Thermo‑mechanical and geometric properties of 3D printable shape memory resins for orthodontic aligners

In the last decade rapid prototyping technologies have radically transformed the approach to orthodontic treatments. This is particularly evident with clear aligners, an appliance to move the teeth without bands, brackets, or wires which is based on vacuum-formed dental contour devices. Accordingly, several efforts have been made to develop advanced materials that are suitable for the CAD/CAM technology. Currently, various thermoplastic materials such as polyethylene terephthalate glycol (PETG), polypropylene, polycarbonate, thermoplastic polyurethanes (TPU), and copolyester, are being used by different actors to manufacture clear aligners. Furthermore, multi-hybrid materials have been developed to overcome the limitations of a single material by improving the physical properties of maximum tensile load-bearing capacity . The development of new materials and enhancement of the material properties have significantly improved the performance of the clear aligners. However, these aligners are still manufactured by the conventional method of vacuum thermoforming biocompatible thermoplastic transparent materials on a dental model, which is a complex manufacturing process requiring considerable time and effort. In addition, geometric inaccuracies are induced during the thermoforming process. The shrinkage and expansion of the material that occurs during the thermoforming process affects the orthodontic force and fit of the clear aligners to the dentition, in addition to the changes in various physical properties such as transparency, water absorption, surface hardness, and elastic modulus. To overcome the limitations of the conventional manufacturing method, direct 3D printing of clear aligners with biocompatible materials has been recently attempted. This method requires less time and materials waste, and also results in fewer geometric inaccuracies. However, the mechanical properties and behavior of 3D printing materials composed of cross-linked polymers are expected to be different compared with conventional thermoplastic materials composed of non-cross-linked polymers used for thermoforming. The purpose of the present project was to evaluate the thermo-mechanical, viscoelastic and geometric properties of the currently available orthodontic 3D direct printing materials, to test their biocompatibility and to implement a preliminary computational model for the assessment of the obtained orthodontic forces.