Effect of treated filler loading on the photopolymerization inhibition and shrinkage of a dimethacrylate matrix.

This study shows how treated filler loading influences the photopolymerization of a dimethacrylate comonomer mixture, regarding, in particular, shrinkage and inhibition under atmospheric oxygen, present in oral environment. Bis-GMA/TEGDMA (75/25 wt.%) resins were loaded with hybrid filler (Ba aluminosilicate glass and pyrogenic silica), treated with gamma-methacryloxy(propyl)trimethoxysilane, at 0-50 wt.% and light cured over a total of 30 s (45 mW/cm2). Degree of double-bond conversion (DC), obtained using FTIR, decreased with filler content. 1H MAS spectra (293-340 K) and STRAFI images (293 K) were obtained as a function of irradiation time and filler concentration. 1H signals of unreacted methacrylate groups were more intense for higher loaded resins and resonances from -CH2SiO2(OH) (T2) and -CH2SiO3- (T3) units, also observed by 29Si NMR, were resolved and suggest the presence of T2-resin bonds. 1D images show a reduction on polymerization contraction and reaction inhibition at the composite resin surface with filler loading. 2D resin images present a highly mobile surface layer, hence with lower DC.

EVA copolymer matrix for intra-oral delivery of antimicrobial and antiviral agents.

Biocompatible ethylene vinyl acetate copolymer (EVA) was utilized to study the release of an antiviral drug (acyclovir (ACY)) and an antimicrobial drug (doxycycline hyclate (DOH)). Release of both drugs from EVA was measured individually and in combination. The effect of drug combination of DOH and ACY is presented. Additionally, the release rate of DOH after coating of the matrix with a different copolymer, in drug-loading with increasing loads of DOH, and with increases in temperature are also presented. The drugs incorporated in EVA films were prepared from the dry sheet obtained by solvent evaporation of polymer casting solutions with drugs. Drug release from the films was examined for about 12 days in distilled water at 37 degrees C. Changes in optical density were followed spectrophotometrically. The combination of ACY and DOH resulted in an increased release of ACY by about three times (P < 0.001) while DOH showed a decrease in rate of about two times compared to the individual release rates (P = 0.008). Increases in drug levels of DOH resulted in increases in drug release rates (P = 0.001). The release rate of DOH increased with temperature (P = .001; 27, 32, 37 and 42 degrees C were studied) and the energy of activation (DeltaE ( not equal) = 56.69 kJ/mol) was calculated using the Arrhenius equation for the diffusion of DOH molecules. Thus, the release rates of drugs were influenced by many factors: drug combination, coating the device, drug-loading, and temperature variation. Therefore it is proposed that controlling these variables should make it possible to obtain therapeutic levels of drugs released from drug loaded polymer, which may be beneficial in treating oral infections.

Novel light-cured resins and composites with improved physicochemical properties.

OBJECTIVES: The aim of this study was to determine the effect of two new diluent agents (Bis-GMA analogues), at different dilution levels and filler contents on relevant physicochemical properties of several novel resins and composites containing Bis-GMA as matrix. Composites using TEGDMA as diluent were used as control. METHODS: Twenty formulations were prepared combining three monomer mixtures (Bis-GMA/TEGDMA, Bis-GMA/CH(3) Bis-GMA and Bis-GMA/CF(3) Bis-GMA), at three dilution levels (85/15, 10/90, 0/100) and two percentages of filler loading (silanated barium aluminosilicate glass): 0%, 10%, 35%. Preliminary rheological testing was performed in order to obtain the viscosity of the resin samples. Resins and composites were then inserted into molds and light-cured (500mW/cm(2)). The properties evaluated were: (1) homogeneity of curing (HC), using FTIR or Vickers microindentor, (2) microhardness, by a Vickers microindentor, (3) depths of cure and oxygen inhibitor effect (OIE), quantified by scraping, (4) water contact angle on the materials surface, (5) water sorption and solubility, performed by the Oysaed-Ruyter method and (6) scanning electron microscopy analysis of the specimens surfaces. Data were analyzed by ANOVA and Student-Newman-Keuls tests (p<0.05). RESULTS: Materials with CH(3) Bis-GMA and CF(3) Bis-GMA exhibited less hydrophilicity, water sorption and solubility. Bis-GMA dilution induced an increase in depth of cure and promoted a higher OIE, particularly when the diluent was TEGDMA. Filler loading reduced the OIE and increased hydrophobicity of the resins. SIGNIFICANCE: CH(3) Bis-GMA may be considered as good candidate to be used as diluent because when replacing TEGDMA-induced lower hydrolytic degradation and increase in HC.

Stability and release of antiviral drugs from ethylene vinyl acetate (EVA) copolymer.

The use of polymer based drug delivery systems in dentistry is a relatively new area of research with the exception of the inhibition of secondary caries by the release of fluoride ions from polyalkenoate cements and their predecessors silicate cements. The present study was to test on orally biocompatible material, ethylene vinyl acetate copolymer (EVA), for release of antiviral drugs at oral therapeutic levels over extended periods of time. We also determined their stability during film casting and release. Materials studied include gancyclovir (GCY), acyclovir (ACY), dichloromethane (DCM), and ethylene vinyl acetate (EVA). The square films (3 x 3 x 0.1 cm) were prepared from the dry sheet obtained by solvent evaporation of polymer casting solutions. These solutions were made of EVA and the drug (40:1) in 70 ml of dichloromethane at 38 degrees C. Then drug release characteristics from the drug loaded films were examined at 37 degrees C for a minimum of 14 days in 10 ml medium (ddwater) replaced daily. Kinetics of drug release were followed by spectral measurements using previously determined lambda(max) values (GCY = 250 nm; ACY = 253 nm). A minimum of three samples was tested and reproducible results were obtained. Drug stability (ACY) during film casting and its release was determined using 1H NMR spectrometer (Bruker DRX-500 and 400). Rate of drug release was determined from the part of the curve (rate vs. time) after the onset of the "burst." Although GCY has a larger molecular weight (255) than ACY (225), GCY exhibited about three times higher rate of release than ACY. This difference in rate values may be explained due to its relatively greater solubility in EVA, facilitating faster diffusion of the molecules through the channels present in EVA. This is consistent with the observation that the rate at which drug molecules diffuse through the channels of the polymer, can be increased by decreasing the molecular weight. In the case of ACY, the molecules may be undergoing molecular associations, perhaps dimerization or trimerization in addition to its lower solubility in EVA. The diffusion of ACY tends to be slower under these circumstances compared to GCY resulting in lower rate value than in the case of GCY. Biological studies revealed that ACY exhibited a remarkable decrease in a number of viral organisms present in virus infected cell culture system using real-time polymerase chain reaction (RT-PCR). NMR analysis indicates that the chemical structure of the drug remains stable during film casting process and release.

Drug release from cast films of ethylene vinyl acetate (EVA) copolymer: Stability of drugs by 1H NMR and solid state 13C CP/MAS NMR.

The study utilizes an oral biocompatible material based on ethylene vinyl acetate copolymer (EVA) designed to release drugs in vitro at therapeutic levels over several days. We examined the drug stability during film casting process using proton and solid state NMR techniques. The drug-loaded EVA films were prepared from the dry sheet obtained by solvent (dichloromethane) evaporation of polymer casting solutions. Drugs tested include chlorhexidine diacetate (CDA), doxycycline hydrochloride (DOH), tetracycline hydrochloride (TTH) and nystatin (NST). Drug release from the films was examined for at least 14 days in 10 ml ddH2O (NST in water/ethanol (4:1)) which was replaced daily. Changes in optical density were followed spectraphotometrically. Effect of temperature on rate measurements was studied and the energies of activation (E*) were calculated using Arrhenius plots. Effect of EVA copolymer composition on CDA release rate was also investigated. The enhanced rates with temperature increase may be attributed to the formation of channels with increased geometry in the polymer. The highest E* observed for CDA compared to DOH and TTH may be related to their average molecular weights. Spectral analyses for CDA and NST revealed that the chemical and physical structures of the drugs remained unaffected during the film casting process.

A polymeric device for delivery of anti-microbial and anti-fungal drugs in the oral environment: effect of temperature and medium on the rate of drug release.

OBJECTIVES: The use of drug delivery systems in dentistry is a relatively new area of research with the exception of fluoride ion release from polyalkenoate cements and their predecessor silicate cements. The present study is based on the use of a bio-compatible material ethylene vinyl acetate copolymer (EVA) that enables constant release of drugs of therapeutic levels over extended periods of time at doses suitable for the treatment of oral conditions. METHODS: Polymer casting solutions were made by dissolving EVA and the drug in the ratio of 40:1 in 70 ml of dichloromethane at 38 degrees C for 6 h. Thin square films of 3 x 3 cm2 with a thickness of 1 mm were cut from the dry sheet obtained by solvent evaporation technique. Drug loaded samples were extracted for a minimum of 14 days in 10 ml medium (double distilled water or water/ethanol (4:1)) which was replaced daily. Spectral measurements were made to follow changes in optical densities (OD) during release kinetics. Effect of temperature (24 and 37 degrees C) on the rate of drug release was studied and the energies of activation (DeltaE not equal ) were calculated using Arrehenius equation for the diffusion (translocation) of molecules of tetracycline hydrochloride (TTH), doxycycline hydrochloride (DOH), and chlorhexidine diacetate (CDA) in water as extracting medium. Effect of extracting medium (water and water/ethanol (4:1)) was also investigated on the rate of drug release measurements at 24 degrees C. RESULTS: Analysis of variance of the data revealed that significantly enhanced rates were observed at the higher temperature (37 degrees C) and when extracting medium was changed to water/ethanol (4:1) for TTH, DOH and CDA (p<0.0015). The enhanced rate values seem to be due to the formation of channels in the polymer. The largest activation energy (21.83 kcal mol(-1)) observed for CDA was interpreted as due to the highest average molecular weight (626) compared to TTH (481) and DOH (481).Significance. These in vitro rate of drug release measurements will provide a basis for establishing a novel approach (treatment modality) for sustained intra-oral drug delivery over extended time periods using laboratory methods and materials that are readily available to dentists.

Low viscosity dimethacrylate comonomer compositions [Bis-GMA and CH3Bis-GMA] for novel dental composites; analysis of the network by stray-field MRI, solid-state NMR and DSC & FTIR.

The effect of dilution of 2,2-bis (4-(2-hydroxy-3-methacryloxyprop-1-oxy) phenyl) propane (Bis-GMA) with 2,2-bis(4-(2-methacryloxyprop-1-oxy)phenyl) propane (CH3Bis-GMA) on extent of polymerization (Ep) was investigated by FTIR and NMR. The results correlate well with Ep values of the system and its flexibility probed by Tg (obtained by DSC). Spatially resolved photopolymerization kinetics and polymerization shrinkage (PS, i.e. volumetric polymerization contraction) was also studied by stray-field imaging (STRAFI-MRI). PS was obtained in the presence of oxygen from the atmosphere as the total volumetric contraction. All the results were compared with those obtained for the Bis-GMA/TEGDMA (triethylene glycol dimethacrylate) system. Use of Bis-GMA mixed with hydrophobic low viscosity CH3Bis-GMA comonomer, as a substitute for the commonly used TEGDMA, resulted in improving significantly properties such as PS, water sorption and Ep, thereby reducing the unreacted double bond concentration.

Development of butyl elastomer/methacrylate monomer systems as denture soft lining materials.

Previous studies have shown elastomer/methacrylate monomer formulations to have good strength; however, water uptake was high and they also suffered from oxidation. This study has looked at the use of three different butyl elastomers, well known for their oxidation resistance, butyl (PB), chlorobutyl (PCB) and bromobutyl (PBB). The tensile and water uptake properties of the three elastomers gelled with ethyl hexyl methacrylate containing 1% ethylene glycol dimethacrylate and 1% lauryl peroxide (5+ formulation from previous studies) were studied. Water uptake of the pure elastomers was also measured. Tensile strengths were low (PCB5+ = 3.09+/-0.12 MPa and PBB5+ = 3.90+/-0.36 MPa); however, elongation to break values were high (PCB5+ = 797+/-17% and PBB5+ = 599+/-13%). Water uptake was high and protracted with none of the formulations reaching equilibrium. The PCB5+ had the highest uptake (approximately 6% at 203 days) with that for PBB5+ and PB5+ at a similar level (approxiamtely 4% at 203 days). None of the materials showed any sign of oxidation. The PBB proved to be the most suitable of the three elastomers for further development in soft lining formulations.

Controlled drug release for oral condition by a novel device based on ethylene vinyl acetate (EVA) copolymer.

The application of drug delivery systems in oral environment is relatively a new area of research with the exception of release of fluoride ions from polyalkenoate cements and their predecessor silicate cements. The present study addresses development of a novel device based on ethylene vinyl acetate copolymer (EVA), a biocompatible material which enables constant drug release over several days to treat oral infections. Drugs incorporated in EVA included tetracycline, minocycline and nystatin together with combinations (C) of nytatin-tetracycline (1 : 1) and nystatin-minocycline (1 : 1). Polymer casting solutions were prepared by dissolving EVA and the drugs in the ratio of 10 : 1 in 70 ml of dichloromethane at 38 degrees C for 6 h. Thin square films of 3x3 cm and 1 mm thickness were cut from the dry sheet obtained by solvent evaporation. Drug loaded samples were extracted for a minimum of 15 days in 10 ml medium (water or water/ethanol (1 : 2) or 0.9% saline solution) which is replaced daily. Spectral measurements were made to follow changes in optical densities (OD) during release kinetics. Analysis of the data revealed that among all the drugs tested tetracycline exhibited the highest release rate (56.15microg/cm2/day) and % cumulative release (27.92). The observed enhanced values may be interpreted as due to the channels formed due to changes in free volume (microvoids). In case of nystatin-minocycline combination, the observed increased values of release rates and percent cumulative release, may be attributed to the swelling component or channels or relative hydrophobic interactions. Initial "burst" effects due to liberation of surface-bound drug molecules were observed with reference to all the three drugs and the combinations of drugs studied. Among all the drugs, minocycline exhibited the least "burst" effect suggesting that the drug is more homogeneously distributed in the copolymer. Drug loaded EVA thermoplastic copolymer may provide a favorable therapeutic material for the development of a novel, local treatment for oral, mucosal and periodontal infections.

Development of novel elastomer/methacrylate monomer soft lining materials.

An earlier study identified a formulation comprising a butadiene/styrene copolymer (PBS) gelled with ethyl hexyl methacrylate (5 + formulation) as a potential denture soft lining material. It had good mechanical properties but water uptake was high as a result of the presence of a separating agent. This study has compared the tensile and water absorption properties of four elastomers free from separating agent (three butadiene/styrene, HBS, EBS, SBS, and one isoprene/styrene, SIS) with those of PBS all using the 5 + formulation. HBS is emulsion polymerized; the others are solution polymerized. SIS5 + had the better tensile properties whereas HBS5 + had the lowest water uptake. All the other 5 + formulations had higher uptakes than PBS5 +, which is thought to be due to the presence of hydrophilic groups from the solution polymerization process. All materials showed some sign of oxidation. Emulsion polymerized elastomers are regarded as less suitable for medical uses than the solution polymerized alternatives. Of these, SIS5 + seems to be less prone to oxidation and has the better mechanical properties so was considered to be the most suitable material for further development.

Analysis of a dimethacrylate copolymer (bis-GMA and TEGDMA) network by DSC and 13C solution and solid-state NMR spectroscopy.

In this work the effect of dilution with TEGDMA on the kinetics of Bis-GMA polymerization and on the extent of polymerization or degree of conversion was studied using (a) DSC and (b) NMR. The systems with lower viscosity and lower Tg exhibited higher extent of polymerization. For Bis-GMA/TEGDMA mixtures the calculated Tg values were found to be higher than the experimental values suggesting that a dilution effect is predominant rather than intermolecular hydrogen bonding. Solid state NMR has been shown to be a convenient method for measuring the total amount of conversion in a mixed monomer system. The disappearance of the NMR solution spectrum was used to reveal overall polymerization kinetics.

Relationship between composite matrix molecular structure and properties.

The synthesis of three novel dimethacrylate esters for use as the monomer phase in dental composites was studied. The monomers were prepared by the reaction of glycidyl methacrylate with phthalic, isophthalic and terephthalic acids, respectively, and the reaction products were found to be mixtures of isomers. The monomers obtained from the reaction of phthalic and isophthalic acids were low-viscosity liquids and the relationship between viscosity and molecular structure was studied. The low viscosity of these di-hydroxyl containing monomers was shown to be due to intramolecular hydrogen bonding. This reduced monomer viscosity compared with BisGMA allowed a reduction in the quantity of diluent monomer triethyleneglycoldimethacrylate (TEGDMA) required to give standard viscosities of 1 and 2 Pas resulting in a modest decrease (ca. 10%) in polymerization shrinkage compared with that exhibited by BisGMA/TEGDMA solutions of equivalent viscosities. A number of properties relevant to the use of these as potential core monomers in dental composites were determined for both monomer and polymer systems.

Relationship between filler and matrix resin characteristics and the properties of uncured composite pastes.

The relationships between filler type, filler content, matrix resin composition and viscosity and the flow characteristics of composite paste formulations have been investigated. BIS-GMA and 10 experimental BIS-GMA analogues were diluted as needed with TEGDMA to produce 1000 and 2000 cp solutions corresponding to the viscosity range of commercial composite matrices. All of these resins were mixed with a silanated hybrid-filler and a silanated micro-filler. For each combination the maximum filler content was determined, as well as the paste consistencies (plasticities) at a series of filler contents. Paste consistencies were measured by a modification of the ADA specification methods developed for determining standard test consistencies for zinc phosphate and silicate cements. For each monomer-filler combination increasing filler contents resulted in monotonically reduced plasticities. The maximum filler contents appeared to be an attribute of the particular filler rather than matrix resin characteristics. For the materials used, the maximum contents were approximately 86 wt% (83 vol%) filler for the hybrid filler, and 36% wt% (24 vol%) for the microfilled material. At lower filler contents, plasticity differences unexpectedly existed even at equal filler contents and matrix viscosities. Evidence was found that the plasticity varied both with the base monomer composition and the amount of TEGDMA present. The results imply that compositional interactions between the filler and matrix influence the results when all known mechanical factors are controlled.

Evaluating the effect of soft lining materials on the growth of yeast.

STATEMENT OF PROBLEM: Soft lining materials continue to have a place in clinical removable prosthodontics. However, there is an increased probability of yeast colonization on soft lining materials. PURPOSE: This study (1) assessed a method of evaluating the effect of long-term soft lining materials on the growth of yeast and (2) investigated the effect five soft lining materials had on the growth of three species of yeast. MATERIAL AND METHODS: Coe Supersoft, Novus, and three experimental soft lining materials were investigated together with Candida albicans, Candida tropicalis, and Issatchenkia orientalis (formerly Candida krusei) yeasts. Strips of soft lining material incubated on blood agar plates were examined for inhibition of the growth of yeast. Soft lining materials soaked in sterile trypticase soya broth or water were inoculated with yeast and incubated. The change in colony forming units per milliliter from the initial load of yeast at 3 days was measured. Statistical analysis was performed with an independent paired Student t test. RESULTS: Inhibition of yeast growth occurred for two soft lining materials. Despite the presence of sufficient viable organisms, differences between the initial load of yeast and the 3-day results were mostly small, both for the test and control groups, suggesting that the material does not support the growth of the tested yeast during this period. CONCLUSIONS: The often described increased prevalence of yeast associated with soft lining materials in the oral environment is likely related to readily available nutrients in the mouth and the difficulty in maintaining and cleaning these materials.

Characterization of some aromatic dimethacrylates for dental composite applications.

Several novel dimethacrylates have been developed as alternative matrix materials for dental composite applications. For the cured bulk polymers the equilibrium water uptake, reduction of glass transition temperatures (Tgs) by water sorption, refractive indices and the surface hardness have been determined. The properties were then compared with those of the control Bis-GMA. These properties correlated well with the structures of the polymers. Polar groups were found to increase the water sorption and thus reduce surface hardness.

Structure property relationship among novel dental composite matrix resins.

Several novel dimethacrylate monomers of propoxylated diphenols have been synthesized to establish a correlation between their structure and the important properties of viscosity, curing shrinkage and wetting behaviour. These dimethacrylates were designed to possess linear and flexed structures. These new monomers exhibited very much lower viscosities compared to the conventionally used Bis-GMA. Good correlations were obtained for variations of the monomer structures with bulk viscosities and curing shrinkages. However, the wetting behaviour of these resins did not show any trend.

Water uptake of soft lining materials from osmotic solutions.

OBJECTIVES: The water uptake characteristics of soft lining materials are of obvious importance in that they are expected to function in the oral environment. Results for Novus (Hygenic Corp., Akron, OH, USA) show a very high uptake from distilled water. Despite this high uptake, Novus appears to function satisfactorily in the mouth. High water uptake of soft lining materials has been attributed to the presence of water soluble impurities that, on immersion, form solution droplets; the driving force for the uptake being the osmotic gradient between the droplets and the external solution. Uptake should therefore be less from ionic solutions. The object of this study was to test the applicability of this theory to Novus and two experimental soft lining materials. METHODS: Water uptake of two experimental materials and Novus has been determined from distilled water and two saline solutions (0.45 and 0.9 M). After 196 days specimens were desorbed to constant weight and then subjected to a second sorption cycle. RESULTS: Novus had the highest uptake from distilled water at approximately 18%, the experimental materials having an uptake approximately 7%. Desorptions were all rapid, minimum weight being reached within 1-2 days. Uptakes of the second sorptions from water were all higher. Uptake from saline solutions was approximately 12% for all materials, uptake from 0.9 M saline being the lowest. Second sorption results from solution were similar to the first. CONCLUSION: The results obtained support the theory that the high water uptake of elastomeric materials is osmotically driven.

Influence of additives on the water uptake of hydrosilanized silicone rubbers.

Water uptake characteristics of a silicone polymer containing additives of varying solubility have been investigated. A hydrophobic silica, a sparingly soluble agent and a soluble catalyst were added to a stoichiometrically balanced hydrosilanized silicone polymer (the inherent absorption of which was 0.09 wt%). The diffusion coefficients for desorption of all materials were of the order 10(-6) cm2s-1, whereas the diffusion coefficients for absorption showed a decrease with increasing hydrophilicity and solubility of the additive. The greater the solubility of the additive, the more prolonged the uptake and the greater the deviation from classic diffusion theory.

Influence of hydrogen bonding on properties of BIS-GMA analogues.

The influence of chemical structure on the important properties of composite matrix resins is being systematically investigated. This study addresses the relationships between pendent side chain structures, viscosity and curing shrinkage. In particular, viscosity is known to be greatly influenced by intermolecular interactions, such as hydrogen bonding, and free volume effects. In order to establish the relative importance of these factors, analogues of BIS-GMA were synthesized in which the pendent hydroxyl groups were replaced by trimethyl siloxyl, and by dimethyl, isopropyl siloxyl groups. The viscosities were determined with a cone and plate viscometer and curing shrinkages were determined gravimetrically. They were compared to previously determined values for BIS-GMA and its methyl and hydrogen substituted analogues. The high viscosity of BIS-GMA is drastically reduced by replacement of the hydroxyl group, or its substitution by silylation. The relatively smaller effects produced by varying the bulk of the substituted side chains indicates that the main effect on viscosity is due to the presence or absence of hydrogen bonding. Conversely, increasing the bulk of the side chain substituent has less effect on viscosity, but significantly reduces the curing shrinkage. Changes in curing shrinkages were explained in terms of effects of free volumes associated with the monomers.

Effect of dilution on the kinetics of cross-linking thermal polymerization of dental composite matrix resins.

The effect of dilution of 2,2-bis-(4-(2-hydroxy-3-methacryloxyprop-1oxy) phenyl) propane (bis-GMA) with triethylene glycol demethacrylate (TEGDMA) on the extent of polymerization, Ep, has been studied using differential scanning calorimetry. The isothermal and dynamic measurements indicate that the Ep is affected dramatically by dilution. The residual polymerization was also found to be strongly influenced by dilution. However, the apparent activation energy, Eapp, was found to be independent of dilution.