Evaluation of surface roughness, microhardness, staining, and antimicrobial properties of zinc oxide–coated orthodontic thermoformed retainers – an in vitro study

Vacuum-formed retainers (VFRs) are the most commonly used removable orthodontic retention appliances owing to their esthetics, comfort, and ease of fabrication. However, they are prone to discoloration, microbial colonization, and reduced mechanical durability when used intraorally for extended periods of time. Surface modification with biocompatible nanoparticles, such as zinc oxide (ZnO), has shown the potential to enhance antibacterial activity and prolong material longevity in dental applications; however, this approach has not yet been explored for orthodontic retainers. This in vitro study aimed to evaluate the effects of zinc oxide nanoparticle (ZnO NPs) coating on the surface characteristics, color stability, microhardness, and antimicrobial efficacy of thermoformed polyethylene terephthalate glycol (PETG) retainers. A total of 60 PETG sheets were evaluated, comprising ZnO nanoparticle-coated ( n = 30) and non-coated ( n = 30) specimens. Scanning Electron Microscopy (SEM) and Fourier Transform Infrared Spectroscopy (FTIR) were performed to evaluate the coating morphology and characterization, Atomic Force Microscopy (AFM) was used to assess the surface roughness parameters (Ra, Rq, Ry), and nanoindentation was used for microhardness evaluation. A spectrophotometer was used to assess the staining by measuring the color change (ΔE) values after immersion in coffee, turmeric, Coca-Cola, and artificial saliva for 24 h and 7 days. Antimicrobial activity against Streptococcus mutans and Candida albicans was assessed using the disc diffusion method for ZnO NP suspension and the colony-counting method for ZnO-coated retainer sheets. Descriptive statistics and intergroup comparisons were performed using SPSS software (version 26.0; IBM Corp., Armonk, NY, USA). FTIR confirmed the presence of ZnO NPs coating after thermoforming. The coated retainers exhibited significantly lower surface roughness ( p = 0.032) and higher microhardness ( p = 0.045) than the uncoated specimens. Color stability significantly improved in the ZnO Np-coated samples, with reduced staining observed in all staining media except coffee, which exhibited an increased color change ( p < 0.05). ZnO nanoparticle suspension exhibited higher antimicrobial activity for S. mutans (Zone of Inhibition = 18.7 mm and 21.2 mm at 50 µg and 100 µg concentration) than positive control antibiotics but exhibited negligible antifungal activity against C. albicans . However, the coated retainer did not show significant antimicrobial activity against Streptococcus mutans and C.albicans. ( p > 0.05) ZnO nanoparticle coating significantly enhanced the hardness of PETG-based vacuum-formed retainers and improved color stability in all staining media except coffee. Despite the antibacterial activity of the ZnO nanoparticle suspension against S. mutans , the coated retainers showed no significant antimicrobial effect. The observed improvements were demonstrated under laboratory conditions, and further in vivo and clinical studies are required to validate the long-term performance, safety, and clinical applicability of ZnO-coated PETG retainers.