866 final The Impact of Thermocycling on the Physio- mechanical Proper6es of SLA and DLP 3D-Printed Dental Ceramics: A Compara6ve Study Vol 13 No 1 (2025) DOI 10.5195/d3000.2025.866 h#p://den*stry3000.pi#.edu The Impact of Thermocycling on the Physio- mechanical Properties of SLA and DLP 3D-Printed Dental Ceramics: A Comparative Study Nadia H Hasan College of Dentistry, University of Mosul, Mosul, Iraq Abstract Objec&ve: Three-dimensional (3D) prin4ng technology is highly promising for producing nanoceramic resin dental restora4ons. However, the effects of environmental stressors on the structural integrity and clinical performance of these restora4ons require further elucida4on. To inves4gate the effects of Stereolithography (SLA) and digital light processing (DLP) 3D prin4ng technologies on the physical- mechanical proper4es of a 3D-printed resin material used in dental applica4ons. Methods: A total of 120 resin specimens (Senertek P-Crown V2) were fabricated using SLA and DLP technologies. The microhardness, flexural strength, and surface roughness of the specimens were evaluated under control and thermocycling condi4ons to evaluate their long-term performance. To assess sta4s4cal significance a two independent sample t-tests (P < 0.05) were used to analysis the data. Results: SLA samples exhibited significantly higher microhardness (P = 0.001) and flexural strength than DLP samples, both in the control state and aQer thermocycling. AQer thermocycling, the microhardness of SLA samples increased, whereas that of DLP samples decreased. Surface roughness values increased significantly in both SLA and DLP samples aQer thermocycling, with SLA samples exhibi4ng higher roughness values. Conclusion: SLA-printed resin demonstrated superior microhardness and flexural strength compared to DLP-printed resin. However, its long-term durability is affected by immersion and thermocycling. This study highlights the impact of water sorp4on, polymeriza4on mechanisms, and surface morphology on material performance. Keywords: 3D prin4ng, dental materials, digital light processing, flexural strength, microhardness, stereolithography, surface roughness, thermocycling. Cita:on: Hasan NH. (2025) The Impact of Thermocycling on the Physio- mechanical Proper:es of SLA and DLP 3D- Printed Dental Ceramics: A Compara:ve Study. Den:stry 3000. 1:a001 doi:10.5195/d3000.2025.866 Received: February 27, 2025 Accepted: March 31, 2025 Published: April 21, 2025 Copyright: ©2025 Hasan NH. This is an open access ar:cle licensed under a Crea:ve Commons AWribu:on Work 4.0 United States License. Email: nadiahameed@uomosul.edu.iq. Introduction Stereolithography (SLA) and digital light processing (DLP) are 3D printing dental techniques requiring high resolution and accuracy. Despite their similarities in using volume-accelerated technology (VAT) to create accurate dental restorations, but differ in speed, resolution, and cost-effectiveness [1]. Ultraviolet light uses to solidify a resin in both SLA and DLP but different method [2,3]. A resistance to daily exposure to an intense condition of the oral environment, is a significant factor for the successful clinical use of a nano ceramic materials that impact it’s the morphological, physical, mechanical properties cause surface integrity degradation [4,5]. A higher microhardness enhances wear resistance, while excessive surface roughness can weaken flexural strength by creating stress concentration points [6,7], and an optimal balance through material selection and technique- processing is essential to improving ceramic durability, structural reliability [8]. The study aimed to compare the SLA and DLP methods with regards to their physical- mechanical properties and the influence of artificial aging. The The Impact of Thermocycling on the Physio- mechanical Proper6es of SLA and DLP 3D-Printed Dental Ceramics: A Compara6ve Study Vol 13 No 1 (2025) DOI 10.5195/d3000.2025.866 h#p://den*stry3000.pi#.edu null hypothesis stated that both 3D printing techniques showed no difference in such properties. Material and Methods The samples used in this study were divided as depicted in Figure 1. Ethical considerations All study participants provided informed consent, and the study design was performed solely on laboratory models; therefore, informed consent was not required confidentiality, and procedures for withdrawal from the research. Measures A total of 120 rectangular bars (25 × 5 × 1 mm) were prepared using the Senertek P-Crown V2 resin material (Libral Traders, India). To design the bar geometry, AutoCAD software was used and saved as a standard tessellation language (STL) file. The STL file was utilized in the Anycubic Photon Mono 2 and Anycubic Photon D2 3D printers (Lux Creo Inc., Chicago, IL, USA), which use SLA and DLP technologies, respectively, to produce 60 bars from each technique (Figure 1). The STL file was processed using a slicer program that converted it into G-code for machine readability [9]. The samples were constructed with resolutions as fine as 50 microns, ensuring precise detailing. 120 Bars Preperd By 3D-Additive Manufacturing Technologies Stereolith ography (SAL) n=60 Control Group n=30 Microhardness Test n=10 Fluxral Strength Test n=10 Surface Ruoghness Test n=10 Thermocycling Group n=30 Digital light processing (DLP) n=60 Figure 1. Schematic diagram of the study design. The Impact of Thermocycling on the Physio- mechanical Proper6es of SLA and DLP 3D-Printed Dental Ceramics: A Compara6ve Study Vol 13 No 1 (2025) DOI 10.5195/d3000.2025.866 h#p://den*stry3000.pi#.edu After printing, the samples were cured in an ultraviolet oven for 8 minutes in a clear jar filled with glycerin, to improve the mechanical properties and ensure complete solidification. Post- processing steps included sanding, painting, polishing, tumbling, high- pressure air cleaning, and coloring to make the 3D model ready for application [10]. In SLA, a perforable platform was positioned adjacent to the VAT containing liquid polymer resin. A laser beam was directed onto the platform, solidifying the first layer of resin. The platform was then incrementally lowered to form subsequent layers, repeating until the 3D object was fully constructed, taking approximately 38 minutes. SLA requires support structures to stabilize the model during printing. DLP is like SLA, with the primary distinction being the light source. DLP employs a conventional light source and digital micromirrors to print all the layers simultaneously. This approach allows faster printing compared to SLA, where the laser traces the cross-sectional area point by point. [11,12]. Sixty rectangular bars (30 from each technique) underwent thermocycling in a thermocycling unit (Dorsa apparatus, Tehran, Iran) from 5 °C to 55 °C for 1,000 cycles with a 30 second dwell time, to simulate approximately 1 year of intraoral use [13]. The microhardness, flexural strength, and surface roughness of the nanoceramic resin material were evaluated in 40 rectangular bars per test (20 bars from each 3D printing technique: 10 as control group and 10 after thermocycling). The microhardness of the nanoceramic resin bars was evaluated by subjecting the centre of each bar to three indentations using a diamond pyramid indenter. A load of 300 g was applied for 10 s to determine the Vickers Hardness Number in kg/mm² [13]. The mean values of these measurements were calculated using a DM 8/DM 2 microhardness tester (Yang Yi Technology Co., Ltd, Tainan City 70960, Taiwan) (Figure 2). Using a universal testing machine, flexural strength of the nanoceramic resin bars was assessed. On two supporting bearers each bar was placed with its intaglio surface facing downward, following ISO standards [14]. A 1.6 mm spherical-tipped indenter applied force centrally on a 5-mm wide face, perpendicular to the bar’s longitudinal axis. The load required to fracture each bar was recorded to the nearest 0.1 N, with a force application rate not exceeding 0.5 N/s (Figure 3). a b Figure 2: (a) Vickers micro-indentation testing machine, (b) image of indentation. a b b Figure 3: (a) schematic diagram of the three-point flexural strength test apparatus, (b) image of universal testing machine and testing bar. The Impact of Thermocycling on the Physio- mechanical Proper6es of SLA and DLP 3D-Printed Dental Ceramics: A Compara6ve Study Vol 13 No 1 (2025) DOI 10.5195/d3000.2025.866 h#p://den*stry3000.pi#.edu Tests were conducted in a dry environment, and flexural strength was calculated using the standard equation [14,15]. σ = 3 FL/2 wd2 N/mm2 or MPa Where σ (sigma) represents stress in MPa; F, the applied force (N); L, length or distance over which the force is applied (mm); w, width of the element affected by the stress (mm); and d, depth or thickness of the element (mm). Using the touch approach of an atomic force microscope (FlexAFM, Nanosurf AG, Liestal, Switzerland ) , an intaglio surfaces of the nanoceramic resin bars were analysed for surface topography. The three central readings per bar were recorded, and surface roughness value (Sa) was calculated. A 3D image covering an area of 20,000 × 20,000 nm was captured (Figure 4). Statistical analysis For statistical analysis of data distribution, the microhardness, flexural strength, and surface roughness values were statistically analyzed using SPSS Statistics 27.0 (IBM Corp., Armonk, NY, USA). Parametric independent sample t-tests were used to compare the effect of each variable and determine statistical differences between groups, with 95% confidence level and P ≤ 0.05 considered statistically significant. Results Participant characteristics To determine the sample size a software (G*Power) used, at α = 0.05 and 1-β = 0.80, with an expected effect size of 0.3. Descriptive statistics The impact of thermocycling on the physical-mechanical Figure 4. Roughness parameters. The Impact of Thermocycling on the Physio- mechanical Proper6es of SLA and DLP 3D-Printed Dental Ceramics: A Compara6ve Study Vol 13 No 1 (2025) DOI 10.5195/d3000.2025.866 h#p://den*stry3000.pi#.edu properties of 3D-printed Senertek P-Crown V2 resin material was determined. Tables 1 and 2 present the descriptive statistics, including mean, standard deviation, and campers’ mean, of the microhardness, flexural strength, and surface roughness of 3D-printed resin samples to assess the impact of control and thermocycling states on SLA and DLP technologies and vice versa, respectively. Two independents sample t-tests (P < 0.05) were conducted to assess the significance of the results. Correlation among variables As shown in Table 1, after thermocycling, the microhardness values were higher in SLA samples (0.71153 kg/mm²) compared to DLP samples, whereas DLP samples had significantly higher values in the control state (0.32602 kg/mm²). Higher mean flexural strength values (24.5080 MPa, 5.3940 MPa) were obtained for SLA samples compared to DLP samples, respectively, in both the control state and after thermocycling. Finally, the highest mean value for surface roughness (Sa value) was obtained for SLA samples that underwent thermocycling. DLP, digital light processing; SLA, stereolithography TEST N Mean Std. Deviation Std. Error Mean T-test P value Flexural Strength value DLP Control 10 19.7040 0.95789 0.30291 0.298 0.772 Thermocycling 10 20.1170 4.27474 1.35179 SLA Control 10 24.5080 1.04583 0.33072 1.258 0.225 Thermocycling 10 25.3940 1.96681 0.62196 Microhardness value DLP Control 10 18.6250 1.03098 0.32602 3.463 0.004** Thermocycling 10 16.3200 1.83473 0.58019 SLA Control 10 16.9720 0.73976 0.23393 2.351 0.039* Thermocycling 10 18.7330 2.25007 0.71153 Surface Roughness Value DLP Control 10 4.8087 0.66203 0.20935 3.974 0.002** Thermocycling 10 7.1051 1.71642 0.54278 SLA Control 10 5.3585 0.70850 0.22405 9.148 0.000** Thermocycling 10 22.5296 5.89325 1.86361 Table 1. Descriptive statistics and camper mean of flexural strength, microhardness, and surface roughness values for both stereolithography and digital light processing 3D printing technologies. The Impact of Thermocycling on the Physio- mechanical Proper6es of SLA and DLP 3D-Printed Dental Ceramics: A Compara6ve Study Vol 13 No 1 (2025) DOI 10.5195/d3000.2025.866 h#p://den*stry3000.pi#.edu Additionally, in shown in Table 2, among DLP samples, higher microhardness values were observed in the control state (18.6250 kg/mm²), whereas the SLA group showed higher values after thermocycling (18.7330 kg/mm²). No significant differences in flexural strength were observed between the control and thermocycling groups for both SLA and DLP technologies. Finally, the increase in Sa values was more pronounced after thermocycling than in the control state for both DLP and SLA technologies (5.3585 µm and 22.5296 µm, respectively). DLP, digital light processing; SLA, stereolithography Discussion In this in vitro comparative study, we investigated the effects of SLA and DLP 3D printing on the physical- mechanical properties of a 3D- printed resin material used in dental applications. In clinical dentistry, SLA and DLP have become pivotal for fabricating indirect restorations, offering efficiency and material conservation. Both processes enable the production of intricate designs with minimal material waste compared to traditional subtractive manufacturing techniques, yet each exhibits distinct advantages and TEST N Mean Std. Deviation Std. Error Mean T-test P value Flexural strength value Control DLP 10 19.7040 0.95789 0.30291 10.712 0.000** SLA 10 24.5080 1.04583 0.33072 Thermocycling DLP 10 20.1170 4.27474 1.35179 3.546 0.002** SLA 10 25.3940 1.96681 0.62196 Microhardness value Control DLP 10 18.6250 1.03098 0.32602 4.119 0.001* SLA 10 16.9720 0.73976 0.23393 Thermocycling DLP 10 16.3200 1.83473 0.58019 2.628 0.017* SLA 10 18.7330 2.25007 0.71153 Surface roughness value Control DLP 10 4.8089 0.66178 0.20927 1.793 0.09 SLA 10 5.3585 0.70850 0.22405 Thermocycling DLP 10 7.1051 1.71642 0.54278 7.947 0.000** SLA 10 22.5296 5.89325 1.86361 Table 2. Descriptive statistics and camper mean of flexural strength, microhardness, and Sa surface roughness values for each 3D printing technologies in control and thermocycling states. The Impact of Thermocycling on the Physio- mechanical Proper6es of SLA and DLP 3D-Printed Dental Ceramics: A Compara6ve Study Vol 13 No 1 (2025) DOI 10.5195/d3000.2025.866 h#p://den*stry3000.pi#.edu limitations depending on the application [16,17]. For instance, challenges such as weak interlayer adhesion and anisotropic material behavior underscore the importance of understanding the nuances of each technology. Choosing between SLA and DLP requires not only technical expertise but also the guidance of knowledgeable partners to optimize production outcomes and achieve high-quality results [18]. During daily use within oral environment, restorations are subjected to thermal stresses and evaluating the effects of 3D-printed resins in simulated oral conditions are important studies to assess the clinical performance of these newly introduced technologies. Bardia et al. [12], reported that when a resin restoration subjecting a to 1,000 cycles of a thermal variation to simulates 1 year of clinical dental use. A plasticizer effect of water sorption of resin affects its mechanical properties. Moreover, increased temperature accelerates water sorption, thereby interfering with its mechanical properties [19]. Salivary pH usually ranges from 6.8 to 7.2; however, dietary acids can cause a drop in salivary pH . A dietary acids hydroxyl ion may diffuse through saliva water molecules, react with resin ceramic oxygen and caused ceramic degradation [5]. Surface hardness is an indicator of a material’s abrasion resistance and surface strength. Restorations with low surface hardness are more prone to scratches, damage to the resin surface, and dimensional changes during mechanical dental brushing or chewing hard foods [20]. In our study, DLP samples exhibited low hardness values after thermocycling, probably due to water sorption and alterations in the composition of the material and printing layers under thermal stress. In contrast, the SLA group exhibited a significant increase in hardness with aging compared to the bulkier DLP group, which may be attributed to its layering technique and enhanced polymerization rate, depth of polymerization, and degree of conversion. The higher flexural strength of the SLA group compared to that of the DLP group indicates its greater resistance to surface deformation and higher flexibility, which may be explained by the differences in the curing mechanisms of DLP and SLA, coupled with differences in surface morphologies [21]. Additionally, a non-significant positive effect in flexural strength after artificial aging was observed for both the DLP and SLA groups, possibly owing to reduced water sorption, which alters a material’s characteristics by creating internal tensions that are damaging to a resin’s long-term usefulness [22]. Reducing water sorption can prevent breaches and cracks and enhance the longevity of dental restorations. Moreover, remnant monomers in the resin have been linked to dimensional fragility of restorations and may adversely affect oral tissues [23]. Surface roughness is one of the most important properties affecting the longevity of dental restorations. Rougher surfaces increase microbial adhesion, leading to dental stomatitis, and promote surface staining and discoloration, which can compromise dental aesthetics [18]. The surface topography of additively manufacturing materials requires further investigation, as the layering technology impacts the The Impact of Thermocycling on the Physio- mechanical Proper6es of SLA and DLP 3D-Printed Dental Ceramics: A Compara6ve Study Vol 13 No 1 (2025) DOI 10.5195/d3000.2025.866 h#p://den*stry3000.pi#.edu porosity and morphological features of printed objects [24]. Significant difference in surface roughness existed between the control and thermocycling groups for both the DLP and SLA technologies. According to Al-Dulaijan et al., this difference may be attributable to the deterioration that occurs in the organic matrix or at the matrix-filler interface of dental resins during thermocycling and to the layering technology, which involves multiple steps between adjacent layers[25]. Therefore, the null hypothesis, which posited no differences in microhardness, flexural strength, and surface roughness between SLA- and DLP-printed nanoceramic resin bars even after artificial aging, was rejected. Conclusion To conclude, SLA printed resins are preferable for strength, but their roughness value increases with time and their long-term performance needs further study focusing on reducing water sorption and improving resin stability. Conflicts of interest The author declares no competing interest. References 1. Mechanical and biocompapbility properpes of 3D-printed dental resin reinforced with glass silica and zirconia nanoparpcles: In vitro study. Alshamrani A, Alhotan A, Kelly E, Ellakwa A Polymers 2023; 15:2523. 2. An overview of modern metal additive manufacturing technology. Armstrong M, Mehrabi H, Naveed N. J Manuf Process 2022; 84:1001- 1029. 3. Additive manufacturing by digital light processing: a review. 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