The objective of this study was to evaluate the antimicrobial activity, bacterial adhesion, and surface morphology of orthodontic elastomeric ligatures coated with titanium dioxide nanoparticles (TiO ₂ NPs) at different concentrations over 4 weeks.

TiO ₂ NPs were synthesized via hydrothermal synthesis and applied at 5% and 10% on elastomeric ligatures (Ormco®) using a sol-gel dip-coating method. In all, 720 ligatures were divided into control (uncoated), 5%, and 10% TiO ₂ NPs groups. The primary objectives were to assess antimicrobial activity against Streptococcus mutans via agar diffusion immediately after coating and after 48 hours, 2 weeks, and 4 weeks of immersion in artificial saliva at 37 °C, and to quantify bacterial adhesion as CFU. The secondary objectives included evaluation of surface morphology and roughness using SEM. Statistical analysis was performed using two-way ANOVA with Bonferroni correction.

Both TiO ₂ NP coated groups showed significantly higher antibacterial activity and lower bacterial adhesion than controls ( P < 0.001). In the 10% group, inhibition zones decreased by 6.8 mm (95% CI: 4.9–8.7) at 48 hours, 10.6 mm (95% CI: 8.7–12.5) at 2 weeks, and 24.7 mm (95% CI: 22.8–26.6) at 4 weeks; 5% group showed smaller reductions (8.3 mm, 13.9 mm, 21.5 mm). Bacterial adhesion declined by 9933 CFU (95% CI: 9113–10,754) for 10% and 6477 CFU (95% CI: 5656–7297) for 5%. The 10% coating consistently had the strongest antibacterial effect and lowest CFUs, though efficacy decreased over time. SEM revealed increased surface roughness, especially in the 10% group.

TiO ₂ NPs coatings, particularly at 10%, enhance short-term antibacterial properties of orthodontic ligatures. However, the antimicrobial benefits decrease over time due to coating degradation and exposure to saliva. Higher nanoparticle concentrations increase surface roughness, which may affect long-term biofilm resistance. Optimizing nanoparticle dispersion and coating durability is essential for sustained clinical effectiveness.