Cytotoxic, Elastic-Plastic and Viscoelastic Behavior of Aged, Modern Resin-Based Dental Composites

Comparative analysis of conversion and crosslinking degree in bulk-fill, monochromatic, and ORMOCER composites

Bulk-fill, monochromatic, and ORMOCER composites were introduced in restorative dentistry with the aim of reducing clinical time and/or alleviating contraction stresses at the interface between the tooth and restoration. While the conversion and immediate properties of these materials are comparable to conventional composites, studies evaluating their long-term properties and the structure of the polymer matrix are lacking. The objective of this study was to evaluate the degree of conversion and, indirectly, the crosslink density of conventional, bulk-fill, monochromatic, and ORMOCER resin composites. The degree of conversion was measured using Fourier-transform infrared spectroscopy (FTIR). Flexural strength, elastic modulus, and hardness were measured both dry (24 h) and after ethanol storage (24 h) using three-point bending and Knoop microhardness testing, respectively. Sorption and solubility were determined by changes in mass before and after immersion in water and drying. Data were analyzed using one-way or two-way ANOVA (material or material/storage conditions). The degree of conversion was statistically similar across all materials. For composites from the same manufacturer, no significant differences were observed in the reduction of mechanical properties after ethanol storage between bulk-fill and monochromatic composites, although differences were noted for the ORMOCER material. Sorption and solubility varied among most composites from the same manufacturer, suggesting subtle differences in composition. Therefore, it can be concluded that, for composites from the same manufacturer, the degree of crosslinking was similar between bulk-fill and monochromatic composites, indicating comparable polymeric structures. The same was not observed for the ORMOCER material, although it exhibits properties similar to other conventional composites.

Effects of testing speed on the tensile and mode I fracture behavior of specimens printed through the Fused Deposition Modeling technique

Additive Manufacturing (AM) processes are known as revolutionary manufacturing processes that fabricate a part using a 3D model layer upon layer. These techniques gained more attention from various industries due to their advantages like low waste material. Also, these processes can produce any part with high degrees of complexity in a short period of time. The Fused Deposition Modeling (FDM) process is a material extrusion-based technique which works by extruding a fine molten polymeric filament through a heated nozzle on the heated platform named printer bed. In this method, some important manufacturing parameters play a crucial role in controlling the mechanical properties and quality of the final fabricated part. However, all printed specimens through the FDM process should be tested based on the standards under some critical circumstances. Thus, in the current research paper, five and three test speeds are considered in tensile and fracture testing procedures, respectively to evaluate how these speeds can affect the mechanical and mode I fracture properties. Also, as the FDM specimens present elastic-plastic behavior, the critical value of J-integral is assumed as a fracture assessment and calculated from the finite element analysis. Among the mechanical properties, ultimate tensile strength is affected significantly by the test speed. For instance, the ultimate tensile strength of FDM specimens is 39.02, 38.58, 42.33, 48.09, and 52.11 for test speeds of 2, 4, 6, 8, and 10 mm/min, respectively. But vice-versa results are detected for the mode I fracture behavior and corresponding values of J for the FDM-PLA specimens. Finally, experimental and numerical results together with comprehensive discussions about the considered speeds and obtained results are reported.

Shade, Aging and Spatial-Dependent Variation of Elastoplastic and Viscoelastic Characteristics in a Dental, Submicron Hybrid CAD/CAM Composite

This article reports the elastoplastic and viscoelastic response of an industrially cured CAD/CAM resin-based composite (Brilliant Crios, Coltene) at different scales, spatial locations, aging conditions, and shading. Mechanical tests were performed at the macroscopic scale to investigate material strength, elastic modulus, fracture mechanisms and reliability. An instrumented indentation test (IIT) was performed at the microscopic level in a quasi-static mode to assess the elastic and plastic deformation upon indentation, either by mapping transverse areas of the CAD/CAM block or at randomly selected locations. A dynamic-mechanical analysis was then carried out, in which chewing-relevant frequencies were included (0.5 to 5 Hz). Characteristics measured at the nano- and micro-scale were more discriminative in identifying the impact of variables as those measured at macro scale. Anisotropy as a function of the spatial location was identified in all shades, with gradual variation in properties from the center of the block to peripheral locations. Depending on the scale of observation, differences in shade and translucency are very small or not statistically significant. The aging effect is classified as low, but measurable on all scales, with the same pattern of variation occurring in all shades. Aging affects plastic deformation more than elastic deformation and affects elastic deformation more than viscous deformation.