In contrast to previous studies that used dumbbell-shaped specimens, this research employed stick-shaped specimens to minimize experimental bias due to variations in specimen width. In the previous experiments, the locations of the fracture during the elongation process under the tensile force were determined by the dumbbell shape in some cases. With the stick-shape specimens, such factors affecting the location of fracture could be eliminated. However, fractures still occurred at the junction between the specimen and the fixed region. This phenomenon is attributable to the material’s significant elasticity and yield strength. However, strain at such high values are not relevant to the clinical use of the clear orthodontic devices, thus there is no issue in deriving a clinical interpretation from these experimental results.
The initial hypothesis applied to the previous study was that variations in the thickness of clear aligner devices could lead to unwanted forces on the teeth, with the assumption that aligner thickness significantly affects the orthodontic forces applied. However, results from the previous studies focused on the differences in physical properties between PETG and TC-85 rather than addressing the differences in physical properties based on thickness variations. In this study, only one material was used, and the hypothesis was empirically tested by fabricating TC-85 specimens at three thicknesses of 0.5 mm, 0.65 mm, and 0.8 mm.
One of the most significant findings from previous research is that the physical properties of materials, especially TC-85, demonstrate very particular viscoelastic behaviors and temperature-dependent shape memory properties 9,22,28. Experiments were conducted at 37°C and 80°C to test the properties near body temperature. However, temperature such as 80°C above Tg was a condition too extreme to compare the changed properties to draw clinical meaning. The current study sought to rectify this by evaluating material properties at temperatures that are more representative of conditions within the oral cavity on a daily basis: 30°C, 35°C, 40°C, and 45°C.
Theoretically, tensile strength and elastic modulus are derived from maximum tensile and cross-sectional area of the material, thus are independent of the thickness. The current study corroborated this, showing no significant variance in these measurements across different thicknesses. However, a clear trend was observed whereby an increase in temperature led to a decrease in both tensile strength and elastic modulus, which in turn made the material more ductile, requiring less force for deformation. Unlike vacuum formed clear aligners made of materials such as PETG, direct printed aligners can be designed to fit dental anatomy precisely regardless of undercuts. This allows close fit along all surface between the device and teeth, providing more effective orthodontic force. However, the perfect fit requires the device to pass through the height of contour and reach the undercut area. For optimal placement, the device must be able to adapt to dental contours and undercuts with minimal force and have adequate flexibility to avoid discomfort or breakage during the application or removal process. Therefore, lower tensile strength and elastic modulus are considered preferable. Additionally, the temperature-dependent flexibility can be used in advantage to ease the fitting and removal process of the device. For example, the device can be heated in hot water before fitting and heated by drinking warm water above body temperature before removing in order to increase flexibility instantly and ease the tight fit to fit and remove with minimal force required.
Maximum standard force is measured in order to investigate the physical properties with thickness factor as an added factor. It was discovered that actual tensile strength varies with the thickness of the device, implying that thinner devices requires smaller force for deformation. In clinical settings where the aligner is to be applied to areas with poor periodontal health or sever undercuts, thinner device would be more advantageous for avoiding applying large force. Although a trend of decreasing force required with increasing temperatures was noted across all thicknesses, no substantial differences were observed above 40°C. Also, the force difference between 0.65 mm specimen and 0.8 mm specimen was relatively small at 30°C and 35°C. Therefore, while the thickness of the device does have some effect on the orthodontic corrective force exerted on teeth, its clinical significance seems insignificant.
A stress-strain curve was created to visually demonstrate force variations relative to temperature (Fig. 3). As previously explained, force required for same strain decreased with increased temperature for a given thickness. Although the graph shows how the material behaves until its elongation break, only the initial values of the curve is clinically meaningful, because actual tooth movement occurs within a narrow range of elongation, making values beyond the initial changes less critical in clinical relevance. Similar to the results from previous studies, all specimen reached its peak force in the beginning stage of deformation. Compared to peaks of PETG, TC-85 showed significantly lower peak force. This result corroborates the potential of TC-85 to facilitate efficient tooth adjustment with fewer steps and biologically more suitable force. Applying this finding to clinical settings, the thicker the device is, the more movement per step one give. However, thickness over 0.65 mm is expected to have no significant effect. Also, since the material’s flexibility is temperature-dependable, it is possible to minimize side effects and discomfort especially when using an aligner designed for large tooth movement by utilizing warm temperature over body temperature and introducing physiological force to the teeth.
As shown in Fig. 4, the temperature increases by 5°C from 30°C to 45°C, storage modulus decreases and tan delta increases. This phenomenon is very natural because it is based on the softening of polymers due to temperature change and also aligns with previous works 9,22. However, in case of an orthodontic device, the temperature at which the viscoelastic properties are maximized must be around 35°C near human body temperature. In this study, as the temperature rises from 30°C to 40°C, the decrease of storage modulus and increase of tan delta occur at a constant rate, but the rate is hindered when the temperature rises above 40°C. Such phenomenon generally occurs around the Tg (glass transition temperature) of polymer materials. It can be inferred that the viscoelastic properties of the material are maximized in an environment at 35°C in the oral cavity near its Tg.
Stress relaxation and creep of TC-85 were examined at 2% strain rate to mimic the material deformation during the application of the device (Fig. 5). In the previous study, experiments were conducted under a 1% strain rate condition, which was an inevitable condition due to the low elasticity of PETG. However, focus of this study was to investigate the effect of temperature by using only the TC-85 material, which has high flexibility and correspondingly high elastic range, so the experiments could be conducted at a strain rate of 2%. As the cycles progressed, stress relaxation became evident when the temperature was higher, and the static force also tended to decrease immediately. According to the results of the previous experiment, the amplitude of the initial force tended to be larger at lower temperature, and the static force in stress relaxation condition was also relatively higher at lower temperature. In addition, the creep occurred more clearly at lower temperatures and static force continuously increased as the cycle progressed. From a biomechanical perspective, creep could be seen as beneficial since it minimizes force decay and sustains the orthodontic force applied by the aligners.
These results show that in a clinical situation where the orthodontic device is worn and placed under continuous strain, the force exerted from the device to the teeth can be changed with temperature. When a patient wears the aligner, appropriate orthodontic force must be applied to the teeth, and it must occur at a temperature close to oral environment approximately at 35°C. Combined with the results from the previous studies, the higher strain rate resulted in greater force. Therefore, by individualizing movement of each tooth in the set-up process and adjusting the expected strain rate, the ideal orthodontic force can be achieved with specifically controlled applied force 29,30. If the patient experiences pain when installing the device, using water at a temperature higher than body temperature can help by immediately lowering the applied force to 0 N.
There may be concerns about permanent deformation of the device due to creep phenomenon and repeated use. However, these concerns could be dismissed by the previous research which confirmed the shape memory property of the newly developed material at 37°C and 80°C followed by this study attempted to look at the shape memory pattern under more detailed conditions. Likewise, the material exhibited quicker recovery at an elevated temperature (Table 2). This study provides detailed pattern of shape memory and suggests that warming the clear aligner may be used as a guideline for first-timers and patients in transitions between orthodontic stages to lessen the discomfort.
With TC-85, it is possible to control the applied force by distance adjustments rather than thickness control. In the case of thermoformed aligners made with PETG materials, 1 kg tension was generated at a strain rate of 1%, but direct printed aligner generated a force of 100g at 1% and 200g at 2%. Such orthodontic-friendly force range allows the operator to freely plan the orthodontic treatment without concerns on excessive amount of force. This method can be expressed as a force driven system. Even if the oral temperature rises by just 5 degrees, the orthodontic corrective force is clearly released and returns in moderate measures that is more biomechanically suitable.
In conclusion, the immediate force when applying the device at body temperature may surpass physiological force. This is a common phenomenon among existing clear aligners, and materials such as PETG exert much greater initial force 31. However, more efficient tooth movement is ensured with the property of TC-85 which allows stress relaxation to occur immediately and exert the most physiological force. Therefore, when using the device for the first time, it is advised to use it at a higher initial temperature to minimize discomfort and prevent excessive forces. Then, gradual adjustment to oral cavity temperature will ensure a smooth transition to physiological force range for optimal orthodontic corrective force applications.
Although the study examined material properties across various temperatures, additional experiments considering different clinical conditions, simulation of installation and removal of the device, and the orthodontic force under fluctuating temperature rather than a fixed temperature in order to derive more clinically meaningful results. In addition, further investigation is necessary on various teeth morphologies and health conditions of periodontal tissues to determine how much tooth movement per set is appropriate to achieve the 1% and 2% strain as assumed in this and previous studies.