Mechanical Properties of Thermoformed and Direct- Printed Aligner Materials After Immersion in 37 °C Water: A 14-Day In Vitro Study

<title>Abstract</title> This study compared the mechanical properties of direct-printed dental aligner materials made from 3D-printed resins TC-85, TR-07, and TA-28 with those of two conventional thermoformed materials—Zendura-A and Zendura-Flx—to evaluate their performance under simulated physiological conditions. Test specimens were immersed in a 37°C water bath for 12 different durations: 0, 5, and 30 min; 1, 3, 6, and 9 h; and 1, 3, 7, and 14 d. Tensile tests were performed using a universal testing machine (Zwick Z010, Zwick, Ulm, Germany) to measure the Young’s modulus (MPa), elongation at break (%), and tensile force (N) at strains of 1%, 2%, and 3%. After 14 d of immersion, TC-85, TA-28, and TR-07 exhibited forces in the range of 4.04–7.24 N at 1% strain and 7.30–13.48 N at 3% strain, while Zendura A and Zendura FLX exhibited forces of 26.26–32.91 N at 1% strain and 32.91–65.23 N at 3% strain. The Young’s modulus and UTS results exhibit a trend similar to that of the tensile force. Direct-printed aligners exhibited a 25.3% (TC-85) increase in elongation at break after 30 min, whereas thermoformed aligners exhibited a 5.5% reduction. Direct-printed resins, such as TC-85, TA-28, and TR-07, with temperature-responsive viscoelastic behavior, exhibited statistically significant differences from thermoformed aligner materials, delivering lower mechanical loads that could favor a more suitable orthodontic force profile for clear aligners.