## 4 Discussion  
The development of new materials has significantly improved the characteristics of clear aligners. However, the vast majority of these are produced with the conventional thermoforming method during which the shrinkage and expansion of the material affects the orthodontic strength and the fit of the aligners to the teeth [10]. Polymer blending is an effective method for improving the mechanical properties of polymers. Polymers exist in amorphous, crystalline, and liquid crystalline forms. In the first case, the structure of the polymer is disordered, while in the second case, it is organized and regular. From a structural point of view, the crystalline domains act as a reinforcing grid, thus improving the performance of the polymer in a wide range of temperatures. These are known as semi-crystalline polymers because they retain amorphous regions in their structure [32]. Exceed30 (EX30) was the polymer material used to make Invisalign(r) aligners from 2001 to early 2013. It was a medical-grade polymer made up of polyurethane methylene diphenyldiisocyanate 1,6-hexanediol [32]. From the first quarter of 2013, EX30 was replaced by a new polymer called Smart Track (LD30), a multilayer aromatic thermoplastic polyurethane/copolyester. LD30 denotes increased flexibility providing gentle and constant force, reducing pain, duration, pressure upon insertion, and long-term clinical action. It also features better adaptability and adhesion to the dental arch improving comfort and ease of use [32]. Align Technology is considered the leader in the clear aligner market, producing the world's most advanced clear aligner system (Invisalign(r)). Meanwhile, Eon(r) Holdings designs and manufactures clear removable aligners with a special form of polyurethane. Then, 3M TM launched its own clear aligner system called Clarity(r), made up of a resistant and practically invisible material. Finally, the SureSmile(r) aligners were designed by Dentsply-Sirona; these are manufactured from Essix plastic, which is a thermoformed polyurethane material [31; 33]. While most clear aligners are polyurethane thermoset polymer products, there is some variation among different companies. These differences are attributed to processing variations in manufacturing techniques incorporating various additives and dimensional characteristics [31]. The creation work begins through a virtual software with the use of plaster impressions or digital 3D intra-oral scanners. For each individual aligner in the set, a physical 3D model is required, either by 3D printing, stereolithography, or casting. Next, the aligners are fabricated by modelling the clear material on the 3D model of the patient's teeth and finally cut [5; 34] (Figure 3). All tested materials showed good mechanical, chemical, physical, thermo-mechanical, viscoelastic, stress decay, and structural characteristics. The materials were also tested following thermoforming and reported a reduction in properties [13; 35]. Specifically, the analysis of the mechanical and chemical properties highlighted, according to the study by [PERSON], that there are two differences between the PETG materials: the first is due to the different molecular weight, while the second is due to the influence of thermoforming on the mechanical properties. However, these data need further analysis [13]. The high hardness values in Smart Track indicate better wear resistance in clinical conditions, while the high moduli of elasticity indicate an increased force delivery capacity in orthodontic devices, however causing greater fragility. The lower resistance to sliding also means that, under constant occlusal forces, it is more likely to deform [13]. Despite the differences noted, there is no clinical evidence of a different influence on treatment or intra-oral behavior. Future controlled clinical studies are therefore also needed to determine the wear resistance and in vivo deterioration of materials [13].