Investigation of the influence of disinfection procedures on the surface properties and in vitro behavior of Trogamid-coated metal brackets using Fourier transform infrared spectroscopy and force spectroscopy

This study investigates the influence of disinfection procedures on the surface chemistry, mechanical integrity and in vitro behavior of Trogamid® CX-based orthodontic hybrid brackets. Brackets were fabricated by injection molding onto micro-structured nickel free stainless-steel inserts and subjected to three disinfection procedures: UV-C irradiation, 70% ethanol immersion or steam autoclaving prior to surface analysis and in vitro testing. Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM) with force spectroscopy were employed to assess changes in the polymer's chemical structure, morphology, and mechanical response. In vitro biocompatibility and bacterial adhesion were also investigated. FTIR spectra indicated that UV irradiation weakened the amide hydrogen-bond network without forming new degradation bands, whereas ethanol disinfection and autoclaving were associated with a reorganization of the hydrogen-bond network. AFM showed a progressive reduction in surface roughness from UV irradiation to ethanol disinfection to autoclaving. Force spectroscopy revealed a lower elastic modulus after UV irradiation (1.2 vs 1.9 GPa for untreated brackets) and higher moduli after ethanol disinfection and autoclaving (2.3 and 2.4 GPa). Streptococcus mutans colonization did not differ significantly between untreated and treated brackets, and UV-disinfected hybrid brackets showed no cytotoxic effect on human gingival cells, comparable to stainless-steel brackets. Overall, none of the treatments left the bracket surface unchanged: UV disinfection best preserved the original topography but softened the surface layer, whereas ethanol and autoclave disinfection smoothed and stiffened the surface, with ethanol causing the smallest change in surface chemistry. The choice of procedure for in vitro studies should therefore be guided by the surface property most relevant to the intended investigation.