DENTAL AND MEDICAL MATERIALS
The Dental and Medical Materials Program provides basic materials science, engineering, test methods, and standards to sectors of the health care industry for the development of new or improved materials and delivery systems. The objective of this program is the development of improved dental and medical materials that have greater durability, wear resistance and clinical acceptability, with the main program focus on dental restorative materials.
Dental restorative composites are heterogeneous materials having three essential phases: (1) a polymeric matrix which comprises the continuous phase, (2) fillers of various types, sizes, shapes and morphologies which constitute the disperse phase and (3) an interfacial phase that, in varying degree, bonds the continuous and disperse phases into a unitary material rather than a simple admixture. While all three phases are important in determining the properties of the composites, this program is focused primarily on the interfacial and polymer matrix phases. Since the polymerization shrinkage that occurs in the matrix phase is one of the most commonly cited deficiencies of dental restorative composites, resources are allocated to develop high conversion, durable, low shrinkage polymeric materials for use in dental resin and composite applications. The polymeric matrix of a dental composite typically is formed by free radical polymerization of a resin which contains one or more vinyl monomers, usually of the methacrylate class. Polymerization is started either by the formation of initiating radicals from chemical reduction-oxidation (redox) reactions or by photochemical redox reactions.
Although only a minor component of these composites, the interfacial phase that develops from the interaction of the silane coupling agent with the polymer matrix and the siliceous filler exerts a profound effect on the properties of the composites. Because these composites are used in an aggressive, aqueous environment that constantly challenges the vulnerable silane mediated polymer-filler bond, understanding of this critical interfacial phase is being acquired so that strategies can be developed for its improvement.
The occupational and environmental hazards associated with the use of mercury-containing dental alloys are a recurring source of public concern. Since dental amalgams have performed exceedingly well over more than one hundred years, the development of a direct filling material still based on the common constituents of dental amalgams, other than mercury, is desirable. This project is focused on acid-assisted consolidation of chemically precipitated silver powders and property measurements of hand consolidated test compacts prepared with the tools and procedures normally employed by dentists. The observed values of flexural strength for the silver compacts were equal or superior to mercury amalgams. Corrosion resistance, microleakage and marginal toughness values of the compacts were found to be superior to those of amalgams. Wear and biocompatibility studies on the hand-consolidated compacts are in progress.
Besides the dental materials projects, efforts are directed toward the development of improved bone fixation materials and the evaluation of biomaterials. A project, carried out in collaboration with the American Dental Association and the National Institute of Dental Research, is directed at enhancing the biocompatibility and mechanical properties of composite bone cements. The biomaterials evaluation effort centers on the NIST Orthopedic Wear Consortium which consists of four companies to develop accelerated wear test procedures for rapid screening of materials used in hip and knee replacements. This will accelerate the introduction of new biomaterials into practice.
Dental and medical research directions in support of the goals are established in collaboration with the American Dental Association (ADA), the National Institute of Dental Research, the National Heart, Lung and Blood Institute, the US Food and Drug Administration, and guest scientists from the U.S. Navy, and the U.S. Public Health Service. NIST has hosted research associates from ADA since 1928. Currently, the ADA Health Foundation sponsors 30 research associates at NIST. The collaborative relationship between that professional association and the federal government is unique, and continues to develop and transfer important new technologies to dentistry and medicine.
Significant Accomplishments
- A series of highly fluorinated dimethacrylate monomers were prepared and evaluated in homopolymerization studies. Dental composites with enhanced durability and color stability may result from several of the new fluoropolymers that combine excellent mechanical strength with extreme hydrophobicity.
- A near-infrared analytical technique was developed to allow the determination of bulk polymerization conversion on dental resin and composite specimens. This provides a nondestructive method to analyze conversion directly on a variety of test specimens such as flexural strength bars or water sorption disks.
- An effective dental bonding system for both enamel and dentin was developed based on mineral acid-modified phenyliminodiacetic acid. In addition, it was demonstrated that methoxy ring substitution significantly enhanced the polymerization-initiating properties of phenyliminodiacetic acid without adversely affecting its self-etching and priming abilities. The ability to enhance both the etching and polymerization-initiating ability of phenyliminodiacetic acid permits the development of simplified, highly effective adhesive systems for the entire tooth structure.
- Further modification in the microbond test has led to the successful development of a micro-shear bond test for studying dental adhesion of extremely small bonded areas. This new test facilitates surface and depth profile mapping of tooth structure, and reduces the need for extracted teeth in the evaluation of dental adhesive systems.
- Fluorescent probes were successfully used for cure monitoring of dental bonding resins and bone cements. Improved products will result from the addition of one of the probes to dental bonding resins or bone cements.
- A new accelerated wear resistance machine was developed in collaboration with the Ceramics Division and the University of Maryland under a NIST research consortium formed by CRADAs with four orthopaedic companies: Biomet Inc. and Zimmer Inc, both of Warsaw, Ind.; Johnson & Johnson Professional Inc., Raynham, Mass.; and Oesteonics Inc., Allendale, New Jersey. The new test machine allows for evaluation of new orthopaedic joint materials in five days or less, under a variety of loads, load-cycle and motion conditions promises to hasten the introduction of longer lasting hip, knee, and other orthopaedic joint implants to the market. As a result of this successful development, the four companies elected to seek extension of their CRADAs to September 30, 2000.
Dental Resins Based on Fluorinated Monomers and Oligomers
Jeffrey W. Stansbury
Objective
The objective of this project is the development of new resins and coupling agents that will provide dental composite restoratives with significantly improved durability and reduced polymerization shrinkage compared with current dental materials.
Technical Description
Through appropriate monomer design, the resin phase of dental composites can undergo efficient photopolymerization with minimal shrinkage. The use of oligomeric multimethacrylates, cyclopolymerizable methacrylate monomers or spiro orthocarbonate monomers that undergo double ring-opening polymerization all provide routes to low shrinkage polymerization processes. A reduction in the volume contraction associated with polymerization is necessary to avoid excessive internal stresses that result in both micro and macro defects in dental composites. The lack of long-term durability of dental composites compared with amalgam restorations requires that material modifications be made in the polymeric and interfacial phases of composites, which are the weak links in this regard. Thus, there is need for more hydrophobic, more hydrolytically stable polymers and interfacial layers to enhance the resistance of composite restoratives to the challenging oral environment.
The use of ring-opening spiro orthocarbonate (SOC) monomers, which can produce expansion during polymerization, and monomers with bulky spacer groups that limit the density of reactive groups can provide photocurable resins that offer low shrinkage and good mechanical strength properties. The introduction of organofluorine substituents in the monomers allows the formation of inert polymers that repel not only water, but a wide range of chemicals that can potentially degrade or stain polymer-based dental restorations. Structural alterations to increase the hydrophobicity of the silane coupling agent used to fuse the resin matrix with the reinforcing filler particles can also serve to improve the strength and stability of dental composites.
External Collaborations
Dr. Kyung Choi, who had been working under a NIDR-supported contract between NIST and the University of California, Irvine, completed her portion of the project to develop new fluorinated resins and composites.
Dr. Chetan Khatri is presently working on urethane-containing methacrylate monomers under a contract between NIST and the University of Massachusetts.
Professor Chris Bowman of the Chemical Engineering Department at the University of Colorado in Boulder is collaborating on projects involving photopolymerization cure kinetics and development of new methacrylate monomers.
Dr. Hidekazu Takahashi of Tokyo Medical and Dental University is involved with durability studies of fluorinated resins and composites.
Planned Outcomes
- Development of visible light photocurable dental resins with less than half the polymerization shrinkage of conventional materials. Expect these new monomers to provide low shrinkage polymers with less residual stress and minimized polymerization-induced defects.
- Development of organofluorine-containing dental composites with less than 1/10th the water uptake of conventional materials. Expect more durable composites that retain their excellent initial properties during prolonged exposure to challenging environments.*
- Improve understanding of structural factors of methacrylate monomers that control their photopolymerization kinetics, comonomer interactions and network development.
Accomplishments
Fluorinated resins and composites:New fluorinated monomers were prepared and evaluated in homopolymerization studies. Amorphous dimethacrylate monomers with perfluoroalkane segments of C4 to C10 had viscosities suitable for use as hydrophobic diluent comonomers but with low mechanical strength properties. Novel fluorinated dimethacrylates with aromatic core groups produced photocured homopolymers with high flexural strengths as well as low levels of water uptake. Unlike prior examples of fluorinated oligomers, which showed significant mechanical reinforcement due to urethane group hydrogen bonding interactions, the addition of urethane functionality in fluorinated dimethacrylate monomers did not provide significant mechanical property improvements. Further, the presence of the polar urethane groups led to unacceptable levels of water uptake in the polymers.
Photopolymerization kinetics of methacrylate resins: Resin viscosities and hydrogen bonding interactions were examined using rheometric and FT-IR analyses. The photopolymerization reaction rate shows a clear dependence on resin viscosity, which originates primarily from hydrogen bonding interactions. In addition, structural variations among the different methacrylate monomers examined influence other factors, such as diffusion, cross-linking potential and possibly polymerization reaction pathways. These factors combine to provide significantly different kinetic profiles for the various dental resins studied.
Variations in coreactivity of dental monomers: A study of Bis-GMA/TEGDMA and urethane dimethacrylate/TEGDMA resins was conducted to determine the compositional drift that occurs during the photo-activated, cross-linking copolymerization of these widely used materials. The photopolymerization of unfilled resins was controlled by varying the irradiation time to give polymers with conversions ranging from 20 % to 80 %. The extraction residues from the resulting polymers were analyzed by FT-IR and NMR to determine the composition of the sol fraction (and by difference the composition of the gel fraction). The results show that at all stages of the copolymerization, TEGDMA is more reactive than either Bis-GMA or UDMA but that the reactivity bias with Bis-GMA, which has a more limited diffusion potential than UDMA, is particularly pronounced. This has implications as to the network structures of the resulting cross-linked polymers. It is also critical information for evaluation of data concerned with the evolution of physical properties during the course of the polymerization. This extraction technique can provide useful information on the coreactivity of fluorinated monomers with conventional dimethacrylate comonomers.
Near-infrared analyses of dental resins: A near-infrared (NIR) analytical technique was developed to determine bulk polymerization conversion of dental resins and composites with specimen geometries of approximately 0.5 mm to >5 mm rather than the typical thin film or KBr pellet sampling approaches. There is a significant exotherm that accompanies the rapid photopolymerization of methacrylates in bulk samples that is not accounted for in the thin film conversion measurements dictated by the traditional mid-IR technique. The NIR technique can be applied to samples with dimensions that are practical as models for actual dental restorations. This approach offers a new method for direct study of the influence of filler on conversion in composites. At the same time, a separate region of the near-IR spectrum provides valuable information about the water content and the hydrogen bonding interactions of water within the polymeric network.
Polymerization kinetics of urethane-based methacrylate monomers and oligomers: Photo- differential calorimetry was used to examine the polymerization kinetics of resins based on either a commercially available urethane dimethacrylate monomer (UDMA) or a series of experimental urethane-containing monomers and oligomers. The results demonstrated that the smaller UDMA monomer had slightly higher polymerization reactivity than the experimental monomers with the bulky aliphatic core urethane group based on the high molecular mass diisocyanate DDI. Addition of an aromatic substituent near the terminal methacrylate groups did not decrease reactivity. Resins from the experimental urethane monomers had significantly greater degrees of conversion than UDMA resins. The resins based on a multifunctional urethane oligomer had similar reactivities to those of the urethane monomers. This is preliminary work that can be applied to the fluorinated urethane monomers and oligomers in our NIDR proposal.
Impact
The provisional patent filed in 1996 on the new fluorinated resins was converted to a full patent filing in November 1997. A dental materials producer has expressed interest in the fluorinated monomers as alternatives to Bis-GMA based dental resins. This same company is currently conducting internal evaluations of some spiro orthocarbonate oligomers for low shrinking dental resins. The work on the coreactivity of dimethacrylate monomers in cross-linking polymerizations is the first in this area.
Outputs
Publications
J. W. Stansbury, K. M. Choi, C. A. Khatri, B. B. Reed, and S. H. Dickens, Photopolymerization Kinetics of Methacrylate Dental Resins, J. Dent. Res. 77, 155, Abstract 393 (1998).
J. M Antonucci, B. O. Fowler, and J. W. Stansbury, Facile Synthesis of Silanated Derivatives of Bis-GMA, J. Dent. Res. 77, 260, Abstract 1233 (1998).
J. W. Stansbury, S. H. Dickens, and C. A. Khatri, Compositional Drift During the Copolymerization of Dental Resins, J. Dent. Res. 77, 818, Abstract 1495 (1998).
J. W. Stansbury, and K. M. Choi, Homopolymerization Studies of New Fluorinated Dimethacrylate Monomers, Amer. Chem. Soc., Polym. Prepr. 39(2), 878 (1998).
J. M. Antonucci, B. O. Fowler, and J. W. Stansbury, Facile Syntheses of Oligomeric Organofluorosilsesquioxanes, Amer. Chem. Soc., Polym. Prepr. 39(2), 810 (1998).
L. G. Lovell, D. C. Syrpes, J. W. Stansbury, and C. N. Bowman, Dimethacrylate Dental Resins: The Effect of Comonomer on the Polymerization Kinetics, Amer. Chem. Soc., Polym. Prepr. 39(2), 260 (1998).
Presentations
J. W. Stansbury, New Polymers for Use in Dental Materials, Chemical Engineering Department, University of Colorado, Boulder, CO, March 9, 1998.
J. W. Stansbury, and K. M. Choi, Homopolymerization Studies of New Fluorinated Dimethacrylate Monomers, American Chemical Society, Fluoropolymer Symposium, Boston, MA, August 26, 1998.
J. M. Antonucci, B. O. Fowler, and J. W. Stansbury, Facile Syntheses of Oligomeric Organofluorosilsesquioxanes, American Chemical Society, Fluoropolymer Symposium, Boston, MA, August 25, 1998.
J. W. Stansbury, K. M. Choi, C. A. Khatri, B. B. Reed, and S. H. Dickens, Photopolymerization Kinetics of Methacrylate Dental Resins, American Association for Dental Research Meeting, Minneapolis, MN, March 5, 1998.
J. W. Stansbury, S. H. Dickens, and C. A. Khatri, Compositional Drift During the Copolymerization of Dental Resins, International Association for Dental Research Meeting, Nice, France, June 25, 1998.
Patents
J. W. Stansbury, J. M. Antonucci, and K. M. Choi, High strength polymeric networks derived from (meth)acrylate resins with organofluorine content and process for preparing same, US patent application filed November 12, 1997.
J. M. Antonucci, J. W. Stansbury, and B. O. Fowler, Novel methods and compositions for the preparation of silyl derivatives of resins. Provisional patent application filed December 1997.
Dental Composites With Improved Interfaces
Joseph M. Antonucci, Walter G. McDonough, and Joy P. Dunkers
Objective
The objective is to develop polymeric dental composites with enhanced interfacial strength and durability.
Technical Description
Microbond Test: It is anticipated that improvements in the quality of the interfacial phase, along with similar improvements in the polymer matrix and filler phases will result in dental composites of improved performance and service life. In addition to the use of traditional mechanical tests as a means of measuring interfacial strength, two single fiber tests, the microbond test and the single fiber fragmentation test, have also been used to assess interfacial behavior. Previously it was shown that the microbond test can be a useful method for assessing polymer-fiber shear bond strengths and for exploring the interaction of silane coupling agents with silica surfaces and dental polymers. The microbond test also has been used successfully in durability studies and, most recently, has been adapted to assess the interfacial shear strength between dental polymers and flat substrates such as quartz slides and sliced specimens of dentin and enamel.
Development of a Micro-Shear Test for the Study of Dental Adhesion: It is common in many dental adhesion studies to employ a shear bond test as a method of assessing the strength of the interfacial bond between enamel or dentin and adhesive dental materials. In these tests, bonding areas of about 12-14 mm2 are typically used. With the modified microbond test, bonded areas of 1.5-3.2 mm2 can be studied. Recent experiments with both commercial and experimental adhesive materials have shown that it is feasible to employ the microbond test apparatus to study bonding at multiple sites on one flat tooth specimen of enamel and dentin. Such adhesive studies are difficult, if not impossible, with conventional shear adhesion tests. The ability of the micro-shear test to perform adhesive mapping of heterogeneous substrates such as dentin has obvious advantages. It can also be used to study adhesion in small, less accessible regions of tooth structure, e.g., proximal areas. The micro-shear bond test also is expected to facilitate depth profile and durability adhesion studies. A major advantage of the micro-shear bond test is that it reduces significantly the need for extracted teeth to conduct in vitro adhesion studies. Future studies are planned with more homogeneously characterized substrates (e.g., metals, porcelain) and to explore the relationship between the size of the bonding area and interfacial strength. Surface analysis studies of debonded surfaces using scanning electron microscopy and FT-IR microscopy also are planned.
Single-Fiber Fragmentation Test: Thus far, attempts to use the single-fiber fragmentation test to determine the strength and durability of the interface derived from dental polymers and variously treated glass fibers have met with limited success due to the premature fracture of the brittle polymer matrices. Preliminary experiments with more flexible resin systems suggest that this approach may obviate this problem.
Synthesis of Oligomeric Organosilsesquioxanes: There has been increasing interest in organo-silsesquioxanes, especially those that are oligomeric or polymeric, and those that can be further polymerized. The generic structure of organosilsesquioxanes can be represented by the empirical formula (RsiO1.5)x, where R is an organic substituent and x designates the number of repeat units in what is often a complex molecular structure. In previous studies, controlled hydrolysis-condensation reactions of trialkoxysilanes such as 3-methacryloxypropyltrimethoxysilane were used to synthesize reactive oligomers having basically (RsiO1.5)x structures. These oligomers were characterized by FT-IR and nuclear magnetic resonance (NMR) and also shown by gas permeation chromatography (GPC) to have a broad distribution of molecular weights (polydispersity = 3.51) and Mw ~ 7000. Recently, the complex, three dimensional structure of a methacrylic organosilsesquioxane has been studied by MALDI (Matrix Assisted Laser Desorption Ionization) time-of-flight (TOF) mass spectroscopy. Analysis of the mass spectral data suggest that this (RSiO1.5)x has significant elements of ladder structure with some linear branched and polyhedral structure as well (see Mass Spectrometry of Polymers, Charles Guttman et al.) The methacrylic functionalized oligo-silsesquioxanes are readily polymerized by free radical methods to yield clear, hard, cross-linked polymers and are undergoing evaluation in dental resin systems and composites.
In an effort to understand the influence of the organic substituent, R, on the molecular structure and properties of (RSiO1.5)x, several types of organosilsesquioxanes have been synthesized and characterized by FT-IR and 1HNMR spectroscopies. MALDI TOF mass spectrometric analysis of these new (RSiO1.5)x are planned. Of especial interest has been the synthesis of oligomeric organofluorosilsesquioxanes from commercially available fluoroorganosilane agents. By co-oligomerization of fluoroorganosilanes with methacrylic silane agents, it appears feasible to synthesize methacrylic organofluorosilsesquioxanes.
Synthesis of Silylated Resins: In a related research initiative, it was demonstrated that simple exchange (transetherification) reactions between hydroxylated monomers such as BIS-GMA and organotrialkoxysilanes such as 3-methacryloxypropyltrimethoxysilane were feasible routes to silylated resins. Characterization by FT-IR and 1HNMR spectroscopies indicate that a significant number of hydroxyl groups of BIS-GMA have been converted to silyl ether groups. The extent of the transetherification appears to be amenable to control by proper selection of reactant composition and reaction conditions. This facile silylation technique can be adapted for the preparation of new types of silane coupling agents. Other applications include self-adhesive sealants, coatings and matrix resins. An invention disclosure has been submitted.
External Collaboration
B. O. Fowler, a Guest Scientist from the Craniofacial and Skeletal Diseases Branch of the National Institute of Dental Research is assisting in the characterization of the silsesquioxanes and silylated resins.
Dr. Hidekazu Takahashi of the Department of Dental Technology of the Tokyo Medical and Dental University is assisting in the mechanical property evaluation of experimental composites.
Dr. Yasushi Shimada of the Department of Operative Dentistry of the Tokyo Medical and Dental University is actively assisting in the evaluation of new dental adhesive systems and the development of a microshear bonding test.
Accomplishments
By further refinements of the microbond test, a micro shear bond test capable of studying dental adhesion of extremely small bonded areas, and that permits surface and depth profile mapping of mineralized tissue substrates has been developed.
Facile syntheses of organofluorosilsesquioxanes and methacrylic organo-fluorosilsesquioxanes from readily available silane coupling agents were developed. These reactive oligomers are readily polymerized by free radical methods and have potential for use in dental composites, coatings, sealants and adhesives. These fast curing resins are expected to yield polymers with low dielectric properties and high resistance to aggressive chemical environments. Polymers with these properties also may be suitable for electronic packaging applications.
Facile silylation techniques for converting hydroxylated monomers to their silyl derivatives were developed. There novel resins are expected to have self-adhesive properties and be suitable for potential applications in composites, coatings, sealants and adhesives.
Impact
An invention disclosure based on the synthesis of silylated resins has been submitted. These novel resins have potential as self-adhesive matrices for use in composites, coatings and sealants. This technology also permits the facile preparation of improved types of silane coupling agents.
Outputs
Publications
W. G. McDonough, J. M. Antonucci, and J. P. Dunkers, Evaluation of Dental Adhesion and Durability by Means of the Microbond Test, Proceedings of the 21st Meeting of the Adhesion Society, Savannah, GA, Feb. 22-25, 1998, pp. 129-131.
W. G. McDonough, J. M. Antonucci, G. E. Schumacher, and J. P. Dunkers, Shear Strengths of Dentin-Polymer Interfaces by the Microbond Test, J.Dent Res. 77, 259 Abstract 1228 (1998).
J. M. Antonucci, B. O. Fowler, J. W. Stansbury, and H. Takahashi, Synthesis and Evaluation of an Acrylic Organosilsesquioxane, Transactions of the Third International Congress on Dental Materials, Waikiki, Hawaii, November 4-8, 1997, p. 314.
J. M. Antonucci, B. O. Fowler, and J. W. Stansbury, Facile Synthesis of Oligomeric Organofluorosilsesquioxanes, Polymer Preprint 39 (2), 810-811 (1998).
J. M. Antonucci, B. O. Fowler, and J. W. Stansbury, Facile Synthesis of Silanated Derivatives of Bis-GMA, J.Dent Res. 77, 260 Abstract 1233 (1998).
Presentations
J. M. Antonucci, B. O. Fowler, J. W. Stansbury, and H. Takahashi, Synthesis and Evaluation of an Acrylic Organosilsesquioxane, Third International Congress on Dental Materials, Waikiki, Hawaii, November 4-8, 1997.
J. M. Antonucci, W. G. McDonough, and J. P. Dunkers, Evaluation of Dental Adhesion and Durability by Means of the Microbond Test, 21st Meeting of the Adhesion Society, Savannah, GA, Feb. 22-25, 1998.
W. G. McDonough, J. M. Antonucci, G. E. Schumacher, and J. P. Dunkers, Shear Strengths of Dentin-Polymer Interfaces by the Microbond Test, AADR Meeting, Minneapolis, MN, March 4-7, 1998.
J. M. Antonucci, B. O. Fowler, and J. W. Stansbury, Facile Synthesis of Silanated Derivatives of Bis-GMA, AADR Meeting, Minneapolis, MN, March 4-7, 1998 .
J. M. Antonucci, B. O. Fowler, and J. W. Stansbury, Facile Systhesis of Oligomeric Organofluorosilsesquioxanes, Fluoropolymer Symposium at the ACS Meeting, Boston, MA, August, 1998.
Improved Adhesive Systems for Bonding to Tooth Structure
Joseph M. Antonucci
Objectives
The overall objective is to develop effective, less technique-sensitive adhesive systems for bonding to dentin and other dental substrates. An ancillary objective is to elucidate the mechanisms involved in bonding to dentin by means of N-aryl-?-amino acids and N-arylimino acids.
Technical Description
Previously it was shown that the application of aqueous acetone solutions of N-phenyliminodiacetic acid (PIDAA) to dentin both etches and activates the dentinal surface for subsequent polymerization of photoactivated adhesive resins. With this simple two-step bonding protocol strong composite-to-dentin bonds were routinely obtained. Recent studies indicate that these unique adhesive systems can be modified by the addition of silane coupling agents without adversely affecting their adhesion to dentin. The presence of silane coupling agents has potential for mediating adhesion to other dental substrates such as porcelain. In order to make the PIDAA bonding system applicable to enamel as well as dentin, a study was initiated to determine the effect of adding mineral acids to this self-etching primer. With phosphoric acid-modified PIDAA, relatively poor bonding to both enamel and dentin occurred. By contrast, nitric acid-modified PIDAA gave excellent bonding to both enamel and dentin. Other types of acid-modified PIDAA systems are currently under study. In addition, in order to gain further insight into the etching, priming and polymerization-initiating properties of PIDAA, several analogs of PIDAA have been synthesized and characterized. It was found that methoxy ring substituents enhanced the polymerization-initiating properties of PIDAA without adversely affecting its self-etching and priming properties. Presumably the electron-donating-methoxy group enhances the electron density of the nitrogen of the substituted PIDAA thereby enhancing its initiating potential.
External Collaborations
Dr. Eichmiller, Dr. Farahani and Dr. G. E. Schumacher, Guest Scientists from Paffenbarger Research Center are assisting in the evaluation of new adhesive systems for dentin and enamel.
Dr. T. Nikaido a guest scientist from the Tokyo Medical and Dental University is assisting in the evaluation of new dental adhesive systems.
Dr. Chetan A. Khatri, a Guest Scientist from the University of Massachusetts, is actively advancing the synthetic aspects of the adhesion studies under an NIDR supported contract between NIST and the University of Massachusetts.
Accomplishments
N-phenliminodiacetic acid (PIDAA), modified with dilute nitric acid, was shown to be a multi-functional dental primer with the ability to etch effectively not only dentin but also enamel at the same time. In addition, PIDAA with methoxy ring substituents demonstrated enhanced polymerization-initiating properties compared to PIDAA, probably due to the electron-donating effect of the methoxy group.Impact
Two patents (U.S. Patent 5,498,643 and U. S. Patent 5,690,840) have been issued based on self-etching primers of the N-aryliminodiacetic acid type. The American Dental Association has licensed this invention and Caulk/Dentsply has recently sublicensed this technology. A new patent based on acid-modified PIDAA has been applied for in conjunction with the ADA.
Outputs
Publications
M. Farahani, J. M. Antonucci, and L. R. Karam, A GC-MS Study of the Addition Reaction of Arylamines with Acrylic Monomers, J. Appl. Poly. Sci. 67, 1545-1551 (1998).
J. M. Antonucci, C. A. Khatri, G. E. Schumacher, T. Nikaido, and J. E. Code, Dental Adhesion with Self-Etching, Polymerization-Initiating Primers, Proceedings of the 21st Meeting of the Adhesion Society, 126-128 (1998).
C. A. Khatri, G. E. Schumacher, and J. M. Antonucci, Self-etching, Polymerization-Initiating Primers for Dental AdhesionPolymer, Preprints 39(2), 238-239 (1998).
J. M. Antonucci, J. E. Code, P. S. Bennett, and G. E. Schumacher, Photoactivated Dentin Bonding with N-Phenyliminodiacetic Acid, Dental Materials, in press.
C. A. Khatri, and J. M. Antonucci, New Self-Etching, Polymerization-Initiating Primers for Dental Adhesion, J. Dent. Res 77, 225, Abst. 957 (1998).
Presentations
J. M. Antonucci, C. A. Khatri, G. E. Schumacher, T. Nikaido, and J. E. Code, Dental Adhesion with Self-Etching, Polymerization-initiating Primers, 21st Meeting of the Adhesion Society, Savannah, GA., February 22-25, 1998.
J. M. Antonucci, C. A. Khatri, and G. E. Schumacher; Self-etching, Polymerization-Initiating Primers for Dental Adhesion, American Chemical Society Meeting, Boston, MA, August 23-27, 1998.
P Bioactive Polymeric Dental Materials with Remineralization Potential Based on Amorphous Calcium Phosphate
Joseph M. Antonucci
Objective
The objective is to develop improved polymeric composities with remineralization potential using various forms of amorphous calcium phosphate (ACP) as the filler phase.
Technical Description
ACP, a highly soluble, non-crystalline calcium phosphate with a propensity for facile conversion to hydroxyapatite (HAP), has potential as remineralizing agent for defective mineralized tissues. Previous studies in this laboratory have shown that polymeric composites derived from dental acrylic polymers and stabilized ACP powders give sustained release of calcium and phosphate ions over extended periods of time. Composite sealants were effective in remineralizing caries-like lesions in extracted bovine enamel.
In the present study, potentially adhesive composites made of pyrophosphate-stabilized ACP and various new photocurable resin mixtures containing carboxylic acid monomers are being screened for their ion releasing potential. It is thought that the presence of the carboxylic acid functional group in the resin will confer adhesive properties on these composites. Those that exhibit adequate ion release will be further tested for mechanical and adhesive strength. Of the composites examined to date, those containing triethylene glycol dimethacrylate and the adhesive monomer PMGDM released Ca and PO4 at rates exceeding those of previously examined non-adhesive Bis-GMA resin mixtures.
External Collaborations
Dr. E.D. Eanes and Mr. A. W. Hailer of NIDR and Dr. D. Skrtic of the American Dental Association Health Foundation's Paffenbarger Research Center are actively assisting in the further development of bioactive polymeric composites utilizing ACP as a remineralizing filler.
Accomplishments
The results of this study demonstrated that experimental, visible-light curable ACP resin composites with remineralizing properties and potentially good adhesive properties can be prepared with certain carboxylic acid monomers as components of the resin phase.
Impacts
A patent has been issued based on the use of ACP as a bioactive filler in polymeric dental materials. Several companies have expressed interest in this technology.
Outputs
Publications
J. M. Antonucci, E.D. Eanes, and D. Skrtic, Bioactive Polymeric Composites Based on Hybrid Amorphous Calcium Phosphates, Polymer Preprints 39(2), 236-237 (1998).
M. S. Park, E. D. Eanes, J.M. Antonucci, and D. Skritic, Mechanical Properties of Bioactive ACP/Methacrylate Composites, Dental Materials, in press.
Presentations
J. M. Antonucci, E. D. Eanes, and D. Skrtic, Bioactive Polymeric Composites Based on Hybrid Amorphous Calcium Phosphates, 3rd International Biorelated Polymer Symposium and Polymeric Drugs and Drug Delivery Systems, ACS Meeting, Boston, MA, August 23-27, 1998.
Cure Monitoring of Dental and Medical Resins by Fluorescence Spectroscopy
Francis W. Wang
Objective
The objective is to monitor the cure of resins by fluorescence spectroscopy, to establish correlations between fluorescence properties and performance properties for curing resins, and to transfer technology in photo-physics of fluorescent probes to manufacturers of resins for dental and medical applications.
Technical Description
Cure Monitoring of Bonding Resin: The durability and the physical properties of resin-infiltrated demineralized dentin depend primarily on the completeness of the cure of the primer and the bonding resin that infiltrate the demineralized dentin. It is therefore important to develop a technique to determine the degree of cure of the resin in resin-infiltrated demineralized dentin. Fluorescence techniques are particularly useful for cure monitoring because they are sensitive and adaptable to nondestructive, in-situ monitoring. Furthermore, with the use of fluorescence microscopy, the depth dependence of the degree of cure can be observed.
A wavelength-shift fluorescent probe, whose peak fluorescence wavelength changes with the polarity and the mobility of the probe's environment, is used in this project to monitor the degree of cure of the bonding resin. In this method, a trace amount of the wavelength-shift fluorescent probe was dissolved in a bonding resin, and the fluorescence spectrum of the probe was recorded as a function of cure time during the photo-initiated cure of the bonding resin. Because the polarity and the mobility of the bonding resin change during the cure, the peak fluorescence wavelength of the fluorescent probe decreases with the cure time. The change in the peak fluorescence wavelength of the probe can be correlated the degree of cure of the bonding resin. This correlation is established by relating the change in the peak fluorescence wavelength with the degree of cure determined by FTIR. An absolute value for the degree of cure can then be obtained from the change in the peak fluorescence wavelength.
Cure Monitoring of Bone Cement: The ultimate success of methyl methacrylate bone cements in surgical arenas requires application at an appropriate viscosity. Long term durability could be compromised by the empirical nature of the approach now used to determine the setting of a particular sample. The literature from one manufacturer states that, in addition to the concentration effects one would expect in a bi-phasic free-radical system, ambient temperature and humidity can substantially affect the setting time of a sample. It suggests that "..the working time ....is best determined by the experience of the surgeon..." In this project, a trace amount of a fluorescent probe is used as an in situ indicator of the viscosity of a bone cement sample and its suitability for surgical application. In addition, the project capitalizes on a novel technique- the impedance of quenching- for monitoring the increase in local viscosity accompanying polymerization.
A bone cement is made by mixing its liquid component [97% by volume methyl methacrylate and 3% by volume N,N- dimethyl- p-toluidine] and its solid component [15% by mass poly(methyl methacrylate), 75% by mass styrene-methyl methacrylate copolymer, 10% by mass BaSO4, and 2% by mass benzoyl peroxide], often at a powder to liquid ratio of 2 to 1 by mass. After the mixing, the viscosity of the bone cement increases as the polymers dissolve in the liquid and methyl methacrylate polymerizes. If a trace amount of a fluorescent probe is dissolved in the liquid component, the probe initially fluoresces faintly because N,N- dimethyl- p-toluidine from the liquid component quenches the fluorescence. However, the fluorescence intensity of the probe increases as the increase in the viscosity of the cement suppresses the diffusion-controlled fluorescence quenching. Consequently, the increase in the fluorescence intensity of the probe is a measure of the viscosity of the bone cement. Once a correlation is established between the increase in the fluorescence intensity and the viscosity, an absolute value for the viscosity can be obtained from the change in the fluorescence intensity.
External Collaborations
Dr. Deborah G. Sauder, a Guest Scientist from Hood College, Frederick, MD, and Dr. Koichi Komatsu, a Guest Scientist from Nihon University, School of Dentistry at Matsudo, Matsudo, Chiba, Japan, are actively advancing the efforts to monitor curing of dental and medical resins by fluorescence spectroscopy.
Planned Outcome
Technology transfer, to manufacturers of dentin bonding resins and bone cements, of quality control methods for the development of bone cements and dentin bonding resins.
Accomplishments
A fluorescent probe, 4-(N,N-dibutylanilino)hexatrienyl-4'-pyridinium butylsulfonate (AHPBS), has been evaluated for cure monitoring of a bonding resin. The peak fluorescence wavelength of the fluorescent probe was shown to decrease with the cure time. A correlation was established between the peak fluorescence wavelength and the Vickers hardness of the bonding resin.
It was demonstrated that anthracene or Re(CO)3ClL, (where L=4,7-diphenyl-1,10-phenanthroline) can be used as a fluorescent probe in a bone cement. The decrease of fluorescence quenching by N,N- dimethyl- p-toluidine (DMT) was measured to monitor the change in cement viscosity. With anthracene as a probe, the ratio of the fluorescence intensity from anthracene-DMT exciplex to that from anthracene decreased linearly with time until it leveled off at 40 min while, with Re(CO)3ClL, the probe's fluorescence intensity, initially quenched, increased linearly from 5 min to 10 min after which the intensity leveled off. Since, for these probes, the linear change in the intensity ratio or the probe's intensity occurred over the working time of a bone cement, they are suitable as in-situ indicators of the viscosity of the bone cement.
Outputs
Presentations
F. W. Wang, and D. G. Sauder, Application of Fluorescent Molecules As In Situ Probes of Bone Cement Cure, Polymer Preprints 39(2), 256 (1998).
K. Komatsu, and F. W. Wang, Fluorescent Probe for Cure Monitoring of Bonding Agent on Surface, Proceedings of the Dental Materials Group, International Association for Dental Research Meeting, #1410, June, 1998.
F. W. Wang and D. G. Sauder, Cure Monitoring of Bone Cement by Fluorescence Spectroscopy, Proceedings of the Dental Materials Group, International Association for Dental Research Meeting, #64, June, 1998.
Presentations
F. W. Wang, Fluorescence monitoring of polymerization, 3M Engineering Systems and Technology, St. Paul, MN, November 20, 1997.
F. W. Wang, Collaborative research on dental and medical materials at Dental and Medical Materials Group, Polymers Division, Dental Products Division, 3M Pharmaceutical and Dental Products Group, St. Paul, MN, November 20, 1997.
F. W. Wang, and D. G. Sauder, Application of Fluorescent Molecules As In Situ Probes of Bone Cement Cure, American Chemical Society Meeting, Boston, MA, August 25, 1998.
K. Komatsu, and F. W. Wang, Fluorescent Probe for Cure Monitoring of Bonding Agent on Surface, Dental Materials Group, International Association for Dental Research Meeting, Nice, France, June, 1998.
F. W. Wang, and D. G. Sauder, Cure Monitoring of Bone Cement by Fluorescence Spectroscopy, Dental Materials Group, International Association for Dental Research Meeting, Nice, France, June, 1998.
Patent:
F. W. Wang, and D. G. Sauder, Non-destructive method for determining the extent of cure of a polymerizing material and the solidification of a thermoplastic polymer based on quenching of fluorescence. Provisional patent filed November 19, 1997.
Support for the Biomaterials Integrated Products Industries
J. A. Tesk, Steve Hsu1, C. Johnson2, Jeffrey Stansbury, Ming Tung3, Drago Skrtic3, and Alan Litsky 4
1Ceramics Division, NIST
2Metallurgy Division, NIST
3American Dental Association Research Associate, NIST
4Ohio State University, Columbus, Ohio
Objective
The objective is to provide support for the American biomedical industry via development of test methods and standards needed by industry, provision of reference materials, and exploratory research into new materials and properties with useful benefits, such as: employment of cyclopolymerizable monomers as substitutes for poly-(methylmethacrylate) in orthopaedic bone cement and denture base applications, and metallic glasses for coatings of orthopaedic implants for bearing surfaces or boney fixation.
Technical Description
Objectives will be met by forming cooperative relationships between industry, NIST, other government agencies (NIH and FDA), and standards bodies to identify needs and develop strategies to meet needs. This is done by sponsoring workshops, developing CRADAs and other agreements, and actively supporting standards related activities. New materials and needed property data will be explored within NIST, and with universities and industry.
External Collaborations
Biomet Inc. (Warsaw Ind.), Johnson & Johnson Professional, Inc. (Raynham, Mass.), Ostenoics Inc. (Allendale, N.J.), and Zimmer, Inc., (Warsaw, Ind.), are members of the Cooperative Research and Development Agreement (CRADA) Consortium, established to develop a new wear test machine and test for accelerating the evaluation of the wear properties on orthopaedic joint implant materials and to provide partial support.
Poly Hi Solidur (Fort Wayne, Ind), and Teledyne Allvac/Vasco (Monroe, NC) have or will be contributing materials to the consortium.
Professor Alan Litsky (Ohio State University) is contributing to research on use of cyclopolmerizable monomers as acrylic biomaterials.
American Association of Orthopaedic Surgeons, Hip and Knee Society is contributing to release of public information of the CRADA Research.
National Institutes of Health (Member,Biomaterials Implant Science Coordinating Committee) & Food and Drug Administration helped plan two industry workshops, one on Biomedical Reference Materials and the other on Heart Valves with Carbomedics Inc., Carbon Research Inc., Medtronic, Inc., St. Jude Medical, Inc., Baxter , Inc., and Alliance, Inc.
Planned Outcomes
Accomplishments
An accelerated-wear tester and test methodology was developed to evaluate the wear properties of orthopaedic joint materials, developed during year two of Cooperative Research & Development Agreements (CRADAs). Four (4) orthopaedic companies joined together as a CRADA-formed consortium and based on the successful development of the test machine these four companies requested extension of the CRADAS (to September 30, 2000) to support research on evaluation of new materials. They have opened the opportunity for research to others who may wish to join the consortium as the benefits are perceived useful to the industry as a whole for standards and regulatory purposes and in the development of basic understanding of the wear resistance of implant joint materials.
Through its standards and reference materials effort, this program facilitated a NIST-NIH-FDA-Industry collaboration for NIST dissemination of Industry Developed Polyetherurethane, low density polyethylene, and silica free poly(dimethylsiloxane) reference materials, useful for development of cardiovascular and other devices. A workshop on Biomedical Reference Materials was planned for November 13, 1998, with cooperation of the NIH, FDA, Industry, and the Society for Biomaterials.
A workshop on Carbon Heart Valves was held on February 23 with the heart valve industry and a follow-up workshop is scheduled for December 2, 1998.
An ASTM Workshop on Standards for Tissue-Engineered Medical Products was held at NIST on December 16, 1997.
A method was found for using poly(methyl methacrylate) powders with co-monomers of cyclopolymerizable and methylacrylate types to produce doughs and heat curing in times comparable to those used for dental denture base (DB) formulations, but which can persist for a over one day. Extended-time dough state could be expected to be labor saving as a DB material. Other potential benefits include reduced polymerization shrinkage (Bone Cement, DB, & DB reline); for improved fits and, due to low vapor pressure, elimination of noxious odor. In the heat cured condition (105 °C) the fraction of double bonds converted was 96 %.
Outputs
Publications
J. A. Tesk, and Christian E. Johnson, Glassy Alloys as Potential Coatings for Orthopaedic Applications, Program Abstract for ASTM Symposium on Alternative Bearings for Orthopaedic Joints, San Diego, November, 1997, P. 25.
J. A. Tesk, Jeffrey W. Stansbury, and Alan Litsky Cyclopolymerizable Monomers as Acrylic Biomaterials, Transactions of the Society for Biomaterials, Vol 21, Abstr, 373, San Diego, CA., Apr. 22-26, 1998.
G. L. Picciolo, K. B. Hellman, J. A. Tesk, R. Valentini, A. Coury, and P.C. Johnson, Living Standards, Biomaterials FORUM, Vol 20, #2, March-April, 1998, pp. 17-18.
G. L. Picciolo, J. Tesk, R. Valentini, A. Coury, and P. C. Johnson, Tissue Engineering Products Standards: Living Standards for Living Tissue, on Web at www.pittsburgh-tissue.net/brochure/Outreach/TEMPS.html.
J. A. Tesk, and Christian E. Johnson, Glassy Alloys as Potential Coatings for Orthopaedic Applications, ASTM Special Symposium Publication Alternative Bearings for Orthopaedic Joints, in press.
J. A. Tesk, S Hsu, and M. Shen, Quarterly Reports (confidential) to Orthopaedic CRADA Research Consortium, September, December, March, June, FY 1998.
Presentations
J. A. Tesk, and Christian E. Johnson, Glassy Alloys as Potential Coatings for Orthopaedic Applications, ASTM Symposium on Alternative Bearing Surfaces in Total Joint Replacement, Sheraton Harbor Hotel, San Diego, CA., November 11-12, 1998.
J. A. Tesk, Jeffrey W. Stansbury, and Alan Litsky Cyclopolymerizable Monomers as Acrylic Biomaterials, Annual meeting of the Society for Biomaterials, San Diego, CA., Apr. 22-26, 1998.
J. A. Tesk, Jeffrey W. Stansbury, and Alan Litsky Cyclopolymerizable Monomers as Acrylic Biomaterials, Johns Hopkins University Symposium, Biological Response to Orthopaedic Implants, Renaissance Harborplace Hotel, Baltimore, MD., May 1 -2, 1998.
J. A. Tesk, Traditional Methods by which The Laboratories of The National Institute of Standards & Technology may help meet the needs of the Biomedical Devices Industry, presented to the Board of the Alliance for Medicine and Biology in Engineering, Washington Marriot Hotel, Washington DC, May 2, 1998.
OTHER PROJECTS
In response to requests from other federal agencies, the Division conducts research on a reimbursable basis to solve specific needs of the requesting agency. The nature of these activities changes from year to year depending on national priorities. During the past year, work has been conducted on one such project for the Bureau of consular Affairs, U.S. Department of State.
Development and Utilization of Test Methods for Qualifications of Passport Laminates
W.G. McDonough, and D.L. Hunston
Objective
The objective is to perform testing and to provide consultation to assist the Bureau of Consular Affairs at the U.S. Department of State in selecting and purchasing the plastic laminate that is used to protect the photograph and descriptive information on the U.S. Passport.
Technical Description
NIST has been under contract for the past ten years to test and evaluate the laminate security film that is used to cover the data page in the U.S. Passport. This laminate protects the data page from fraud and counterfeiting attempts as well as protecting the data page from dirt, abrasion and accidental damage. Prior testing and evaluations performed at NIST on the passport laminates revealed limitations in the current technology's ability to protect the data page. This finding prompted the Department of State to investigate and develop alternatives to the processing and design of the U.S. Passport. The role of NIST has been to evaluate proposed products and to identify areas of vulnerability and strength. In fact, NIST has been and continues to be the primary agency to evaluate new products.
External Collaborations
John Mercer, and Richard McClevey of the U.S. Department of State
Accomplishments
The NIST part of the Next Generation U. S. Passport project was successfully commpleted. Future passports shall incorporate digital imaging and a new high security laminate to protect against tampering or forgery. The production method for this new passport shall be made in a "one-step" process that shall greatly reduce production times and costs. The work done at NIST to assess the quality of submitted products and to evaluate the test results was a critical factor in deciding the winning proposal. In fact, it was work at NIST examining the level of security in the current passport system that led to the decision to pursue the novel approach of digital imaging.
Impact
The procurement was a $63 million award over the next five years, and the implementation of this one-step, digitized image passport is the first of its kind for a large country. Several countries are following the United States' technical lead in this field and are planning to issue similar passports. It is expected that the technology incorporated into the new U.S. Passport shall eventually become an international standard.
Outputs
Report on the test results for the technical evaluation of submitted products was sent to the Department of State.
