Stability of silorane dental monomers in aqueous systems.

UNLABELLED: Siloranes (silicon-based monomers with oxirane functionality) are investigated as matrix resins for new low shrinkage/stress dental composites. Compounds containing oxirane groups are known to be reactive with water, which could impart instability to the composite. OBJECTIVE: To test the stability of siloranes by measuring changes in the chemical structure of the oxirane group in aqueous environments. METHODS: Two siloranes (PH-SIL and TET-SIL) and their 1:1 mixture (SIL-MIX) were evaluated (n=2-3). Siloranes were mixed in aqueous solutions with and without 1% tetrahydrofuran (THF) containing either liver esterase or epoxide hydrolase at pH 7.4, or dilute HCl at pH 1.4. The stability of conventional dioxiranes 3,4-epoxycyclohexyl-methyl-3,4-epoxycyclohexane carboxylate (ECHM-ECHC), and bisphenol A diglycidyl ether (BADGE) were also monitored under similar conditions. NMR was used to estimate the extent of reaction and give structural information about reaction products. RESULTS: Siloranes were found to be stable for 24h in all aqueous environments tested. In contrast, ECHM-ECHC reacted at pH 1.4 to form species containing oxirane, ester, hydroxyl and carboxylic acid groups. Water hydrolyzed the ester group of ECHM-ECHC in the presence of liver esterase. In the presence of epoxide hydrolase, BADGE oxirane groups were hydrolyzed to diols, hydrolysis ranged from 0 to 34% depending on the aqueous environment. CONCLUSION: The stability and insolubility of siloranes in biological fluid simulants suggests that these may be more suitable for use in the oral environment than conventional oxirane-functional monomers.

Photopolymerization of developmental monomers for dental cationically initiated matrix resins.

OBJECTIVES: The objectives were to investigate the structure and selected physical properties of products resulting from the photopolymerization of a binary mixture containing an aliphatic dioxirane, 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate (ECHM-ECHC) and a potential expanding monomer, 3,9-bis(oxiranylcyclohexylmethyl)-1,5,7,11-tetraoxaspiro[5.5]undecane (BOCHM-TOSU). METHODS: Reaction mixtures were irradiated with a dental curing lamp at room temperature. Some reactions were quenched prior to gel point. Oligomeric products were separated from unreacted monomers by column chromatography, and analyzed by NMR. Physical properties of polymeric solids were measured using accepted standard methods. Protonation energies for monomers were calculated using semi-empirical quantum mechanical methods. RESULTS: Types of oligomers found included poly(ether)s and poly(carbonate)s. Quantum mechanical calculations indicated preferential attack at the more nucleophilic oxaspirocyclic ring sites. For cured solid polymer samples, the elastic modulus was 2.39 +/- 0.24 GPa and the fracture toughness was 0.73 +/- 0.10 MPa m(1/2). These values were similar to those measured for a cured conventional BISGMA/TEGDMA matrix resin. SIGNIFICANCE: The room-temperature photopolymerization of an aliphatic dioxirane and a potential expanding monomer demonstrates the possibility of making cross-linked copolymer resins with improved polymerization shrinkage characteristics for use in dental composites.

Genotoxicity assessment of oxirane-based dental monomers in mammalian cells.

The potential use of oxirane (epoxy) monomers in dental composite development raises the concern to test their genetic safety. Oxiranes can interact with DNA resulting in DNA damage, mutations, and possibly carcinogenesis. Our objective was to evaluate DNA damage and cell-cycle disruption in mammalian cells after exposure to epoxy monomers. The experimental oxiranes were Araldite trade mark GY 281, Cyracure trade mark UVR 6105 and 1,3-dioxane-2,2'-1,3-dioxane-5',4'-bicyclo[4.1.0] heptane (DECHE-TOSU). L929 fibroblast cells were incubated with the monomer for 7 and 24 h at 37 degrees C/5% CO(2). After incubation, cells were subjected to DNA damage alkaline unwinding assay and flow cytometry cell-cycle analysis. Lack of DNA damage and cell-cycle effects were observed with DECHE-TOSU. Exposure to subtoxic doses of Araldite trade mark GY 281 or Cyracure trade mark UVR 6105 caused DNA damage and cell cycle disruption. A significant (p < 0.01) effect for Araldite trade mark GY 281 was observed with cell populations in G1 and G2/M when compared to DMSO solvent control. Similar comparisons revealed significant differences in G2/M cell cycle population after 24-h exposure to 100 microM Cyracure trade mark UVR 6105. For comparison, BISGMA was evaluated to produce DNA damage but without cell-cycle effects suggesting DNA repair mechanisms were effective. Our findings with DECHE-TOSU, Araldite trade mark GY 281 and Cyracure trade mark UVR 6105 indicated cell-cycle disruption followed DNA damage.

In vitro effect of light-cure dental adhesive on IL-6 release from LPS-stimulated and unstimulated macrophages.

The objective of this study was to measure IL-6 release from LPS-stimulated and -unstimulated macrophages exposed to extracts from fresh and aged Scotchbond Multipurpose Plus adhesive disks (5 mm in diameter by 2 mm in thickness) light cured for 10, 20, or 40 s. One set of disks was aged for 16 weeks at 4 degrees C. Extracts were prepared by incubating three disks in 1 mL of serum-free culture medium for 72 h at 37 degrees C. Then macrophages (RAW 264.7) were exposed to the extracts (6.25-50 microL) for 72 h at 37 degrees C/5% CO(2). Supernatants were analyzed for cytokine levels (ELISA), and the monolayer of cells was assessed for viability (MTT assay). Unlike adhesive disk age, curing time affected cell viability. Disk extracts cured for 10 s were more cytotoxic (p < 0.05) than were extracts from 20- or 40-s cured disks. Macrophage release of IL-6 was stimulated significantly (p < 0.01) by extracts from fresh 10-s cured disks, up to 777 pg/mL and by 2 microg/mL of LPS (1174 pg/mL). The LPS response was significantly (p < 0.05) suppressed by 50 microL of extracts, which may be related to the enhanced cytotoxicity exhibited by LPS in combination with extracts. This study has demonstrated the possibility that IL-6 release is stimulated by light-cure dental adhesive applications using 10-s curings.

In vitro mutagenicity and metabolism of the cycloaliphatic epoxy Cyracure UVR 6105.

Cyracure UVR 6105 is a cycloaliphatic epoxy monomer and has both carboxylate and epoxy groups, with the potential for rapid polymerization. It is widely used in industry for the preparation of inks, resins, coatings, and was proposed for incorporation into dental composites. The objective of this study was to determine the mutagenic potential of this chemical related to its metabolite products. Several doses of Cyracure UVR 6105 were dissolved in DMSO and subjected to the Ames Salmonella mutagenicity assay. A metabolic activation system (S9-mix) was used consisting of Arochlor-induced liver S9 homogenate enriched with NADP and glucose-6-phosphate cofactors. In contrast to studies without S9-mix, Cyracure UVR 6105 exhibited enhanced genotoxic activities with strains TA100 and TA1535 in the presence of liver S9-mix. From in vitro metabolism of Cyracure UVR 6105 with S9-mix, as used in the Ames assay, several metabolites were identified. The alcohol metabolite, 3,4-epoxycyclohexylmethanol, containing intact epoxy group was identified in the organic solvent extract. This metabolite was synthesized and proved to be mutagenic against TA100 when assayed in the presence and absence of S9-mix. Results showed that the increased mutagenicity of Cyracure UVR-6105 in the presence of liver enzymes is due to the formation of the mutagenic metabolite 3,4-epoxycyclohexylmethanol.

Dynamic mechanical analysis and esterase degradation of dentin adhesives containing a branched methacrylate.

A study of the dynamic mechanical properties and the enzymatic degradation of new dentin adhesives containing a multifunctional methacrylate are described. Adhesives contained 2-hydroxyethyl methacrylate, 2,2-bis[4-(2-hydroxy-3-methacryloxypropoxy) phenyl]-propane, and a new multifunctional methacrylate with a branched side chain-trimethylolpropane mono allyl ether dimethacrylate (TMPEDMA). Adhesives were photopolymerized in the presence of 0, 8, and 16 wt % water to simulate wet bonding conditions in the mouth and compared with control adhesives. The degree of conversion as a function of irradiation time was comparable for experimental and control adhesives. In dynamic mechanical analysis, broad tan delta peaks were obtained for all samples, indicating that the polymerized networks are heterogeneous; comparison of the full-width-at-half-maximum values obtained from the tan delta curves indicated increased heterogeneity for samples cured in the presence of water and/or containing TMPEDMA. The experimental adhesive showed higher T(g) and higher rubbery modulus indicating increased crosslink density when compared with the control. The improvement in esterase resistance afforded by adhesives containing the TMPEDMA is greater when this material is photopolymerized in the presence of water, suggesting better performance in the moist environment of the mouth. The improved esterase resistance of the new adhesive could be explained in terms of the densely crosslinked network structure and/or the steric hindrance of branched alkyl side chains.

Enzymatic biodegradation of HEMA/bisGMA adhesives formulated with different water content.

Dentin adhesives may undergo phase separation when bonding to wet demineralized dentin. We hypothesized that adhesives exhibiting phase separation will experience enhanced biodegradation of methacrylate ester groups. The objective of this project was to study the effect of enzyme-exposure on the release of methacrylic acid (MAA) and 2-hydroxyethyl methacrylate (HEMA) from adhesives formulated under conditions simulating wet bonding. HEMA/bisGMA(2,2-bis[4(2-hydroxy-3-methacryloyloxy-propyloxy)-phenyl] propane), 45/55 w/w ratio, was formulated with different water content: 0 Wt % (A00), 8 wt % (A08), and 16 wt % (A16). After a three day prewash, adhesive discs were incubated with/without porcine liver esterase (PLE) in phosphate buffer (PB, pH 7.4) at 37 degrees C for 8 days. Supernatants were collected daily and analyzed for MAA and HEMA by HPLC. For all formulations, daily MAA release in the presence of PLE was increased compared to MAA release in PB. HEMA release in the presence of PLE was not detected while HEMA release was consistently measured in PB. A08 and A16 released significantly larger amounts of HEMA compared to A00. Analysis of the cumulative release of analytes showed that the leachables in PLE was significantly increased (p < 0.05) as compared with that released in PB indicating that MAA release was not only formed from unreacted monomers but from pendant groups in the polymer network. However, the levels of analytes HEMA in PB or MAA in PLE were increased in A08 and A16 as compared with A00, which suggests that there could be a greater loss of material in HEMA/bisGMA adhesives that experience phase separation under wet bonding conditions.

Lipid vesicles as membrane models for toxicological assessment of xenobiotics.

Traditionally animals and cell cultures have been used to assess the toxic potential of xenobiotics on cell membranes. In search for more reproducible, quantitative, cost- and time-effective assays, toxicologists have recently become interested in biomimetic lipid vesicle-based test systems. Lipid vesicles (liposomes) have long been appreciated as simple cell membrane models in biochemical and biophysical studies providing a good understanding of the physicochemical properties of liposome systems. More recently a number of reports have been published on the interactions of toxic substances with vesicles. Literature reports on liposome assays have appeared for widely different classes of xenobiotics, such as dental materials, antibiotics, detergents, and peptides. In this review we focus on those reports that contain a quantitative and significant correlation with more established toxicological tests like cell culture assays. We provide an introduction to the structure and main characteristics of vesicles and related lipid aggregates. The two main assays presented are leakage of fluorescence dyes and differential scanning calorimetry (DSC) measurements of the solid-ordered/liquid-disordered main phase transition temperature (Tm).

Preparation and Properties of Novel Dentin Adhesives with Esterase Resistance.

A new methacrylate monomer, trimethylolpropane mono allyl ether dimethacrylate (TMPEDMA), was synthesized and evaluated. This branched methacrylate was designed to increase esterase-resistance when incorporated into conventional HEMA (2-hydroxyethyl methacrylate)/BisGMA (2,2-bis[4(2-hydroxy-3-methacryloyloxy-propyloxy)-phenyl] propane) dental adhesives. The new adhesives, HEMA/BisGMA/TMPEDMA in a 45/30/25 (w/w) ratio were formulated with H(2)O at 0 (A0T) and 8 wt % water (A8T) and compared with control adhesives (HEMA/BisGMA, 45/55 (w/w), at 0 (A0) and 8 wt % (A8) water). Camphoroquinone (CQ), 2-(dimethylamino) ethyl methacrylate and diphenyliodonium hexafluorophosphate were used as photoinitiators. The new adhesives showed a degree of conversion comparable with the control and improved modulus and glass transition temperature (T(g)). Exposure of photopolymerized discs to porcine liver esterase for up to eight days showed that the net cumulative methacrylic acid (MAA) release in adhesives formulated with the new monomer and 8% water (A8T: 182 µg/mL) was dramatically (P < 0.05) decreased in comparison to the control (A8: 361.6 µg/mL). The results demonstrate that adhesives made with the new monomer and cured in water to simulate wet bonding are more resistant to esterase than conventional HEMA/BisGMA adhesive.