Introduction
This study aimed to evaluate the physical properties of directly 3-dimensional–printed clear aligners (DPA), fabricated from TA-28 resin (Graphy Inc, Seoul, Republic of Korea), subjected to thermal cycling, focusing on color stability and surface wettability as indicators of clinical performance and material durability.
Methods
Ninety disc-shaped specimens (15.0 × 2.0 mm) were fabricated using the manufacturer-recommended TA-28 resin (Graphy Inc). For color stability assessment, samples (n = 60) were immersed in distilled water, black tea, coffee, or red wine and evaluated at 1, 3, 7, and 14 days using a spectrophotometer based on the International Commission on Illumination L∗a∗b∗ and International Commission on Illumination color difference 2000 (ΔE00) systems. For surface wettability, specimens (n = 30) were subjected to thermal cycling at 5°C-55°C for 7 and 14 days, and the water contact angle was measured using the sessile drop method. Statistical analyses were conducted using the Kruskal-Wallis test for overall comparisons of nonparametric data, with post-hoc pairwise tests performed using the Mann-Whitney U test. For normally distributed variables, paired-samples t tests were applied. A significance level of P <0.05 was considered statistically meaningful.
Results
Colorization tests revealed clinically unacceptable discoloration (ΔE 00 >2.25) in specimens immersed in tea, coffee, and red wine, with red wine producing the most severe changes (ΔE00 >30 at 14 days). A time-dependent increase in discoloration was observed across all staining solutions. Water contact angle values were consistently <90°, confirming a hydrophilic surface profile. Although values decreased after 7- and 14-day thermal cycling, the differences were not statistically significant ( P = 0.143 and P = 0.599), indicating stable surface wettability over the aging period.
Conclusions
DPAs fabricated from TA-28 resin demonstrate hydrophilic surface characteristics and are highly susceptible to discoloration when exposed to pigmented beverages. These findings represent preliminary laboratory observations rather than clinical performance outcomes. Further studies simulating intraoral conditions, including mechanical wear and daily cleaning, are required to confirm the long-term esthetic stability of DPAs.
Highlights
-
•
Directly printed aligners show a hydrophilic surface profile.
-
•
Red wine caused the highest discoloration among all beverages.
-
•
Tea and coffee also led to severe and unacceptable color changes.
-
•
Surface wettability remained stable after 7-14 days of aging.
-
•
The esthetic durability of TA-28 aligners remains a clinical concern.
In recent years, the growing need for esthetics has significantly increased the desire for orthodontic treatment among adults, resulting in the widespread acceptance of clear aligner therapy. Despite being one of the most prevalent treatment modalities, thermoformed clear aligners have specific limitations in their physical and mechanical properties. The heat and pressure involved in the thermoforming process can cause these aligners to undergo structural deformation, potentially leading to inconsistencies between the intended therapy and the actual clinical results. In addition, intraoral factors, including temperature variations, saliva, and dietary practices, further influence discoloration and changes in material hardness. These drawbacks frequently result in poor outcomes for both professionals and patients. Maintaining physical qualities, including color stability, microhardness, and surface wettability, is essential for guaranteeing the clinical longevity and esthetic efficacy of clear aligners. Inadequate color stability may lead to unfavorable outcomes in this patient population, in which esthetic considerations are of utmost significance. Materials exhibiting inadequate color stability are hence inappropriate for prolonged intraoral use. Water absorption causes both physical and chemical changes in the material, resulting in permanent degradation of the polymer’s mechanical characteristics. In the oral environment, moisture absorption and hygroscopic expansion can cause dimensional changes that compromise the fit of the aligner. Previous studies have reported that the physical and mechanical characteristics of clear aligners are affected by saliva exposure, and force transmission may differ based on the material composition. In this context, thermocycling has emerged as a prevalent aging protocol, offering a clinically pertinent approach for assessing the long-term efficacy and integration of clear aligners.
The recent advent of 3-dimensional (3D) printing technologies and innovative resin materials has facilitated the integration of directly printed aligners (DPAs) into orthodontic practice. This method, in contrast to conventional thermoforming, obviates the necessity for model creation and pressure molding, enabling the manufacture of aligners that accurately fit the patient’s dental structure. The removal of these intermediate phases provides multiple benefits, such as a more efficient workflow, reduced reliance on clinical personnel, reduced material waste, and shorter overall production times. Furthermore, aligners can be tailored with different thicknesses based on the clinician’s treatment plan. The manufacturer-recommended Tera Harz TC-85 resin has obtained biocompatibility certification and is deemed appropriate for intraoral application owing to its superior flexibility and shape-memory characteristics. Recently, the same manufacturer has introduced another shape-memory resin, TA-28. This resin demonstrates reduced water solubility, enhanced flexural strength, and an elevated flexural modulus, facilitating the application of stronger orthodontic tensions. A study of the existing literature found no studies assessing the physical properties of TA-28 resin. This study aimed to assess the physical properties—specifically, color stability and water contact angle—of DPAs produced from TA-28 resin after 7 and 14 days of thermal cycling. The initial null hypothesis of the study posited that there are no statistically significant variations in color change among various colorization beverages. Thermal cycling will not yield any statistically significant alterations in the water contact angle of DPA specimens.
Material and methods
The experimental workflow is illustrated in Figure 1 . Ethical approval was considered unnecessary as the study did not include human participants or employ human-derived data. The necessary sample size was calculated using G∗Power software (version 2.3.21.0, Sydney, Australia; The Jamovi Project, 2022), employing an effect size of 0.20 and a significance threshold of 0.05, resulting in a statistical power greater than 95%. The calculation determined that 15 specimens each group were required, culminating in a total of 90 samples prepared for the study.
The workflow of the study.
The study groups are shown below:
-
1.
Group I: Colorization with different beverages (specimens: 15.0 × 2.0 mm; n = 60; TA-28 resin, Graphy Inc, Seoul, Republic of Korea)
-
I.A: Distilled water group (n = 15)
-
I.B: Black tea group (n = 15, Yellow Label tea; Lipton, Hefei, China)
-
I.C: Coffee group (n = 15, Nescafe Classic)
-
I.D: Red wine group (n = 15, Buzbağ Regional series; Elazığ, Turkey)
-
-
2.
Group II: Water contact angle after thermal cycling (Specimens: 15.0 × 2.0 mm; n = 30; TA-28 resin, Graphy Inc)
-
II.A: seventh-day thermal cycling (5°C-55°C, n = 15)
-
II.B: Fourteenth-day thermal cycling (5°C-55°C, n = 15)
-
This study used TA-28 resin (Graphy Inc), as advised by the manufacturer for direct 3D printing, to produce the specimens. Disc-shaped specimens measuring 15.0 × 2.0 mm were designed using computer-aided design software (Autodesk Meshmixer, version 3.5.35; Autodesk, Inc, San Rafael, Calif) and exported in ZSLR file format. Printing was conducted using a Uniz NBEE 3D printer (Uniz, San Diego, Calif). The resin was mixed for several minutes to ensure homogeneity and preheated to approximately 30°C before printing. After fabrication, the specimens were removed from the build platform and centrifuged at 500-600 rpm for 5-6 minutes to remove surplus uncured resin. The printed discs underwent postcuring in a specialized UV curing equipment (Tera Harz Cure, Graphy Inc) for 14 minutes in a nitrogen atmosphere, adhering to the manufacturer’s guidelines. The use of nitrogen facilitates the displacement of oxygen within the curing chamber, thus establishing an oxygen-free environment that reduces polymerization inhibition.
The resin specimens were randomly allocated into 4 groups and submerged in various staining solutions to assess color stability: distilled water (n = 15), black tea (Yellow Label tea; Lipton; n = 15), coffee (Nescafe Classic; n = 15), and red wine (Buzbağ Regional series; n = 15). Thermocycling was omitted from the staining process. In accordance with the manufacturer’s instructions, the coffee solution was formulated by dissolving 2 g of coffee powder in 200 mL of boiling water. The tea solution was prepared by infusing a single tea bag in 200 mL of boiling water. To mitigate the potential of bacterial contamination, all staining solutions were produced fresh each day and gently stirred twice daily. Throughout the experimental duration, the specimens were maintained in an incubator at 37°C in the absence of light. Color alterations were delineated using the International Commission on Illumination L∗a∗b∗ color system. Before measurement, each specimen was cleaned with running water and dried using an air spray. The L∗, a∗, and b∗ values of the specimens were documented at baseline (T0), day 1 (T1), day 3 (T2), day 7 (T3), and day 14 (T4). Measurements were conducted in the center region of each specimen under D65 standard illumination using a spectrophotometer (CM-3600A, Konica Minolta, Osaka, Japan). Each specimen was measured twice, and the mean values were documented by the same investigator for further investigation. The color differences (ΔE00) were calculated using the International Commission on Illumination color difference 2000 (CIEDE2000) formula, based on the mean L∗, a∗, and b∗ parameters:
In the CIEDE2000 formula, ΔL′, ΔC′, and ΔH′ represent the differences in lightness, chroma, and hue, respectively. R T is the rotation function that considers the interplay between chroma and hue variations in the blue region of the color space. S L , S C , and S H are weighting functions that modify the overall color difference based on alterations in the L∗, a∗, and b∗ coordinates as well as the parametric factors. K L , K C , and K H are correction factors established based on the experimental conditions. In the present study, all parametric factors were set to 1, as recommended in previous literature. The perceptibility threshold was established at ΔE00 = 1.30 units, whereas the clinical acceptability threshold was set at ΔE00 >2.25 units. Here, L∗ denotes lightness, a∗ represents the red (+)/ green (−) coordinate, and b∗ represents the yellow (+)/ blue (−) coordinate. Although this classification is based on the perceptibility thresholds described by the National Bureau of Standards, it has been reformulated for clarity in the present study ( Table I ).
Table I
National Bureau of Standards ratings
| ΔE 00 range | Level of color change | Interpretation |
|---|---|---|
| 0.0 to <0.5 | Minimal | The color change is extremely slight or barely perceptible. |
| 0.5 to <1.5 | Mild | The color change is slight, but may be noticed. |
| 1.5 to <3.0 | Moderate | The change is perceptible to the average observer. |
| 3.0 to <6.0 | Marked | Obvious and clearly visible color change. |
| 6.0 to <12.0 | Severe | Highly noticeable and significant shift in color. |
| ≥12.0 | Extreme | The material appears to have changed to a different color. |
The specimens were classified into 2 groups, and initial water contact angle measurements were recorded before aging. The samples underwent thermocycling for durations of 4 and 8 hours to replicate clinical application. The 4-hour aging time equated to roughly 7 days of intraoral application, whereas the 8-hour period represented 14 days of intraoral application. The selected time intervals corresponded to the aligner replacement durations most frequently favored in clinical orthodontic practice. Thermocycling was conducted with a specialized thermocycler (SD Mechatronik Thermocycler; SD Mechatronik, Westerham, Germany), programmed to operate at temperatures ranging 5°C-55°C, with a dwell duration of 5 seconds.
The sessile drop technique was employed to assess the water contact angle of the TA-28 DPA specimens. Approximately 4 μL of distilled water was applied to the middle surface of each disk. Measurements were conducted using a confocal microscope (Smartproof 5, Zeiss, Germany) linked to contact angle goniometer software (OneAttension, Biolin Scientific, Sweden). The analysis mode was configured to contact angle (Young-Laplace fit) with automatic baseline identification. Air was characterized as the gaseous phase, whilst distilled water at 20°C and diiodomethane served as probing liquids. The contact angle measurements were determined using the OneAttension program. According to the criteria applied in the previous study, a measured contact angle <10° indicated a superhydrophilic surface, values between 10°-90° represented hydrophilic surfaces, and values >90° indicated hydrophobic surfaces.
Statistical analysis
The data regarding color change and water contact angle were organized in Microsoft Excel (Microsoft, Redmond, Wash), and statistical analyses were performed using the open-source platform Jamovi (The Jamovi Project, version 2.3.21.0, 2022; www.jamovi.org ). The Shapiro-Wilk test revealed that the color change values were not normally distributed; therefore, the Kruskal-Wallis test was employed for intergroup comparisons, with Mann-Whitney U tests conducted for post-hoc pairwise analysis. The water contact angle data had a normal distribution and were consequently evaluated using a paired-samples t test for both intra and intergroup comparisons. A significance threshold of P <0.05 was adopted for all statistical evaluations.
Results
Statistically significant differences were observed in all groups, except group I.A, when comparing the intragroup values of the specimens treated with the colorization test across all time intervals ( P <0.001). The most significant color change in groups I.B, I.C, and I.D was observed at 14 days. In groups I.B and I.C, the 14-day period exhibited statistically significant variations relative to the preceding intervals; however, in group I.D, no significant difference was detected between the 7- and 14-day intervals ( Table II ). Table II presents intergroup comparisons across several time intervals, indicating statistically significant differences across the groups at all periods ( P <0.001). At 1-, 3-, 7-, and 14-day intervals, the most significant discoloration was noted in the red wine group, followed by the black tea, coffee, and distilled water groups, in that order. The red wine group demonstrated the most significant discoloration at every time interval, with ΔE00 values >30 by day 14, indicating a severe and clinically intolerable degree of color alteration.
Table II
Inter- and intragroup comparisons of the colorization test
| Group I | Group I.A | Group I.B | Group I.C | Group I.D | P value | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Mean ± SD | Median | Min-Max | 25%-75% | Mean ± SD | Median | Min-Max | 25%-75% | Mean ± SD | Median | Min-Max | 25%-75% | Mean ± SD | Median | Min-Max | 25%-75% | ||
| ΔE 00 1 | 1.94 ± 1.13 | 1.80 a | 0.72-5.37 | 1.35-2.09 | 11.6 ± 1.22 | 11.6 Ab | 9.33-13.7 | 11.1-12.2 | 8.56 ± 1.53 | 8.75 Ec | 6.73-12.0 | 7.22-9.45 | 32.7 ± 4.16 | 34.1 Hd | 24.8-39.3 | 30.6-35.0 | <0.001 MW |
| ΔE 00 3 | 2.73 ± 1.51 | 2.46 e | 0.64-5.99 | 1.57-3.92 | 14.0 ± 1.89 | 13.8 Bf | 9.51-17.0 | 13.3-14.9 | 9.55 ± 1.33 | 9.72 Eg | 7.12-12.5 | 8.72-10.2 | 39.2 ± 22.81 | 40.1 Ih | 31.3-42.9 | 381-40.8 | <0.001 MW |
| ΔE 00 7 | 2.15 ± 0.85 | 1.94 i | 0.95-4.42 | 1.73-2.33 | 16.7 ± 1.14 | 17.0 Cj | 13.4-18.3 | 16.4-17.1 | 12.9 ± 1.32 | 13.1 Fk | 10.7-15.5 | 12.3-13.4 | 43.7 ± 1.77 | 44.2 Jl | 40.8-46.2 | 42.7-44.7 | <0.001 MW |
| ΔE 00 14 | 1.92 ± 0.814 | 1.76 m | 1.08-4.17 | 1.37-2.14 | 19.9 ± 1.40 | 20.3 Dn | 17.6-22.9 | 19.0-20.5 | 15.2 ± 1.29 | 15.6 Go | 13.1-17.8 | 14.3-15.7 | 45.7 ± 2.39 | 45.6 Jp | 40.7-50.7 | 44.3-46.9 | <0.001 MW |
| P value | 0.335 MW | <0.001 MW | <0.001 MW | <0.001 MW | |||||||||||||
Stay updated, free dental videos. Join our Telegram channel
Leave a Reply
VIDEdental - Online dental courses
