Properties of Bis-GMA free bulk-filled resin composite based on high refractive index monomer Bis-EFMA

With the aim to prepare Bis-GMA-free bulk-filled dental resin composite (DRC), Bis-GMA-free resin matrix was prepared by mixing Bis-EFMA with TEGDMA at two mass ratios (Bis-EFMA/TEGDMA = 50 wt/50 wt and 60 wt/40 wt), and the bulk-filled resin composites were then obtained by mixing resin matrix with silanated glass fillers at a mass ratio of 30 wt/70 wt. Bis-GMA based resin composites were used as control. Refractive indexes of resin matrixes were measured. Besides the depth of cure mentioned in ISO standard, double bond conversion (DC) and bottom/top Vickers hardness (VHN) ratio of resin composites were investigated to evaluate the curing depth. Physicochemical properties, such as flexural properties, volumetric shrinkage (VS), shrinkage stress (SS), water sorption (WS) and solubility (SL), and cytotoxicity of resin composites were tested and statistically analyzed (ANOVA, Tukey's, p = 0.05). The results showed that Bis-EFMA/TEGDMA resin matrixes had higher refractive indexes than Bis-GMA/TEGDMA resin matrixes. Viscosities of Bis-EFMA based DRCs were higher than Bis-GMA based DRCs. Bis-EFMA-based (50/50) DRC had comparable depth of cure, DC, and VHN as Bis-GMA-based (50/50) DRC (p > 0.05). Though Bis-EFMA/TEGDMA (60/40) had the highest refractive index in all resin matrix, the corresponding DRCs had the lowest depth of cure, DC, and bottom/top VHN ratio in all groups (p < 0.05). Replacing Bis-GMA with Bis-EFMA had no negative effect on flexural properties, WS and SL of DRCs, and could reduce VS and SS of DRCs. Results of CCK8 assay showed that all of DRCs had the same cytotoxicity (p > 0.05), and the thickness of sample had no influence on the cytotoxicity (p > 0.05). All the results indicated that Bis-EFMA could be used to replace Bis-GMA to prepare bulk-filled dental resin composites. According to the results of depth of cure, DC, and bottom/top VHN ratio, 50 wt/50 wt was more appropriate than 60 wt/40 wt as the mass ratio of Bis-EFMA and TEGDMA in the resin matrix for bulk-filled dental resin composites.