Quaternary tannic acid-based colorless coatings with outstanding antimicrobial properties for clear aligners

Surface degradation of clear aligners promotes bacterial adhesion and biofilm formation, which increases the risk of enamel demineralization and compromises aesthetics. This study aimed to improve the antimicrobial performance of clear aligners by applying a hydrophobic quaternary tannic acid (QTA) coating that does not alter the mechanical or optical properties.

Aligners were dip-coated with QTA, and the coating characteristics were evaluated using scanning electron microscopy (SEM)-energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and Fourier transform infrared analysis. Properties including water sorption, mechanics, optical transmittance, and color stability were evaluated. Antimicrobial efficacy was tested using mono- and polymicrobial biofilm models with colony forming unit (CFU) counts, crystal violet staining, and SEM imaging. Biocompatibility was assessed using a cell-counting kit-8, hemolysis tests, and a hamster oral-irritation model.

QTA was successfully grafted onto aligners without compromising their mechanical or optical properties. Coated aligners exhibited strong antimicrobial activity, with > 99% and 82.9% (p < 0.05) reductions in Streptococcus mutansand Candida albicans CFU counts, respectively. Furthermore, they effectively inhibited mono- and polymicrobial biofilm formation. The antimicrobial effects remained stable after exposure to artificial saliva and storage for 3 months. Biocompatibility was confirmed based on low cytotoxicity, minimal hemolysis, and no mucosal irritation in vivo.

QTA-based coatings endow clear aligners with potent and sustained antimicrobial activity. Moreover, the favorable biocompatibility and preserved functional properties of QTA-coated aligners suggest a strong potential for clinical orthodontic applications.