More than 60% of the more than one hundred million dental restorations performed each year involve the use of photopolymerizable polymeric composites. Despite their ubiquitous presence in modern dentistry, these composites suffer from significant problems that limit their applicability and utility. Based on resin chemistry developed and implemented nearly 50 years ago, the problems with these materials include issues associated with the presence of extractable, unreacted monomer following cure, degradation of the monomers and composite, polymerization shrinkage and shrinkage induced stresses, the long timeframe for polymerization, lack of toughness and other mechanical behavior of the resin material, thermal and moisture uptake by the sample following polymerization. With less than an 8 year average service life, the result of these problems is often the premature failure of composite restorations, resulting either from secondary caries or from mechanical failure within the bulk or at the interface. We propose to develop and evaluate a composite restorative system based on the now-classic click reaction, that is the copper catalyzed azide-alkyne (CuAAC) reaction. Its characteristics include achieving high conversion without side reactions, being robust and readily performed at ambient, and forming a product that is not readily degradable by acids, water, or enzymes. Moreover, the product of the reaction is a triazole ring structure that is capable of secondary molecular interactions (i.e., non-covalen bond formation) that enhance toughness, glass transition, and modulus of the crosslinked polymer material. Thus, this research will address the critical shortcomings of methacrylate composite systems by (i) developing a completely new approach to the resin portion of these composites that implements the CuAAC reaction in a manner that will lead to achievement of near-quantitative functional group conversions, limit extractable monomers, eliminate the potential for hydrolytic and enzymatic degradation, improve the mechanical properties through secondary molecular interactions, and dramatically reduce shrinkage and stress; (ii) combining this novel CuAAC-based resin phase with appropriately functionalized fillers to achieve the desired mechanical performance, improve fracture toughness, extend the lifetime of these restorations, and achieve enhanced dimensional stability of the composite; and (iii) analyzing the adhesion, degradation, extraction, and other long-term performance metrics for these resins and composites. The photo-induced CuAAC polymerization system is ideally suited for the next generation of dental restoratives and our goal is the development of a composite system that is compatible with current dental practices and adhesives and yet yields at least a two fold increase in the service life of these restoratives.
The proposed work represents a new paradigm in dental restorative materials that aims to significantly improve the performance of composite restorative materials used in more than 100 million restorations each year. By introducing a novel photoinitiated, copper-catalyzed azide- alkyne-based polymerization approach into dental restorative materials, we will dramatically reduce the level of extractables, swelling, shrinkage, stress and degradation of the composite while also improving the mechanical behavior through the presence of the triazole reaction product. This approach proposes to yield a dental composite system that will at least double the service life of composite restorations over the course of this grant.
