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 focus of this program is the development of improved dental restorative materials with greater durability, wear resistance and clinical acceptability. 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 mass 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 is 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 mass 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. Dental research directions in support of the goals are established in collaboration with the American Dental Association(ADA), the National Institute of Dental Research (NDIR), 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 32 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
Dental Resins Based on Fluorinated Monomers and Oligomers Jeffrey W. Stansbury Objective The objective 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 and 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 mass. 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, a Guest Scientist from the University of California, Irvine, is actively advancing the work with fluorinated resins and modified coupling agents under an NIDR-supported contract between NIST and the University of California, Irvine. Planned Outcomes
Accomplishments Fluorinated resins and composites: New fluorinated urethane monomers and oligomers have been prepared and several have been identified that produce polymerized resins with acceptably high flexural strengths as well as high water contact angles and very low levels of water uptake that are indicative of improved long-term stability. Studies with composites have shown that the introduction of a more hydrophobic silane coupling agent improves compatibility between the fluorinated resins and fillers while allowing for significantly higher filler loading levels. Photo-DSC studies have been used to compare the polymerization inhibition effects of oxygen on fluorinated and conventional resins. Air inhibition effects were demonstrated in the bulk rather than just at the surface of both fluorocarbon and hydrocarbon polymers. A separate study showed a modest polymerization inhibition effect associated with low to moderate levels of filler, while high filler levels resulted in more significant reductions in monomer conversion. Dimethacrylate comonomers for fluorinated resins: An evaluation of homopolymers of commercially available hydrocarbon dimethacrylate diluent monomers was made to select an appropriate comonomer for use in fluorinated resin formulations. While ether-containing dimethacrylates produce high strength polymers, water uptake was judged to be excessive for these polymers and previous experience has indicated problems with phase separation in fluorinated resins. Based on photopolymerization rate, water contact angle, water uptake and flexural strength results, 1,12-dodecanediol dimethacrylate was chosen as an acceptable, compatible comonomer for use with a wide variety of fluorinated monomers and reactive oligomers. Photopolymerization kinetics of methacrylate resins: A photo-differential calorimetry-based investigation was undertaken with the following conventional dental resins: 1) Bis-GMA/triethylene glycol dimethacrylate (TEGDMA); 2) ethoxylated bisphenol A dimethacrylate/TEGDMA; and 3) urethane dimethacrylate (UDMA)/TEGDMA. A broad range of comonomer compositions were evaluated for each resin. In each resin system, the more mobile TEGDMA monomer tended to increase the overall conversion but the more viscous monomers were responsible for enhanced resin reactivity. Resin viscosities were measured and correlated with the photopolymerization reactivities. In addition, hydrogen bonding interactions in the Bis-GMA and UDMA resin systems were determined by FT-IR. For Bis-GMA/TEGDMA resins in particular, the compositions with the highest photopolymerization rates cured much more rapidly than either of the individual comonomers alone. Bis-GMA/TEGDMA resins were most reactive at an approximately equimolar composition while the other resins were most reactive when only a small amount of TEGDMA was included. The UDMA/TEGDMA resin system had the highest overall optimized resin reactivity. The photopolymerization reaction rate is clearly related to resin viscosity. Optimum reactivity occurs when resin viscosity allows sufficient mobility of small monomer molecules but limits termination reactions between macromolecular radicals. The result is an essentially instant onset of the autoacceleration effect. Expanding polymers: Water sensitivity problems were demonstrated during the homopolymerizations of spiro orthocarbonate (SOC) monomers and oligomers. While the ring-opened polymers are relatively hydrophobic, the bicyclic monomers are hydrophilic and absorb water. With appropriate SOC structures, the water does not degrade the monomer; however, during cationic polymerization with strong acid catalysts, hydrolysis of the SOC linkage to diol and cyclic carbonate competes quite effectively with the desired ring-opening polymerization process. Under dry conditions, the new oligomeric SOC=s produce glassy, cross-linked polymers with little if any soluble fraction. Dual cure (radical/cationic) copolymerization studies with methacrylate and SOC monomers gave primarily interpenetrating network structures with minor amounts of crossover due to hydrogen abstraction. Outputs Publications K.M. Choi and J.W. Stansbury, New Families of Photocurable Oligomeric Fluoromonomers for Use in Dental Composites, Chem. Mater. 8, 2704 (1997). J.M. Antonucci and J.W. Stansbury, Molecularly Designed Dental Polymers, in: Desk Reference of Functional Polymers; Syntheses and Applications, Arshady, R., ed., Amer. Chem. Soc. Books, pp 719 (1997). J.W. Stansbury, D.-W. Liu, and S.I. Kim, Polymer-supported Quinuclidinyl Catalysts for Synthesis of Cyclopolymerizable Monomers via the Aldehyde-acrylate Coupling Reaction, Macromolecules 30, 4540 (1997). J.W. Stansbury, K.M. Choi and J.M. Antonucci, Considerations in the Development of Semifluorinated Methacrylate Dental Resins and Composites, Amer. Chem. Soc., Div. Polym. Chem., Polym. Prepr. 38, 96 (1997). J.M. Antonucci, B.O. Fowler, J.W. Stansbury and H. Takahashi, Facile Syntheses of Acrylic Organosilsesquioxanes for Dental Applications, Amer. Chem. Soc., Div. Polym. Chem., Polym. Prepr. 38, 118 (1997). B.B. Reed, K.M. Choi, S.H. Dickens and J.W. Stansbury, Effect of Resin Composition on Kinetics of Dimethacrylate Photopolymerization, Amer. Chem. Soc., Div. Polym. Chem., Polym. Prepr. 38, 108 (1997). H. Takahashi, J.M. Antonucci, J.W. Stansbury, F.W. Wang, Relationship Between the Tensile and Flexural Strength of Composite Resins, Amer. Chem. Soc., Div. Polym. Chem., Polym. Prepr. 38, 163 (1997). Presentations J.W. Stansbury, Development of fluorinated resins as matrix materials for improved composites, NIST Polymers Division Seminar, October 3, 1996. J.W. Stansbury, Designing monomers to minimize polymerization shrinkage, AT&T Bell Labs, Lucent, Murray Hill, NJ, February 3, 1997. J.W. Stansbury, Alternative polymers for use in dental materials, Institute of Materials Science, University of Connecticut, Storrs, CT, March 28, 1997. J.W. Stansbury, K.M. Choi and J.M. Antonucci, Considerations in the Development of Semifluorinated Methacrylate Dental Resins and Composites, Symposium on Dental Materials and Composites, American Chemical Society Meeting, Las Vegas, NV, September 8, 1997. H. Takahashi, J.M. Antonucci, J.W. Stansbury and F.W. Wang, Relationship Between the Tensile and Flexural Strength of Composite Resins, Symposium on Dental Materials and Composites, American Chemical Society Meeting, Las Vegas, NV, September 9, 1997. B.B. Reed, K.M. Choi, S.H. Dickens and J.W. Stansbury, Effect of Resin Composition on Kinetics of Dimethacrylate Photopolymerization, Symposium on Dental Materials and Composites, American Chemical Society Meeting, Las Vegas, NV, September 9, 1997. J.W. Stansbury and M.H. Dermann, Radical/cationic Photopolymerization of SOC-modified Methacrylate Resins, International Association for Dental Research Meeting, Orlando, FL, March 20, 1997. Patents A provisional patent was filed on November 14, 1996 (NIST #96-042PA): High strength polymeric networks derived from (meth)acrylate resins with organofluorine content and process for preparing same. Full patent filing is currently in progress. A dental materials manufacturer, has expressed interest in licensing due to potential problems with bisphenol-A based monomers, among other reasons. Dental Composites With Improved Interfaces Joseph M. Antonucci, W. G. McDonough, J. P. Dunkers and B.O. Fowler1 1National Institute of Dental Research, Bethesda, MD Objective The objective is to develop polymeric dental composites with enhanced interfacial strength and durability. Technical Description It is anticipated that improvements in the quality of the interfacial phase along with similar improvements in the polymer matrix and fiber 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. The microbond test has proved to 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 modified to assess the interfacial shear strength between dental polymers and primed dentin and enamel substrates. 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 suggests that this approach may obviate this problem. There has been increasing interest in the development of reactive organosilsesquioxanes for composite, coating, adhesive and other applications. The feasibility of synthesizing reactive organosilsesquioxanes from trialkoxysilanes such as 3-methacryloxypropyltrimethoxysilane (MPTMS) by controlled hydrolysis-condensation reactions has been investigated. A variety of solvent and catalyst systems have been studied in an effort to control the extent of oligomerization, functionality and residual silyl ether and/or silanol content. Thus far from MPTMS, both viscous liquid, multi-methacrylic oligomers (MW > 6,000) and fluid liquids consisting of mainly dimeric products have been obtained. Both types, in contrast to the starting MPTMS, are easily photopolymerized using visible light irradiation. As expected from their silsesquioxane structure, polymers derived from the viscous oligomer exhibited high thermal stability. In addition to their potential use in dental composite, sealant, and adhesive applications, these fast-curing resins also may find use in other fields such as electronic packaging. External Collaboration B. O. Fowler, a Guest Scientist from National Institute of Dental Research, characterized reactive organosilsesquioxanes for dental applications. Accomplishments
Outputs Publications W.G. McDonough, J.M. Antonucci and J.P. Dunkers, Durability of Polymer-Glass Interfaces by the Microbond Test, J. Dent. Res., 76, 38 Abstract 198 (1997). W.G. McDonough, J.M. Antonucci and J.P. Dunkers, A Study of Dental Adhesion by Means of the Microbond Test, Polymer Preprints 38, 112-113 (1997). J.M. Antonucci, B.O. Fowler and J.W. Stansbury, Facile Synthesis of Reactive Organosilsesquioxanes for Dental Applications, J. Dent. Res., 76, 40 Abstract 214 (1997). J.M. Antonucci, B.O. Fowler and J.W. Stansbury, Facile Synthesis of Acrylic Organosilsesquioxanes for Dental Applications, Polymer Preprint 38 118 (1997). Presentations W.G. McDonough, J.M. Antonucci and J.P. Dunkers, Shear Strength Measurements of Dental Polymer-Glass Fiber Interfaces Using The Microbond Test, Tokyo Medical and Dental University, Tokyo, Japan, September, 1996. W.G. McDonough, J.M. Antonucci and J.P. Dunkers, Determining Mechanical Strength Mass of Dental Composites, Tokushima University, Tokushima, Japan, September, 1996. W.G. McDonough, J M. Antonucci and J.P. Dunkers, Durability of Polymer-Glass Interfaces by the Microbond Test, 75th General Session of the International Association for Dental Research, Orlando, FL, March 19-23, 1997. W.G. McDonough, J.M. Antonucci and J.P. Dunkers, A Study of Dental Adhesion by Means of the Microbond Test, Symposium on Dental Materials and Composites at the American Chemical Society Meeting, Las Vegas, NV September 7-11, 1997. J.M. Antonucci, B.O. Fowler and J.W. Stansbury, Facile Synthesis of Reactive Organosilsesquioxanes for Dental Applications, 75th General Session of the International Association for Dental Research, Orlando, FL, March 19-23, 1997. J.M. Antonucci, B.O. Fowler and J.W. Stansbury, Facile Synthesis of Acrylic Organosilsesquioxanes for Dental Applications, Symposium on Dental Materials and Composites at the American Chemical Society Meeting, Las Vegas, NV September 7-11, 1997. 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 Recently 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-resin 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 addition, mass spectrometry was used to aid in elucidating the mechanism of initiating polymerization with N-aryl-"-amino acids. Also, gas chromatography-mass spectrometry has been used to study the Michael addition of primary and secondary aryl amines with several types of acrylic monomers. This type of reaction can provide a facile synthetic method to a variety of arylimino diacids. In order to gain further insight into the etching, priming and polymerization-initiating mass of aryliminodiacetic acids, several analogs of PIDAA have been synthesized and characterized. Evaluation of these potential adhesive agents is in progress. Studies are in progress that seek to elucidate the role(s) of a variety of primers in dentin bonding. External Collaborations Dr. Farahani, a Guest Scientist from the American Dental Association Health Foundation=s Paffenbarger Research Center, investigated mechanisms of the addition reaction of arylamines with acrylic monomers. Dr. G.E. Schumacher and P.S. Bennett, Guest Scientists from Paffenbarger Research Center, Dr. J.E. Code, a Guest Scientist from the U.S. Public Health Service, and Dr. T. Nikaido, a Guest Scientist from the Tokyo Medical and Dental University, carried out experiments on N-phenyliminodiacetic acids as an etchant/primer for dentin bonding. Accomplishments
Impact A patent (U.S. Patent 5,498,643) has 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. Outputs Publications M. Farahani, J.W. Stansbury, J.M. Antonucci and C.S. Phinney, The Addition Reaction of Aryl amines with Acrylic Monomers. A Gas Chromatography-Mass Spectrometry Study, J. Dent. Res. 76, 40 Abstract #213 (1997). J.E. Code, J.M. Antonucci, G.E. Schumacher and P.S. Bennett, Shear Bond Strengths of a Silane Modified Bonding System with Intermediary Bonding Resins, J. Dent. Res. 76, 21 Abstract #59 (1997). G.E. Schumacher and J.M. Antonucci, Acid-Modified N-Phenyliminodiacetic Acid as a Self Etching Primer in Dentin Bonding, J. Dent. Res., 76, 21 Abstract # 60 (1997). T. Nikaido and J.M. Antonucci, Effect of HEMA-Solvent Systems on Shrinkage of Demineralized Collagen, J. Dent. Res., 76, 20 Abstract #55 (1997). M. Farahani, J.M. Antonucci, L.R. Karam and C.S. Phinney, Mass Spectrometric Analysis of Polymers Derived from N-Aryl-a-Amino acid Initiators, J. Appl. Poly. Sci. 65, 561 (1997). J.M. Antonucci, G.E. Schumacher and P.S. Bennett, N-Phenyliminodiacetic Acids as an Etchant/Primer for Dentin Bonding, J. Dent. Res., 76, 602 (1997). T. Nikaido, J.M. Antonucci, G.E. Schumacher, J.E. Code and J. Tagami, Efficacy of N-Aryl-and N-alkyliminodiacetic Acids in the Stabilization of Demineralized Dentin, Polymer Preprints 38, 141 (1997). Presentations M. Farahani, J.W. Stansbury, J.M. Antonucci and C.S. Phinney, The Addition Reaction of Aryl amines with Acrylic Monomers. A Gas Chromatography-Mass Spectrometry Study, 75th General Session of the International Association for Dental Research, Orlando, FL, March 19-23, 1997. T. Nikaido, J.M. Antonucci and J. Tagami, Effect of HEMA-Solvent Systems on Shrinkage of Demineralized Collagen, 75th General Session of the International Association for Dental Research, Orlando, FL, March 19-23, 1997. J.E. Code, G.E. Schumacher, J.M. Antonucci and P.S. Bennett, Shear Bond Strengths of Silane-Modified Dentin Bonding Systems with Intermidiary Bonding Resins, 75th General Session of the International Association for Dental Research, Orlando, FL, March 19-23, 1997. G.E. Schumacher and J.M. Antonucci, Acid Modified N-Phenyliminodiacetic Acid as a Self-Etching Primer for Dentin Bonding, 75th General Session of the International Association for Dental Research, Orlando, FL, March 19-23, 1997. T. Nikaido, J.M. Antonucci, G.E. Schumacher, J.E. Code and J. Tagami, Efficacy of N-Aryl- and N-Alkyliminodiacetic Acids in the Stabilization of Demineralized Dentin, Symposium on Dental Materials and Composites, American Chemical Society Meeting, Las Vegas, NV, September 7-11, 1997. Polymer-Modified Porous Ceramics: Interpenetrating Phase Composites for Dental Applications Joseph M. Antonucci Objectives The objective is to develop strong, tough interpenetrating phase composites by the in situ polymerization of resin-infiltrated porous ceramic structures that are easily formed by low temperature pyrolysis of ceramic powders coated with pre-ceramic polymers such as polysiloxanes. Technical Description Conceptually, interpenetrating phase composites are expected to have enhanced mass compared to either dental ceramics or traditional dental composites. Previously, it was shown that a polymer-reinforced interpenetrating composite (IPC) with high strength mass could be prepared by the in situ polymerization of methyl methacrylate (MMA) within a silanized, porous ceramic network formed by the low temperature pyrolysis (<750/C) of feldspathic porcelain or alumina powders that had been coated with a poly(dimethylsiloxane) binder. The effects of silanization versus no silanization of the porous ceramic and the influence of resin composition on the biaxial tensile strength of the IPCs also have been investigated. Silane treatment of the porous ceramic network had a positive effect on the IPC, but the effects of varying resin composition were less clear cut. During the current phase of this research, the effect of using fluorinated monofunctional methacrylates as part of the infiltrating monomer system was explored. Both the strength and durability (after aqueous challenge) of the IPCs were enhanced compared to a control IPC infiltrated with methyl methacrylate. External Collaboration Dr. J. R. Kelly, a Guest Scientist from the Naval Dental Research Institute and the Naval Dental School, Bethesda, prepared porous ceramic structures by low temperature pyrosis of ceramic powders coated with pre-ceramic polymers such as polysiloxanes. Accomplishments
Impacts Two patent applications have been filed and several companies have shown interest in the technology. Outputs Publications C.R. Fahncke, J.R. Kelly and J.M. Antonucci, Influence of Veneering Material on Failure of In-CERAM FPD Analogs, J. Dent. Res., 76, 311 Abstract #2378 (1997). J.R. Kelly and J.M. Antonucci, Processing and Properties of Interpenetrating-phase Composites, Polym. Prepr. 38, 125 (1997). Patents J.R. Kelly and J.M. Antonucci, U. S. Patent Applications - 08,487,557 and 08,660,000. Presentations C.R. Fahncke, J.R. Kelly and J.M. Antonucci, Influence of Veneering Material on Failure of In-CERAM FPD Analogs, 75th General Session of the International Association for Dental Research, Orlando, FL, March 19-23, 1997. J.R. Kelly and J.M. Antonucci, Processing and Properties of Interpenetrating-phase Composites, Symposium on Dental Materials and Composites, American Chemical Society Meeting, Las Vegas, NV, September 7-11, 1997. Measurement Methods for Process Control of Cell Encapsulation Francis W. Wang Objective The objective is to develop measurement methods for monitoring the encapsulation of cells and enzymes. Technical Description In a commonly used process for encapsulating cells with sodium alginate, reaction conditions, such as temperature, reaction time, and amounts of reactants, are controlled to produce functionalized alginate with varying degrees of attachment of photopolymerizable functional groups. In addition, the extent of photo-induced cross-linking of the functionalized alginate is controlled to produce microcapsules of required porosity. The method of wavelength-shift fluorescence is used in this project to monitor the degree of functionalization. This is accomplished by: (1) synthesizing a mobility-sensitive fluorescent probe which becomes covalently attached to sodium alginate at a reaction rate similar to the rate for the attachment of photopolymerizable functional groups to sodium alginate; (2) adding a trace amount of the probe to the reaction mixture containing sodium alginate and other reactants; and (3) measuring the fluorescence spectrum of the probe as a function of the reaction time. The fluorescence wavelength of the probe decreases with decreasing mobility of the probe. Since the mobility of the probe molecules decreases upon their attachment to sodium alginate the fluorescence spectrum is a measure of the increase in the attachment of the probe molecules to sodium alginate. Consequently, the change in the fluorescence wavelength of the probe is a measure of the degree of attachment of the photopolymerizable functional groups to sodium alginate, if the probe has been designed such that its reactivity with the secondary hydroxy groups of sodium alginate is comparable to the reactivity of the reactant for attaching photopolymerizable functional groups to the secondary hydroxy groups of sodium alginate. Once a correlation is established between the change in the fluorescence wavelength and the degree of attachment determined by H-NMR, an absolute value for the degree of attachment can be obtained from the change in the fluorescence wavelength. Alternatively, the measured value of the fluorescence wavelength can be used to compare or maintain the degree of attachment throughout the functionalization process. Photo-induced crosslinking of the functionalized alginate can be monitored in the manner similar to the method described for monitoring the functionalization. This will be accomplished by: (1) synthesizing a mobility-sensitive fluorescent probe with a photopolymerizable moiety which has the same reactivity as the photopolymerizable functional groups covalently attached to sodium alginate; (2) adding a trace amount of the fluorescent probe to the reaction mixture containing the photo-crosslinkable alginate and other reactants; and (3) measuring the fluorescence spectrum of the probe as a function of the reaction time. Since the mobility of the probe molecules is reduced due to their reaction with the functional groups on sodium alginate, the fluorescence wavelength of the probe decreases with the increase in the degree of crosslinking of the alginate. Consequently, the change in the fluorescence wavelength of the probe is a measure of the degree of crosslinking. Once a correlation is established between the change in the fluorescence wavelength and the porosity of the microcapsules, the porosity can be obtained from the change in the fluorescence wavelength. Alternatively, the measured value of the fluorescence wavelength can be used to compare or maintain the porosity throughout the photo-induced crosslinking process. Measurement methods described above can also be used to monitor other functionalization processes and other crosslinking reactions. For example, the reaction of tetrahydroxy polyethylene glycol with acryloyl chloride, the photo-induced crosslinking reaction of polyethylene glycol functionalized with acrylate moieties, and the crosslinking reaction of acrylamide and bis-acrylamide can all be monitored in the manner described above. Planned Outcome Technology transfer, to small biotechnology companies, of process control methods for functionalization and cross-linking of sodium alginate and other water-soluble polymers for cell encapsulation. Accomplishments Mobility-sensitive fluorescent probes for monitoring functionalization and cross-linking of sodium alginate have been synthesized. It was demonstrated that, when the mobility-sensitive probe was dissolved in the reaction mixture for functionalization of sodium alginate, the peak fluorescence wavelength of the fluorescent probe decreased with the extent of functionalization. It was demonstrated that when a bifunctional fluorescent probe was dissolved in a buffer solution containing acrylamide and a bis-acrylamide, the crosslinking reaction can be monitored by measuring the change in the excitation spectrum of the probe. Outputs Presentations F.W. Wang, Applications of Fluorescence Spectroscopy in Polymer Systems. Center of Molecular and Macromolecular Studies, Polish Academy of Sciences, Lodz, Poland, November, 1996. F.W. Wang, Wavelength-shift Fluorescent Probes for Monitoring of Polymerization, Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, Poland, November, 1996. F.W. Wang, Applications of Fluorescence Spectroscopy in Polymer Systems, Institute of Polymer Chemistry, Polish Academy of Sciences, Zabrze, Poland, November, 1996. Bioactive Polymeric Dental Materials with Remineralization Potential Based on Amorphous Calcium Phosphate. Joseph M. Antonucci Objective The objective is to develop bioactive polymeric composites 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. It was demonstrated that caries-like lesions in extracted bovine enamel that were covered with a sealant layer of a photocured ACP-composite regained considerably more of their lost mineral than did lesions coated with a similar HAP-filled composite. It also was demonstrated that glass forming agents such as tetraethoxysilane and zirconyl chloride can be used to prepare modified ACP fillers for bioactive composites. These new bioactive composites exhibited both adequate release of calcium and phosphate ions on exposure to oral-simulative environments sufficient for remineralization, and also had improved mechanical strength compared to an unmodified ACP composite control. In the present study the mechanical properties of a BisGMA/ACP composite were evaluated to determine its applicability as a base/liner material. Biaxial flexure, diametral tensile and compression tests were performed on this experimental composite comprising a BisGMA resin that was filled with 40 mass % of stabilized ACP. External Collaborations Dr. E.D. Eanes and Mr. A.W. Hailer of NIDR, Dr. D. Skrtic of the American Dental Association Health Foundation=s Paffenbarger Research Center and Dr. M.S. Park of Phil Dental Clinic, Seoul, Korea carried out the experimental development of bioactive polymeric composites with remineralization potential. Accomplishment The results of this study demonstrated that experimental, visible-light curable ACP-BisGMA resin composites with remineralizing properties and with mechanical properties suitable for use as base/liners or pit and fissure sealants can be prepared. Impact 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 D. Skrtic, J.M. Antonucci and E.D. Eanes, Improved Properties of Amorphous Calcium Phosphate Fillers in Remineralizing Resin Composites, Dent. Mater., 12:295-301(1996). M.S. Park, E.D. Eanes, J.M. Antonucci and D. Skrtic, Mechanical Properties of Bioactive ACP/methacrylate Composites, J. Dent. Res. 76 :76 (Abstract No. 504) (1997). Presentations M.S. Park, E.D. Eanes, J.M. Antonucci and D. Skrtic, Mechanical Properties of Bioactive ACP/methacrylate Composites, 75th General Session of the International Association for Dental Research, Orlando, FL, March 19-23, 1997. Support for the Biomaterials Integrated Products Industries J.A. Tesk, Steve Hsu1, Kenzo Asaoka2, Alan Litsky3 1 Ceramics Division, NIST; 2 Tokushima University; 3 Ohio State University Objective The objective is to provide support for the American Biomedical industry via development of test methods, provision of reference materials, development of standards useful to the industry, and exploratory research into new materials and properties with useful benefits, such as: employment of cyclopolymerizable monomers as substitutes for poly(methymethacrylate) in orthopaedic bone cement; metallic glasses for wear resistant coatings of orthopaedic implant bearing surfaces, and modeling of processing variables on the mechanical behavior of dental/medical materials. 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 of workshops, development of CRADAs and other agreements, and active support of standards related activities. New materials and properties benefits will be explored via collaborations within NIST and with universities and industry. External Collaborations Biomet Inc. (Warsaw Ind.), Johnson & Johnson Professional, Inc. (Raynham, Mass.), Ostenoics Inc. (Allendale, N.J.), Smith & Nephew Richards, Inc. (Memphis, Tenn.), Wright Medical Technology, Inc. (Arlington, Tenn.), and Zimmer, Inc. (Warsaw, Ind.), which are members of the Cooperative Research and Development Agreement (CRADA) Consortium established to evaluate test methods for the wear properties of orthopedic materials, provided insight into the needs of the orthopedic industry and partial financial support. Poly Hi Solidur (Fort Wayne, Ind) and Teledyne Allvac/Vasco (Monroe,NC) contributed materials to the CRADA Consortium. Professor K. Asaoka of Tokushima University modeled the effects of processing variables in dental investments. Planned Outcomes Accelerated tests for evaluation of wear of orthopaedic joint materials; Reference materials for implantable materials and explanted devices; National and international standards for medical devices; New materials with features of interest for potential applications to improve biomedical products; Understanding of processing effects on mechanical behavior of dental/medical materials; Foreshortening the time for acceptance of device submissions to the FDA (an industry estimate is that shortening acceptance time by two weeks for only one device would recoup the research expenditures). Accomplishments Completed one year of Cooperative Research & Development Agreement (CRADAs) research with six (6) orthopaedic companies, formed as a CRADA consortium, to identify accelerated methods for evaluation of the wear properties of orthopaedic joint materials. Based on results from first year, four (4) of the six (6) companies voted to amend the CRADAs to change the direction of the research according to a proposal recommended by NIST. Amended the CRADAs, with additional support provided by the companies. Facilitating NIST-NIH-FDA-Industry collaboration for NIST dissemination of Industry Developed Polyetherurethane Reference Materials, useful for development of Cardiovascular devices. A Memorandum of Understanding (MOU) was signed by three government agencies--- NIST (Polymers Division and Standard Reference Materials Program), the Food and Drug Administration, and the National Institute of Health---for the development of reference materials for biomaterials. The MOU may facilitate the cooperative efforts of these agencies to produce reference materials that are intended for use in quality assurance of biomaterials for medical devices. Developed method for use of methacrylate powders with co-monomers of cyclopolymerizable and methylacrylate types to produce doughs in times comparable to those used for orthopaedic bone cement (BC) and dental denture bases (DB), but which can persist for a couple of days. Extended-time dough state could be expected to make insertion time of a BC less critical and the use as a DB material potentially labor saving. Other potential benefits include: lower exotherm, less tissue damage (BC; DB reline material), higher conversion, less residual monomer, (BC; DB & DB reline), large monomer size (BC, no blood pressure drop due to monomer absorption; DB & DB reline), Reduced polymerization shrinkage (BC, DB, & DB reline; improved fit). Outputs/Interactions Publications/Reports K. Asaoka and J.A. Tesk, Simulation of Transient Thermal Stress in Gypsum-bonded Investment, Dent. Mater., December 1996. J.A, Tesk and O. Okuno, High-Temperature Investments, In: Dental Materials and Their Selection, 2nd ed., W.J. O=Brien Editor, Qu J.A.Tesk and E.P. Mueller, Reference Materials in Standards for Tissue-Engineered Products, Proceedings of ASTM Workshop on Tissue Engineering: The Role of ASTM, May 1997. J.A. Tesk, Review of Dental School Departmental Programs, Report of Expert Advisory Panel on 20 Years of Dental Research at Tokushima University, February 1997. J.A. Tesk, S. Hsu, and M. Shen, Quarterly Reports (confidential) to Orthopaedic CRADA Research Consortium, September 1996, December 1996, March 1997, June 1997. Presentations J.A. Tesk, Implant Retrieval: Reference Materials, Standards, and Analysis, Tokushima University Tokushima, Japan, March 1997. J.A. Tesk, Needs for Reference Materials in Standards, ASTM Workshop on Tissue Engineering, St. Louis MO, May 6-7, 1997. |
