Alternative monomer for BisGMA-free resin composites formulations

Abstract

Objective:

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.

Methods:

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 wt% 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 wt% and 5 wt%, respectively. All photocuring procedures were carried out by a mercury arc lamp filtered to 320–500 nm at 800 mW/cm². 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 analysed after 24h by Luciferase Assay. Data were statistically analyzed by one-way ANOVA and Tukey’s test (α ≤ 0.05).

Results:

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.

Conclusion:

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.

1. Introduction

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 1960’s, 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 [1], 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 [2]. 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 [3–6]. It is true that alternative synthetic routes exist for the production of BisGMA which do not utilize BPA as a starting material [7], 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 [3–6]. TEGDMA, in turn, also presents major drawbacks associated with its higher hydrophilicity and polymerization shrinkage, susceptibility to cyclicization [8], and potential cytotoxic effects [9]. 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 [10]. 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 [11–14]. 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 [14]. However, the urethane methacrylates designed and tested presented some issues such as high viscosity [11, 14] and decrease in refractive index as the size of the side alkyl chain increases [12]. 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 (LogP = 5.33), high reactivity, and low viscosity (averaged in 0.05 Pa.s) [15], 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 [16]. 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 comonomers, 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.

2. Materials and Methods

2.1. 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 (Figure 1) at 60 wt% 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- socyanato-1-methylethylbenzene) – BDI), according to the procedures described previously [15]. 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%.

Figure 1.

Base (bisphenol A-glycidyl methacrylate (BisGMA) and urethane dimethacrylate (UDMA)) and co-monomer (triethylene glycol dimethacrylate (TEGDMA) and 2-hydroxy-1-ethyl methacrylate (2EMATE-BDI)) monomers tested.

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/cm²) was being delivered perpendicularly to the specimen.

2.2. Polymerization Kinetics

The kinetics of the polymerization reaction was assessed by near-IR spectroscopy in real time for 300s 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 300s 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 the equation (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.

$$DC(\%)=\left(1-\frac{6165c{m}^{-1}\mathit{\text{cured}}}{6165c{m}^{-1}\mathit{\text{uncured}}}\right)\times 100$$ Equation (1):

2.3. 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 [17]. In brief, the mass of the discs were measured before incubation (m1), after 1 week-immersion in Millipore water (m2), and until the mass stabilized during desiccation under house vacuum (m3). Water sorption (WS) and solubility (SL) were calculated using the following equations:

$$WS=\frac{(m2-m1)}{v}$$ Equation (2):

$$SL=\frac{(m3-m1)}{v}$$ Equation (3):

Where, v is the volume of the disc in mm³.

2.4. Polymerization Stress

For the polymerization stress test, uncured composite was loaded into a gap (5 mm in diameter × 1 mm in thickness) between a vertical cylindrical steel piston connected to a load cell and a fused silica slide (n=3), in a cantilever Bioman system [18]. In order to prevent any debonding during the test, piston and fused silica slide surfaces were treated with Z-Prime Plus (Bisco INC) and Ceramic Primer (3M ESPE), respectively. Samples were irradiated for 300s through the bottom glass slide and the force data was recorded for 600s. The stress was calculated as the ratio of force and cross-sectional area of the sample, and plotted as a function of time.

2.5. Flexural Strength and Flexural Modulus

The mechanical properties were assessed by three-point bending test according to ISO 4049 [17]. In brief, beam-shaped specimens (25 mm in length by 2 mm in width and thickness) were made in metal molds sandwiched between two glass slides. The light guide tip was placed 7 cm away from the molds in order to create a spot size that was able to cover the entire sample, and the irradiance was adjusted to ensure that 800 mW/cm² would reach the surface of the bars. The samples were photocured for 120 seconds on each side and stored for 48 hours in dry conditions or for 7 days in Millipore water (n=6). At the end of the storage period, bars were subjected to three-point bending at 0.5 mm/min on a 20 mm span support (Criterion, MTS, EdenPrairie, MN, USA) to obtain flexural strength and flexural modulus according to the equations described in the standard. Deflection of the beam was not measured directly, but rather estimated from the cross-head movement.

2.6. Luciferase Assay

Bacterial viability was assessed using a renilla reporter assay, as described previously [19]. Briefly, six discs (6 mm in diameter by 2 mm in thickness) were prepared from silicone molds and photoactivated for 120 s on each side. After 24 hours, the top surface was sanded with #600 sandpaper in order to obtain a standard surface roughness of 0.3 μm on average, as assessed by a surface roughness tester (TR-200, SaluTron Messtechnik GmbH). Samples were sterilized immediately prior to use by immersion in isopropyl alcohol for 20 minutes.

A derivative of wild type UA159, the bioluminescent S. mutans strain IdhRenGSm, was selected for this assay (Merritt, Kreth, Qi, Sullivan, & Shi, 2005). From a frozen stock, the bacteria was streaked out onto an agar plate and grown in an incubator at 37°C, under humidified atmosphere of 5% CO₂ in air for 1 day. At the end of this period, planktonic cultures were grown for 16 hours in TH culture medium supplemented with 10% yeast extract at the same conditions described above. The discs were incubated in a 48-well plate containing 1 ml of TH (Todd-Hewitt broth) medium supplemented with 1% (w/v) sucrose and 1:500 dilution of the inoculum for 24 hours in an incubator with humidified atmosphere of 5% CO₂. In the next day, discs were moved to 24-well black plates (Black Visiplate TC, Wallac, Finland) containing 0.5 ml of fresh TH medium and 5 μl of Coelenterazine-h ethanol solution per well. Bioluminescence was measured immediately by a spectrophotometer (GloMax Discover Multimode Microplate Reader, Promega Corporation) in relative light units (RLU) at 420 nm wavelength.

2.7. Statistical Analysis

Data were analyzed by one-way ANOVA and Tukey’s test after normality (Anderson-Darling) and homoscedasticity (Bartlett/Levene) tests. Statistical significance was accepted at p≤0.05. The analyses were performed using the software GraphPad Prism 8.3.1 for Windows (GraphPad Software, San Diego, California, USA).

3. Results

Polymerization kinetics results are shown in Table 1 and Figure 2. In terms of maximum rate of polymerization (RP_MAX), the BisGMA control group showed the highest values (17.6 ± 0.37 %.s⁻¹) followed by the UDMA-based formulations copolymerized with 40% and 60% TEGDMA (12.03 ± 0.88 and 9.28 ± 0.35 %.s⁻¹, respectively). The formulations containing 60% and 40% of 2EMATE-BDI as co-monomer presented the lowest values of RP_MAX (6.46 ± 0.27 and 5.91 ± 0.26 %.s⁻¹, respectively), degree of conversion at the maximum rate of polymerization (DC at RP_MAX) (10.9 ± 2.1 and 7.4 ± 1. 0%, respectively) and final degree of conversion (Final DC) (72.3 ± 0.6 and 70.8 ± 1.0%, respectively). In general, the UDMA-based compositions copolymerized with 60% and 40% TEGDMA showed the highest values of DC at RP_MAX (22.3 ± 3.5 and 18.7 ± 4.4 %, respectively) and final DC (83.2 ± 0.6 and 81.8 ± 0.4 %, respectively), followed by the BisGMA control (DC at RP_MAX = 14.3 ± 0.8% and final DC = 77.3 ± 0.4 %).

Table 1.

Maximum rate of polymerization (RP_MAX), degree of conversion at the maximum rate of polymerization (DC at RP_MAX) and final degree of conversion (final DC) for all experimental resin composite formulations. Averages (n = 3) followed by different letters within the same test indicate significant differences (p ≤ 0.05).

Groups	RPMAX (%s−1)	DC at RPMAX (%)	Final DC (%)
BT 60–40	17.6 (0.37) a	14.3 (0.8) b	77.3 (0.4) b
UT 40–60	9.28 (0.35) c	22.3 (3.5) a	83.2 (0.6) a
UT 60–40	12.03 (0.88) b	18.7 (4.4) ab	81.8 (0.4) a
U2E 40–60	6.46 (0.27) d	10.9 (2.1) c	72.3 (0.6) c
U2E 60–40	5.91 (0.26) d	7.4 (1.0) d	70.8 (1.0) c
p	<0.0001	0.0004	<0.0001

Figure 2.

Degree of conversion (%) as a function of time (s), and rate of polymerization (%.s⁻¹) as a function of the degree of conversion (%) for the experimental resin composites. Unless otherwise noted, the base monomer was UDMA, mixed with 60 or 40 wt% of either TEGDMA (controls) or 2EMATE-BDI. BisGMA/TEGDMA 60–40 wt% was tested as an additional control.

The water sorption (WS) and solubility (SL) results are shown in Figure 3. In general, there was no significant difference in WS between the BT control and the other formulations (ranged between 21.06 ± 0.99 and 14.12 ± 2.01 μg/mm³), except for 60% 2EMATE-BDI which showed the lowest values (12.42 ± 2.06 μg/mm³). There were no significant differences in SL among the groups.

Figure 3.

Water sorption (WS) and solubility (SL) results for the experimental resin composites after 7-day water incubation. In regards to water sorption, there was a significant difference between the formulations (p = 0.0079), whereas the groups showed similar results in terms of solubility (p = 0.0978).

The polymerization stress results are presented in Figure 4. UDMA-based compounds copolymerized with 60% and 40% of TEGDMA showed the highest results (7.50 ± 0.20 and 6.15 ± 0.49 MPa, respectively), followed by the BisGMA control group (5.10 ± 0.24 MPa). The 2EMATE-BDI-contaning composites showed reduction in 30% in relation to the BisGMA control and 50% in comparison to UDMA-TEGDMA formulations.

Figure 4.

Polymerization stress results at the end of 10 minutes and polymerization stress generated as a function of time curves for all tested groups. The bump on the curves around 300s corresponds to the moment at which the photocuring light was turned off and the system started cooling down. There was a statistically significant difference between the experimental resin composites (p<0.0001).

Mechanical properties results assessed by 3-point bending test are shown in Figure 5. With respect to flexural strength (FS), for dry storage, the results ranged between 84.02 ± 26.77 and 132.86 ± 28.15 MPa and all tested composites were statistically similar to the BisGMA control group. After 7-day wet storage, all groups were statistically similar. Regarding flexural modulus (FE), under dry storage, the values ranged between 8.93 ± 0.81 and 11.20 ± 2.29 GPa and all tested groups were statistically similar to the BisGMA control, except the composite containing 40% 2EMATE-BDI which was significantly lower. After 7-day water storage, 40% 2MATE-BDI presented similar results to the BisGMA control (9.10 ± 0.49 and 8.81 ± 0.83 GPa, respectively). BisGMA control and 60% TEGDMA showed significant reduction after the water incubation, with average decrease of 23.0% and 40.8%, respectively.

Figure 5.

Flexural strength (FS) and flexural modulus (FE) obtained from 3-point bending test after 24h dry storage and 7-day water incubation. Different uppercase letters indicate statistically significant differences between the groups within the same storage time (p < 0.05). For FS, there was a significant difference between the tested groups under dry conditions (p = 0.0103) but not after the water incubation (p = 0.0803). For FE, under both storage conditions, there were significant differences among the tested resin composites (p = 0.0304 and <0.0001 for dry and wet storages, respectively).

The biofilm formation assessed by bioluminescence is presented in Figure 6. The results ranged between 1.22E+07 ± 4.20E+06 and 2.29E+07 ± 6.12E+06 RLU, and there is no statistically significant difference among all tested resin composites formulations.

Figure 6.

Results of streptococcus mutans biofilm formation after 24h incubation assessed by Luciferase Assay. All groups showed statistically similar performance (p = 0.1138).

4. Discussion

The organic matrix of dental resin composites combines multi-methacrylate monomers to optimize the handling properties, polymerization rate, final degree of conversion, and mechanical properties. The most traditional organic system is composed of the highly viscous BisGMA as base monomer and TEGDMA as co-monomer [1]. However, despite the satisfactory mechanical properties, degree of conversion and rheological properties, this system shows some major drawbacks, which may contribute to compromise the clinical lifespan of dental restorations. In the present study, a newly synthesized hydrophobic, rigid core and low viscosity diurethane dimethacrylate – 2EMATE-BDI, was tested as alternative co-monomer used in combination with UDMA base monomer, to ultimately achieve BisGMA-free formulations. The viscosity was measured as 0.05 Pa.s [15], which is mainly due to the weaker intermolecular hydrogen bonding of the urethane groups (N-H⋯N = 13 KJ/mol and N-H⋯O = 8 KJ/mol) in comparison to the stronger hydrogen bonding interactions established by the hydroxyl groups present in the BisGMA chemical structure (O-H⋯N = 29 KJ/mol and O-H⋯O = 21 KJ/mol) [11, 12, 14].

The group containing BisGMA as the base monomer, as expected, showed the highest polymerization rates. This monomer has been shown to be very reactive when combined with the low viscosity TEGDMA, which ensures enough mobility within the system to promote an optimal balance between propagation and termination events prior to vitrification [1]. In the early stages of the reaction, diffusion limitations hinder termination, increasing propagation and the overall polymerization rate (autoacceleration). As the reaction progresses, the system becomes more hindered due to crosslinking, and eventually the propagation also becomes diffusion-controlled, leading to deceleration of the reaction. The onset of deceleration (RP_MAX) can be used to determine the conversion at which the onset of vitrification is observed (DC at RP_MAX). These events can be appreciated in the curve of rate of polymerization as a function of degree of conversion (Figure 2). The UDMA-TEGDMA material showed higher DC at RP_MAX than the BisGMA-TEGDMA formulation, and had the highest values of DC at RP_MAX and final DC. This may due to the fact that the incorporation of the low molecular weight and flexible TEGDMA into the mixtures is able to delay the diffusion control of propagation to later stages in conversion, which ultimately maximizes the final double bond conversion. The addition of TEGDMA also decreases the termination rates in the early stages of the polymerization reaction, which reduces the values of RPMAX. However, since the UDMA-TEGDMA system lacks the stiff central core and strong intermolecular hydrogen bonding found with BisGMA, the network formed with UDMA is less sterically hindered, allowing higher molecular mobility to the reaction environment and, consequently, promoting segmental movement of radicals up to later stages in conversion. In other words, the incorporation of TEGDMA into the system allows higher degrees of conversion before the propagation becomes diffusion controlled and the polymerization reaction decelerates, which is evidenced by the broad profile of the rate of polymerization as a function of degree of conversion curves (Figure 2). In addition, the higher final DC of the mixtures containing TEGDMA may be due to this molecule’s tendency to cyclization, which translates to increased overall double bond conversion, albeit without contribution to network formation [8].

The mixtures containing 2EMATE-BDI showed the lowest values for all the three polymerization kinetics parameters evaluated. Despite the low initial viscosity of the newly synthesized diurethane [15], this monomer has high molecular weight (560.69 g/mol) and a central aromatic ring, which makes the molecule less mobile. Therefore, it is possible to assume that the copolymerization of the rigid 2EMATE-BDI with the higher viscosity UDMA greatly hindered the mobility of the reacting chains, which decreased the overall reactivity. Additionally, also due to 2MATE-BDI’s low viscosity, it is possible to speculate that the termination was not suppressed in the beginning of the reaction, since the reaction environment probably maintained enough mobility, which might cause the propagation to be diffusion-controlled at earlier stages, limiting the reactivity of the mixture and the final double bond conversion.

In terms of WS, while the incorporation of TEGDMA resulted in higher values, the addition of 2EMATE-BDI led to lower ones. This could be foreseen due to the marked difference in the octanol-water partition coefficient (LogP) between the tested organic matrices (Table 2). Since the hydrophilicity of a system increases as the LogP values decreases [20], it was expected that the formulation based on UDMA copolymerized with 60% TEGDMA would be prone to absorb more water. In addition, since TEGDMA is a long and flexible molecule, it is possible to assume that the polymer network resulting from the copolymerization with UDMA is more loosely packed, which facilitates water absorption and retention. On the other hand, since 2EMATE-BDI is the most hydrophobic of the species tested here, and it was expected that its incorporation in high concentrations would reduce the water sorption of the composite. This was true when mixtures containing 60 wt% of the co-monomers TEGDMA and 2EMATE-BDI are compared. However, in compositions that are mainly comprised of UDMA, the effect of the addition of 2EMATE-BDI is overshadowed and there is actually a trend for it to absorb more water than the TEGDMA-containing compositions. In relation to SL, there was no significant difference between the tested groups, which may means that in spite of showing different susceptibilities to absorbing water, all composites were equally resistant to the leaching of components/unreacted monomers [21].

Table 2–

Octanol–water partition coefficient (log P) and the molecular weight (MW – g/mol) of the tested formulations considering the wt% of each monomer in the mixtures, and vinyl bond concentration ([C=C] – mmol) in 1g of each tested organic matrix formulation.

Groups	log P	MW (g/mol)	[C=C] (mmol)
BT 60–40	3.622	421.6	5.14
UT 40–60	2.604	359.6	5.90
UT 60–40	3.196	396.4	5.35
U2E 40–60	4.950	524.4	3.84
U2E 60–40	4.760	506.3	3.99

The MW (g/mol) and log P values obtained from the Chem-BioDraw Software for the monomers were: BisGMA = 512 and 5.09, UDMA = 470 and 4.38, TEGDMA 286 and = 1.42,and 2EMATE-BDI = 560 and 5.33, respectively).

However, one of the greatest advantages of 2EMATE-BDI incorporation into dental resin composite formulations is highlighted by the marked reduction in polymerization stress, which ranged between 30 and 50%. This may be due to the fact that 2EMATE-BDI formulations show the highest molecular weight among the tested materials (Table 2). 2EMATE-BDI mixtures show significant lower molar concentration of C=C in comparison to BisGMA control and UDMA-TEGDMA formulations (23% and 30.5% lower [C=C], respectively), which may have led to lower volumetric shrinkage and, ultimately, reduced polymerization stress [22]. In fact there is a strong, linear correlation between the degree of conversion and the shrinkage stress (r²=0.9329), which may indicate that the lower stress is likely explained by the lower C=C double bond conversion. In addition, the lower concentration of vinyl bonds in 2EMATE-BDI mixtures leads to longer crosslinks and looser polymer network structure which, associated to the slow polymerization reaction and the flexible nature of the urethane bonds, leads to the formation of a system that is potentially better able to reduce and relax the generated polymerization stress.

As with any material that proposes a reduction in polymerization stress, it is important to verify that this benefit is not produced to the detriment of mechanical properties. With respect to flexural strength in dry storage, the highest values were shown by the UDMA composite containing 60% TEGDMA, which may be related to the higher vinyl concentration (Table 2) allied with the high final conversion, leading to shorter crosslinks and, ultimately, tighter and stronger polymer network structure [11]. However, despite the statistical difference between 60 or 40 wt% 2EMATE-BDI formulations, all groups were statistically similar to the traditional BisGMA/TEGDMA control. Interestingly, after 7-day water incubation, the differences were even less pronounced and all groups showed statistically similar results. The hydrophilic character of TEGDMA would have been expected to impact the water absorption and, ultimately, its plasticization effect [23], but this was not observed. One possible explanation is that the 7-day storage period (used according to ISO4049) was not sufficient to fully saturate the network, and might actually have caused enough mobility to somewhat increase conversion [24]. In fact, though not statistically significant, for 40 wt% 2EMATE-BDI, for example, the flexural strength values numerically increased. In terms of flexural modulus, in dry storage, the BisGMA/TEGDMA control showed the highest numeric values, which is probably related to the bulky and rigid structure of BisGMA and its strong intermolecular hydrogen bonding interactions [10]. However, despite the weaker hydrogen bonding in UDMA, the comparable results exhibited by most of the UDMA mixtures are likely related to the more efficient crosslinking and higher final DC due to the flexibility of its structure [10]. However, after water incubation, the BT control, and 60% and 40% TEGDMA co-monomer mixtures showed a reduction of 23%, 43% and 22%, respectively, in modulus, while the 60% and 40% 2EMATE-BDI-contaning formulations showed 20% and no reduction in modulus, respectively. These differences ensured that most of the tested groups presented results comparable to the BT control, except for the resin containing 60% TEGDMA. This reinforces once again the drawback in using high concentrations of a hydrophilic monomer as a co-monomer for dental resin composites.

And finally, the alternative BisGMA-free resin composites were tested against Streptococcus mutans in order to check the potential for biofilm formation. This assay was not intended to find any antimicrobial effect, but merely in order to ensure that newly developed 2EMATE-BDI systems would not make the polymers more susceptible to biofilm formation, which would represent a significant limitation to the clinical application. The Luciferase assay using a bioluminescent strain showed no difference among groups, which allows us to speculate that the leachates, if any, from the newly synthesized diurethane dimethacrylate do not lead to the upregulation of the growth of cariogenic species [25, 26]. For an antibacterial effect, it would be interesting to introduce an antibacterial methacrylate agent [27] in this BisGMA free formulation. More studies are needed to find a compromise between this new monomer and other antibacterial monomers in order to reduce the formation of biofilm on the surface of the composite and thus improve the longevity of dental composite restorations.

5. Conclusion

The newly developed diurethane dimethacrylate 2EMATE-BDI used as co-monomer for BisGMA-free dental resin composites formulations led to reduced polymerization stress without compromising mechanical properties. The hydrophobicity and lack of effect on biofilm formation, together with the reduction in stress, make this monomer a viable option for novel composite formulations, especially when incorporated in higher concentrations into the resin composite formations.