Putting the Bite on New Materials
New polymer composites could replace traditional dental restorative materials
Materials scientists at Southwest Research Institute (SwRI) are developing a novel restorative material for use in making the composites used by dentists for tooth fillings, inlays, and crowns. The material, a polymer composite made from tantalum oxide and silica nanoparticle fillers incorporated in a liquid crystal monomer matrix, retains the desirable properties of existing composite restoratives but avoids their major shortcomings.
The development of this material contributed to the award in 1997 of a five-year, $3.4 million, multidisciplinary research grant to the University of Texas Health Science Center at San Antonio (UTHSCSA) School of Dentistry and SwRI from the National Institute of Dental Research, a division of the National Institutes of Health. Scientists from SwRI's Chemistry and Chemical Engineering and Materials and Structures Divisions are collaborating in this project. Dr. H. Ralph Rawls, professor in the UTHSCSA department of restorative dentistry, is director of the overall research program that also includes a dentin-stimulating project designed to increase the longevity of fillings as well as the strength of the tooth.
Improving the material properties of fillings, inlays, and crowns can reduce trips to the dentist, lower health costs, and provide better oral hygiene. Traditional composite restorative materials that are in popular use last only about ten years, while amalgam and ceramic materials can last twice as long
The engineering challenge of teeth
Teeth are primarily made of a natural ceramic, enamel, and a natural composite, dentin. The thin surface layer of enamel mainly consists of a calcium-based mineral called hydroxyapatite. Beneath the enamel, the bulk of the tooth is formed of dentin, a mixture of hydroxyapatite, collagen, water, and salts. A third type of tissue, cementum, lines the dentin under the gum line.
The mouth presents a harsh environment, both chemically and mechanically. Acids from foods, plaque-forming bacteria, and mouth secretions erode both teeth and tooth repairs. The act of chewing subjects the tooth and any associated filling or crown to what amounts to a series of hard blows. To choose an appropriate material for restorative purposes, dentists have to carefully assess the stresses that the material will encounter.
Molars, for example, bear the brunt of mastication and experience greater stress than teeth at the front. And, because the center of a tooth surface flexes more than its edges, a filling located in the center runs a greater risk of loosening if not properly bonded.
The ideal restorative material would produce no shrinkage during polymerization, provide wear resistance similar to enamel, adhere to dentin and enamel, be biocompatible and tooth colored, seal easily at the interface, and develop high early strength. To date, no commercially available restorative material meets all these ideal requirements. As a result, worn or cracked fillings are a common clinical problem resulting from the shortcomings of the materials and the procedures by which they are placed.
Problems in current restorative materials
Dental amalgam, the most widely used material for filling teeth at the back of the mouth, is an alloy of silver, tin, copper, zinc, and mercury. It is reasonably priced, strong, and dependable. However, its future seems uncertain. The silver color is no longer considered aesthetically acceptable, and getting rid of millions of potential environmentally hazardous old fillings provides a substantial disposal problem. The European Economic Union, for example, has already passed legislation to phase out the use of amalgam.
Composites used in the front of the mouth are generally made of silica or glass particles bound with a polymer resin. They have aesthetic advantages over amalgams in that color and texture can be matched to the patient's teeth by the addition of particulate fillers. However, they are particularly subject to shrinkage and loosening and are currently not strong enough to withstand the chewing forces in the back of the mouth. In addition, both dental caries and existing composite fillings have insufficient X-ray contrast with their surroundings, making it hard for the dentist to identify new areas of decay.
Current dental procedures using composites are lengthy and exacting. The dentist applies an adhesive to the tooth cavity after roughening the dentin with a mild acid to improve bonding. Next, the cavity is filled by layering and photocuring several successive composite coats. The light causes the monomer molecules to react with one another, linking them together to form a solid resin. Overlaying several increments and photocuring each is necessary to permit relaxation of the interfacial stress generated by polymerization shrinkage.
In addition, there are other problems associated with current composites. If the coating is too thick the polymer may separate from the surface during the setting process, weakening the bond to the tooth. Tiny gaps or cracks can occur at the interface between the filling and the tooth and harbor cavity-causing bacteria. The acid-etching technique also removes minerals from the dentin and can weaken the tooth. Dentin is wet and has a high protein content, complicating the search for appropriate bonding materials. Finally, polymer fillings are susceptible to staining and discoloration, making them sometimes unsightly.
The primary goal in the NIH research project is to find a viable alternative for both current amalgams and composites by producing a tough and wear-resistant polymer that can be used anywhere in the mouth. This alternative material should not shrink as it sets, so that the adhesive bond between the tooth and the restorative material is retained. The material also needs to be gel-like at zero applied stress, and sufficiently malleable so that the cavity can be filled prior to photocuring.
These objectives require that the new material have a combination of properties, the synthesis, formulation and testing of which demand a multidisciplinary approach. The cooperation of scientists at SwRI and UTHSCSA is well suited for this. Active research cooperation and participation with commercial manufacturers is also anticipated, to ensure the material meets practical industrial needs and to encourage acceptance in the dental community.
Accomplishments: metal oxide-polymer composites
In the late 1980s, scientists in the SwRI Chemistry and Chemical Engineering Division were asked by the National Aeronautics and Space Administration (NASA) Johnson Space Flight Center to help solve a materials problem with astronaut face shields. Early astronauts on the moon had their vision disturbed by strange flashes of light caused by high energy particles passing through their non-radio-opaque face shields and stimulating their optic nerves.
The new face shield material had to be transparent, tough, and resistant to scratching and corrosion, as well as sufficiently radio-opaque to protect the astronauts from bombardment by the particles. SwRI chemists began to evaluate a group of materials known as polymer-ceramic composites.
Polymer-ceramic composites have the potential to combine the properties of polymers and ceramics -- particularly oxides -- in useful ways. Of special use are the so-called nanocomposites that consist of alloys of polymers with ceramic particles with diameters much smaller than the wavelength of visible light. These nanocomposites had potential for use as face shield material because of their high refractive index and their resistance to scratching and corrosion.
Using these materials, SwRI scientists were able to develop clear (transparent) polymer glasses with a heavy loading of tantalum oxide which successfully blocked the high energy particles. They achieved this by a sol-gel process using non-aqueous chemistry that allows the oxide particles to be blended with polymers to make a compatible and homogeneous system. Although there were some initial problems with the mechanical properties of the glasses, these were overcome by incorporating additional ductile reinforcing polymers into the material.
News of the availability of these optically clear but radio-opaque composite materials with high fractions of oxide particles was of great interest to the dental community. A clear, strong composite to which one can add pigments to give a tooth-like appearance seemed close to a dental ideal.
The next step in developing a restorative was the formulation of a matrix material which, when photocured, exhibits little or no shrinkage. Careful research in the chemical literature suggested the choice of a liquid crystal monomer. (Monomers are molecules that can be chemically linked to form a polymer). The literature indicated that, when carefully designed, these monomers expand on polymerization, offsetting the natural shrinkage that occurs when liquid monomers are converted to solid polymers. Liquid crystal monomers also have a desirable rheology; they are almost rigid when they are put in the tooth, yet are sufficiently malleable for dentists to work with. Using photoinitiators the monomers also harden when exposed to low-intensity blue light.
Liquid crystal monomer matrix
The synthesis and testing of the liquid crystal monomers proved time-consuming. Problems encountered included separation and purification difficulties and a tendency to crystallize into hard materials at room temperatures. For this reason it was necessary to initiate a chemical synthesis project in which dozens of monomers were created and various groups substituted onto the base molecule in an effort to make a monomer that is liquid-crystalline at room temperature. Two LC monomers resulted which appear to have the critical properties of transforming from a nematic, highly organized state at room or mouth temperature to an isotropic amorphous state when photocured, with sufficient expansion to offset the contraction that accompanies the formation of covalent bonds.
Mechanical material testing
A number of simple property measurements of the material have already been carried out by staff at UTHSCSA. The Health Science Center will also be responsible for performing future tests that characterize the color, shrinkage, and wear of the material. Some evaluations will be carried out in an environment that simulates the mouth, using artificial saliva.
The bulk of the mechanical and physical property characterizations will be carried out at SwRI by technical staff working under Dr. James Lankford, director of Materials Engineering. Lankford will conduct the extensive physical and mechanical property characterizations needed to ensure satisfactory performance. Testing will be more rigorous as the materials tested move in the direction of zero shrinkage. The Materials Engineering department will use standard technologies developed for similar materials, as well as certain tests specified in American Dental Association Standards. Some samples will be tested under tension for elongation and ductility, others for fracture toughness. Other required tests include wear characterization, stress-strain properties, fracture strength, and stiffness.
This mechanical and structural characterization stage of the project is vital for final acceptance of the product. Non-shrinkage is of little use if a material is too weak. The project has specific objectives; to meet them requires the achievement of a demanding combination of properties. Once these requirements are met, using standard small coupons, testing will move into full-scale mode. As production of the research material is scaled up over the next two years, a priority will be to ensure that all the original desirable material qualities are maintained.
The author acknowledges the significant contribution of Dr. Hong Dixon and Ms. Sunshine Leamon and Mr. Jacob Moore for their work in materials preparation for the project and Daniel Nicolella for the mechanical properties measurement.
S.T. Wellinghoff, Metal Oxide-Polymer Composites, U.S. Patent No. 5,372,796, 12/13/94.
S.T. Wellinghoff, Metal Oxide-Polymer Composites, U.S. Patent No. 5,670,583, 9/23/97.
S.T. Wellinghoff, Metal Oxide Compositions and Methods, U.S. Patent applied, 9/27/96.
Published in the Spring 1998 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.