO2-coated stainless-steel orthodontic brackets: An in vitro study

Background This study aimed to compare the effects of three nanoparticle coatings, silver (Ag), silver/zinc oxide (Ag/ZnO), and silver/titanium dioxide (Ag/TiO 2 ), on cytotoxicity, antibacterial activity, and surface roughness of stainless-steel (SS) orthodontic brackets. Materials and methods A total of 148 SS brackets were divided into four equal groups ( n = 37): Ag-coated, Ag/ZnO-coated, Ag/TiO 2 -coated, and uncoated control. Coatings were applied by physical vapor deposition to a thickness of 50 nm. Surface coatings were analysed by energy-dispersive X-ray spectroscopy and atomic force microscopy (AFM). Cytotoxicity was assessed using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay on oral epithelial cells. Antibacterial activity against Streptococcus mutans was evaluated using the disk diffusion method. Surface roughness was measured using AFM. Statistical analysis was performed using one-way analysis of variance followed by Tukey's post hoc test ( P ≤ 0.05). Results Cell viability differed significantly among groups ( P = 0.001), with Ag/TiO 2 -coated brackets showing the highest viability among coated groups (86.81 ± 2.92%), followed by Ag/ZnO (73.02 ± 2.09%), whereas Ag-coated brackets showed the lowest viability (57.23 ± 5.84%), indicating the highest cytotoxicity. Antibacterial testing demonstrated significant intergroup differences ( P < 0.001), where Ag/ZnO-coated brackets showed the highest inhibition zone against S. mutans (10.60 ± 1.65 mm), followed by Ag-coated (9.10 ± 1.01 mm) and Ag/TiO 2 -coated brackets (7.60 ± 0.52 mm), while controls showed no inhibition. Surface roughness analysis showed significant differences among groups for both average roughness (Ra) and root mean square roughness (Rq) ( P = 0.001), with Ag-coated brackets exhibiting the lowest roughness values (Ra: 2.12 ± 1.18 nm; Rq: 3.02 ± 1.71 nm). Conclusion Ag/TiO 2 demonstrated the highest cell viability among the coated groups, whereas standalone Ag coatings exhibited the highest cytotoxicity. The ZnO/Ag combination showed the greatest antibacterial efficacy against S. mutans . All nanoparticle coatings reduced the surface roughness of stainless-steel brackets, with Ag coatings producing the smoothest surfaces.