Received 2025-05-16

Revised 2025-07-14

Accepted 2025-10-18

Introduction

In the past, dental materials were mainly chosen for their chemical stability and lack of interaction with oral tissues. Nowadays, the focus has shifted toward biocompatible options that combine safety with enhanced mechanical and chemical performance [1, 2]. Clear aligner fabrication is the process of making transparent orthodontic appliances to align teeth [3]. Biocompatible materials are generally grouped into bioinert, bioactive, and smart (bioresponsive) types based on how they interact with the biological environment [1-3]. In recent years, growing interest has emerged in replacing passive dental materials with smart alternatives capable of responding to external stimuli by altering their shape, color, or size [4, 5]. Smart materials are capable of altering one or more of their properties in response to external stimuli such as energy absorption or environmental changes, which classifies them as responsive materials [6, 7]. In the dynamic oral environment with fluctuations in pH, humidity, and microbial activity, there is increasing demand for such materials that can adapt beneficially to these variations [8]. Their application has notably transformed orthodontics, particularly through the use of shape memory alloys and polymers [9, 10]. 4D printing involves three spatial dimensions plus time, allowing objects to change shape or function in response to stimuli, unlike 3D printing, which creates static objects with only three spatial dimensions. Smart materials play a key role in 4D printing, in which structures get created that respond to changing conditions. However, in dentistry and medicine, this technology is still under development [11]. Although direct 3D printing of clear aligners is an emerging field, with limited clinical studies available, challenges related to surface roughness and material properties remain, as experimental studies consistently show that the surface quality of printed aligners often fails to meet clinical standards without optimized post-processing [12]. In this review, we attempted to reveal the possibility of using smart materials in the field of orthodontic clear aligner treatments as a beneficial material for use in 3D or 4D printing approaches.

Clear Aligner Materials and 3D-printing

Clear aligners can be fabricated using two primary approaches: the conventional technique, which utilizes vacuum thermoforming of thermoplastic sheets over either 3D-printed or cast dental models, or the direct 3D-printing method, which bypasses the need for physical intermediary models altogether [13, 14]. The materials used in clear aligner thermoforming fabrication have evolved from "polyurethane" and "polyethylene terephthalate glycol-modified (PETG)" to materials like "polypropylene (PP)," "polycarbonate (PC)," and "thermoplastic polyurethanes (TPU)"[15]. However, none of these materials possess all the ideal characteristics, indicating the need for a new material to optimize orthodontic treatment [16]. Thermoforming variations affect thermoplastic properties, impacting aligner fit and performance [17]. The process also raises environmental concerns like plastic waste, high energy use, and toxic emissions (benzene from PETG, tetrahydrofuran from polyurethane) [18, 19]. Mechanical friction may release microplastics, but current evidence suggests this is minimal during the short wear time of aligners [20]. To address the drawbacks of traditional vacuum thermoforming, direct 3D printing of clear aligners has emerged as a promising alternative [21]. This approach eliminates mechanical distortions and material degradation caused by thermoforming, resulting in superior dimensional accuracy, better fit, increased mechanical strength, and enhanced reproducibility [22]. Consequently, digital technologies such as 3D printing have become favored methods for producing dental aligners when applicable that offers more precision, customization, and manufacturing efficiency [23]. Direct 3D printing circumvents the negative effects associated with thermoforming, including changes in mechanical, dimensional, and aesthetic properties, thereby providing improved geometric accuracy, fit, efficacy, and consistency. Common materials used in orthodontic 3D printing include acrylonitrile-butadiene-styrene (ABS), epoxy-based stereolithography resins, polylactic acid (PLA), polyamide (nylon), glass-filled polyamide, as well as metals like silver, steel, titanium, photopolymers, wax, and polycarbonates [24-26].

Additive Manufacturing (AM) refers to technologies that create objects directly from digital models, beginning with CAD files converted into STL format to guide the printing process [26]. AM, also referred to as 3D printing, rapid prototyping, or direct digital manufacturing, provides significant advantages by enabling the fabrication of complex geometries and highly customized components tailored to specific applications or patients [27, 28]. The advent of smart materials has enabled AM to evolve into “4D printing,” which incorporates structural transformation over time [29, 30]. Unlike traditional 3D printing, 4D printing allows fabricated objects to change shape or function in response to external stimuli such as temperature, light, moisture, pH, or electromagnetic fields [31-33]. These changes can be pre-programmed and precisely controlled that gives material capability of the development of getting into adaptable structures with dynamic geometries and time-responsive functionality [34].

The 3D printing (Direct Printing) workflow for clear aligners using the direct printing approach includes five main steps: Acquisition of Digital Files (Digital impressions are obtained via intraoral scanning technologies following clinical and radiographic evaluations), Digital Treatment Planning (Treatment objectives are defined, and necessary components, such as attachments, are digitally designed using specialized software), 3D Printing of Clear Aligners (The digitally designed models are fabricated through 3D printing using appropriate resins and printer technologies), Post-Curing Processing (Printed models are detached from the build platform, and any excess structures are removed. At this stage, residual uncured resin is eliminated using isopropyl alcohol (IPA) or centrifugal methods), Finishing and Polishing (Sharp or rough edges are refined to ensure proper fit, patient comfort, and aesthetic quality of the final appliance) [35]. Direct 3D printing of clear aligners allows precise in-house fabrication that improves accuracy and efficiency by removing thermoforming steps [36, 37]. Combining this with 4D printing and smart materials enables adaptive, programmable orthodontic devices [38]. Figure-1 presents a comparative workflow between the thermoforming process (indirect printing) and direct 3D printing. Table-1 summarizes the commonly used polymers for 3D-printed clear aligners, while Table-2 compares the advantages and disadvantages of thermoplastic versus directly 3D-printed clear aligner materials.

Smart Materials

The clinical success of clear aligners heavily depends on the physical and mechanical properties of the materials used in their fabrication [39]. An ideal aligner material should exhibit a balance of resilience, elasticity, biocompatibility, and transparency while withstanding mechanical, thermal, and chemical stresses. Additionally, it must possess sufficient stiffness to deliver the forces required for effective tooth movement. Materials with excessively high modulus of elasticity may result in rigid, inflexible aligners, whereas insufficient stiffness leads to inadequate force generation for tooth repositioning [40]. Table-3 outlines the essential characteristics of an optimal aligner material.

Improvements in the biochemical composition of aligner materials can significantly enhance their therapeutic effectiveness; without such advancements, aligners remain constrained by biomechanical limitations and may underperform compared to fixed orthodontic appliances [41]. The next section explores the primary factors that affect the critical properties of smart materials, a key step toward optimizing their design.

Printing Process, Post-printing Process, 3D Printer Machine

The mechanical properties of 3D-printed materials are influenced by the printing technique, post-processing protocols, and printer type, potentially impacting clinical outcomes [42-44]. Aligners fabricated directly from Stereolithography (STL) files have demonstrated superior accuracy and precision compared to those produced via vacuum thermoforming [25]. While some studies report that print orientation has minimal impact on mechanical properties such as flexural strength [45, 46] and only a slight effect on overall accuracy [47], another study has found that it significantly influences the dimensional accuracy of directly printed orthodontic aligners [48]. Horizontal print orientation, in particular, is associated with optimal mechanical performance [49]. Moreover, the printing angle affects both resin consumption and production costs [50]. A study by Mattle et al. found that the mechanical properties of 3D-printed resin aligners were not significantly affected by either post-curing in a nitrogen atmosphere or by heat treatment [51]. Similarly, one study reported that eliminating oxygen during the printing process did not influence the mechanical properties of the aligners [52]. However, another study has reported conflicting results regarding the impact of oxygen exposure during printing, leaving this issue open to further investigation [53]. Research has also demonstrated that optimal polymerization times significantly affect both the mechanical and thermal properties of dental resins, with materials like Tera Harz benefiting from both very short and extended curing durations [54]. Regarding post-processing, a 2-minute centrifugation at 55°C has been proposed as an effective method to remove uncured resin without adversely affecting the aligners’ physical or optical qualities, making it practical for clinical applications [55, 56]. A minimum post-curing time of 60 minutes is essential to enhance the clinical performance of 3D-printed resins [57]. Ultraviolet post-curing is also critical for achieving the necessary rigidity in directly printed aligners, though prolonged curing has limited impact on accuracy [58]. Furthermore, the type of 3D printer plays a crucial role in determining the precision of orthodontic models and the mechanical properties of printed aligners, which in turn impact clinical outcomes and treatment effectiveness [42, 43]. On the other hand, 3D printing resins are highly toxic before polymerization, but their toxicity decreases after curing, making proper post-processing essential to reduce toxicity to safe levels [22]. An in-vivo studies confirm the general safety of several 3D-printed materials [44], though material choice and post-processing significantly affect in vitro cytotoxicity [59]. Another trial found no cytotoxic differences using various UV-polymerization units or rinsing solvents [60]. Prolonged UV exposure and extended curing increase cytotoxicity which might be showing the need for standardized curing protocols [61]; for example, a 20-minute UV cure ensures safety for Tera Harz TA-28 up to 6 mm thick [62]. Among common polymers, TC-85A showed no cytotoxicity, while E-Guard and Dental LT exhibited slight effects [63, 64].

Material Composition

The introduction of 3D-printed aligners with shape memory properties (4D aligners) marks a significant advancement in orthodontics. These materials demonstrate mechanical characteristics better suited for orthodontic applications compared to traditional thermoforming materials [65]. The composition of aligner materials plays a crucial role in determining the forces and moments they can apply [66]. For instance, Dental LT resin, when adequately cured, produces geometrically precise aligners with improved mechanical strength and elasticity that enables efficient and accurate in-house fabrication [36]. TC-85 3D-printed aligners have proven effective in delivering forces appropriate for tooth movement [67, 68]. This advanced material exhibits exceptional shape memory at oral temperatures, enhancing aligner fit, minimizing force decay, and permitting greater tooth movement per step thanks to its increased flexibility. Moreover, TC-85 maintains microhardness comparable to conventional thermoformed sheets, ensuring durability and clinical effectiveness over time [67]. Its wider elastic range and enhanced flexibility also allow for more extensive tooth movements without causing permanent deformation [13].

Viscosity

The quality of 3D prints, including accuracy, durability, and aesthetic appeal, depends significantly on the viscosity of the resin used [68]. The 3D printing resin showed a significant decrease in viscosity and a 2.34-fold increase in flow when heated to 55 °C. This behavior, consistent with typical liquid resins, highlights a direct relationship between temperature, viscosity, and flow. When combined with shear force, the resin’s viscosity approached zero, suggesting that raising the temperature can enhance the efficiency of centrifugal cleaning systems [55]. Increasing the oligomer content in the resin system effectively improves mechanical properties; however, it also significantly raises the viscosity of the UV-curable resin, which negatively impacts its printability [69]. IV. Aligner Thickness Shape-memory 3D-printed materials (4D aligners) have introduced a transformative shift in aligner fabrication, offering superior mechanical properties, optimized workflow, and improved control over thickness and design compared to conventional thermoforming techniques [67]. Direct 3D printing offers enhanced precision by eliminating intermediate steps, enabling full control over aligner thickness, coverage, and attachment placement [70]. With 0.4 mm-thick aligners delivering forces between 3.1 N and 15.8 N, and thicker aligners producing higher forces [66]. While increasing aligner thickness can affect force and movement generation, the relationship is complex and tooth-specific [55, 71, 72]. Multilayer materials tend to produce lower initial forces than single-layer ones [73], and increased thickness (e.g., 0.7 mm vs. 0.5 mm) correlates with improved bending resistance [45]. Although some researchers suggest that thickness has minimal influence under specific settings [74]; others report significant impacts on mechanical behavior, color stability, and surface roughness [75]. Additionally, thermoforming typically reduces the original material thickness, whereas direct 3D printing may inadvertently increase thickness, potentially compromising clinical performance [76, 77]. Increased thickness also improves retention, making thicker, and single-layer rigid materials preferable for effective bodily tooth movement [78]. Thicker aligners produce greater forces and possess a higher modulus of elasticity with reduced deformation, making them more suitable for complex tooth movements such as root translation. Conversely, thinner aligners offer increased flexibility and deformability but are more prone to fracture. Notably, in 3D-printed aligners, reducing the printing layer thickness enhances strength that shows the critical influence of thickness on force generation and material performance [68]. Direct-printed aligners provide biologically compatible and more consistent forces for tooth movement compared to thermoformed ones under in vitro conditions [79].

Thermomechanical aging reduces these forces within the first 48 hours [73, 80]. Moreover, direct 3D printing allows precise customization of thickness and addition of ridges, enhancing biomechanical efficiency and potentially reducing the need for attachments [81].

Aging

Orthodontists should consider that aligner materials may deteriorate over time, affecting mechanical performance and guiding optimal replacement intervals [73]. However, one study reported no significant mechanical changes after one week of intraoral use of in-house 3D-printed aligners [82]. A 15-day wear period is recommended for a better fit and minimal gaps [83]. Thermoforming and aging both influence the mechanical properties of aligner materials, with thermoforming having a more pronounced weakening effect [73]. Despite this, thermoformed aligners have demonstrated good thickness retention and dimensional stability following intraoral aging in healthy adult subjects [84]. Nonetheless, biocompatible 3D-printed resins maintain sufficient strength to endure occlusal forces even after aging, making them suitable for intraoral appliances [85].

Water Absorption

Moisture, specifically a simulated oral conditions, has a more pronounced impact on the mechanical properties of direct 3D-printed aligners than on thermoformed ones, potentially compromising their ability to deliver consistent orthodontic forces [86]. In contrast, thermoplastic materials generally demonstrate lower water absorption and solubility, along with smoother surfaces, resulting in improved transparency and color stability compared to evaluated 3D-printing resins [87].

Surface Patterns and Abrasion

Aligner materials must resist degradation in the oral environment to withstand the forces generated during chewing [88]. A research has shown that intraoral exposure and functional use can significantly impact the surface roughness of “in-house” fabricated aligners across different regions [89]. Although minor surface defects often appear after two weeks of clinical use, these changes do not seem to markedly affect the mechanical properties of the aligners [90]. The surface characteristics of clear aligners are influenced by both manufacturing techniques and process parameters. Thermoformed and 3D-printed aligners differ notably in thickness and fit depending on tooth type and location, with thermoformed materials generally being stiffer, harder, and sometimes rougher [14, 91]. However, surface texture alone does not appear to significantly influence the force delivery characteristics in either thermoformed or directly printed aligners [79]. Recent research by Goracci et al. highlights the importance of print orientation on surface roughness and gloss, with vertical printing yielding significantly rougher and glossier surfaces compared to horizontal printing. The material type mainly affects gloss, with TC material showing higher gloss than LT. Moreover, polishing enhances the specimens’ resistance to aging, which may contribute to improved clinical longevity [92, 93].

Discoloration

Orthodontic aligners are prone to staining from beverages like coffee, cola, and red wine [94, 95]. Patients should limit such intake during treatment [96]. Optical properties vary by brand and material, and degrade with aging [97]. The optical properties of orthodontic aligners vary across different brands and materials but tend to degrade with in vitro aging. Studies indicate that a 30-minute post-curing period can achieve clinically acceptable color stability, highlighting the need to optimize post-curing protocols according to clinical requirements. Additionally, increased material thickness has been linked to greater yellowing in the samples [98]. Comparative studies show that 3D-printed aligners, especially those made from polyurethane, undergo more discoloration than thermoformed ones, while PETG-coated aligners demonstrate greater resistance to staining and degradation [99].

Permanent Deformations

An ideal orthodontic aligner should exhibit sufficient rigidity, high yield strength, and deliver forces within the elastic range. Common aligner materials, however, have an elastic modulus 40–50 times lower than Ni-Tiarchwires, making them more prone to permanent deformation [100]. Force decay in clear aligners arises from viscoelastic behavior and repeated use. TC-85 aligners, with shape memory properties, maintain consistent force at body temperature and support greater tooth movement due to their superior flexibility and wider elastic range [13, 101].

Conclusion

The integration of smart materials, particularly shape memory polymers like TC-85, introduces a promising paradigm shift by enabling clear aligners to actively respond to the oral environment. These materials exhibit improved elasticity, force consistency, and shape retention, which could allow for fewer aligner stages and better patient outcomes. Factors such as material composition, thickness, aging resistance, and water absorption critically influence performance and must be carefully considered during material selection and processing. Despite the promising advances, the clinical use of smart materials in clear aligners remains in its early stages. More in vivo studies and long-term evaluations are required to validate their effectiveness, biocompatibility, and safety. Future research should focus on standardizing printing protocols, enhancing material transparency and stain resistance, and developing environmentally friendly formulations. Ultimately, the convergence of digital workflows, additive manufacturing, and smart materials can revolutionize orthodontic care, making treatment more efficient, personalized, and biologically harmonious.

Conflict of Interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.

GMJ Copyright© 2025, Galen Medical Journal. This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/) Email:gmj@salviapub.com

 Correspondence to: Hossein Ebrahimi, Department of Orthodontics, School of Dentistry, Shahed University of Medical Sciences, Tehran, Iran. Telephone Number: 09124417468 Email Address: dr.hosseinebrahimi@gmail.com

Oral and Maxillofacial Disorders (SP1)

GMJ.2025;14:e3918

www.salviapub.com

Ebrahimi H, et al.

4D-Printed Smart Materials in Clear Aligner Fabrication

2	GMJ.2025;14:e3918 www.gmj.ir

4D-Printed Smart Materials in Clear Aligner Fabrication

Ebrahimi H, et al.

GMJ.2025;14:e3918 www.gmj.ir

3

Table 1. Common Smart Polymers in 3D printing Clear Aligners.

Brands	Manufacturer	Composition
E-Guard	Envision TEC (Rockhill, SC, United States)	Photo-polymeric clear methacrylate-based resin
Dental LT	Form Labs (Somerville, Massachusetts)	Photopolymers methacrylate-based resin
TC-85A (Tera Harz TC-85)	Graphy (Seoul, South Korea)	Aliphatic vinyl ester-polyurethane polymer
3D:1M	Okamoto chemicals	Aliphatic vinyl ester-polyurethane polymer
Accura 60 SLA	3D systems (Rockhill, South Carolina)	Polycarbonate-based photopolymer

Ebrahimi H, et al.

4D-Printed Smart Materials in Clear Aligner Fabrication

4	GMJ.2025;14:e3918 www.gmj.ir

Table 2. Comparison of the advantages and disadvantages of thermoplastic and directly 3D-printed clear aligner materials [102, 103]

3D Printed Materials

Thermoplastic Materials

Advantages: - High production efficiency - Customizable thickness according to requirements - Environmentally friendly manufacturing process - Reduction in material waste Disadvantages: - Requires post-curing processing - Limited availability of dedicated design software - Not widely available commercially - Materials have not undergone sufficient clinical trials - Inaccurate printing direction may affect outcomes - Final thickness greater than the designed specifications

Advantages: - Good biocompatibility - Wide range of approved materials - Accessibility Disadvantages: - Generation of waste materials - Potential environmental pollution - The final product has thinner dimensions than originally designed - Long production process - Alterations in material properties during processing

4D-Printed Smart Materials in Clear Aligner Fabrication

Ebrahimi H, et al.

GMJ.2025;14:e3918 www.gmj.ir

5

Table 3. Desirable properties and characteristics of a material for clear aligner fabrication.

Category	Property/Characteristic	Clinical Relevance
Mechanical	- Optimal stiffness and elasticity	Sufficient force for tooth movement without compromising comfort
	- High resilience and flexibility	Maintains shape while adapting to tooth morphology
	- Adequate stress and distortion resistance	Withstands chewing and occlusal forces without permanent deformation
	- Sustained force delivery	Delivers continuous, gentle force for effective tooth movement
	- Optimal Fitting And Accuracy	Provides a precise fit for accurate and predictable tooth movement
Structrual	- Dimensional stability	Retains form during treatment duration and under thermal stress
	- Appropriate aligner thickness	Provides effective force while maintaining comfort and aesthetics
	- Stain and color resistance	Resists stains and maintains high transparency for aesthetic appearance
	- High transparency	Aesthetically acceptable and less visible during wear
	- Thermal resistance	Withstands heat from fabrication and oral temperature variations
Chemical	- Resistance to oral chemicals, enzymes, and beverages.	Prevents degradation and maintains integrity in the oral environment
	- Low cytotoxicity and high biocompatibility	Ensures safe intraoral use and minimizes adverse tissue responses
	- Antimicrobial Properties	Inhibits microbial growth, enhancing hygiene and aligner material longevity.

Ebrahimi H, et al.

4D-Printed Smart Materials in Clear Aligner Fabrication

6	GMJ.2025;14:e3918 www.gmj.ir

4D-Printed Smart Materials in Clear Aligner Fabrication

Ebrahimi H, et al.

GMJ.2025;14:e3918 www.gmj.ir

7

Ebrahimi H, et al.

4D-Printed Smart Materials in Clear Aligner Fabrication

8	GMJ.2025;14:e3918 www.gmj.ir

References

1. Almulhim KS, Syed MR, Alqahtani N, Alamoudi M, Khan M, Ahmed SZ, Khan AS. Bioactive inorganic materials for dental applications: a narrative review. Materials. 2022 Oct 2;15(19):6864.
2. McCabe JF, Yan Z, Al Naimi OT, Mahmoud G, Rolland SL. Smart materials in dentistry. Aust Dent J. 2011;56 Suppl 1:310.
3. Bichu YM, Alwafi A, Liu X, Andrews J, Ludwig B, Bichu AY, Zou B. Advances in orthodontic clear aligner materials. Bioactive materials. 2023 Apr 1;22:384403.
4. Vasiliu S, Racovita S, Gugoasa IA, Lungan MA, Popa M, Desbrieres J. The Benefits of Smart Nanoparticles in Dental Applications. Int J Mol Sci. 2021;22(5):12278.
5. Subramanian P, Dutta B, Arya A, Azeem M, Pavithra B, Balaji D. Smart Material for Smarter Dentistry. Journal of Pharmacy and Bioallied Sciences. 2024;16:S17S9.
6. Kök M, Qader İN, Dagdelen F, Aydoğdu Y. A review of smart materials: researches and applications. ElCezeri. 2019;6(3):75588.
7. Rathi HP, Chandak M, Reche A, Dass A, Sarangi S, Thawri SR. Smart Biomaterials: An Evolving Paradigm in Dentistry. Cureus. 2023;15(10):e47265.
8. Saleem R, Sahoo A, Gupta S. Application of Smart Materials in Dental Sciences. Smart Materials for Science and Engineering. 2024; :7588.
9. Inamdar M, Chole D, Bakle S, Gandhi N, Hatte N, Rao M. Comparative evaluation of BRIX3000, CARIE CARE, and SMART BURS in caries excavation: An in vivo study. Journal of Conservative Dentistry. 2020;23:163.
10. Fernandes D, Peres R, Mendes A, Elias C. Understanding the ShapeMemory Alloys Used in Orthodontics. ISRN dentistry. 2011;2011:132408.
11. AlTaweel A. role of 4D printing materials, process and potentials for dentistry. International journal of health sciences. 2022:735664.
12. Jungbauer R, Sabbagh H, Janjic Rankovic M, Becker K. 3D Printed Orthodontic Aligners—A Scoping Review. Applied Sciences. 2024 Nov 5;14(22):10084.
13. Lee SY, Kim H, Kim HJ, Chung CJ, Choi YJ, Kim SJ et al. Thermomechanical properties of 3D printed photocurable shape memory resin for clear aligners. Scientific Reports. 2022;12(1):6246.
14. Park SY, Choi SH, Yu HS, Kim SJ, Kim H, Kim KB et al. Comparison of translucency, thickness, and gap width of thermoformed and 3Dprinted clear aligners using microCT and spectrophotometer. Sci Rep. 2023;13(1):10921.
15. Dupaix RB, Boyce MC. Finite strain behavior of poly (ethylene terephthalate)(PET) and poly (ethylene terephthalate)glycol (PETG). Polymer. 2005;46(13):482738.
16. Nicita F, Donato L, Scimone C, Alibrandi S, Mordà D, Costa A et al. Polymer materials for orthodontic use: an update on the properties and critical issues. Life Saf Secur. 2022;10:20914.
17. Staderini E, Chiusolo G, Guglielmi F, Papi M, Perini G, Tepedino M et al. Effects of Thermoforming on the Mechanical, Optical, Chemical, and Morphological Properties of PETG: In Vitro Study. Polymers. 2024;16(2):203.
18. Macrì M, D’Albis V, Marciani R, Nardella M, Festa F. Towards sustainable orthodontics: environmental implications and strategies for clear aligner therapy. Materials. 2024 Aug 23;17(17):4171.
19. Peter E, Monisha J, Sylas VP, George SA. How environmentally friendly is the disposal of clear aligners A gas chromatographymass spectrometry study. American Journal of Orthodontics and Dentofacial Orthopedics. 2025;167(1):3946.
20. Quinzi V, Orilisi G, Vitiello F, Notarstefano V, Marzo G, Orsini G. A spectroscopic study on orthodontic aligners: First evidence of secondary microplastic detachment after seven days of artificial saliva exposure. Science of The Total Environment. 2023;866:161356.
21. Lombardo L, Martines E, Mazzanti V, Arreghini A, Mollica F, Siciliani G. Stress relaxation properties of four orthodontic aligner materials: A 24hour in vitro study. Angle Orthod. 2017;87(1):118.
22. Bichu YM, Alwafi A, Liu X, Andrews J, Ludwig B, Bichu AY et al. Advances in orthodontic clear aligner materials. Bioactive Materials. 2023;22:384403.
23. Jindal P, Worcester F, Siena FL, Forbes C, Juneja M, Breedon P. Mechanical behaviour of 3D printed vs thermoformed clear dental aligner materials under nonlinear compressive loading using FEM. J Mech Behav Biomed Mater. 2020;112:104045.
24. Ergül T, Güleç A, Göymen M. The Use of 3D Printers in Orthodontics A Narrative Review. Turk J Orthod. 2023;36(2):13442.
25. Spangler T, Ammoun R, Carrico CK, Bencharit S, Tüfekçi E. The effect of crowding on the accuracy of 3dimensional printing. Am J Orthod Dentofacial Orthop. 2023;164(6):87988.
26. Wong KV, Hernandez A. A review of additive manufacturing. International scholarly research notices. 2012;2012(1):208760.
27. Abdulhameed O, AlAhmari A, Ameen W, Mian SH. Additive manufacturing: Challenges, trends, and applications. Advances in Mechanical Engineering. 2019;11(2):1687814018822880.
28. Jiménez M, Romero L, Domínguez IA, Espinosa MdM, Domínguez M. Additive manufacturing technologies: an overview about 3D printing methods and future prospects. Complexity. 2019;2019(1):9656938.
29. Khoo Z, Teoh J, Liu Y, Chua C, Yang S, An J et al. 3D printing of smart materials: A review on recent progresses in 4D printing. Virtual and Physical Prototyping. 2015;10:10322.
30. Antezana PE, Municoy S, Ostapchuk G, Catalano PN, Hardy JG, Evelson PA, Orive G, Desimone MF. 4D printing: The development of responsive materials using 3Dprinting technology. Pharmaceutics. 2023 Dec 7;15(12):2743.
31. Chu H, Yang W, Sun L, Cai S, Yang R, Liang W, Yu H, Liu L. 4D printing: a review on recent progresses. Micromachines. 2020 Aug 22;11(9):796.
32. Li YC, Zhang YS, Akpek A, Shin S. 4D bioprinting: The nextgeneration technology for biofabrication enabled by stimuliresponsive materials. Biofabrication. 2016;9:012001.
33. Kuang X, Roach D, Wu J, Hamel C, Ding Z, Wang T et al. Advances in 4D Printing: Materials and Applications. Advanced Functional Materials. 2018;29:1805290.
34. Fu P, Li H, Gong J, Zengjie F, Smith A, Shen K et al. 4D Printing of Polymeric Materials: Techniques, Materials, and Prospects. Progress in Polymer Science. 2022;126:101506.
35. Thakkar D, Benattia A, Bichu Y, Zou B, Aristizabal J, Fadia D et al. Seamless Workflows for InHouse Aligner Fabrication. Seminars in Orthodontics. 2023;29: 1724.
36. Jindal P, Juneja M, Siena F, Bajaj D, Breedon P. Mechanical and geometric properties of thermoformed and 3D printed clear dental aligners. American Journal of Orthodontics and Dentofacial Orthopedics. 2019;156:694701.
37. Tartaglia G, Mapelli A, Maspero C, Santaniello T, Serafin M, Farronato M et al. Direct 3D Printing of Clear Orthodontic Aligners: Current State and Future Possibilities. Materials. 2021;14:1799.
38. Lee J, Kim HC, Choi JW, Lee IH. A review on 3D printed smart devices for 4D printing. International Journal of Precision Engineering and ManufacturingGreen Technology. 2017;4:37383.
39. Srinivasan B, Padmanabhan S, Srinivasan S. Comparative evaluation of physical and mechanical properties of clear aligners a systematic review. Evid Based Dent. 2024;25(1):53.
40. Dorr L. Understand the Differences Among Clear Aligner Materials. Dental Products Report. 2023;57(4): .
41. Upadhyay M, Arqub SA. Biomechanics of clear aligners: hidden truths & first principles. Journal of the World Federation of Orthodontists. 2022;11(1):1221.
42. Bhardwaj A, Mishra K, Pandey A, Kirtiwar D, Kharche A, Agaldiviti P. Mechanical Properties of 3D Printed Orthodontic Aligners Produced by Different Commercially Available Printers An InVitro Study. J Pharm Bioallied Sci. 2024;16(Suppl 2):S1431s2.
43. Venezia P, Ronsivalle V, Rustico L, Barbato E, Leonardi R, Lo Giudice A. Accuracy of orthodontic models prototyped for clear aligners therapy: A 3D imaging analysis comparing different market segments 3D printing protocols. Journal of Dentistry. 2022;124:104212.
44. Yu X, Li G, Zheng Y, Gao J, Fu Y, Wang Q et al. ‘Invisible’ orthodontics by polymeric ‘clear’ aligners molded on 3Dprinted personalized dental models. Regenerative Biomaterials. 2022;9: rbac007.
45. Khalil AS, Zaher AR. Effect of printing orientation and resin thickness on flexural strength of direct 3Dprinted aligners. BMC Oral Health. 2025;25(1):238.
46. Camenisch L, Polychronis G, Panayi N, Makou O, Papageorgiou SN, Zinelis S et al. Effect of printing orientation on mechanical properties of 3Dprinted orthodontic aligners. Journal of Orofacial Orthopedics/Fortschritte der Kieferorthopädie. 2024:18.
47. McCarty MC, Chen SJ, English JD, Kasper F. Effect of print orientation and duration of ultraviolet curing on the dimensional accuracy of a 3dimensionally printed orthodontic clear aligner design. American Journal of Orthodontics and Dentofacial Orthopedics. 2020;158(6):88997.
48. Koenig N, Choi JY, McCray J, Hayes A, Schneider P, Kim KB. Comparison of dimensional accuracy between directprinted and thermoformed aligners. Korean J Orthod. 2022;52(4):24957.
49. Alghauli MA, Alqutaibi AY, Aljohani R, Almuzaini S, Saeed MH. Influence of different print orientations on properties and behavior of additively manufactured resin dental devices: A systematic review and metaanalysis. The Journal of Prosthetic Dentistry. 2025;133(3):736.e1.e12.
50. Williams A, Bencharit S, Yang IH, Stilianoudakis SC, Carrico CK, Tüfekçi E. Effect of print angulation on the accuracy and precision of 3Dprinted orthodontic retainers. American Journal of Orthodontics and Dentofacial Orthopedics. 2022;161(1):1339.
51. Mattle M, Zinelis S, Polychronis G, Makou O, Panayi N, Papageorgiou SN et al. Effect of heat treatment and nitrogen atmosphere during postcuring on mechanical properties of 3Dprinted orthodontic aligners. Eur J Orthod. 2024;46(1): cjad056.
52. Zinelis S, Panayi N, Polychronis G, Dionysopoulos D, Papageorgiou SN, Eliades T. Effect of nitrogen atmosphere during 3D printing on mechanical properties of orthodontic aligners. European Journal of Oral Sciences. 2025:e70008.
53. Šimunović L, Jurela A, Sudarević K, Bačić I, Haramina T, Meštrović S. Influence of PostProcessing on the Degree of Conversion and Mechanical Properties of 3DPrinted Polyurethane Aligners. Polymers. 2023;16(1):17.
54. Šimunović L, Marić AJ, Bačić I, Haramina T, Meštrović S. Impact of UV light exposure during printing on thermomechanical properties of 3Dprinted polyurethanebased orthodontic aligners. Applied Sciences. 2024 Oct 21;14(20):9580.
55. Kim JE, Mangal U, Yu JH, Kim GT, Kim H, Seo JY et al. Evaluation of the effects of temperature and centrifugation time on elimination of uncured resin from 3Dprinted dental aligners. Scientific reports. 2024;14(1):15206.
56. Neoh SP, Khantachawana A, Santiwong P, Chintavalakorn R, Srikhirin T. Effect of postprocessing on the surface, optical, mechanical, and dimensional properties of 3Dprinted orthodontic clear retainers. Clinical Oral Investigations. 2025;29(1):48.
57. Kim D, Shim JS, Lee D, Shin SH, Nam NE, Park KH, Shim JS, Kim JE. Effects of postcuring time on the mechanical and color properties of threedimensional printed crown and bridge materials. Polymers. 2020 Nov 23;12(11):2762.
58. Kanase A, Misra V, Sobti R. The effect of postultraviolet light curing on the accuracy of directprinted aligners: an in vitro study. Australasian Orthodontic Journal. 2024;40:8594.
59. Wulff J, Schweikl H, Rosentritt M. Cytotoxicity of printed resinbased splint materials. Journal of Dentistry. 2022;120:104097.
60. Lopez S, Pecci Lloret MR, Pecci Lloret MP, GarcíaBernal D, OñateSánchez R. Influence of the postprocessing protocol on a biocompatible 3Dprinted resin. Journal of prosthodontics : official journal of the American College of Prosthodontists; 2024.
61. Iodice G, Ludwig B, Polishchuk E, Petruzzelli R, Di Cunto R, Husam S et al. Effect of postprinting curing time on cytotoxicity of direct printed aligners: A pilot study. Orthod Craniofac Res. 2024;27 Suppl 2:1416.
62. Jungbauer R, Sabbagh H, Janjic Rankovic M, Becker K. 3D Printed Orthodontic Aligners—A Scoping Review. Applied Sciences. 2024;14(22):10084.
63. Pratsinis H, Papageorgiou SN, Panayi N, Iliadi A, Eliades T, Kletsas D. Cytotoxicity and estrogenicity of a novel 3dimensional printed orthodontic aligner. Am J Orthod Dentofacial Orthop. 2022;162(3):e116e22.
64. Niu C, Li D, Zhang Y, Wang Y, Ning S, Zhao G et al. Prospects for 3Dprinting of clear aligners—a narrative review. Frontiers in Materials. 2024;11:1438660.
65. Atta IMAM. Materialtechnische Untersuchungen von Alignermaterialien aus Formgedächtnispolymeren [dissertation]. Bonn: Rheinische FriedrichWilhelmsUniversität Bonn; Available from: https://hdl.handle.net/20.500.11811/11774
66. Wendl T, Wendl B, Proff P. An analysis of initial force and moment delivery of different aligner materials. Biomedical Engineering/Biomedizinische Technik. 2025 Jun 26;70(3):25967.
67. Atta I, Bourauel C, Alkabani Y, Mohamed N, Kim H, Alhotan A et al. Physiochemical and mechanical characterisation of orthodontic 3D printed aligner material made of shape memory polymers (4D aligner material). Journal of the Mechanical Behavior of Biomedical Materials. 2024;150:106337.
68. Narongdej P, Hassanpour M, Alterman N, RawlinsBuchanan F, Barjasteh E. Advancements in Clear Aligner Fabrication: A Comprehensive Review of Direct3D Printing Technologies. Polymers. 2024;16(3):371.
69. Huang X, Peng S, Zheng L, Zhuo D, Wu L, Weng Z. 3D Printing of High Viscosity UVCurable Resin for Highly Stretchable and Resilient Elastomer. Adv Mater. 2023;35(49):e2304430.
70. Rajasekaran A, Chaudhari P. Integrated manufacturing of direct 3Dprinted clear aligners. Frontiers in Dental Medicine. 2023;3:1089627.
71. Spanier C, Schwahn C, Krey KF, Ratzmann A. Fused filament fabrication (FFF): influence of layer height on forces and moments delivered by aligners—an in vitro study. Clinical Oral Investigations. 2023;27(5):216373.
72. Iliadi A, Koletsi D, Eliades T. Forces and moments generated by alignertype appliances for orthodontic tooth movement: A systematic review and metaanalysis. Orthod Craniofac Res. 2019;22(4):24858.
73. Elshazly TM, Keilig L, Nang D, Golkhani B, Weber A, Elattar H, Talaat S, Bourauel C. Effect of thermomechanical aging on force system of orthodontic aligners made of different thermoformed materials: an in vitro study. Journal of Orofacial Orthopedics/Fortschritte der Kieferorthopädie. 2025 Sep;86(5):33746.
74. Lyu X, Cao X, Yan J, Zeng R, Tan J. Biomechanical effects of clear aligners with different thicknesses and gingivalmargin morphology for appliance design optimization. American Journal of Orthodontics and Dentofacial Orthopedics. 2023;164(2):23952.
75. Wu C, Mangal U, Bai N, Kim H, Hwang G, Cha JY, Lee KJ, Kwon JS, Choi SH. Effects of printing layer thickness and build orientation on the mechanical properties and color stability of 3Dprinted clear aligners. The Angle Orthodontist. 2025 Jul 1;95(4):35561.
76. Edelmann A, English JD, Chen SJ, Kasper FK. Analysis of the thickness of 3dimensionalprinted orthodontic aligners. American Journal of Orthodontics and Dentofacial Orthopedics. 2020;158(5):e91e8.
77. Bandić R, Vodanović K, Vuković Kekez I, Medvedec Mikić I, Galić I, Kalibović Govorko D. Thickness Variations of Thermoformed and 3DPrinted Clear Aligners. Acta Stomatol Croat. 2024;58(2):14555.
78. Elshazly TM, Bourauel C, Ismail A, Ghoraba O, Aldesoki M, Salvatori D et al. Effect of material composition and thickness of orthodontic aligners on the transmission and distribution of forces: an in vitro study. Clin Oral Investig. 2024;28(5):258.
79. Hertan E, McCray J, Bankhead B, Kim KB. Force profile assessment of directprinted aligners versus thermoformed aligners and the effects of nonengaged surface patterns. Progress in Orthodontics. 2022;23(1):49.
80. Remley ML, Miranda GFPC, Bankhead B, McCray J, Kim KB. Force assessment of thermoformed and directprinted aligners in a lingual bodily movement of a central incisor over time: a 14day in vitro study. Journal of Korean Dental Science. 2023;16(1):2334.
81. Bae BG, Kim YH, Lee GH, Lee J, Min J, Kim H et al. A study on the compressive strength of threedimensional direct printing aligner material for specific designing of clear aligners. Sci Rep. 2025;15(1):2489.
82. Can E, Panayi N, Polychronis G, Papageorgiou SN, Zinelis S, Eliades G et al. Inhouse 3Dprinted aligners: effect of in vivo ageing on mechanical properties. European Journal of Orthodontics. 2022;44(1):515.
83. Linjawi AI, Abushal AM. Adaptational changes in clear aligner fit with time. Angle Orthod. 2022;92(2):2205.
84. Bucci R, Rongo R, Levatè C, Michelotti A, Barone S, Razionale AV et al. Thickness of orthodontic clear aligners after thermoforming and after 10 days of intraoral exposure: a prospective clinical study. Prog Orthod. 2019;20(1):36.
85. Topsakal KG, Aksoy M, Duran GS. The effect of aging on the mechanical properties of 3dimensional printed biocompatible resin materials used in dental applications: An in vitro study. American Journal of Orthodontics and Dentofacial Orthopedics. 2023;164(3):4419.
86. Shirey N, Mendonca G, Groth C, KimBerman H. Comparison of mechanical properties of 3dimensional printed and thermoformed orthodontic aligners. Am J Orthod Dentofacial Orthop. 2023;163(5):7208.
87. Neoh SP, Khantachawana A, Chintavalakorn R, Santiwong P, Srikhirin T. Comparison of physical, mechanical, and optical properties between thermoplastic materials and 3dimensional printing resins for orthodontic clear retainers. American Journal of Orthodontics and Dentofacial Orthopedics. 2025;167(1):95109.e1.
88. Hartshorne J, Wertheimer MB. Emerging insights and new developments in clear aligner therapy: A review of the literature. AJODO Clinical Companion. 2022;2(4):31124.
89. Koletsi D, Panayi N, Laspos C, Athanasiou AE, Zinelis S, Eliades T. In vivo aginginduced surface roughness alterations of Invisalign(®) and 3Dprinted aligners. J Orthod. 2023;50(4):35260.
90. Fang D, Li F, Zhang Y, Bai Y, Wu B. Changes in mechanical properties, surface morphology, structure, and composition of Invisalign material in the oral environment. American Journal of Orthodontics and Dentofacial Orthopedics. 2020;157:74553.
91. Albilali AT, Baras BH, Aldosari MA. Evaluation of Mechanical Properties of Different Thermoplastic Orthodontic Retainer Materials after Thermoforming and Thermocycling. Polymers. 2023;15(7):1610.
92. Goracci C, Bosoni C, Marti P, Scotti N, Franchi L, Vichi A. Influence of Printing Orientation on Surface Roughness and Gloss of 3D Printed Resins for Orthodontic Devices. Materials. 2025;18:523.
93. ParadowskaStolarz A, Wezgowiec J, Malysa A, Wieckiewicz M. Effects of polishing and artificial aging on mechanical properties of dental LT clear® resin. Journal of functional biomaterials. 2023 May 25;14(6):295.
94. Venkatasubramanian P, Jerome MS, Ragunanthanan L, Maheshwari U, Vijayalakshmi D. Color stability of aligner materials on exposure to indigenous food products: An invitro study. J Dent Res Dent Clin Dent Prospects. 2022;16(4):2218.
95. Olteanu ND, Taraboanta I, Panaite T, Balcos C, Rosu SN, Vieriu RM, Dinu S, Zetu IN. Color stability of various orthodontic clear aligner systems after submersion in different staining beverages. Materials. 2024 Aug 12;17(16):4009.
96. Erçin Ö, Kurnaz M, Kopuz D. Evaluation of the color stability of attachments made with different resin composites. American Journal of Orthodontics and Dentofacial Orthopedics. 2023;164(4):e121e8.
97. Lombardo L, Arreghini A, Maccarrone R, Bianchi A, Scalia S, Siciliani G. Optical properties of orthodontic alignersspectrophotometry analysis of three types before and after aging. Prog Orthod. 2015;16:41.
98. Topsakal KG, Aksoy M, Sükut Y, Duran GS. Effect of postcuring parameters and material thickness on the color stability of 3Dprinted dental resins: An in vitro study. International Orthodontics. 2025;23(2):100985.
99. Šimunović L, Čekalović Agović S, Marić AJ, Bačić I, Klarić E, Uribe F, Meštrović S. Color and chemical stability of 3Dprinted and thermoformed polyurethanebased aligners. Polymers. 2024 Apr 11;16(8):1067.
100. Upadhyay M, Arqub SA. Biomechanics of clear aligners: hidden truths & first principles. J World Fed Orthod. 2022;11(1):1221.
101. Cenzato N, Di Iasio G, Martìn CarrerasPresas C, Caprioglio A, Del Fabbro M. Materials for Clear Aligners—A Comprehensive Exploration of Characteristics and Innovations: A Scoping Review. Applied Sciences. 2024;14(15):6533.
102. Elsaharty MA, Hafez AM, Abdelwarith A. Accuracy of 3D printing versus milling in fabrication of clear aligners dental models. AlAzhar Journal of Dental Science. 2023;26(2):26376.
103. Suganna M, Kausher H, Tarek Ahmed S, Sultan Alharbi H, Faraj Alsubaie B, Ds A et al. Contemporary Evidence of CADCAM in Dentistry: A Systematic Review. Cureus. 2022;14(11):e31687.

4D-Printed Smart Materials in Clear Aligner Fabrication

Ebrahimi H, et al.

GMJ.2025;14:e3918 www.gmj.ir

9

Ebrahimi H, et al.

4D-Printed Smart Materials in Clear Aligner Fabrication

10	GMJ.2025;14:e3918 www.gmj.ir

4D-Printed Smart Materials in Clear Aligner Fabrication

Ebrahimi H, et al.

GMJ.2025;14:e3918 www.gmj.ir

11

Ebrahimi H, et al.

4D-Printed Smart Materials in Clear Aligner Fabrication

12	GMJ.2025;14:e3918 www.gmj.ir

4D-Printed Smart Materials in Clear Aligner Fabrication

Ebrahimi H, et al.

GMJ.2025;14:e3918 www.gmj.ir

13