Comparative Evaluation of Mechanical Properties of Clear Aligner Biosheet Materials -An In Vitro Study

ABSTRACT

Background:

Thermoplastic polymers are extensively used in the fabrication of clear aligners due to their biocompatibility, flexibility, and mechanical properties. Evaluating the mechanical performance of these materials under clinical conditions is crucial for optimizing their application.

Materials and Methods:

Two types of thermoplastic polymers, **Duran** and **Zendura**, were selected for analysis. Specimens with uniform dimensions (10 mm × 50 mm × 1 mm) were prepared from sheets of each material. Tensile strength, elastic modulus, and flexural strength were measured using a universal testing machine (UTM) under controlled conditions. Cyclic loading tests were conducted to simulate intraoral forces over 1000 cycles at 37°C in an artificial saliva solution. Data were statistically analyzed using one-way ANOVA with a significance level of P < 0.05.

Results:

Zendura exhibited higher tensile strength (average: 62 ± 3 MPa) and elastic modulus (average: 2400 ± 100 MPa) compared to Duran (tensile strength: 55 ± 2 MPa; elastic modulus: 2100 ± 90 MPa). Similarly, Zendura showed superior flexural strength. Cyclic loading tests revealed that Zendura retained 96% of its original mechanical properties after 1000 cycles, while Duran retained 93%.

Conclusion:

Zendura outperformed Duran in tensile strength, elastic modulus, and flexural strength, making it a highly suitable material for aligner fabrication. However, Duran demonstrated satisfactory flexibility and durability, which may provide specific clinical advantages.

INTRODUCTION

Clear aligners have become a popular alternative to traditional fixed orthodontic appliances, offering improved aesthetics, comfort, and hygiene. These appliances are predominantly fabricated using thermoplastic polymers, which are chosen for their favorable biocompatibility, mechanical strength, and flexibility.[1] The mechanical properties of these materials play a pivotal role in determining the efficacy of aligners in transferring orthodontic forces to teeth while maintaining their structural integrity under clinical conditions.[2]

Among the widely used thermoplastic materials, Duran and Zendura have gained prominence due to their unique mechanical characteristics. Duran is known for its flexibility and ease of forming, while Zendura has been specifically developed to withstand higher stresses, making it suitable for long-term aligner use.[3,4] The choice of material significantly impacts the aligner’s ability to resist deformation, maintain precise tooth movements, and endure intraoral forces over extended periods.[5]

To optimize the performance of clear aligners, it is crucial to evaluate the tensile strength, elastic modulus, and flexural strength of these materials, as these properties determine their resistance to breakage and deformation.[6] Additionally, cyclic loading tests provide insights into the durability of these polymers under repeated forces, simulating the clinical environment.[7]

MATERIALS AND METHODS

Materials

This study evaluated two commercially available thermoplastic polymers, Duran and Zendura, which are commonly used in the fabrication of orthodontic aligners. For each material, tests were carried out on the material that was not yet thermoformed (as from the supplier, A.S.), after the thermoforming process (no storage in artificial saliva), and after the thermoforming process plus storage in artificial saliva.

Sheets of each material were obtained in standardized thicknesses, and specimens were prepared with dimensions of 10 mm × 50 mm × 1 mm using precision cutting tools to ensure uniformity. The thermoforming process was performed as recommended by manufacturers for pressure, heating, and cooling time and according to the procedure normally adopted to manufacture clear aligners.

Mechanical testing

The mechanical properties of the samples were assessed using a universal testing machine (UTM).

1. Tensile Strength and Elastic Modulus

   Tensile tests were conducted following ASTM D638 standards. Samples were clamped between the grips of the UTM and subjected to tensile loading at a crosshead speed of 5 mm/min. The stress-strain curves obtained were used to calculate tensile strength and elastic modulus.

2. Flexural Strength

   Flexural properties were measured using a three-point bending setup in the UTM, adhering to ASTM D790 standards. A load was applied at the midpoint of the specimen with supports placed at a span length of 40 mm.

Cyclic loading test

To simulate intraoral conditions, cyclic loading tests were performed at 37°C in an artificial saliva solution (pH 7.4). Each specimen underwent 1000 loading cycles at a frequency of 1 Hz, with a load range of 10–50 N, mimicking masticatory forces. After the cyclic loading, tensile and flexural tests were repeated to evaluate any changes in mechanical properties.

Statistical analysis

Data were analyzed using one-way analysis of variance (ANOVA) in SPSS software (Version 26.0; IBM Corp., Armonk, NY).

RESULTS

The results of the mechanical tests are summarized in Tables 1 and 2. The study found that Zendura consistently outperformed Duran in all evaluated parameters, including tensile strength, elastic modulus, and flexural strength, across all testing conditions.

Table 1.

Tensile strength and elastic modulus of materials

Condition	Material	Tensile Strength (MPa)	Elastic Modulus (MPa)
As-supplied	Duran	55.2±2.1	2105±89
	Zendura	62.3±3.2	2407±95
Thermoformed	Duran	52.8±1.8	2007±92
	Zendura	60.1±2.5	2352±88
Thermoformed + saliva	Duran	51.4±2.0	1978±87
	Zendura	59.0±2.9	2320±92

Table 2.

Flexural strength of materials

Condition	Material	Flexural Strength (MPa)
As-supplied	Duran	88.5±3.6
	Zendura	95.2±4.0
Thermoformed	Duran	85.6±3.8
	Zendura	92.7±3.5
Thermoformed + saliva	Duran	83.1±3.5
	Zendura	91.4±3.9

Tensile strength and elastic modulus

Table 1 displays the tensile strength and elastic modulus of both materials. Zendura demonstrated significantly higher tensile strength and elastic modulus compared to Duran. After thermoforming and storage in artificial saliva, a slight reduction in these properties was observed for both materials. However, Zendura retained a greater proportion of its mechanical properties.

Flexural strength

The flexural strength results are shown in Table 2. Zendura exhibited significantly higher flexural strength than Duran under all testing conditions. After exposure to artificial saliva and cyclic loading, a minor reduction in flexural strength was observed, with Zendura retaining a higher percentage of its initial strength.

Effect of cyclic loading

After 1000 cycles of loading, both materials showed minimal degradation in mechanical properties. Zendura retained 96% of its tensile strength and 94% of its flexural strength, while Duran retained 93% and 90%, respectively.

DISCUSSION

The tensile strength of aligner materials is critical as it determines their ability to withstand stretching forces without breaking. In the present study, Zendura exhibited a tensile strength significantly higher than Duran, both in the as-supplied condition and after thermoforming and storage in artificial saliva. Similar findings have been reported in previous research, where Zendura demonstrated enhanced mechanical properties attributed to its advanced polymer formulation designed for durability.[1,2]

Elastic modulus, which indicates the stiffness of a material, was also higher for Zendura across all conditions. A higher elastic modulus is advantageous as it allows the aligner to apply sustained orthodontic forces over time, ensuring effective tooth movement. Duran, while exhibiting lower stiffness, may offer benefits in situations requiring greater material flexibility, as noted in earlier studies.[3]

Flexural strength, reflecting the material’s resistance to bending, is another crucial parameter for clear aligners. Zendura demonstrated superior flexural strength compared to Duran, with minimal degradation after exposure to cyclic loading and artificial saliva. The retention of mechanical properties following cyclic loading is particularly noteworthy, as it simulates intraoral conditions such as masticatory forces and constant exposure to saliva. This durability aligns with the findings of other studies highlighting Zendura’s resilience under repeated stress.[4,5]

The reduction in mechanical properties observed after thermoforming and artificial saliva exposure can be attributed to thermal and chemical stresses encountered during these processes. Although both materials exhibited some degree of degradation, Zendura retained a greater proportion of its original properties, supporting its application in long-term orthodontic treatments.[6,7]

The cyclic loading test provided further insight into the performance of these materials under repetitive forces. Zendura retained 96% of its tensile strength and 94% of its flexural strength after 1000 loading cycles, compared to 93% and 90% for Duran. These results suggest that Zendura is better suited for sustained use in the oral environment, where aligners are subjected to continuous functional and parafunctional forces.[8]

CONCLUSION

In conclusion, Zendura’s superior tensile strength, elastic modulus, and flexural strength make it a highly suitable material for clear aligner fabrication, particularly for cases requiring prolonged wear and higher force resistance. However, Duran’s satisfactory performance in terms of flexibility and durability suggests that it may still be appropriate for specific clinical scenarios.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.