## 1 Introduction  
A novel resin material for direct 3D printing of clear aligners has been recently introduced [1] and tested for cytotoxicity and estrogenicity [2, 3]. Recently, it received approval for biocompatibility from the Korean and United States Food and Drug Administration [4].  
Tera Harz TC-85 (Graphy Inc., Seoul, Republic of Korea) exhibited the capability of delivering light forces, with a reduced force decay induced by repeated insertion of the clear aligners and geometric stability at high temperatures [5].  
In order to maintain biocompatibility and to obtain an effective appliance, the production process needs to carefully follow the processing protocol indicated by the manufacturing company, which disciplines not only the printing stage, but also the resin excess removal, the support removal, and the post-curing phase; each of these passages can have an impact on the surface characteristics and on the final mechanical properties of the product.  
Since a standardised production workflow can avoid errors and lead to an efficient and time-saving in-office aligner production, several studies have been conducted to test the effect of different variables on the final product.  
[PERSON] et al. compared the mechanical properties of orthodontic aligners among different commercially available 3D printing devices and reported that the mechanical properties of 3D-printed orthodontic aligners were dependent on the 3D printer used [6].  
[PERSON] et al. concluded that the print orientation affected the dimensional accuracy of directly 3D-printed orthodontic aligners. Particularly, they recommended the three-dimensional printing of orthodontic aligners at a 90\({}^{\circ}\) angle owing to an improved dimensional accuracy of the intaglio surface, and also because a maximum number of aligners could be printed per job [7]. However, it has to be noted that this recommendation is not in line with the manufacturer's recommendations.  
Another study took into consideration the fidelity of 3D printing technology and concluded that direct 3D printing was associated with an increased wall thickness of approximately 0.2 mm [8].  
[PERSON] et al. report that the dimensional accuracy of thermoformed and 3D-printed aligners ranged from 0.1 to 0.3 mm and from 0.1 to 0.4 mm, respectively [9]. [PERSON] and colleagues reported that 40 vs. 20 min of curing time had no impact on the dimensional accuracy of direct-printed passive clear aligners, as revealed by comparing surface distances among optical scans of the printed ones and the digital master [10].  
However, difficulties were encountered in all studies involving optical scanning and superimposition techniques, because a spray application is required to collect the image of the transparent and reflective aligner surface, potentially leading to errors in the accuracy evaluation. On the other hand, a non-demolishing and precise method to assess thickness is available by means of micro-computed tomography, which has already been used to assess the aligner's geometrical characteristics [11; 12; 13; 14]. To the best of our knowledge, no study has already used it to assess the impact of curing conditions on the final thickness of 3D-direct-printed aligners.  
The aim of the present study was to investigate the effect of different curing times on the final thickness of direct 3D-printed aligners via micro-CT scanning. The null hypothesis was that different curing times have no impact on the final thicknesses of direct 3D-printed aligners. As secondary outcomes, the effect of nitrogen usage and variations in thicknesses among different tooth types and locations were evaluated.