Use of (meth)acrylamides as alternative monomers in dental adhesive systems
Introduction
Methacrylates are widely used in dentistry to create bonding between dental substrate and restorative material. The combination of hydrophilic monomers, such as 2-hydroxyethyl methacrylate (HEMA), with mainly hydrophobic dimethacrylate monomers allowed for the hybridization of the collagen on the dentin substrate [1], as well as co-polymerization with the restorative composite material [2]. However, the incorporation of high concentrations of hydrophilic and/or ionic monomers increases water sorption of the system [3,4], and the adhesive interfaces behave as permeable membranes [5]. In the presence of water, the ester linkage of the methacrylate backbone may undergo hydrolytic cleavage, yielding methacrylic acid and alcohol-bearing residues. In conjunction with the degradation of the collagen, this causes the bonding to progressively degrade over time due to the action of water and enzymes [6].
Acrylamides and methacrylamides, with more stable amide bonds, have been postulated as alternative monomers for the design of more hydrolytically stable adhesive systems [7,8] with the rationale of increasing the longevity of the bonded interface. These monomers have been used in at least one commercial product for a number of years, with conflicting results, especially in clinical trials, with some studies showing similar clinical performance compared to methacrylate controls and others showing worse performance [9,10]. Less than ideal results may be a function of the somewhat increased water sorption for some methacrylamides [11], as well as to their potential lower reactivity [12], which has been reported specifically for tertiary methacrylamides [13]. In fact, in depth, systematic analyses of the reaction kinetics of tertiary methacrylamides in co-polymerizations with monomers leading to the formation of glassy networks are lacking. In addition, past concerns over the cytotoxicity of acrylamides have precluded their use in biological applications, but more recently, non-cytotoxic alternatives have been reported [14]. These factors justify the current use of (meth)acrylamides in commercial preparations in combination with other monomers.
Even for pure methacrylates, a mixture of monomers is typically employed to harness the advantages of each individual compound. For example, the basic composition of fifth generation adhesives contains a relatively viscous crosslinking base monomer, such as BisGMA, which is added to improve both the reactivity and the mechanical properties of the adhesive layer. A low-viscosity, hydrophilic co-monomer, such as HEMA is added to decrease the viscosity and improve spreading, but mainly to allow diffusion into the dentin substrate [7,15]. This implies that all compounds need to be miscible with each other, as well as with the solvent of choice, as this affects both the interaction with the substrate and the reaction kinetics. In addition, the copolymerization of the monomers included in the mixture also needs to be considered. For example, it is well known that acrylates present much higher reactivity than their methacrylate counterparts, resulting in the formation of two independently polymerizing networks [16]. In summary, monomer reactivity and copolymerization ability are critical screening tools for monomer selection and adhesive design. Previous studies have tested the compatibility and co-polymerization of (meth)acrylamides with other commonly used monomers in dental adhesive applications [8,17], with reported improvements in bond strength [8,18,19]. Others have demonstrated lower reactivity of certain methacrylamides [15].
Since the presence of water in the hybrid layer is inevitable, several strategies have been proposed to improve the resistance of the adhesive layer to degradation. One attempt has been the elimination or reduction of the ester groups on the polymeric network, such as with the use of (meth)acrylamide-based adhesives systems. The presence of a nitrogen atom in amides, as compared to the oxygen atom in acrylates, leads to steric and electronic effects that reduce the susceptibility to hydrolysis. Nitrogen is less electronegative than oxygen, which makes it more likely to donate non-bonded electrons to the carbonyl carbon, shortening and strengthening the bond, ultimately decreasing the susceptibility to nucleophilic attack [20]. Tertiary methacrylamides present significant lower reactivity, while tertiary acrylamides and secondary methacrylamides present similar or higher reactivity compared to methacrylates [21]. The substitution of an alpha hydrogen atom (next to the carbonyl group) by a methyl group is expected to impart greater hydrophobicity and hydrolytic stability. Therefore, the objective of this study was to evaluate two tertiary acrylamides and two secondary alpha-substituted methacrylamides, copolymerized with BisGMA and UDMA, for dental adhesives formulations with potentially improved hydrolytic stability. The experimental materials were tested for mechanical properties, water sorption/solubility and dentin bond strength. The tested hypothesis was that compared to conventional methacrylate-based systems, (meth)acrylamide-based dental adhesives would: 1. present comparable properties; and 2. produce higher and more stable microtensile bond strengths.
Section snippets
Formulation of experimental adhesives
Experimental adhesive resins were formulated with 60 wt% BisGMA (Bisphenol A glycidyl dimethacrylate) or UDMA (urethane dimethacrylate) purchased from ESSTECH (Essington, Pennsylvania, USA), and 40 wt% of one of the monofunctional monomers listed in Table 1 (synthetic procedures and characterization of the monomers, including 1H NMR spectra and logP calculations obtained with ChemDraw software are detailed in Supplementary Appendix). In all formulations, the photoinitiator system was composed
Results
Polymerization kinetics (rate of polymerization as a function of conversion) for all experimental materials combined with BisGMA and with UDMA are presented in Fig. 1A and B, respectively. Values of maximum rate of polymerization (RPmax), degree of conversion at the maximum rate/onset of deceleration (DC at RPmax, used to estimate the onset of vitrication) and the degree of conversion at 40 s (as an example of the conversion at clinically-relevant exposure times) are presented in Table 2. In
Discussion
Most commercial adhesive systems are based on ester-containing methacrylate polymers, making them susceptible to hydrolysis and enzymatic degradation [26]. (Meth)acrylamides lack ester bonds and are more hydrolytically stable [27], which makes their use for adhesive applications a logical alternative to overcome intraoral degradation issues. However, for these monomers to be useful in dental applications, a balance must exist between their reactivity and susceptibility to degradation. In this
Conclusion
In conclusion, the results of the present study indicate that the alpha-substituted secondary methacrylamides led to more stable bonds after 6 months, which was true regardless of the base monomer (BisGMA or UDMA). Steric hindrance influenced the polymerization kinetics of the monomer mixtures. However, copolymerization is a complex reaction affected by many additional factors, such as initial viscosity, partition coefficient, intermolecular interactions, and reactivity. Overall, UDMA
Acknowledgment
The authors acknowledge the National Institute of Dental and Craniofacial Research for funding (K02 DE025280, R01 DE026113, U01 DE023756).
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