In order to improve the performance and durability of current dental restorative composites, we will develop a novel boron nitride nanosheet (BNNS)-based filler system. Similar to graphene, exfoliated hexagonal boron nitride yields single- or few-layer BNNSs with useful advantages. Like graphene, BNNSs with narrow size distributions have been observed to self-assemble into colloidal liquid crystals that are extremely strong and light, wear resistant, and chemically stable. The lamellar organization of BNNSs as a filler in dental composites would reduce composite viscosity, increase overall filler loading, make the composite shear-thinning, and enhance handling properties and composite placement. BNNSs also have very low coefficient of friction that would reduce composite wear rate. They can act as an optical conduit (depending on orientation) to guide and distribute photon energy during cure and potentially increase depth of cure. They are hydrophobic and can impart hydrophobicity to the composite and protect it from degradation. They can form lamellar layers that could act as water barriers. Finally, unlike graphene, BNNSs (aka ?white graphene?) are less expensive to manufacture and are transparent and colorless, so they have promise as fillers in esthetic composites. The significance of these features is that they should result in substantially increased composite longevity. We have developed novel methods for exfoliating, concentrating, and surface treating BNNSs for use in nanocomposites. These BNNSs were functionalized with oxiranes and incorporated into Oxirane/Acrylate interpenetrating network resin System (OASys) composites (developed under a separate NIH U01 grant). Even at the very low concentration of 0.5 wt%, BNNSs made with an unoptimized exfoliation method, significantly enhanced composite modulus, reduced viscosity, increased compatibility of the hydrophobic fluorinated monomer in the composite, and increased composite translucency. As such, we will further optimize the BNNS exfoliation method and explore the properties that BNNSs could impart on a more common conventional dental composite system at much higher loadings.
Four specific aims are proposed: 1) To determine the effects of BNNS exfoliation, loading and orientation on composite total filler loading, curing, physical and optical properties. 2) To determine the biocompatibility of BNNS-loaded composites following the ISO 7405 and ISO 10993 series for pre-clinical biocompatibility evaluation of medical devices. Composites from Aim 1 with clinically acceptable properties will be tested in the following aims. 3) To determine the effects of BNNS concentration and orientation on composite static and time-dependent mechanical properties, including fracture toughness and three-body wear. 4) To determine the effect of BNNS concentration on biofilm formation.
(PUBLIC HEALTH RELEVANCE) The annual cost for replacement dentistry is about $5 billion in the USA, and accounts for more than 60% of all restorative dentistry (between 9 and 39 million restorations). Thus, a longer-lasting restorative material is an urgent oral health need. The goal of this work is to develop a novel reinforcing filler system for dental restorative materials using extremely strong and chemically-stable boron nitride (aka ?white graphene?) nanosheets that can drastically improve the strength, durability and clinical lifetime of current dental composites.