The significant rise in the use of clear aligners for orthodontic treatment is attributed to their aesthetic appeal, enhancing patient appearance and self-confidence. The aim of this study is to evaluate the aligners’ response to common staining agents (coffee, black tea, Coca-Cola, and Red Bull) in color and chemical stability. Polyurethane-based thermoformed and 3D-printed aligners from four brands were exposed to common beverages to assess color change using a VITA Easyshade compact colorimeter after 24 h, 48 h, 72 h, and 7 days, as well as chemical stability using ATR-FTIR spectroscopy. The brand, beverage, and manufacturing method significantly influence color stability. ATR-FTIR analysis revealed compositional differences, with variations in response to beverage exposure affecting the integrity of polymer bonds. Color change analysis showed coffee as the most potent staining agent, particularly affecting Tera Harz TC85 aligners, while ClearCorrect aligners exhibited the least susceptibility. 3D-printed aligners showed a greater color change compared to thermoformed ones. Aligners with a PETG outer layer are more resistant to stains and chemical alterations than those made of polyurethane. Additionally, 3D-printed polyurethane aligners stain more than thermoformed ones. Therefore, PETG-layered aligners are a more reliable choice for maintaining the aesthetic integrity of aligners.

Vat photopolymerization 3D printing, including stereolithography (SLA), digital light processing (DLP), and masked SLA (mSLA), has transformed dental device fabrication by enabling precise and customizable components. However, the rheological behavior of photopolymer resins is a critical factor that governs the printability, accuracy, and performance of printed parts. This review surveys the role of viscosity, shear-thinning, and thixotropy in defining the “printability window” of dental resins and explores the relationship between these properties and the formulation and final material performance. Rheological characterization using rotational rheometry provides key insights, with shear rate sweeps and thixotropy tests quantifying whether a resin behaves as Newtonian or pseudoplastic. The literature shows that optimal printability typically requires resins with low to moderate viscosity at shear, moderate thixotropy for stability, and formulations balanced between high-strength oligomers and low-viscosity diluents. The addition of fillers modifies the viscosity and dispersion, which can improve reinforcement but may reduce print resolution if not optimized. Thermal and optical considerations are also coupled with rheology, affecting the curing depth and accuracy. In conclusion, controlling resin rheology is essential for bridging material formulation with reliable clinical outcomes, guiding both resin design and printer process optimization in modern dental applications.

The surface similarity phenomenon describes how users apply maintenance habits from one material to another based on perceived similarities. Cleaning protocols for thermoformed aligners are often used for three-dimensional (3D) printed aligners, despite their different material properties. This study aimed to evaluate how such practices impact the physical and aesthetic properties of 3D-printed aligners. A total of 84 polyurethane-based specimens were fabricated using direct light processing. They were divided into three groups: brushing only, brushing with toothpaste, and immersion in an alkaline solution (Corega). Each protocol simulated two weeks of daily cleaning. Surface roughness was assessed via profilometry and AFM, hardness via the Ball Indentation Method, and color change via VITA Easyshade before and after cleaning. Water absorption was measured daily over 14 days during immersion in either Corega or saline. Statistical analyses included Wilcoxon signed-rank tests, paired t-tests, one-way ANOVA with LSD post-hoc tests, and Kruskal-Wallis tests with post-hoc Dunn test. Brushing with toothpaste caused the greatest increase in surface roughness (ΔSa = 37.45 nm), followed by brushing alone (ΔSa = 10.55 nm); Corega slightly reduced roughness (ΔSa = -4.86 nm). Surface hardness increased after brushing but decreased with Corega (p < 0.05). No significant ΔE color differences were found, though “b” values decreased with brushing (p = 0.012). Water absorption stabilized at 8.47% in saline, while Corega immersion led to peak absorption on day 8 before degradation (p < 0.001). Cleaning methods designed for thermoformed aligners can alter surface properties and degrade 3D-printed aligners. Tailored cleaning protocols are essential to preserve their integrity and functionality.

Aim: Polyurethane-based aligners, created through photoinitiated free-radical polymerization, have been the subject of numerous studies focusing solely on their mechanical properties. In contrast, we investigate their thermomechanical properties, which are crucial for their efficacy. This paper aims to investigate the effects of different UV light exposure durations on the complex modulus of elasticity, tan delta, glass transition temperature, and the degree of conversion (DC). Methods: Aligners were printed using Tera Harz TC-85 and NextDent Ortho Flex resin with specific exposure times (2, 2.4, 3, 4, and 4.5 s for Tera Harz; 5, 6, 7, and 8 s for NextDent) and processed per manufacturer guidelines. The degree of conversion was analyzed using Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy, while Dynamic Mechanical Analysis (DMA) characterized the mechanical properties (complex modulus and tan delta) and the glass transition. Results: Tera Harz TC-85 showed a higher degree of conversion (90.29–94.54%), suggesting fewer residual monomers, which is potentially healthier for patients. However, its lower glass transition temperature (35.60–38.74 °C) might cause it to become rubbery in the mouth. NextDent Orto Flex, with a higher storage modulus (641.85–794.55 MPa) and Tg (49.36–50.98 °C), offers greater rigidity and stability at higher temperatures (greater than temperature in the oral cavity), ideal for orthodontic forces, though its lower degree of conversion raises health concerns. Conclusions: Tera Harz TC 85 generally achieves higher DC and more stable polymerization across different UV exposure times than NextDent Orto Flex. Optimal polymerization times significantly impact both the mechanical and thermal properties of these dental resins, with NextDent showing optimal properties at 7 s and Tera Harz benefiting from both very short and extended exposure times.

Per-layer UV exposure during 3D printing may influence polymerization and the biological response of photocurable aligner resins. This study evaluated exposure-dependent cytotoxicity of Tera Harz TC-85 and NextDent Ortho Flex and performed an exploratory screening of selected resin-specific printing conditions for ER-selective estrogen-like responses. TC-85 was printed at 2.0, 2.4, 3.0, and 4.0 s per layer, and Ortho Flex at 5.0, 6.0, 7.0, and 8.0 s per layer. The resin-specific per-layer UV exposure ranges were selected based on our previous physicochemical characterization of the same materials and printing system. Extracts were prepared according to ISO 10993-12. Cytotoxicity was assessed in human gingival fibroblasts using the MTT assay, while selected conditions (TC-85, 2.4 s; Ortho Flex, 7.0 s) were evaluated in estrogen receptor (ER)-positive MCF-7 and ER-negative MDA-MB-231 cells as an exploratory screening assay rather than a direct test of ER activation. For both resins, per-layer exposure condition, extract concentration, and their interaction significantly affected the normalized MTT response. At 50% extract concentration, normalized MTT responses ranged from 89.2% to 111.3%, whereas Ortho Flex ranged from 8.8% to 76.8%, with the 8.0 s condition being the only Ortho Flex condition above the 70% cytotoxicity threshold. Ortho Flex showed pronounced cytotoxicity in concentrated extracts, particularly at shorter exposure times, whereas TC-85 showed a less monotonic exposure-dependent pattern. In the exploratory screening, the 6.25% Ortho Flex extract produced a normalized MCF-7 MTT response of 129.1%, but this was comparable with the corresponding saline vehicle response of 137.0% and was not statistically greater than the response in ER-negative MDA-MB-231 cells. No statistically supported ER-selective response was detected for either tested resin under the specific screening conditions; however, exposure-time dependence of this response was not evaluated. Per-layer UV exposure therefore influenced the HGF-1 MTT response in a resin- and concentration-dependent manner. For dental laboratory practice, these findings support the use of resin-specific, validated printing parameters rather than transferring exposure settings between materials; deviations from manufacturer-specified processing conditions should be independently validated before clinical use.