Hygroscopic behaviour and colour stability of 3D-printed and thermoformed aligner materials: an in vitro study

Directly 3D-printed aligner materials have recently emerged as an alternative to established thermoformed clear aligners. The aim of this in vitro study was to investigate the colour stability, water sorption, and water solubility of directly 3D-printed aligner materials in comparison with an established thermoformed reference, as these properties are essential for clinical performance but currently insufficiently investigated for directly 3D-printed aligners.

Tera Harz TC-85DAC, Freeprint Aligner, Freeprint Aligner with digital polishing, Dental LT Clear V2 and one thermoformed polyethylene terephthalate glycol material (Biolon) were examined. Water sorption and solubility were determined after seven days in distilled water at 37 °C. Colour stability was assessed after 12 h of immersion at 37 °C in turmeric, coffee, black tea, orange juice and distilled water. Total colour differences (ΔE*) were calculated spectrophotometrically using the Commission Internationale de l'Éclairage L*a*b* system and converted into National Bureau of Standards units. Data were analysed by one-way ANOVA with Games-Howell post hoc test and by Kruskal-Wallis test with Dunn's test and Bonferroni correction (p < 0.05).

Biolon exhibited the lowest water sorption (mean 10.70 µg/mm³) and negligible solubility (median 0.00 µg/mm³). Among the printed resins, Tera Harz TC-85DAC showed the lowest water sorption (mean 17.28 µg/mm³) and solubility (median 2.50 µg/mm³), whereas Dental LT Clear V2 displayed the highest sorption (mean 29.53 µg/mm³) and Freeprint Aligner with digital polishing the highest solubility (median 17.38 µg/mm³). Turmeric caused the most pronounced discolouration, followed by coffee and black tea, while orange juice and water produced minimal changes. The thermoformed material remained most colour-stable (mean of the median ΔE* values across all immersion media = 4.21), whereas Dental LT Clear V2 showed the greatest discolouration (mean of the median ΔE* values = 27.35). Among the printed resins, Tera Harz TC-85DAC displayed the lowest staining susceptibility.

The tested aligner materials differed markedly in water sorption, solubility, and colour stability. The thermoformed polyethylene terephthalate glycol material consistently showed the most favourable profile. Among the printed resins, Tera Harz TC-85DAC exhibited the most favourable combination of the investigated properties, supporting its potential for clinical application.