## 1 Introduction
Moreover, resin polymers are not inert: the mechanical properties of these materials are known to highly depend on environmental conditions [12]. In the oral cavity, aligners are subjected to intermittent short- and long-term loads [13] and, as viscoelastic materials, lose a substantial amount of their initial strength in the first few hours [12]. They are also prone to changes due to heat, humidity, and saliva absorption, making tooth movement less predictable [14,15]. To overcome the limitations of the conventional manufacturing method, direct 3D printing of clear aligners has recently been attempted, since the first photocurable dental resin explicitly indicated for this purpose was launched onto the market (TC-85 DAC, Graphy, Seoul, Republic of Korea). This procedure allows to directly print aligners, without the need to print the corresponding dental models and bypassing the thermoforming process [4,6]. As an emerging technology, despite its practical advantages, the biocompatibility, geometrical accuracy, physical and mechanical properties, as well as chemical stability of the 3D-printed aligners are yet to be tested and validated with respect to the conventional thermodormed aligners. [PERSON] and colleagues recently examined, through indentation testing, the mechanical properties of a few samples of aligners 3D-printed with the aforementioned resin after intraoral exposure, reporting no deterioration after an intraoral exposure period of one week [16]. [PERSON] et al. analyzed some static and dynamic mechanical properties of the same photocurable resin and compared the outcomes with a control PETG material [6]. The choice of the resin to be used to print the aligners is a fundamental step because it significantly influences the final product's mechanical properties [17]. In this article, the mechanical properties of a new experimental resin, made by mixing, in different proportions, two commercially available and certified resins, specifically designed for 3D printing of orthodontic aligners, will be examined. Then, it will be compared to the mechanical properties of TC-85 DAC samples (Graphy), and nickel-titanium wires used as control samples. The aim of this research is to determine if the resin blend can produce aligners with improved mechanical properties compared to those made separately with each resin. Furthermore, the purpose is to obtain a resin that meets the specific needs of different clinical cases by blending commercially available and certified resins in different percentages. This approach will enable orthodontic practitioners to select the most suitable resin blend for each case or for each stage of treatment, providing better outcomes for patients and increasing the efficiency of the treatment process. The research hypothesis for this study is that custom-designed 3D-printing resins with varying force properties will exhibit comparable or superior mechanical characteristics to commercially available resins and nickel-titanium wires. By tailoring the properties of the resin-based materials to specific orthodontic requirements, these novel resins can potentially offer alternative solutions in orthodontic treatments. The study aims to investigate the mechanical properties of these custom resins and compare them with commercially available resins and nickel-titanium wires, expecting that the custom resins will demonstrate a favorable performance, such as improved flexibility, strength, and shape-memory capabilities. The results of this study will contribute to the advancement of personalized orthodontic care through 3D-printing technology.