Composite materials that include combinations of metal oxide and silica nanoparticles in polymeric matrices have been invented for primary use as dental fillings, dental and bone adhesives, and the like. There have been previous efforts to develop polymeric replacements for the amalgam used traditionally as dental filling material, but those efforts involved polymers that exhibited unfavorable characteristics, including shrinkage and poor adhesion to bone. Strong adhesion is desirable and zero shrinkage is essential for a dental filling material because accumulated stresses from shrinkage can cause debonding with consequent leakage and attack by microbes. The present materials are formulated to obtain stronger adhesion and less shrinkage.
A somewhat detailed presentation of historical and technical background is prerequisite to a meaningful description of the invention. One polymeric material that had been suggested previously for use in dental fillings is made from the monomer bis-glycidylmethylmethacrylate (bis-GMA). The use of bis-GMA together with other ingredients usually included in dental adhesives or fillings yields materials with desirable physical properties, but these materials exhibit considerable post-shrinkage and adhere poorly to teeth.
Other polymeric filling materials also exhibit less than the desired amounts of adhesion to tooth surfaces. In order to obtain desired bonding on enamel or dentin, the protein coatings on the enamel or the smear level on the dentin must be removed. Traditionally, this has been done by use of such organic acids as phosphoric, citric, lactic, and diamine dicarboxylic acid. Thus, many tooth-filling products contain polyacids as surface-cleaning and priming agents for enamel and dentin. Because bis-GMA is not inherently adhesive to tooth surfaces, similar provisions for etching by acids would have to be made if bis-GMA were to be used.
Another class of candidate dental adhesive and filling materials includes some nematic liquid crystals that can be photopolymerized within seconds, at temperatures in the vicinity of 90 °C. These materials form densely cross-linked networks of reaction extent greater than 95 percent and exhibit very little polymerization shrinkage because of the high packing efficiency that already obtains in the nematic state. However, polymerization at lower temperatures (including room temperature) results in undesirable intervening smectic and crystalline phases that make the materials unsuitable as medical and dental restoratives. This completes the background information.
In a representative composite material of the present invention, the matrix resin preferably consists of, or at least includes, an acrylate or methacrylate-based nematic liquid-crystal monomer that is photopolymerizable at room temperature and exists in the nematic state (with suppression of crystallinity) at room temperature. The resin is formulated to be able to accommodate a high loading of metal oxide or metal oxide and silica nanoparticles, to enable the resin/particle mixture to flow when pushed into cavities to be filled, and to form a high-molecular-weight polymer, with little or no shrinkage upon polymerization.
The metal oxide and silica nanoparticles help to provide strength. The metal oxide particles can also be used to impart opacity for x-ray photography. While any metal capable of forming one or more amphoteric oxide(s) could be used, tantalum is particularly advantageous for imparting x-ray opacity to the composite.
For compatibility with teeth, the metal oxide particles must not exhibit high surface acidity. The surface acidity of tantalum oxide nanoparticles is neutralized by mixing, with the particles, a polymerizable, biocompatible, heterocyclic base (e.g., an alkene-terminated imidazole or phosphate) that can form complexes with the acid sites on the surfaces of the particles.
In many cases, it is desirable to formulate resin/nanoparticle filling mixtures to be translucent or transparent. In a typical dental restorative procedure, a liquid or pasty filling material is placed on a tooth and ultraviolet light used to effect the polymerization (cure) into a high-strength, hard, x-ray-opaque coating or filling, with essentially zero shrinkage. The transparency or translucency makes it possible to effect photopolymerization of a thicker layer of filler than would otherwise be possible, thus making it unnecessary to apply and photocure multiple thinner layers.
This work was done by Stephen T. Wellinghoff of Southwest Research Institute for Johnson Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
Stephen T. Wellinghoff
Southwest Research Institute
6220 Culebra Road
San Antonio, TX 78228
Refer to MSC-22842, volume and number of this NASA Tech Briefs issue, and the page number.