Effects of Hybrid Copper Oxide-Silver and Titanium Dioxide-Silver Nanocoatings on the Surface Roughness and Friction of Stainless Steel Orthodontic Brackets: A Comparative In Vitro Study

IntroductionNanoparticle coatings on orthodontic brackets have antibacterial potential but may alter surface roughness and friction at the bracket-archwire interface.This study compared the surface roughness and static frictional resistance of stainless steel brackets coated with silver-impregnated copper oxide (CuO+Ag) or silver-impregnated titanium dioxide (TiO₂+Ag) nanoparticles with those of uncoated brackets. Materials and methodsThirty stainless steel maxillary central incisor brackets (0.022-inch slot) were allocated into three groups (n = 10 each): uncoated control, CuO+Ag-coated, and TiO₂+Ag-coated.Arithmetic average roughness (Ra), root mean square roughness (Rq), and maximum peak-to-valley height (Rz) were measured by atomic force microscopy as received (T0), after coating (T1), and after friction testing (T2).Static friction was measured using a universal testing machine with a 0.019 × 0.025-inch stainless steel archwire in artificial saliva.Data were analyzed using one-way or Welch analysis of variance with post hoc tests and paired t-tests (p < 0.05). ResultsBaseline roughness was comparable among the groups.At T1, CuO+Ag increased Ra, Rq, and Rz, whereas TiO₂+Ag increased Ra and Rq but decreased Rz (paired t-test, |t(9)| ≥ 10.82, p < 0.001).Static friction differed among the groups (F(2, 27) = 9.15, p < 0.001, η² = 0.404).CuO+Ag showed the lowest friction (36.31 ± 3.90 gf), significantly lower than the control (41.65 ± 4.77 gf; q = 3.82, p = 0.031) and TiO₂+Ag (44.65 ± 4.53 gf; q = 5.97, p < 0.001), whereas TiO₂+Ag did not differ from the control (q = 2.15, p = 0.298).Roughness increased in all groups after friction testing (t(9) ≥ 33.48, p < 0.001), with smaller increases in the coated groups. ConclusionsBoth nanocoatings altered bracket surface roughness.CuO+Ag significantly reduced static frictional resistance and appears promising for further investigation as an orthodontic bracket coating.