In vitro study on the effect of Halloysite Nanotubes (HNTs) on flexural properties of 3D-printed dental resins after thermal aging

Purpose: The purpose is to evaluate the effect of adding Halloysite Nanotubes (HNTs) on the flexural strength (FS), elastic modulus, and hardness of 3D-printed denture base resins (DBRs). Materials and Methods: A total of 160 bar- and disk-shaped specimens were fabricated from 3D-printed resins (DentaBASE-ASIGA and Denture 3D+-NextDent), incorporating three concentrations of HNTs (0.3%-, 0.6%-, and 0.9%) in addition to one control group without HNTs addition. Specimens were designed to the required dimensions and printed according to the manufacturer's recommendations. The printed specimens were finished, polished, and then subjected to thermal cycling (5000 cycles). Flexural properties were measured using the 3-point bending fixture with a universal testing machine, and a Vickers hardness tester was used to assess the hardness. A scanning electron microscope (SEM) was used for fracture surface analysis and HNTs distribution. ANOVA and post hoc Tukey's test were used for data analysis (α = 0.05). Results: Adding HNTs to 3D-printed DBRs increased FS compared to the control group (p < 0.001). Between HNTs-modified groups, 0.6% and 0.9% groups showed a significant increase in FS compared with the 0.3% group, while no significant difference was observed between 0.6% and 0.9% HNTs (p > 0.05). The elastic modulus significantly increased by adding HNTs compared to the control group (p < 0.001), while no significant differences in the elastic modulus were observed between HNTs-modified groups (p > 0.05). SEM analysis revealed a ductile fracture mode for HNTs-modified groups. Compared to the control group, the hardness of 3D-printed resin increased with HNTs addition (p < 0.001). Up to 0.6% HNTs, a significant increase in hardness was reported, while 0.9% significantly decreased the hardness compared with 0.6% HNTs (p < 0.001). No significant differences were found between materials per concentration (p > 0.05) when comparing materials. Conclusion: The FS, elastic modulus, and hardness of 3D-printed resins increased with the addition of HNTs. Regarding HNTs concentrations, 0.3% and 0.6% positively impacted the tested properties and could be recommended as an alternative to pure 3D-printed resins after testing other properties and evaluating the performance of HNTs-3D-printed nanocomposites.