Inhibition of cariogenic biofilms by a zinc-doped phosphate-based glass orthodontic adhesive

Objectives To evaluate the anti-biofilm efficacy of an orthodontic adhesive incorporating 9 wt% zinc-doped phosphate-based glass (Zn-PBG) against single-species and multispecies oral biofilms. Materials and methods Zn-PBG and phosphate-based glass (PBG) powders were synthesized, and their particle size, density, and elemental composition characterized. The release of calcium (Ca), phosphorus (P), and zinc (Zn) ions from both PBG and Zn-PBG powders was evaluated at various immersion time points. Antibacterial activity against Streptococcus mutans ( S. mutans ) was assessed using an insert-based biofilm model comprising three groups: control adhesive without bioactive glass (CTRL), adhesive containing 9 wt% PBG (PG 9), and adhesive containing 9 wt% Zn-PBG (ZnPG 9). Results Zn-PBG exhibited sustained Zn ion release, while Ca and P release was lower than that of Zn-free phosphate-based glass. The ZnPG 9 group demonstrated a significant 30.31% reduction in S. mutans viability compared to the control group ( P < 0.001). In the multispecies biofilm model, ZnPG 9 showed significantly reduced biofilm biomass (58.95 ± 14.48 μm³/µm²) compared to the control (89.39 ± 11.63 μm³/µm²) ( P < 0.05). Additionally, biofilm thickness was significantly reduced (93.71 ± 32.66 μm) compared to the control (172.35 ± 22.64 μm) ( P < 0.05), along with decreased bacterial density throughout the biofilm depth. Conclusions Adding 9 wt% Zn-PBG to orthodontic adhesives effectively prevented both single-species and multispecies biofilm formation by providing ion-mediated antibacterial action. However, these results come from an in vitro model with a short-term (7-day) ion-release assessment, and future research should evaluate long-term effectiveness under clinical conditions. Clinical Relevance Zn-PBG-containing orthodontic adhesives may provide a material-based strategy to reduce biofilm accumulation at the bracket–enamel interface, potentially helping to prevent biofilm-associated enamel demineralization during orthodontic treatment without relying on patient compliance.