## 1 Introduction  
In the last years, the concept of aesthetics has become increasingly relevant in orthodontics, leading to the development of various technologies and materials designed to meet aesthetic demand during orthodontic treatment [1]. Orthodontic aligners are appreciated as they ensure aesthetics during treatment [2]. The most used materials for thermoformed aligners are polyethylene terephthalate glycol (PET-G) [3] and thermoplastic polyurethanes (TPUs), with thicknesses ranging from 0.5 to 1.5 mm. The introduction of multilayer materials has improved the biomechanical properties of traditional single-layer materials [4, 5, 6]. Aligners' fitting is crucial to ensure the accuracy of planned movements. Proper fitting is directly proportional to the degree of aligner retention, defined as the aligners' ability to effectively control orthodontic movement and counteract vertical displacing forces caused by tongue movement, gravity, or the application of intermaximillary elastics [7, 8, 9]. A factor influencing retention is the design of aligners' gingival margin: straight margins positioned 2 mm apically to the gingival zenith have been shown to ensure higher retention forces compared to scalloped margins [10]. The presence of grip points positively affects retention; in addition, their shape, size, and location significantly impact retention [10; 11; 12]. Rectangular grip points are more effective than elliptical ones in maintaining aligner position in the presence of displacing forces, particularly when made of bulk-fill materials, which are less subject to distortion during polymerization [11; 12]. Retention also increases as the thickness and stiffness of the aligner increase [12] and is affected by the aligner material [10; 12]. Additionally, clinical crown dimensions influence retention [11]. The introduction of 3D printing enabled a shift from traditional aligner thermoforming to direct printing, allowing for more efficient, precise, and customizable production. This method facilitates the production of aligners with differentiated thicknesses and advanced biomechanical characteristics, addressing each patient's specific needs [13]. Aligner 3D printing does not require the 3D-printed dental models necessary for thermoforming, reducing environmental impact and steps involved in aligner production, optimizing processing time and productivity [14]. Several studies have shown comparable or superior accuracy of 3D-printed aligners compared to thermoformed ones [14; 15; 16]. Regarding thickness, it has been observed that 3D-printed aligners with Dental LT Clear material were about 0.200 mm thicker than their original digital design specifications [17]. Several different 3D printing techniques are used to print aligners. Popular 3D printing techniques include Digital Light Processing (DLP), which uses a digital light projector to polymerize entire layers of liquid resin, and Continuous Digital Light Processing (cDLP), which allows for continuous polymerization, further reducing production time [18]. Liquid Crystal Display (LCD) is based on photopolymerization and uses liquid crystals in imaging [19]. Another 3D printing method is Selective Laser Sintering (SLS), which uses a laser to sinter powder particles, building the aligner layer by layer with a high level of detail [20]. Most 3D-printed aligners are made from liquid photopolymerizable resins, which are toxic, irritant, and potentially allergenic before polymerization, making them non-biocompatible. They acquire biocompatibility during the 3D printing and UV polymerization stages. Incomplete polymerization can cause irritation or allergic reactions in patients. Post-polymerization is crucial for improving the material's mechanical properties, dimensional stability, and transparency [21]. The mechanical properties of 3D-printed aligners vary depending on the printer and printing technique employed.