The susceptibility of methacrylates to hydrolytic and enzymatic degradation may be a contributing factor limiting the clinical lifespan of resin composite restorations. The elimination of labile ester bonds is a potential advantage of methacrylamides, which have been shown to produce more stable restorative interfaces. The rationale of this study is to design hydrolytically and enzymatically stable adhesive monomers, with the added benefit of being able to form crosslinked networks. The objective of this study was to synthesize difunctional, hybrid methacrylate-methacrylamide monomers, and evaluate them as potential monomers for dental adhesives.
Materials and methods
HEMA, TEGDMA (controls) or secondary methacrylamides (HEMAM – commercially available, 2EM and 2dMM – newly synthesized) either bearing a hydroxyl group or a methacrylate functionality (Hybrids-Hy), were added at 40 mass% to bisGMA. The photoinitiator system consisted of 2-dimethoxyphenyl acetophenone (DMPA) and diphenyl iodonium hexafluorophosphate (DPI-PF6) at 0.2 and 0.4 mass%, respectively. Polymerization kinetics were followed in real-time by near-IR spectroscopy during light activation at 630 mW/cm 2 for 300 s. Water sorption and solubility (WS, SL) were measured according to ISO 4049. Storage modulus in shear ( G ′) for 300 s was obtained by oscillatory rheometry. For the microtensile bond strength (μTBS), fully formulated adhesives containing 40 vol% ethanol were used to restore caries-free human third molars. Bonded specimens with 1 mm 2 cross-sectional area were tested after 48 h and 6 months storage in water at 37 °C. Single bond (SB) was tested as a commercial control. Data were analysed with one-way ANOVA and Tukey’s test and Student’s t -test ( α = 0.05).
In general, hybrid versions showed lower polymerization rate and degree of conversion, whereas the methacrylate controls, HEMA and TEGDMA, showed the highest values. The hybrid versions showed lower values of WS and SL than their monofunctional versions. HEMAM Hy showed the highest values of G′ and TEGDMA, 2EM, and 2dMM-Hy the lowest. The μTBS values between 48 h and 6 months were statistically reduced only for the HEMA and both 2dMM materials. The formulation containing the monofunctional methacrylamide (HEMAM) showed only 9% reduction in μTBS after 6 months of aging, while the other groups showed a decrease ranging between 18% and 33%.
Overall, hybrid monomers showed lower reactivity than their analogous monofunctional versions, but had markedly lower water sorption. Shear storage modulus was affected differently by the addition of the second functionality. HEMAM-containing systems were able to maintain stable long-term dentin bond strength, which demonstrates that bonding stability is a result of the complex interplay among the factors studied.
The novel monomers showed here are potential alternatives to the current methacrylate adhesives, with selected formulations presenting greater bond stability.
Since resin-based restorative materials were developed over 60 years ago, remarkable improvements have been made in the filler particle systems, photoinitiator effectiveness, and light curing devices . These advances have made adhesive restorative materials the most popular option for direct esthetic dental restorations. However, the limited clinical durability of resin-based materials remains as a challenge, and annually results in costly replacements of dental restorations . The premature degradation of these materials has been ascribed to ester-containing monomer breakdown . Despite the high reactivity and satisfactory mechanical properties of methacrylate-based materials, the ester bonds in these monomers are prone to hydrolysis and enzymatic attack in the hostile oral environment, leading to polymer degradation . More recently, it has been demonstrated that esterases produced by microorganisms compound the hydrolytically-mediated methacrylate monomer degradation . In addition, hydrophilic methacrylate monomers, such as HEMA, are more susceptible to water sorption, which further increases the likelihood for mechanical failure .
A suitable alternative to overcome this problem relies on the incorporation of ester-free monomers in the formulation of the organic matrix. Methacrylamides have been shown to be promising options for dental adhesive formulations . The absence of labile ester bonds makes them highly resistant to enzymatic and hydrolytic degradation, which enables the potential for maintaining high dentin bond strengths even after long term aging . Our previous systematic evaluation in which acrylamides and methacrylamides were compared as diluent monomers in bisGMA and UDMA-based dental adhesive formulations indicated that, while some secondary methacrylamides showed minor or no reduction in dentin microtensile bond strength after 6-month storage in water, the fully methacrylate-based compounds showed reductions of up to 30% . However, this work also highlighted two major concerns associated with the amide functionality – lower reactivity and higher water absorption. The lone pair of electrons in the nitrogen atom is responsible for resonance stabilization with the carbonyl, which is translated into higher stability of the amide bond in comparison with an ester bond on a methacrylate. This is an advantage from the standpoint of hydrolysis, but also leads to decreased reactivity of the nearby vinyl group . In addition, the amides are highly susceptible to establishing strong hydrogen bonds with water molecules and becoming hydrated . This is due to the fact that amides are hydrogen-bond acceptors via the oxygen atom in the carbonyl, and hydrogen-bond donors via the –NH moiety .
Considering that any monomer intended for dental applications would ideally present the reactivity and water absorption resistance of the methacrylates, and the degradation resistance of the methacrylamides, the aim of the present study was to use our previous work with mono-functional methacrylamides as a platform to design, synthesize and evaluate crosslinkable difunctional methacrylamide–methacrylate hybrid molecules as diluent monomers for HEMA-free dental adhesive formulations. The rationale of the present study relies on the hypothesis that the incorporation of the methacrylate functionality on the methacrylamide chemical structure will increase the reactivity and the final degree of conversion, and the enhanced crosslinking provided by the di-functional species will make the network more resistant to water absorption, ultimately mechanically reinforcing the interfacial bond.
Materials and methods
Tested monomers and synthesis procedures
The tested monomers are shown in Fig. 1 . All commercially-available monomers were purchased from Sigma–Aldrich (Milwaukee, WI, USA) at 97% purity and used as received. The chemical structure of the secondary methacrylamide N-hydroxyethyl methacrylamide (HEMAM) was modified with ethyl and methyl substituents on the first (alpha) carbon (2EM and 2dMM, respectively), as described previously . The hybrid versions of these monomers were isolated via chromatography, as described in the Supporting information. NMR and IR spectral data can also be found in the Supporting information.
N-hydroxyethyl methacrylate (HEMA) was tested as monofunctional methacrylate control. Triethyleneglycol dimethacrylate (TEGDMA) was tested as difunctional methacrylate control to provide a comparison with the difunctional methacrylamide–methacrylate hybrid monomers. The partition coefficient (log P ) for each monomer was calculated using the software package Chem Draw Ultra 14.1 (Perkin Elmer, San Jose, CA, USA).
Tested formulations and photocuring conditions
The monomers shown in Fig. 1 were mixed at 40 mass% with bisphenol A-glycidyl methacrylate (bisGMA). The photoinitiator system consisted of DMPA (2,2-dimethoxy-2-phenylacetophenone, λ max = 365 nm) and DPI-PF6 (diphenyliodonium hexafluorophosphate) at 0.2 mass% and 0.4 mass%, respectively. Butylated hydroxytoluene (BHT) was added at 0.1 mass% to each formulation as a free-radical inhibitor. For the dentin microtensile bond strength test only, experiments were conducted with fully formulated adhesives containing 40 vol% of ethanol to provide appropriate viscosity for proper dentin penetration and subsequent volatilization by air drying. All photocuring procedures were accomplished by a mercury arc lamp (Acticure 4000 UV Cure, Mississauga, Canada) filtered to 320–500 nm at 630 mW/cm 2 measured directly at the sample surface using a thermopile power meter (Molectron PM100, Portland, OR, USA). The choice of light source was intended to match the initiator system used.
Kinetics of polymerization
The kinetics of polymerization were assessed in near-IR spectroscopy (Nicolet 6700, Thermo Scientific, USA) in real time during the photopolymerization of disk-shaped samples (10 mm in diameter and 0.8 mm in thickness; measured with a digital caliper to 0.01 mm) for 300 s ( n = 3). Each spectrum was collected with 2 scans at 4 cm −1 resolution. This resolution allowed for baseline correction without compromising the sampling rate and signal-to-noise ratio. The final carbon–carbon double bond conversion (final DC) was calculate based on the areas of the peaks (obtained with the processing tool in the OMNIC software) at 6165 and 6135 cm −1 , which correspond to the vinyl overtone for methacrylates and methacrylamides, respectively. The maximum rate of polymerization (RP MAX ), representing the reactivity of the monomers, was determined as the first derivative of the degree of conversion as a function of the time. The degree of conversion at the maximum rate of polymerization (DC at RP MAX ) was used to estimate the time point in conversion at which diffusional limitations lead to deceleration.
Water sorption and solubility
Water sorption (WS) and solubility (SL) were measured according to the ISO 4049:2019. Briefly, the same samples obtained in the polymerization kinetics test ( n = 3), after having their initial mass M1 determined, were immersed in 5 mL of triple distilled water for 7 days. At the end of this period, M2 was measured and the samples were stored in a desiccator containing silica gel and connected to the house vacuum. Sample weights were measured daily until the final mass did not change to the nearest 0.0001 g (M3). WS and SL were calculated in μg/mm 3 according the following equations, where V is the volume of the disk in mm 3 :
Storage modulus in shear
The storage modulus in shear ( G ′, n = 5) was assessed in an oscillatory rheometer (Discovery HR-1 Hybrid Rheometer, TA Instruments, New Castle, DE, USA), using an 8-mm diameter aluminum plate attached to the upper fixture and an acrylic plate mounted to the UV-Vis accessory on the bottom. Approximately 0.02 g of each material (the exact mass was recorded for each specimen and used to calculate G ′) was placed between the parallel plates, and the light was delivered through the acrylic via the optical apparatus in the UV-Vis accessory. Samples were tested in oscillation mode (sine wave) at 10 Hz and 0.1% strain with a gap of 0.3 mm during the photopolymerization for 300 s ( n = 3).
Dentin microtensile bond strength
Selected formulations with the highest G ′ and lowest WS and SL were subjected to dentin microtensile bond strength testing (μTBS). Sound human dentin of extracted caries-free third molars was used as the substrate. The study was approved by the Oregon Health & Science University IRB (IRB00012056). The enamel was removed and the resulting surface was roughened by hand with light pressure and one pass across wet #600 silicon carbide paper to simulate smear layer formation. The dentin surface was etched for 15 s with 37% phosphoric acid (3M ESPE), rinsed and dried with the aid of gentle air stream for about 10 s. Two layers of the adhesive were applied and, after solvent evaporation, the second layer was photocured for 60 s at 630 mW/cm 2 by the mercury arc lamp. Restorative procedures consisted of a block of Filtek Supreme (shade A2 – 3M ESPE) built in 2 increments of 2 mm each, photoactived with the light guide directly over the surface for 20 s at 1200 mW/cm 2 with an Elipar™ DeepCure-S LED (3M ESPE). Adper Single Bond (3M ESPE) was tested as a commercial adhesive control, in two consecutive layers, air-dried to remove excess solvent, and photoactivated for 20 s using the same light curing unit settings ( n = 6).
24 h after the restorative procedures, teeth were sectioned under water in a slow speed diamond saw (Accutom-50, Struers) to obtain sticks of 1 mm 2 cross-sectional area (checked with a digital caliper to 0.01 mm). The sticks were tested after 24 h or 6 months water storage at 37 °C. Sticks were glued with cyanoacrylate (Zap-it, Dental Ventures of America, Corona, CA, USA) onto custom-made jigs (Odeme Equipment, Luzerna, SC, Brazil – pictured) attached to a universal testing machine (Criterion MTS, Eden Prairie, MN, USA) and tested in tension until failure (0.5 mm/min).
Data was statistically analysed by one-way ANOVA and Tukey’s test ( α = 0.05), after assessment of normality and homoscedasticity. For μTBS, Student’s t -test was carried out to compare the effect of the storage time ( α = 0.05). In the instances where the normality tests failed, the nonparametric Kruskal–Wallis test was carried out ( α = 0.05).
Kinetics of polymerization curves (average of three curves) are depicted in Fig. 2 and results shown in Table 1 . RP MAX ranged between 0.11 and 0.03% s −1 , with TEGDMA and HEMAM Hy showing the highest and lowest values, respectively. The other groups were statistically similar ( Table 1 ). A similar trend was found for the DC at RP MAX results, which ranged between 21.0% and 8.7%, with the methacrylates TEGDMA and HEMA showing the highest values and HEMAM Hy the lowest. In terms of final DC, the monofunctional HEMA and HEMAM showed the highest values (89.0% and 83.2%, respectively) and the hybrid versions HEMAM Hy, 2EM Hy and 2dMM Hy the lowest (63.5%, 63.3%, and 59.4%, respectively). In general, the alpha-substituted methacrylamides 2EM and 2dMM showed lower values than the monofunctional methacrylate control HEMA (73.6%, 76.7% and 89.0%, respectively).