Absorption and desorption dynamics of thermoformed and 3-dimensional-printed orthodontic aligners

The mechanical performance and fit of orthodontic aligners are influenced by their interaction with moisture in the oral environment. This study aimed to evaluate and compare the water absorption, desorption kinetics, and diffusion behavior of thermoformed (Invisalign [Align Technology, Santa Clara, Calif] and ClearCorrect [Institut Straumann AG, Basel, Switzerland]) and 3-dimensional (3D)-printed (Tera Harz TC-85, Clear A; Graphy Inc, Seoul, South Korea) orthodontic aligners.

Thirty-two aligners were divided by brand and fabrication method. The samples were immersed in distilled water at 37°C for 14 days and weighed daily. Desorption was measured at hourly intervals for 8 hours and once at 24 hours after removal from immersion. Diffusion kinetics were modeled using log-log transformed data and linear regression based on the Fickian diffusion theory.

All materials followed Fickian diffusion profiles. Thermoformed aligners showed significantly higher water absorption than 3D-printed aligners by day 14 (4.91% for Invisalign vs 2.76% for Clear A; P <0.001). Invisalign also exhibited the highest diffusion slope (k = 0.74) and the fastest desorption (93.7% in the first hour). Three-dimensional-printed aligners displayed slower early desorption but reached comparable release at 24 hours. A strong correlation was observed between water absorption and early desorption (r = 0.903; P <0.001).

The aligner fabrication method and material composition significantly affected the moisture kinetics. Thermoformed aligners, especially those with outer thermoplastic polyurethane layers, demonstrate greater and faster water interactions, whereas 3D-printed aligners offer lower overall uptake but delayed release. These differences may influence the aligner fit and performance in clinical settings.