Intraoral Aging of 3D Printed Orthodontic Aligners

Participants received a directly 3D-printed TC-85 aligner in the maxillary arch and a directly 3D-printed TA-28 aligner in the mandibular arch. Both aligners were worn simultaneously for 2 weeks (22 hours/day).

Aligner directly 3D-printed from Tera Harz TC-85 shape-memory resin [add printer, layer thickness and post-curing protocol]. Worn 22 hours/day for 2 weeks in the maxillary or mandibular arch, according to a computer-generated randomization sequence.

Participants received a directly 3D-printed TA-28 aligner in the maxillary arch and a directly 3D-printed TC-85 aligner in the mandibular arch. Both aligners were worn simultaneously for 2 weeks (22 hours/day).

Aligner directly 3D-printed from Tera Harz TA-28 shape-memory resin [same printer, layer thickness and post-curing protocol as above]. Worn 22 hours/day for 2 weeks in the maxillary or mandibular arch, according to a computer-generated randomization sequence.

Indentation hardness (HIT, GPa) of directly 3D-printed TC-85 and TA-28 aligners, measured by instrumented nanoindentation using a Nano Indenter XP (MTS Systems Corporation, USA) with a Berkovich diamond indenter. Tests were displacement-controlled to a maximum depth of 2000 nm at a constant strain rate of 0.05 s-¹, with 25 indentations per specimen spaced 30 μm apart. Testing followed ISO 14577-1:2015 and data were analyzed by the Oliver and Pharr method. Retrieved aligners are compared with unused aligners printed from the same digital design.

Indentation elastic modulus (EIT, GPa) of directly 3D-printed TC-85 and TA-28 aligners, measured by instrumented nanoindentation using a Nano Indenter XP (MTS Systems Corporation, USA) with a Berkovich diamond indenter. Tests were displacement-controlled to a maximum depth of 2000 nm at a constant strain rate of 0.05 s-¹, with 25 indentations per specimen spaced 30 μm apart. A Poisson's ratio of 0.4 was assumed. Testing followed ISO 14577-1:2015 and data were analyzed by the Oliver and Pharr method. Retrieved aligners are compared with unused aligners printed from the same digital design.

Color of directly 3D-printed TC-85 and TA-28 aligners measured with a UV-visible spectrophotometer (V-770, JASCO Corporation, Tokyo, Japan). Each aligner was sectioned along the right central incisor-canine region, and the labial surface was positioned in a standardized manner facing the incident-light window. CIE Lab* coordinates were recorded according to ISO 11664-4:2019, and the color change between unused and retrieved aligners was calculated as ΔE₀₀ using the CIEDE2000 equation (parametric factors set to 1). Values are interpreted against 0.8 (50:50% perceptibility) and 1.8 (50:50% acceptability).

Light transmittance and absorbance of TC-85 and TA-28 aligners recorded across the visible range (400-700 nm) with a UV-visible spectrophotometer (V-770, JASCO Corporation, Tokyo, Japan) using Spectra Manager II software, after calibration according to the manufacturer's instructions. Values at 400 and 700 nm were extracted from each spectrum and compared between materials and between unused and retrieved aligners.

Thickness of TC-85 and TA-28 aligners measured by confocal laser scanning microscopy (Leica DMi8, Leica Microsystems, Wetzlar, Germany) with a 10× objective and 1.5× digital zoom under standardized imaging conditions. Standardized 4 × 4 mm specimens were taken from the labial surface of the left central incisor region. Z-stack images were reconstructed in 3D using Leica LAS AF software (version 1.1.0.12420), and thickness was calculated as the perpendicular distance between the upper and lower specimen surfaces. Each specimen was measured twice and the mean was used.

Surface roughness of the inner surface of directly 3D-printed TC-85 and TA-28 aligners, assessed on the same 4 × 4 mm specimens used for thickness measurement, imaged by confocal laser scanning microscopy (Leica DMi8, Leica Microsystems, Wetzlar, Germany). Height maps generated from the z-stacks with Leica LAS AF software were analyzed in ImageJ (National Institutes of Health, USA) with the SurfCharJ-1q plugin to calculate Sa (arithmetical mean height), Sq (root mean square height), Sp (maximum peak height) and Sv (maximum valley depth), in µm. Analysis was repeated twice per specimen and the mean was used. Identical acquisition and analysis settings were applied to all specimens.

Degree of conversion (%) of TC-85 and TA-28 aligners, calculated from attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectra as the reduction of the aliphatic C=C band near 1638 cm-¹ relative to, referenced to the corresponding unset resin. Retrieved aligners are compared with unused aligners.

Qualitative comparison of ATR-FTIR spectra of unused and retrieved TC-85 and TA-28 aligners, focusing on the O-H/N-H stretching bands (3400-3200 cm-¹), aliphatic C-H bands (about 2920 and 2850 cm-¹), the carbonyl (C=O) band (about 1700 cm-¹) and the C-O/C-O-C region (1200-1000 cm-¹).