Chemical, optical, and morphological properties of TPU and PET-G samples after aging in artificial saliva: an in vitro study

Abstract

Background: Thermoplastic materials, such as glycol-modified polyethylene terephthalate (PET-G) and thermoplastic polyurethane (TPU), undergo alterations due to environmental factors in the oral cavity, which can affect their composition and surface properties over time. While previous studies have explored these changes, a comprehensive characterization of TPU and PET-G properties, particularly after immersion in artificial saliva, remains limited. This study aimed to evaluate the aging process of 24 TPU and 24 PET-G dumbbell-shaped specimens before and after exposure to artificial saliva. The analysis focused on the morphological, chemical, and optical properties of the samples, including thickness, weight, and surface roughness.

Methods: The study examined 48 thermoplastic samples, equally divided between PET-G and TPU. The samples were thermoformed into standardized shapes and analyzed at three time points: after thermoforming (T0), after 7 days (T1), and after 14 days (T2) of immersion in artificial saliva at 37 °C. Measurements included weight, thickness, surface roughness, absorbance, and Fourier transform infrared spectroscopy (FTIR). Data were analyzed using one-way ANOVA to identify significant changes over time, with a significance level of p < 0.01.

Results: Both materials exhibited significant reductions in surface roughness, with TPU showing a decrease in average roughness (Ra) from 99.43 nm at T0 to 76.53 nm at T2 (-23.02%) and PET-G decreasing from 33.25 nm to 20.19 nm (-39.27%). The root mean square roughness (Rq) in TPU declined by 41.67% (from 126.91 nm to 74.02 nm), while PET-G showed a reduction of 28.06% (from 44.98 nm to 32.35 nm). Peak-to-valley roughness (Rt) also decreased by 10.5% in TPU and 27.96% in PET-G. No statistically significant changes were observed in thickness, weight, optical density, or chemical composition (p > 0.01). The roughness disparity between TPU and PET-G persisted even after immersion in saliva.

Conclusions: Following the simulated intraoral aging process, significant changes in surface roughness were observed in TPU and PET-G specimens. The reduction in roughness, particularly a 39.27% decline in PET-G and 23.02% in TPU, has been clinically associated with decreased plaque accumulation and reduced friction between the aligner and the teeth.

Keywords: Artificial saliva aging; Clear aligners; Morphological properties; Surface roughness; Thermoplastic materials.

Fig. 1

PLA customized supports

Fig. 2

Average weight of the TPU and PET-G samples at T0, T1, T2

Fig. 3

Average thickness of the TPU and PET-G samples at T0, T1, T2

Fig. 4

(a) Peak-to-valley roughness TPU charts at T0, T1, T2; (b) Average roughness TPU charts at T0, T1, T2; (c) RMS roughness TPU charts at T0, T1, T2; (d) Peak-to-valley roughness PET-G charts at T0, T1, T2; (e) Average roughness PET-G charts at T0, T1, T2; c) RMS roughness PET-g charts at T0, T1, T2

Fig. 5

Representative surface of the TPU and PET-G samples at T0 and T2 made with the atomic force microscope

Fig. 6

(a) Average Roughness TPU and PET-G Cartesian graph; (b) RMS roughness TPU and PET-G Cartesian graph; (c) Peak-to-valley TPU and PET-G Cartesian graph

Fig. 7

Absorbance spectra of the TPU samples at T0, T1 and T2

Fig. 8

Absorbance spectra of the PET-G samples at T0, T1 and T2

Fig. 9

Average FTIR spectroscopy spectrum of the TPU samples at T0, T1 and T2

Fig. 10

Average FTIR spectroscopy spectrum of the PET-G samples at T0, T1 and T2