Biocompatibility of variable thicknesses of a novel directly printed aligner in orthodontics

Abstract

Direct printed aligners (DPAs) offer benefits like the ability to vary layer thickness within a single DPA and to 3D print custom-made removable orthodontic appliances. The biocompatibility of appliances made from Tera Harz TA-28 (Graphy Inc., Seoul, South Korea) depends on strict adherence to a standardized production and post-production protocol, including UV curing. Our aim was to evaluate whether design modifications that increase layer thickness require a longer UV curing time to ensure biocompatibility. Specimens with varying layer thickness were printed to high accuracy using Tera Harz TA-28 and the Asiga MAX 3D printer (Asiga SPS ™ technology, Sydney, Australia). UV curing durations were set at 20, 30 and 60 min. Cytotoxicity was evaluated using the AlamarBlue assay on human gingival fibroblasts. Cell viability decreased with increasing specimen thickness (significant for 2 mm [p < 0.001], 4 mm [p < 0.0001], and 6 mm [p < 0.01]) under the manufacturer-recommended 20-min UV curing. Extending the curing time did not improve cell viability. However, cell viability never decreased by more than 30%, meeting EN ISO 10993-5 standards for non-cytotoxicity. The standard 20-minute UV curing protocol ensures the biocompatibility and patient safety of Tera Harz TA-28 for material thicknesses up to 6 mm.

Keywords: 3D printed aligners; 3D printing; Aligner therapy; Biocompatibility; Clear aligner; Cytotoxicity; Digital orthodontics; Direct printed aligners; Resin.

Fig. 1

Specimen testing to evaluate the influence of the aligner material on human gingival fibroblasts (HGF).

Fig. 2

Measured thickness of specimens (n = 12) printed in different thicknesses (0.5, 1, 2, 4, 6 mm) using Graphy’s aligner resin Tera Harz TA-28 after UV curing with Tera Harz Cure for 20, 30 and 60 min respectively.

Fig. 3

Cell viability of HGF exposed to “Tera Harz TA-28”. (A) Specimens of various thicknesses (0.5, 1, 2, 4, 6 mm), all post-cured for 20 minutes; (B) Proliferation of HGFs exposed to “Tera Harz TA-28”, monitored over time as a fold increase in confluence using life cell imaging. Statistical significance was analyzed using one way Anova, p < 0.05*, p < 0.01**, p < 0.001***, p < 0.0001****.

Fig. 4

Variation in cell viability assessed following post processing (UV curing) for different durations (20, 30, 60 min). Specimens in media for 12 days and treatment of HGFs with the conditioned media (1:2) for 72 h. Data are presented as individual values (n = 6) relative to the control mean (100%), expressed as mean ± SD. Statistical significance was analyzed using one way Anova, p < 0.05*, p < 0.01**, p < 0.001***; p < 0.0001****.

Fig. 5

Cell viability of HGFs exposed to “Tera Harz TA-28” in saliva. (A) 50% saliva conditioned with specimens of varying thicknesses (0.5, 1, 2, 4, 6 mm), all post cured for 20 min, for 12 days. (B) Proliferation of HGFs under “Tera Harz TA-28” with 50% saliva monitored over time as fold increase in confluence by life cell imaging. (C) HGFs in media conditioned with specimen of 1 mm thickness. (D) HGF in 50% salvia conditioned with specimen of 1 mm thickness.