3D Printed Denture Materials: Key Aspects of their Physico-Mechanical and Biological Properties

Abstract

Problem statement: The advent of 3D-printing technology has brought a revolution in the field of dentistry, offering a quick, reliable, and economical method for fabricating various dental appliances, including denture bases. Nevertheless, the mechanical and physical properties of 3D-printed denture base materials tend to be relatively poor compared to conventional denture base materials. On the other hand, the printing parameters such as printing orientation, layer thickness, light wavelength, curing time etc. can affect the properties of the printed resin. Furthermore, the denture base materials are prone to attract micro-organism resulting in oral diseases like denture stomatitis. To counter this, metal or metal oxide nanoparticles were incorporated into the denture base materials to create an innovative nanocomposite material that displays enhanced physical, mechanical and biological characteristics. Aim: This research aimed to optimise the printing parameters, and to develop a novel denture base nanocomposite material by infusing varying concentrations of TiO2 nanoparticles with 3D-printed denture base resin. The study also intended to evaluate their mechanical, physical, and biological properties, and to investigate the effect of hydrolytic ageing in artificial saliva on the mechanical properties. Methods: Initially, various printing parameters were examined to ascertain the ideal print settings encompassing different printing orientations (0°, 45°, 90°) and different curing times (20, 30, 50 min) in relation to the unmodified 3D-printed denture base resin (NextDent). Following this, the optimal printing parameters were applied to evaluate the physical and mechanical properties of the unmodified 3D-printed denture base resin in terms of degree of polymerisation, sorption, solubility, flexural strength, flexural modulus, Vickers hardness, Martens hardness, and impact strength to compare the 3D-printed resins (NextDent® and Formlabs®) with conventional heat-cured resin (Schottlander®) as a control. The specimens were subsequently exposed to hydrolytic ageing in artificial saliva for a three-month duration. After this period, the mechanical properties were re-examined to assess the effect of ageing. Next, the 3D-printed resin (NextDent®) was reinforced with different concentrations (0.10, 0.25, 0.50, and 0.75 wt.%) of silanated TiO2 nanoparticles. The resultant nanocomposite materials were then characterised in terms of degree of conversion, sorption/solubility, Vickers hardness, Martens hardness, flexural strength/modulus, impact strength, antifungal properties, cytotoxicity, and biocompatibility. These properties were compared with the unmodified 3D-printed resin (NextDent®) and conventional heat-cured resin (Schottlander®) materials as controls. Following exposure to artificial saliva ageing, the nanocomposites were re-evaluated in terms of the mechanical properties. The fractured surface was also analysed using an optical and scanning electron microscopes. Results: The results showed that 90° printing orientation produced significantly higher values of flexural strength, Vickers hardness, and water sorption compared to a 0° orientation (p <0.05). However, alterations in the post-curing time did not result in significant variations in the properties tested (p>0.05). Notwithstanding, a post-curing time of 30 minutes yielded marginally improved characteristics compared to 20 min, with no detectable difference observed with 50 min. After establishing the optimal printing parameters and using them to compare 3D-printed materials with conventional heat-cured resin, the data demonstrated that 3D-printed resins had significantly higher sorption values than the control (p<0.05). Higher solubility was also evident, although the difference was not statistically significant (p>0.05). There were no significant differences in Vickers and Martens hardness, or impact strength, among the materials tested. However, the 3D-printed materials exhibited significantly greater flexural strength values (88-94 MPa) compared to the heat-cured material (73 MPa) (p<0.05). The degree of conversion of 3D-printed resins were lower than that of the control group, but this variation was not statistically significant (p>0.05). The 3D-printed materials also contained a significantly higher filler content compared to the control (p<0.05). Additionally, three months of artificial saliva ageing had a significant effect on the Vickers hardness for all groups tested, and the Martens hardness of the heat-cured control group only (p<0.05). However, it did not significantly affect the mechanical properties (p>0.05). The integration of TiO2 nanoparticles (NPs) into 3D-printed resin significantly improved flexural strength/modulus, impact strength, Vickers hardness, and DC (p<0.05), and marginally enhanced Martens hardness (p>0.05) compared to the unmodified resin. Sorption values showed no significant enhancements (p>0.05), while solubility reduced significantly (p<0.05). The addition of 0.10 wt.% NPs yielded the best performance among the different concentrations of the nanocomposite groups. Although ageing slightly diminished the materials' performance, the overall trend remained the same. Scanning electron microscope images revealed a homogeneous dispersion of the nanoparticles at lower concentrations (0.10 and 0.25 wt.%), but disclosed agglomeration at higher concentrations (0.50 and 0.75 wt.%). As for the biological properties, 3D-printed denture base resin reinforced with TiO2 NPs up to a concentration of 0.5 wt.% demonstrated statistically significant anti-fungal properties against Candida albicans (p<0.05), without any cytotoxic effect on human gingival fibroblasts. Nevertheless, a concentration of 0.75 wt.% exhibited a minor, and statistically insignificant reduction (p>0.05) in the number of fungal colonies. The optimal concentration of TiO2 NPs for achieving a balance between anti-fungal properties and compatibility was found to be 0.25 wt.%. Significance: 3D-printed unmodified resins for denture base have shown to be a viable substitute for traditional heat-cured materials in prosthetic dentistry, as they exhibited mechanical, physical, and surface properties that meet the established standards. The newly created nanocomposite material demonstrated further improved mechanical, physical, and also antimicrobial properties, effectively limiting fungal growth and thus reducing the incidence of denture stomatitis.