Comparison of three dental resins for 3D printing of orthodontic appliances: Comparison of leaching, biocompatibility, and thermo-mechanical properties after post-curing and aging

Photo-polymerizable resins can be used to print clear orthodontic aligners with several advantages compared to thermoformed ones. However, the safety of these new printed devices, their efficacy, and the impact of aging on safety and performance are still insufficiently known. This study aims to evaluate several 3D printing resins used in dentistry to produce clear gutters, regarding biocompatibility, leached compounds (release kinetics), surface state, and thermo-mechanical properties before and after simulated intra-oral aging. The impact of post-curing time on their behavior was also evaluated. Three resins - NextDent Ortho Flex, Freeprint Ortho Detax, and Graphy TC-85DAC - were tested using a standardized approach that included the same printer and a standard post-polymerization process. Chemical modifications were analyzed by ATR-FTIR, surface morphology by SEM, thermal properties by TGA and DSC, and cytotoxicity by MTT tests. Mechanical properties were characterized through tensile tests, and substance release in a salivary simulant based on PBS was monitored by HPLC. Aging in simulated saliva was followed for 15 days, corresponding to the duration of use of one aligner. The results showed good biocompatibility for the printed materials, except for the insufficiently post-cured materials. In addition, significant differences in release kinetics and physical properties were observed as a function of the resin used and the post-polymerization time. Intra-oral aging impacts all the properties of the materials tested, particularly the thermo-mechanical (maximum force and T g ) and surface properties, and leads to a release of residual resin and degradation products into the simulant which increases cytotoxicity. This study highlights the importance of strictly standardizing post-polymerization protocols to ensure the safety and performance of 3D-printed dental devices and underlines the risk of the cumulative effect of leached compounds for the patient’s health, even if measured cell viability and photo-initiator exposure dose were within acceptable limits. Moreover, whatever the resin, aging leads to a decrease in thermo-mechanical properties that may affect the device's performance. • Post-curing time affects leachables, cytotoxicity, and mechanical properties. • Cytotoxicity increases with aging. • Residual photo-initiator concentrations are below the critical threshold. • Nature of the leachable impacts biocompatibility more than the amount released.