## 1 Introduction  
Orthodontic treatment has undergone significant advancements in recent years, offering patients more options. The latest advancement is clear aligners, which have revolutionized orthodontic treatment by providing a discreet and convenient alternative to traditional braces. This technique involves using a series of custom-made, transparent aligners that gradually move the teeth into their desired positions [1]. Traditionally, orthodontic aligners were made via the thermoforming technique, which is a widely employed method in orthodontics for the fabrication of orthodontic appliances, such as retainers and aligners. This technique uses heat to soften a thermoplastic material, allowing it to be shaped and molded over a dental model. The process begins with selecting the appropriate thermoplastic material, which is then heated to its specific softening temperature. Once heated, the softened plastic is draped over the dental model and formed into the desired shape using a vacuum or pressure. The thermoplastic material quickly cools and hardens, retaining the molded shape and creating a custom-fitted appliance. The thermodforming technique offers several advantages, including cost-effectiveness, ease of use, and the ability to fabricate appliances relatively quickly. However, it is important to ensure precise adaptation of the material to ensure optimal fit and function. Three-dimensional-printed orthodontic aligners have emerged as a cutting-edge and innovative solution in orthodontics. Utilizing direct 3D-printing technology, these aligners are custom-designed and fabricated for each patient [2,3]. The digitally designed aligners are sent to a 3D printer, which utilizes specially formulated biocompatible resins to produce the aligners without a thermoforming process. The resulting aligners are precise, durable, and exhibit the desired characteristics of flexibility, transparency, and fit. This advanced technology enables orthodontists to achieve highly customized treatment plans, ensuring optimal tooth movement and aligner performance. Among the multiple factors that determine the effectiveness of orthodontic treatment with clear aligners, their design accuracy and mechanical properties are crucial [4]. Currently, various thermoplastic materials, such as polyethylene terephthalate glycol (PETG), polypropylene (PE), polycarbonate (PC), polyurethane (PU), and polypropylene (PP), are being used to manufacture clear aligners since they are biocompatible, transparent, and characterized by low hardness, good elasticity, and resistance in the oral environment [5,6]. Lately, multi-hybrid materials have been developed to overcome the limitations of single materials and to improve their maximum tensile load-bearing capacity [6]. The conventional workflow for manufacturing a set of clear aligners starts with digital scanning of the patient's dental arches and the virtual planning of the treatment by computer-aided design (CAD) software. Thus, a physical dental model is 3D-printed for each stage and arch, fabricated by stretching the clear plastic sheets over the 3D printed model by means of thermoforming (vacuum or high pressure) machines, and finally, trimmed and polished [7]. Ideal orthodontic treatment requires clear aligners to exert constant light forces, leading to a maximum rate of tooth movement with minimal irreversible tissue damage [8]. Despite that both the production efficiency and functional effectiveness of thermoformed aligners have significantly improved over the last years [9], the geometric inaccuracies induced by the shrinkage and expansion of the plastic foil during the thermoforming process have not yet been solved [6], affecting the fit of the aligner to the teeth, the inter-arch occlusal relationship, and the planned distribution of the orthodontic force. The decrease in the thermoplastic sheet thickness with the thermoforming process is well-known [4,6,10,11].