INTRODUCTION

Clear aligners were first introduced by Invisalign in 1998 and have since gained widespread acceptance as an effective alternative to fixed orthodontic appliances for treating moderate malocclusions.[1,2] With the advent of compact 3D printing systems and advanced digital workflows, clinicians have been able to produce thermoformed aligners in-house over the past decade.[3] In recent years, direct 3D-printed aligners have emerged as a viable alternative to traditional thermoformed aligners, eliminating the need for resin dental models and overcoming several physical limitations associated with conventional materials such as polyethylene terephthalate glycol and polyurethane.[4-7]

Studies have shown that 3D-printed polymers possess shape recovery or shape memory properties, enabling them to exert continuous light forces for improved orthodontic efficiency.[8-12] However, thermoformed aligners present several limitations, including a rigid structure prone to water absorption and fatigue, a limited range of action requiring multiple aligners to achieve incremental tooth movements, and reduced elasticity necessitating external attachments to achieve desired results.[13,14] Existing literature has reported that thermoformed aligners often fail to fully achieve complex tooth movements as prescribed in digital treatment plans, leading to prolonged treatment times and the need for additional refinement of aligners.[15-17]

Shape memory polymers (SMPs) have recently been introduced in orthodontics, offering the potential to overcome these challenges.[7-9,18,19] These polymers have the ability to return to their original shape when activated by external stimuli such as heat, and their precision in engaging dental undercuts further reduces the reliance on attachments. This study focuses on evaluating two novel materials, DCA and DCA Plus, developed by LuxCreo. These materials exhibit shape memory characteristics and possess a high glass transition temperature (Tg), making them particularly suitable for orthodontic applications.[19,20]

MATERIAL AND METHODS

Two photo-curable polymers, DCA and DCA Plus, were provided by the company LuxCreo (LuxCreo Inc., Chicago, USA) and tested under controlled laboratory conditions. These resins have gained multiple medical device certifications (Class II) (e.g., FDA: K250343 and K212680, MDR: M.2024.MDR.1057), indicating compliance with biocompatibility based on ISO-10993-1. While these materials are not classified as traditional SMPs, they exhibit shape memory-like properties and are therefore referred to as “active memory materials.”[21] For active memory materials, once creeping occurs under the influence of persistent mechanical stresses, they can completely restore to their original state upon external stimuli, such as thermal excitation. This restoration applies to both three-dimensional geometry and mechanical properties. These materials, if used to fabricate clear aligners, have the capacity to provide recovered and sustained forces necessary for orthodontic tooth movement.

To evaluate their mechanical properties, two primary tests were conducted: Thermomechanical analysis and shape memory evaluation.

Thermomechanical analysis

Dynamic mechanical analysis[18] (DMA) was employed to measure the viscoelastic properties of the materials under varying temperatures, providing insights into their stress response and rigidity. Material samples with dimensions of 40 × 10 × 1 mm were prepared for tensile mode DMA characterization (DMA 1, Mettler Toledo, Swiss). Tests were performed at a constant frequency of 1 Hz with a displacement amplitude of 20 μm, while the temperature was ramped at a rate of 2 K/min. Mechanical response data were recorded at 1-s intervals throughout the testing protocol to ensure sufficient temporal resolution for property determination. The thermomechanical properties of the materials were assessed using tan delta analysis to quantify the energy dissipation and storage capabilities of DCA and DCA Plus, as shown in [Figure 1].

Figure 1: Dynamic Mechanical Analysis (DMA) curves showing tan δ as a function of temperature for the DCA and DCA-plus materials. The glass transition temperature (Tg), identified at the peak of the tan δ curve, occurs at 118 °C for DCA and 130 °C for DCA-plus, indicating that the enhanced formulation retains its rigidity at higher temperatures.

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In addition, the dependence of the DMA modulus on temperature is presented in [Figure 2]. It illustrates how the material’s resilience against deformation varies as the temperature increases.

Figure 2: Dynamic Mechanical Analysis (DMA) curves showing storage modulus versus temperature for DCA and DCA-plus. Both materials display a smooth, gradual decline in modulus with rising temperature, indicating a broad glass-transition region and slow softening rather than an abrupt stiffness loss. Clinically, this translates to steadier force delivery across typical intraoral temperature fluctuations (≈30–40 °C), reducing force spikes with warm/cold exposures and supporting more predictable tooth movement. Across the sweep, DCA-plus maintains a consistently higher modulus than DCA, suggesting improved rigidity and thermal stability that can help preserve planned moment-to-force ratios between aligner changes and during hygiene or cleaning routines.

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Active memory-restoration of geometry

For the shape memory evaluation, a clear aligner printed from an STL file was 3D printed on an iLux Pro Dental printer (LuxCreo Inc., Chicago, USA) using the DCA Plus resin. A thin layer of fine-sized white powder (Jie Chuang Inc., China), especially for scanning clear objects, was applied onto the outer surface of an aligner using a paintbrush. A high-resolution optical scanner (EX Pro, Shining 3D, China) was used to obtain the 3D geometric profile of the aligner. A 3D profile, denoted as Profile A, was obtained and used as the reference file.

The aligner was then immersed in a water bath at 37°C for 24 h and subsequently rescanned to obtain a new profile, denoted as Profile B. The aligner was mounted onto a positioning fixture, as illustrated in [Figure 3]. The sidewall of its second molar was pressed inward by a micrometer-controlled probe with a displacement of 0.5 mm. After another 24 h, the aligner was removed from the fixture and scanned for the 3rd time, generating Profile C. Finally, the aligner was immersed in boiling water for 1 min. After cooling to room temperature, it was scanned for the last time to obtain the final profile (Profile D).

Figure 3: Experimental setup for controlled deformation of an aligner and measurement of elastic force from the aligner.

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The deformation and recovery were analyzed through superimposition of the obtained profiles using specialized software (Geomagic Control X, Version 2023.0, 3D Systems, Rock Hill, SC, USA) to measure dimensional changes [Figure 4]. Profile A was consistently used as the reference file. A section of the aligner subjected to deformation was examined. The matching percentage was defined as the proportion of the area where the deviation between two profiles fell within a specified range (e.g., ±0.05 mm in the left column and ±0.1 mm in the right column). In the rainbow graphs, green represents regions within the deviation tolerance. Blue indicates that the examined profile (Profiles B, C, or D) lies beneath the reference profile (Profile A), while orange denotes the opposite.

Figure 4: Illustration of the geometry restoration of an aligner using profiles superimposition at different states, (a) post-water soaking (top row), (b) post-deformation (middle row), and (c) post-recovery (bottom row).

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Top row (Profile B vs. Profile A): Illustrates aligner deformation after immersion in the water bath, primarily attributed to material water uptake.

Middle row (Profile C vs. Profile A): Visualizes sidewall deformation caused by material creeping under sustained stress.

Bottom row (Profile D vs. Profile A): Demonstrates the reversible geometric changes of the aligner following thermal excitation.

Active memory - restoration of force

Flat samples of the DCA material were subjected to controlled force application. A cantilever sample (30 × 10 × 1 mm) was prepared and loaded onto a DMA test platform placed in a chamber. A probe was applied to bend the cantilever at its end with a displacement of 0.1 mm. The position of the probe was recorded, and the countering force from the cantilever sample was measured [Figure 4]. After 24 h, the probe was removed, and the chamber was heated to 100°C. Once the temperature stabilized at 100°C for 1 min, the heating was discontinued. After the chamber cooled to room temperature, the probe was reapplied to the previously recorded position. Cyclical reset and measurement were conducted every 24 h over a 5-day period [Figure 5]. The recorded force curves were then mapped in a single graph, as shown in [Figure 6].

Figure 5: The setup of the force restoration test using a flat cantilever sample. The red arrow represents the point of application of the force.

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Figure 6: The force restoration every 24 h from a cantilever sample (DCA material) in a 5-day period, showing recovery after thermal excitation at 100°C. The force values at the start and the end of every 24-h cycle are 0.242 ± 0.012 N and 0.078 ± 0.007 N, respectively.

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Then an aligner sample was prepared using DCA Plus material and loaded onto the force test setup [Figure 3]. The micrometer-controlled probe was applied to displace the sidewall of the second molar by 0.5 mm, and the position of the probe was recorded. The countering force of the sidewall was measured. After 24 h, the aligner was removed from the fixture and then immersed in boiling water for 1 min. Once the aligner cooled down to ambient temperature, the probe was reapplied to the pre-recorded position. Cyclical measurements were subsequently carried out over a 5-day period [Figure 7].

Figure 7: The force restoration every 24 h from an aligner sample (DCA Plus material) in a 5-day period, showing recovery after thermal excitation using boiling water. The force values at the start and the end of every 24-h cycle are 11.748 ± 0.531 N and 3.822 ± 0.175 N, respectively.

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RESULTS

As shown in [Figure 1], the tan delta curves for both DCA and DCA Plus are broad with a smooth slope, which is considerably broader than thermoplastic materials as previously reported in the literature.[22,23] This suggests that the direct-printed resins may consist of multiple components. The glass transition temperature (Tg, corresponding to the peak of the curve) is 118°C for DCA and 130°C for DCA Plus. The high Tg indicates excellent thermomechanical stability for both materials. This is further supported by [Figure 2], where the DMA modulus shows a gradual decay as temperature increases. At intraoral temperatures (37°C), both materials maintained a stable modulus. Compared to room temperature (25°C), the modulus decayed by only 8.4% for DCA and 10.0% for DCA Plus. This demonstrates their ability to provide sufficient mechanical forces for orthodontic treatment without significant deformation in the intraoral environment. Even at 100°C, the modulus remained high (74.5 MPa for DCA and 160.7 MPa for DCA Plus). When exposed to boiling water for shape recovery, the aligner retained sufficient rigidity to maintain its geometry. This feature is critical for ensuring the accuracy of the aligner throughout the active memory process, as thermoformed aligners are likely to deform permanently under similar conditions.

The geometry restoration tests revealed that the aligner fabricated from DCA Plus exhibited excellent dimensional stability and recovery ability. After immersion in water, the aligner showed minimal deviation from its original state, indicating low water uptake by the material. Following sidewall deformation for 24 h, significant deviation was observed, as indicated by the blue-colored region in the analysis in [Figure 4]. This was expected, as polymer materials universally creep under sustained static stress. At 100°C, DCA and DCA Plus retained 74.5 MPa and 160.7 MPa of modulus, respectively, demonstrating exceptional thermal stability. Shape recovery tests showed that DCA Plus achieved 99.4% restoration within ±0.1 mm tolerance after thermal activation. These findings highlight the material’s superior resistance to deformation and its ability to maintain orthodontic efficacy over prolonged use.

The force restoration tests, illustrated in [Figures 6 and 7], further demonstrated the materials’ ability to recover their mechanical properties after cyclic loading. Although the force inevitably decayed over time due to stress relaxation and material creeping, it reverted to its initial value after simple thermal excitation. The force recovery also confirmed that the mechanical properties of the material did not deteriorate during thermal excitation. This feature supports the material’s application in orthodontic treatment, where predictable and sustained forces are essential for tooth movement.

DISCUSSION

These findings suggest that DCA and DCA Plus offer superior shape memory and force recovery, a differentiation from conventional thermoform aligners. Unlike traditional aligners, which experience permanent deformation and force decay over time, DCA Plus retains its structural integrity, potentially reducing the need for additional refinement aligners. This could lead to shorter treatment times, fewer aligner sets per patient, and improved patient compliance.

Further clinical validation is necessary to confirm these advantages in real-world orthodontic settings. Their ability to exert continuous forces for extended periods without degradation suggests that they may reduce the number of aligners required throughout treatment, leading to improved efficiency and patient compliance.

Direct 3D printing of aligners using these materials offers several advantages, including improved fit and reduced reliance on attachments. In addition, the absence of resin model fabrication contributes to a more environmentally sustainable process. The findings of this study suggest that the integration of these materials into clinical practice, combined with optimized digital treatment planning software, has the potential to enhance orthodontic outcomes and streamline the treatment process.

One of the weaknesses of the study is that the tests were conducted in the company’s laboratory under strict guidelines. These tests should be repeated in independent laboratories to confirm the findings, as the results should be considered preliminary due to the location where they were conducted.

Further tests investigating the amount of force recovery and the force and pressure exerted by these aligners should be conducted to demonstrate whether the applied force or pressure is sufficient to achieve the desired tooth movement while the material returns to its original shape. Next-stage studies should include investigations on biocompatibility as well as property comparisons among direct print aligner resins from different manufacturers and conventional thermoforming materials.

CONCLUSION

The study on DCA and DCA Plus demonstrates consistent thermomechanical stability, shape memory properties, and force retention, making them a viable alternative to thermoformed aligners. These materials have the potential to improve treatment efficiency, reduce the number of aligners required, and provide more predictable orthodontic outcomes. Future research should focus on independent clinical trials, long-term wear studies, and validation of force application in orthodontic movement to further establish their effectiveness.