Assessment of biofilm resistance, degree of conversion, and mechanical properties of nanomodified 3D-printed orthodontic clear aligner (in-vitro study)

Abstract Background Three-dimensional (3D)-printed orthodontic clear aligners represent a paradigm shift in malocclusion management; however, they remain susceptible to bacterial biofilm accumulation. Therefore, modification strategies that enhance resistance to biofilm accumulation without compromising mechanical performance are required. This study evaluated biofilm resistance, degree of conversion (DC%), flexural strength (FS), and surface microhardness (VHN) of commercially available 3D-printed clear aligner resin (CR) modified with nanozeolite (NZ) and chitosan nanoparticles (Chs NPs). Methods A total of 432 3D-printed specimens were fabricated and allocated into six groups: Group I (control): CR; Group II: (CR + 0.25 wt% NZ); Group III: (CR + 0.5 wt% NZ); Group IV: (CR + 0.25 wt% Chs NPs); Group V: (CR + 0.5 wt% Chs NPs); and Group VI (hybrid): (CR + 0.25 wt% NZ + 0.25 wt% Chs NPs). Surface topography and chemical characterization were performed using scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR) for DC%. Biofilm resistance against Streptococcus mutans was assessed using a crystal violet assay by measuring optical density (OD) after 48-hour and 14-day incubation periods. FS and VHN were evaluated using a universal testing machine and a Vickers microhardness tester, respectively. All properties were evaluated before and after 14-day aging. Data were analyzed using two-way ANOVA and independent samples t -test with Bonferroni adjustment ( α = 0.05). Results Two-way ANOVA indicated that the interaction between nanoparticle content and aging significantly influenced the DC%, OD, FS, and VHN values ( p < 0.001). The 0.25 wt% NZ group showed the lowest OD after 48 h, whereas the 0.5 wt% NZ group showed the lowest OD after 14 days. The hybrid group demonstrated the highest FS and VHN values before and after aging. DC% values remained comparable among most groups, except for the 0.5 wt% Chs NPs group, which exhibited significantly lower DC% values before aging. Overall, the 0.5 wt% NZ group demonstrated the most favorable balance between reduced biofilm biomass and mechanical performance. Conclusions Nanoparticle incorporation improves the functional properties of 3D-printed clear aligners. NZ-containing groups reduced Streptococcus mutans biofilm biomass while preserving acceptable mechanical performance, suggesting potential for development of biofilm-resistant aligner materials.