S. Kalachandra

Influence of fillers on the water sorption of composites.

The main objectives of this work were to characterize the water sorption of dental composites in terms of water uptake, diffusion coefficients (D), and polymer content, and to study how these parameters are influenced by the nature of the filler and the presence of 4-META (4-methacryloxy ethyl trimellitic anhydride). Four anhydrous composites--(1) tribasic calcium phosphate (TCP 50%) and triethylene glycol dimethacrylate (TEGDM), (2) silanated lithium aluminum silicate (SS, 75%) and TEGDM, (3) barium sulfate [BaSO4 (70%) and poly(methyl methacrylate) (PMMA)], and (4) silane-coated barium glass (SBG) (75%)--and PMMA were employed in the water-sorption studies at 37 degrees C. The effect of 5% 4-META on the diffusion and uptake of water was studied at 37 degrees C. The data conformed approximately to Ficks laws of diffusion. The values of D were found to be significantly smaller with SS, suggesting an effective coupling. Polymer contents in the latter two composites were determined by incineration of the samples to constant weight. The uptake of water by the filled specimens was about two-fold that which would be expected on the basis of the PMMA content. These filled specimens took nearly twice as much water as unfilled PMMA. An additional amount of water is perhaps accommodated at the interface between the filler and PMMA matrix. The D values for water in filled specimens were considerably larger than those in the unfilled specimens. It appears that the filler-matrix interface provides paths of facile diffusion similar to grain boundary diffusion.

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.

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.

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.

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.

Water sorption of plasticized denture acrylic lining materials

The objective was to characterize water sorption of plasticized denture acrylic lining materials, with determination both of uptake and diffusion coefficient (D). Feasibility was investigated first for simple binary mixtures of poly(methyl methacrylate), PMMA, and various phthalate plasticizers prepared by radiation polymerization of methyl methacrylate/plasticizer solutions. These were shown to be tractable when samples were saturated with water and then characterized in desorption measurements. Plasticizers were found to decrease the uptake of water. This decrease was attributed to the hydrophobic nature of the plasticizers and also to their ability to fill microvoids in PMMA which, in their absence, would be available to water. Values of D increased monotonically with increasing volume fractions of plasticizer. It appeared that values of D increased more rapidly in cases where large proportions of plasticizer resulted in samples which were above their glass transition temperature, Tg, at the temperature of testing water sorption. Desorption measurements were extended satisfactorily to a model system made from poly(ethyl methacrylate) powder, methyl methacrylate, and dibutyl phthalate, which was polymerized with a conventional redox initiator. Water desorption results were compared with various proprietary materials which were also characterized with respect to Tg and, cursorily, to composition.

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 °C for 6 h. Thin square films of 3×3 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 (56.15ug/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.

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.

Antimicrobial activity and local release characteristics of chlorhexidine diacetate loaded within the dental copolymer matrix, ethylene vinyl acetate

In vitro results are presented for a novel oral drug-delivery system ultimately intended for treatment of oral infections in immunocompromised patients. Test samples of ethylene vinyl acetate copolymer (EVA) containing chlorhexidine diacetate (CDA) showed desirable antimicrobial properties and steady, slow release into aqueous and other media after an initial burst of drug release in the first day of liquid exposure. By washing away this initial burst, the proposed mouthguard device should be capable of sustained delivery of locally effective CDA concentrations far below systemically toxic levels. A prolonged room temperature shelf-life of at least 1 year, and effectivity against a wide range of oral bacteria and Candida species was demonstrated. Drug loaded films showed a top-to-bottom asymmetry in drug release, but good lateral homogeneity, and a linear relationship between initial CDA loading concentration (from 0.63 to 10 wt %) and days 3-14 release rates in a static aqueous environment. The EVA matrix containing CDA appears to possess many suitable properties for localized oral delivery of sustained antimicrobial activity.

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.