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Introduction

Several reports have shown that caries progression and bond degradation within hybrid layer are likely to occur due to degradation of hydrophilic monomers and/or auto-degradation of dentin collagen fibrils . In bond degradation, this mechanism is initiated with the breakdown of exposed acid-demineralized collagen matrix, which is without the protection of the adhesive resin monomers, by host-derived matrix metalloproteinases (MMPs) . With regard to caries progression, the alternating periods between dentin demineralization (pH < 5.5) and neutral pH due to salivary buffers was shown to provide the sequence in which collagen fibers are degraded by MMP activation . These findings may open a new pathway for the prevention of caries and debonding, by the incorporation of MMP inhibitors into different resins that could be advantageously modified in different restorative, dentin bonding and base materials depending on the different clinical needs.

Experiments have demonstrated that chlorhexidine (CHX) is capable of arresting caries when applied to dentin , which was previously attributed to its wide-spectrum antibacterial activity . It has also been demonstrated that CHX inhibits some MMPs , and that the therapeutic mechanism for dental caries arrestment is also very likely to be related to its antiproteolytic properties . In adhesive dentistry, in vivo and in vitro studies have shown that the application of a CHX solution to dentin before the restorative procedure, after acid etching and prior to monomer infiltration, arrests the self-destruction of acid etched dentin organic matrices probably due to MMP inhibition.

In addition, there are many health benefits related to flavonoids (tea polyphenol extracts) being scientifically established. The green tea (GT) plant ( Camellia sinesis ) has flavonoids called catechins. These compounds have been tested, and among all GT catechin polyphenol analogs, (−)-epigallocatechin gallate (EGCG) caused the strongest MMP inhibition . In dentistry, it has been shown that EGCG was able to reduce erosion and abrasion of bovine dentin in situ , this being attributed to the effects of MMP inhibition . Gels containing CHX and EGCG were also shown to prevent dental erosion . Moreover, studies have also reported the effectiveness of EGCG in inhibiting acid production in dental plaque bacteria as well as antimicrobial activity against Streptococcus mutans .

In view of the foregoing discussion, it could be assumed that the long-term durability of resin-dentin bonds and restorations may benefit from drug-loaded methacrylate-based polymeric materials, capable of releasing either CHX or EGCG for extended periods. Drug release from resins is a controlled process affected by many factors such as monomer type, drug type and concentration, and extracting media . It has been shown that at high levels, the incorporation of CHX into copolymers increased water sorption due to the osmotic effect of attracting water into the polymer matrix . In addition, this hindered the polymerization process leading to significant reduction in its mechanical properties . Thus, as a starting point, the relationship between drug type and ratio, and mechanical properties should be investigated in order to analyze the viability of the polymer.

Moreover, bis-GMA (bisphenol glycidyl dimethacrylate) is still the most used monomer in resin composite formulations. It has high viscosity, thus dilution with low viscosity alkoxyalkyl dimethacrylate esters, such as TEGDMA (triethyleneglycol dimethacrylate) is required to obtain adequate filler loading and handling characteristics . Dilution with such a monomer, however, increases polymerization shrinkage and water sorption . Therefore some research has also been directed toward developing more hydrophobic and stable hydroxyl-free monomers with lower viscosities, such as CH ₃ bis-GMA (propoxylated bis-GMA) and CF ₃ bis-GMA (propoxylated fluorinated bis-GMA) as replacements for TEGDMA in resin formulations .

In this sense, the present research was designed to investigate the release rate of two drugs, CHX and EGCG, incorporated in different ratios into bis-GMA-based methacrylates diluted with TEGDMA and the synthesized bis-GMA analogs, CH ₃ bis-GMA and CF ₃ bis-GMA. It also examined the degree of conversion of copolymers, water sorption/solubility, and mechanical properties. The null hypotheses tested were that different ratios of CHX and EGCG would have no effect on the water sorption characteristics, degree of conversion, mechanical properties as well as drug release rates of copolymers.