Bruce O. Fowler

Preparation and Comprehensive Characterization of a Calcium Hydroxyapatite Reference Material

Numerous biological and chemical studies involve the use of calcium hydroxyapatite (HA), Ca10(PO4)6(OH)2. In this study detailed physicochemical characterization of HA, prepared from an aqueous solution, was carried out employing different methods and techniques: chemical and thermal analyses, x-ray diffraction, infrared and Raman spectroscopies, scanning and transmission microscopies, and Brunauer, Emmett, and Teller (BET) surface-area method. The contents of calcium (Ca2+), phosphate (PO43−), hydroxide (OH−), hydrogenphosphate (HPO42−), water (H2O), carbonate (CO32−), and trace constituents, the Ca/P molar ratio, crystal size and morphology, surface area, unit-cell parameters, crystallinity, and solubility of this HA were determined. This highly pure, homogeneous, and highly crystalline HA is certified as a National Institute of Standards and Technology (NIST) standard reference material, SRM 2910.

Synthesis and characterization of dimethacrylates containing quaternary ammonium functionalities for dental applications

OBJECTIVES The widespread incidence of recurrent caries highlights the need for improved dental restorative materials. The objective of this study was to synthesize low viscosity ionic dimethacrylate monomers (IDMAs) that contain quaternary ammoniums groups (antimicrobial functionalities) and are compatible with existing dental dimethacrylate-based monomers. Such monomers have the potential to copolymerize with other methacrylate monomers and produce antibacterial polymers. METHODS Two monomers (IDMA-1 and IDMA-2) were synthesized using the Menschutkin reaction and incorporated at 0-30% (by mass) into a 1:1 (by mass) bisphenol A glycerolate dimethacrylate (BisGMA):triethylene glycol dimethacrylate (TEGDMA) resin. Resin viscosity was quantified using rheology, and polymer degree of conversion (DC) and surface charge density were measured using Fourier transform infrared spectroscopy (FTIR) and fluorescein binding, respectively. Effects of IDMA-1 on initial attachment of Streptococcus mutans and on viability and metabolic activity (via reductase enzymes) of RAW 264.7 macrophage-like cells were quantified. RESULTS IDMA-1 and IDMA-2 were prepared and characterized. IDMA-1 was miscible with BisGMA:TEGDMA and slightly increased the resin viscosity and DC. As expected, polymeric surface charge density increased with increasing IDMA-1. Incorporation of 10% IDMA-1 into BisGMA:TEGDMA reduced bacterial colonization without affecting viability or metabolic activity of mammalian cells. Increasing IDMA-1 up to 30% had no additional effect on bacterial coverage, but ≥20% IDMA-1 significantly reduced macrophage density, viability, and metabolic activity. Leachables from polymers containing IDMA-1 were not cytotoxic. SIGNIFICANCE The Menschutkin reaction provides a facile, convenient means to synthesize new monomers with quaternary ammonium groups for dental and medical applications.

Chemistry of Silanes: Interfaces in Dental Polymers and Composites

The performance and service life of glass-or ceramic-filled polymeric composites depend on the nature of their resin, filler and interfacial phases as well as the efficacy of the polymerization process. The synergy that exists between the organic polymer matrix and the usually inorganic reinforcing filler phase is principally mediated by the interfacial/interphasial phase. This latter phase develops as a result of the dual reactivity of a silane coupling agent, (YRSiX3), a bifunctional molecule capable of reacting with the silanol groups of glass or ceramic fillers via its silane functional group (–SiX3) to form Si-O-Si- bonds to filler surfaces, and also with the resin phase by graft copolymerization via its Y functional group, usually a methacrylic vinyl group. In this paper, we explore some of the chemistry of organosilanes, especially that of functional organosilanes (or silane coupling agents as they are commonly known) that are used to mediate interfacial bonding in mineral reinforced polymeric composites. The chemistry of organosilanes can be quite complex involving hydrolytically initiated self-condensation reactions in solvents (including monomers) that can culminate in polymeric silsesquioxane structures, exchange reactions with hydroxylated or carboxylated monomers to form silyl ethers and esters, as well as the formation of silane derived interfaces by adhesive coupling with siliceous mineral surfaces.

Filler systems based on calcium metaphosphates

Calcium metaphosphates (CMPs)--a unique class of phosphate minerals possessing polymeric structures, [Ca(PO3)2]n, and having refractive indices of approximately 1.54-1.59-- are optically compatible with resins such as BIS-GMA. In this study, several types of CMPs were prepared and evaluated for their potential as fillers for visible-light-activated (VLA) dental composites. The vitreous (V) and beta-crystalline forms of CMP were prepared by controlled thermolysis of monocalcium phosphate monohydrate, Ca(H2PO4)2.H2O. Hybrid fillers were also prepared by thermal methods. Fillers, characterized by IR spectroscopy and optical microscopy, were prepared in several size ranges (e.g., 1-100 microns). VLA composites were formulated by use of both untreated and surface modified CMPs. V-CMP and its hybrids yielded composites which expanded when stored in water but were of low strength, e.g., diametral tensile strength, (DTS) = 8 MPa. beta-CMP composites were more moisture-resistant, had higher DTSs (from 12 to 33 MPa), and showed a tendency to arrest brittle fracture. These novel fillers have potential uses in resin-based materials such as dental composites, cements, and adhesives.

Octacalcium phosphate carboxylates. IV: Kinetics of formation and solubility of octacalcium phosphate succinate

Kinetic and thermodynamic studies of octacalcium phosphate succinate (OCP-SUCC), Ca8(HPO4)2-x(succ)x(PO4)4h. yH2O, where 0.8 < x < 1.0 and 5.5 < y < 6.5, broaden the present knowledge in the chemistry of octacalcium phosphate carboxylates. The kinetics of formation of OCP-SUCC by conversion of α-tricalcium phosphate (α-TCP), α-Ca3(PO4)2, in ammonium succinate solutions was followed at varied initial pH, I ≈0.5 mol dm-3, 37°C. The changes in the liquid phase were monitored by pH measurements. Solid phases were characterized by means of chemical analyses, X-ray diffraction and infrared spectroscopy. The rate of conversion decreased with increasing initial pH. In the solubility experiments the concentrations of calcium, phosphate, succinate, ammonium and pH were determined in solutions equilibrated with OCP-SUCC (6.1< pH <7.4). Utilizing these data and association constants of phosphoric and succinic acid and stability constants of the soluble complexes, the solubility product of OCP-SUCC, Ks = c8(Ca2+)c1.07(H+)c0.93(succ2-)c5.07(PO3-4), was determined: pKs(OCP-SUCC) = 68.4 − 0.9 at I = 0.5 mol dm3-, 37°C.

Composition and Solubility Product of a Synthetic Calcium Hydroxyapatite

Synthetic calcium hydroxyapatites (HA’s) have, in general, excellent biocompatibility with tooth and bone tissues, and they are frequently used as biomedical materials.1,2,3,4 The properties of synthetic HA’s depend strongly on preparative conditions. They are well known to be variable5 and the composition deviates from the stoichiometric HA formula, Ca10(PO4)6(OH)2, that comprises the unit-cell content. In order to properly characterize these HA’s, accurate determinations of complete chemical composition, unit-cell parameters, solubility, surface area, crystallinity, etc., are needed. The focus of this paper is on the procedures and methods for accurate and precise determinations of two important parameters, the Ca/P molar ratio and solubility product, of a HA that was prepared and characterized in our group6 for use as a HA reference material (HA-RM).