Water sorption, high volumetric shrinkage, polymerization stress, and potential estrogenic effects triggered by leached compounds are some of the major concerns related to BisGMA-TEGDMA co-monomer systems used in dental composites. These deficiencies call for the development of alternative organic matrices in order to maximize the clinical lifespan of resin composite dental restorations. This study proposes BisGMA-free systems based on the combination of UDMA and a newly synthesized diurethane dimethacrylate, and evaluates key mechanical and physical properties of the resulting materials.
2EMATE-BDI (2-hydroxy-1-ethyl methacrylate) was synthesized by the reaction between 2-hydroxy-1-ethyl methacrylate with a difunctional isocyanate (1.3-bis (1- isocyanato-1-methylethylbenzene) – BDI). The compound was copolymerized with UDMA (urethane dimethacrylate) at 40 and 60 wt%. UDMA copolymerizations with 40 and 60 wt% TEGDMA (triethylene glycol dimethacrylate) were tested as controls, as well as a formulation based in BisGMA (bisphenol A-glycidyl methacrylate)-TEGDMA 60:40% (BT). The organic matrices were made polymerizable by the addition of DMPA (2.2-dimethoxyphenoxy acetophenone) and DPI-PF6 (diphenyliodonium hexafluorophosphate) at 0.2 and 0.4 wt%, respectively. Formulations were tested as composite with the addition of 70 wt% inorganic content consisting of barium borosilicate glass (0.7 μm) and fumed silica mixed in 95 and 5 wt%, respectively. All photocuring procedures were carried out by a mercury arc lamp filtered to 320–500 nm at 800 mW/cm 2 . The experimental resin composites were tested for kinetics of polymerization and polymerization stress in real time. Flexural strength, elastic modulus, water sorption, and solubility were assessed according to ISO 4049 . Biofilm formation was analyzed after 24 h by luciferase assay. Data were statistically analyzed by one-way ANOVA and Tukey’s test ( α ≤ 0.05).
In general, the addition of 2EMATE-BDI into the formulations decreased the maximum rate of polymerization (RP MAX ), the degree of conversion at RP MAX (DC at RP MAX ), and the final degree of conversion (final DC). However, these reductions did not compromise mechanical properties, which were comparable to the BT controls, especially after 7-day water incubation. The incorporation of 60 wt% 2EMATE-BDI reduced water sorption of the composite. 2EMATE-BDI containing formulations showed reduction in polymerization stress of 30% and 50% in comparison to BT control and TEGDMA copolymerizations, respectively. Biofilm formation was similar among the tested groups.
The use of the newly synthesized diurethane dimethacrylate as co-monomer in dental resin composite formulations seems to be a promising option to develop polymers with low-shrinkage and potentially decreased water degradation.
The organic matrix of current dental resin composites is essentially composed of a base dimethacrylate monomer copolymerized with a low viscosity co-monomer. This association is crucial to allow the incorporation of high percentages of inorganic filler particles and to ensure high reactivity, reasonable mechanical properties, and proper clinical handling. Ever since the resin composites were developed in the 1960s, the most common co-monomer system has been based on BisGMA (bisphenol A-glycidyl methacrylate) and TEGDMA (triethylene glycol dimethacrylate) as base and co-monomer, respectively, with most recent developments including UDMA (urethane dimethacrylate) and BisEMA (ethoxylated bisphenol A methacrylate). The synergistic association of those two multifunctional monomers with different viscosities and chemical structures ensures a favorable balance in terms of reactivity-mobility , which is translated into a copolymer with satisfactory physicochemical properties. While there have been many developments and improvements in the filler systems used in dental composites, the development of alternative co-monomer systems that overcome the significant drawbacks of the traditional BisGMA-TEGDMA system has been limited.
The concerns about BisGMA are related to its high viscosity (1100 Pa s at room temperature) which limits double bond conversion and necessitates the incorporation of high concentrations of co-monomer to enhance conversion and filler particle loading . In addition, in recent years, the potential presence of BPA as an impurity or a degradation product from dental resins, especially sealants, has raised concerns around the use of BisGMA-containing formulations . It is true that alternative synthetic routes exist for the production of BisGMA which do not utilize BPA as a starting material , but it is difficult to know what manufacturers actually use in commercial products. Regardless of the source, studies have found the presence of BPA in patient’s urine and saliva after dental procedures, which is worrisome due to its capability for triggering estrogenic effects, especially in pediatric patients . TEGDMA, in turn, also presents major drawbacks associated with its higher hydrophilicity and polymerization shrinkage, susceptibility to cyclicization , and potential cytotoxic effects . UDMA is an alternative base monomer for dental resin composites that has lower viscosity (11 Pa s at room temperature) and lacks the BPA core. Though UDMA does not establish the same strong intermolecular hydrogen bonding as BisGMA, its flexibility contributes to more efficient crosslinking . Some commercial materials contain UDMA, but still in association with TEGDMA or other hydrophilic low molecular weight co-monomers.
In the past few years, researchers have worked on alternative co-monomer based on urethane methacrylates . The urethanes are especially interesting due to their properties, as discussed above, but mainly because their chemical structure can be easily tailored, making it possible to design and synthesize a wide variety of monomers with different physicochemical properties . However, the urethane methacrylates designed and tested presented some issues such as high viscosity and decrease in refractive index as the size of the side alkyl chain increases . One recent publication has introduced a newly synthesized diurethane dimethacryte, 2EMATE-BDI (2-hydroxy-1-ethyl methacrylate), which is characterized by a stiff molecular structure and shows high average molecular weight (560 g/mol), marked hydrophobicity (log P = 5.33), high reactivity, and low viscosity (averaged in 0.05 Pa s) , making this compound a promising alternative co-monomer for UDMA systems.
Finally, the degradation of BisGMA-based materials leads to the potential formation of by-products, such as BisHPP and methacrylic acid, some of which have been show to up-regulate the activity of biofilm-forming bacteria . Any alternative monomer proposed needs to also be evaluated to guarantee that it either reduces or at least does not potentiate biofilm formation. Therefore, the aim of this study was to produce BisGMA-free resin composite formulations based on the copolymerization of UDMA and 2EMATE-BDI as base and co-monomers, respectively, and test them for kinetics of polymerization, water sorption and solubility, polymerization stress, flexure strength and modulus, and biofilm formation. It was hypothesized that the incorporation of the newly synthesized diurethane into the dental resin composite systems would increase the hydrophobicity and decrease the polymerization stress, without impacting the mechanical properties and the biofilm formation.
Materials and methods
Formulation of the experimental resin composites
The organic matrix of the experimental composites contained UDMA as base monomer, combined with TEGDMA or the newly synthesized 2EMATE-BDI ( Fig. 1 ) at 60 or 40 wt%. The synthesis of the 2EMATE-BDI was performed by the reaction between 2-hydroxy-1-ethyl methacrylate with a difunctional isocyanate (1.3-bis(1-isocyanato-1-methylethylbenzene) – BDI), according to the procedures described previously . As an additional control group, 60 wt% of BisGMA was mixed with 40 wt% of TEGDMA. The photoinitiator system for each monomer mixture consisted of 0.2 wt% DMPA and 0.4 wt% DPI-PF6. Butylatedhydroxytoluene (BHT, in 0.1 wt%) was added as an inhibitor. The inorganic content (70 wt%) was composed of barium borosilicate glass 0.7 μm (Esstech, INC) and fumed silica (Aerosil OX50, Degussa) mixed at 95:5 wt%.
All photocuring procedures were carried out with a mercury arc lamp (Acticure, EXFO Acticure4000 UV Cure; Mississauga, Canada) filtered to 320–500 nm. The light source parameters and positioning was adjusted so that in all test configurations, the same irradiance (800 mW/cm 2 ) was being delivered perpendicularly to the specimen.
The kinetics of the polymerization reaction was assessed by near-IR spectroscopy in real time for 300 s with 2 scans per spectrum at 4 cm −1 and a data acquisition rate of 2 Hz. Samples consisted of disc-shaped specimens (10 mm in diameter × 0.8 mm in thickness) formed in rubber molds sandwiched between glass slides, irradiated continuously for 300 s during the test ( n = 3). The vinyl overtone peak for methacrylate found at 6165 cm −1 was used to calculate the degree conversion at every data point as well as the final conversion, according to Eq. (1) . The rate of polymerization was calculated as the first derivative of the degree of conversion versus time curve, and the degree of conversion at the maximum rate of polymerization was used as a proxy for the onset of vitrification:
Water sorption and solubility
The same samples used in the kinetics of polymerization assay were used to measure water sorption and solubility, according to the procedures described in ISO 4049 . In brief, the mass of the discs were measured before incubation ( m 1), after 1 week-immersion in Millipore water ( m 2), and until the mass stabilized during desiccation under house vacuum ( m 3). Water sorption (WS) and solubility (SL) were calculated using the following equations: