The present study investigated the temperature and time of a centrifugation cleaning method for the removal of uncured monomer fraction from 3D-printed clear aligners, while focusing on their surface characteristics and optical features. The results demonstrated that, while increases in temperature and time did not alter the surface properties, it affected translucency and the internal surface morphology of the aligners.
Recent advancements in clear aligner production have leveraged additive manufacturing due to its high precision and ease of production. Given the importance of aligners’ physical and optical qualities, an organic-solvent-free centrifugation method has been suggested as a method that could be used for the effective removal of any uncured resin. Although a basic centrifugation protocol is established, knowledge of the most effective settings primarily relies on empirical evidence. To explore the optimal physical conditions for effective centrifugation, we modified centrifugal equipment to operate at a standard RPM and added a heating element around the chamber’s edge. We carefully ensured that the heating element avoided contact with the rotating chamber to preserve rotation efficiency (Fig. 2). We achieved temperature control using a digital thermostat, and cycle durations were adjusted manually. Moreover, for efficient centrifugation, aligners were loaded in a balanced manner in each cycle.
To characterize the basic reaction of the supplied resin to heat, we conducted experiments to observe its viscosity and flow. The resin exhibited a predictable increase in flow when heated, which was consistent with viscosity profiles that are typical of most liquid-type resins at elevated temperatures16. There is also a direct correlation between the viscosity of a liquid and its flow rate. In a solvent-free centrifuge setup, this relationship could play a crucial role in effectively removing the uncured resin fraction14. In this study, the 3D printing resin demonstrated an approximately 2.34-fold increase in flow upon heating to 55 °C, which ultimately resulted in increased flowability and decreased viscosity. When evaluating the viscosity change under both temperature and physical shear force, the clear aligner resin exhibited a significant reduction in viscosity, approaching zero Pascal-seconds. This finding further supports the idea that centrifugal cleaning effectiveness can be enhanced by increasing the chamber temperature.
Transparency in clear aligner is the key to the aligner’s clinical compliance and treatment success17. In this study, the IPA group, which had a significantly lower weight, also exhibited significantly lower transparency. This finding aligns with another study that demonstrated significantly lower transparency in the IPA group compared to the centrifuge cleaning method. Additionally, the IPA cleaning group had thinner specimens compared to the centrifuge cleaning method group11. Combining these findings, there appears be a correlation between the weight, thickness, and transmittance of 3D-printed clear aligners; lighter aligners tend to be thinner, and thinner aligners may exhibit lower transmittance. In this study, the weight of the aligners decreased in the following order: NT > RT-2 > RT-6, RT-4, HT-2 > HT-6, HT-4 > IPA. Interestingly, the UV–vis transmittance followed a similar trend, with the groups ordered as NT, RT-2, RT-4, RT-6, HT-2, HT-4, HT-6 > IPA. However, the relationship between weight and transparency was not perfectly proportional. This could be attributed to the limited number of specimens tested or the possibility that the differences in thickness were not substantial enough to significantly impact the transmittance.
The optical properties of materials are determined by how they respond to light in terms of absorbance and transmittance. The higher the transmittance of material, the more transparent it is; conversely, higher absorbance indicates lower transparency18. Contrary to our findings, which suggest a proportional relationship between thickness and transmittance, the Beer-Lambert law (T = exp(− μad), where T is the transmittance, d is the sample thickness, and μa is the absorption coefficient) states that for transparent, non-scattering medium, transmittance is inversely proportional to sample thickness19. These results may be related to the surface morphology of the material. It has been demonstrated that, beyond the rinsing solution, the time and method also significantly affect the surface morphology and roughness of materials3,9. The findings of the present study also add to this evidence corroborating that chemical and non-chemical cleaning techniques differentially affect the surfaces of 3D-printed aligners. Analyzing the SEM images alongside transmittance data, it could be observed that the IPA group, which displayed the lowest levels of light transmittance, had the most irregular surface texture. Conversely, the NT group surface was considerably smoother compared to those of the other groups.
The findings of our study are consistent with these observations. Moreover, SEM revealed noticeable differences in the inner surface of the aligners with increasing temperature. These results may be related to the measurements of the aligner weight, suggesting that the more clearly the layer is visible on the transparent aligner surface, the less monomer remains. This implies that the removal of uncured resin reveals the micro-surface fingerprints characteristic of 3D printing20. Such exposure could indirectly affect light transmission and, in turn, influence the translucency of the aligners, regardless of their thickness. Furthermore, while no significant changes were observed in the net weight of the aligners, variations in translucency were noted with changes in temperature.
In the group where layering was evident in the SEM images, the inter-layer spacing appeared to exceed 50 µm, thus diverging from the predefined layer thickness settings during the printing process. This discrepancy arises because the specimens in this study were not produced as flat shapes but rather as three-dimensional aligners. Further, clear aligners printed at 45 degrees have intermediate options between horizontal and vertical orientations in terms of print layers, print time, and space taken up on the build platform, so these parts are printed at an oblique angle21. The choice to photograph the cervical third of the aligner was deliberate, as this area is most distinct and offers a high contrast between the gum tissue and tooth; this is particularly noticeable when people smile or talk22,23. Enhancing the printing resolution, specifically the layer proximity, might mitigate this effect, which is a relationship that might require further research.
3D-printing resins are highly cytotoxic prior to the 3D-printing process, and cytotoxic levels are significantly reduced after post-polymerization procedures to remove uncured resin24. In this study, there was a statistically significant difference in the cell viability values between experimental groups; however, there were no trends with increasing time or temperature. The NT group showed the lowest cell survival rate among the experimental groups. All groups were found to comply with ISO 10993-5 standards. This suggests that it is biocompatible for oral use regardless of the cleaning method.
The arch-width at the intermolar region is a dynamic value that changes significantly with an individual’s development. In orthodontic treatment, avoiding any unwarranted reduction in intermolar width (IM) is crucial for outcome stability. By analyzing the stress relaxation-driven shape recovery, we confirmed that there was no unwarranted reduction in width, with over 95% recovery observed after 60 min. This finding is consistent with previous studies that demonstrated an increase in recovery rate over time during bending tests. This suggests that the centrifugation cleaning temperature and time do not significantly compromise the shape memory properties of clear aligners, which are crucial for their clinical performance and treatment outcomes25.
Lastly, we assessed the aligner adaptation accuracy by comparing the internal surface morphology to the intended design. The quality of the aligner fit to the tooth surface significantly impacts the delivery of force necessary for effective orthodontic tooth movement21,26. Given that aligners cover all exposed anatomical surfaces of a tooth, any misalignment can substantially disrupt the intended tooth movement. Misalignments could also result in premature occlusal interference, ultimately leading to discomfort and potential adverse effects27.
Considering the poor translucency, the IPA group was excluded from the internal fit assessment. The differences among the centrifugation groups were qualitatively evaluated using color map analysis. In this analysis, a yellow–red gradient indicates an area where the aligner thickness has increased non-uniformly internally, due to aggregation of uncured resin post-centrifugation. Meanwhile, a cyan to blue gradient an area where the surface has become thinner. To provide a baseline for comparison, a morphometric analysis of the NT group was performed. As seen in the 3D color map difference analysis in Fig. 7A inefficient resin removal adversely affects the incisor edge which is a geometrically tapered region. An aligner experiencing this issue is likely to fails in properly engaging the tooth surface, which can lead to an unintentional increase in the incisal dimension or complete loss of fit. This, in turn, may cause severe discomfort.
The centrifugation groups did not exhibit statistically significant differences in RMS values with variations in temperature and time. However, qualitative assessments revealed well-aligned incisal edges for both HT and RT conditions at 2 min. Further, the adaptation at the cingulum surface on the palatal side toward the mesial line angle of the canine showed a sporadic yellow gradient. This indicates that the arch’s curvature might affect the efficiency of removing uncured monomer following 3D printing. A previous study that compared internal fit also identified a potential gap near the cingulum surface in the gingival third of the anterior region11. Although they used micro-computed tomography for their assessment, their findings are consistent with those of the current study. Moreover, the RMS value was derived by creating a 3D model from the internal surface of the aligner, essentially using it as a mold. Given the indirect nature of this assessment method, objectively generalizing the RMS results presents a challenge. In addition, the absence of a definitive, practical guideline for evaluating clear aligner fit complicates the achievement of a high-precision methodology. Despite these challenges, the methodology employed is detailed in such a manner that supports reproducibility in future research.
Despite the comprehensive findings, this study has several limitations. Firstly, there are currently no standardized indicators or benchmarks to define the transparency of clear aligners. As a result, this study could only conduct a relative evaluation of transparency rather than an absolute one. Additionally, a clear aligner produced by scanning a standardized typodont model was used in this study. In clinical practice, clear aligners are prescribed for patients with varying magnitude of malocclusion. Therefore, further research is needed to determine generalization of the proposed cleaning protocol when applied to clear aligners for patients with severe crowding.
Moreover, this study has broader implications beyond orthodontic clear aligners. Recently, the centrifugal cleaning method has been employed in dentistry to produce temporary 3D-printed fixed dental prostheses. Research on centrifugal cleaning using various 3D printing materials has shown that, unlike mechanical cleaning, chemical cleaning impairs flexural strength, and the cleaning strategy recommended by individual manufacturers does not always result in the highest mechanical properties for each material3. Although our study primarily focused on clear aligners, the results may have a wide-ranging impact across dental restorative treatments. We hope that the findings of this study will contribute to the advancement of dental 3D printing technology and the expansion of its clinical applications.
