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Biomedical subjects

K W Davy

Publications and source records attributed to K W Davy.

At least 19 recordsLinked to original sources
PubMed2002

Water absorption characteristics of dental composites incorporating hydroxyapatite filler.

Water uptake characteristics of BisGMA-based composites incorporating untreated and surface-treated hydroxyapatite (HA) with a silane coupling agent have been investigated. The water absorption and desorption behaviour of these composites obeyed the classical diffusion theory. The diffusion coefficients of the composites during first absorption were very similar to that for the base resin, suggesting that the water uptake process occur mainly in the resin matrix. The incorporation of HA reduced the water uptake of the base resin and lower uptake was found for those formulated with surface-treated HA. It was also observed that the equilibrium uptake decreased with increasing filler loading. However, the filled specimens had a higher water absorption than which would be expected on the basis of the resin content. This increase in the water uptake was largely due to the presence of porosity and filler particle aggregates in the microstructure of composites, although the adsorption of some water onto the filler surface has not been ruled out. The experimental composites showed higher solubilities than that obtained for the base resin.

Absorption↗
PubMed1998

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.

Biocompatible Materials↗
PubMed1998

Monomethacrylate co-monomers for dental resins.

Polymerisation shrinkage is widely recognised as a major drawback of resin based dental restoratives. Bis-GMA is often employed as the principal dimethacrylate monomer. Due to its high viscosity, Bis-GMA is normally mixed with large proportions of low viscosity glycol dimethacrylates. The purpose of this study was to determine whether the polymerisation shrinkage of Bis-GMA-based resins would be lower if alternative monomethacrylate co-monomers were used in place of conventional dimethacrylate co-monomers as viscosity modifiers. Conventional resins used were ethyleneglycol dimethacrylate and triethyleneglycol dimethacrylate; the alternative monofunctional co-monomers were tetrahydrofurfuryl methacrylate, hydroxypropyl methacrylate and isobornyl methacrylate. Model resins containing 54% mol/mol of co-monomer in Bis-GMA and 1% w/w of benzoyl peroxide as initiator were heat-cured at 70 degrees C for 8 h. Polymerisation shrinkage, degree of conversion and concentration of remaining methacrylate groups were calculated from density changes obtained gravimetrically. Other properties evaluated were Young's modulus, water uptake and viscosity of the monomer mixtures. The Bis-GMA-based resins exhibited lower shrinkage when mixed using the monomethacrylates rather than with conventional glycol dimethacrylates. Among the alternative co-monomers, tetrahydrofurfuryl methacrylate conferred the best balance of all measured properties.

Absorption↗
PubMed1997

Iodinated methacrylate copolymers as X-ray opaque denture base acrylics.

OBJECTIVES: The purpose of this study was to evaluate novel iodinated methacrylate copolymers as X-ray opaque denture base resins. METHODS: The synthesis of specific monomers and suspension copolymerization with methyl methacrylate to produce copolymer beads. The resulting beads were processed in an identical manner to standard PMMA to produce test-pieces for mechanical testing. RESULTS: Samples prepared from beads containing 25 wt% of the iodinated copolymer exhibited an X-ray opacity equivalent to that exhibited by a similar thickness of aluminium. Furthermore, the appearance and mechanical properties were comparable to standard PMMA, while thermal stability proved superior. CONCLUSION: These novel iodinated methacrylate monomers show promise not only as polymerizable additives to methyl methacrylate to produce an X-ray opaque denture base but also as thermally stable copolymerizable additives to other applications where X-ray opacity would be advantageous.

Contrast Media↗
PubMed1996

Effect of co-monomer composition on the integrity of bioactive growth hormone released from novel PEMA based polymers.

The release of human growth hormone (hGH) from hormone loaded bone cement was previously shown to enhance osteoid formation. hGH is a complex protein and its incorporation into such cements may compromise its bioactivity. We therefore characterized the release of hGH from a series of methacrylate systems based upon poly(ethylmethacrylate) (PEMA). Different mixtures of two monomers, hydroxyethylmethacrylate (HEMA) and n-butylmethacrylate (n-BM) were used to provide polymers with graded water uptakes. Exclusive use of only one of the monomers resulted in enhanced cytotoxicity and also reduced release of the bioactive hormone. Combinations of the monomers improved the recovery of bioactivity from the polymers and reduced their cytotoxicity. hGH released from the polymer with the lowest water uptake (100% n-BM, 0% HEMA) had an exceptionally low bioactivity: immunoactivity ratio, suggesting that the bioactive site of the hormone is particularly susceptible to disruption when it is incorporated into this matrix.

Animals↗
PubMed1996

THFMA in dental monomer systems.

This study investigates the physical properties of a range of dental resins containing up to 30% by weight of tetrahydrofurfuryl methacrylate (THFMA). The principal monomer was 2,2-bis-(4(2-hydroxy-3-methacryloyloxypropoxy)-phenyl)-propane (bis-GMA) or 1,6-bis-(methacryloxy-2-ethoxycarbony lamino)-2,4,4-trimethyl-hexane (UDMA). Heat-cured resins were tested for Young's modulus, flexural strength, refractive index, polymerization shrinkage and degree of conversion. The results indicated that THFMA has negligible effects on the mechanical properties of bis-GMA and UDMA and on the refractive index of UDMA, whilst the other measured properties present a strong relationship with the concentration of the additive. The dental relevance of the results is discussed and it is concluded that there is scope for further research on the use of THFMA as a comonomer in resin-based materials for dental applications.

Acrylic Resins↗
PubMed1992

Refractive index and molar refraction of methacrylate monomers and polymers.

The refractive indices of a number of methacrylate monomers have been measured and corresponding molar refraction data calculated. Similar determinations were made on a number of methacrylate polymers. The molar refraction values determined were in excellent agreement with the values calculated from the published molar refraction values of the chemical groups involved. There was little difference between the molar refraction values of monomers and that of the repeat unit in the corresponding polymers, in marked contrast to the molar volumes. The ratio of refractive index to molar volume was reasonably consistent for all methacrylates studied (0.25-0.30 approximately).

Chemical Phenomena↗
PubMed1992

Reinforcement of polymers of 2,2 bis-4(2-hydroxy-3-methacryloyloxy propoxy) phenyl propane by ultra-high modulus polyethylene fibres.

A study has been made of the reinforcement of 2,2 bis-4(2 hydroxy-3-methacryloyloxy propoxy) phenyl propane/tetra-hydrofurfuryl methacrylate copolymers with ultra-high modulus polyethylene fibres. The fibres were orientated longitudinally, in loadings up to 50% w/w, and both untreated and surface treated fibres were studied. Modulii up to approximately 35 GPa were achieved in the axial direction, and the specimens could not be broken in the simple flexure test employed. Electron microscopy of fractured specimens showed extremely good contact between resin and fibre. No deterioration in properties was observed over 6 months in water.

Biocompatible Materials↗
PubMed1992

Novel acrylic resins for dental applications.

A heat-cured resin based on a copolymer of bis-phenol-A glycidyl methacrylate (bis-GMA) and tetrahydrofurfuryl methacrylate (THFM) was investigated. Workable pastes were made by adding 90/10 w/w bis-GMA/THFM copolymer powder to a 70/30 w/w monomer of the same composition. The organic filler content was 60-64% w/w. Young's modulus, flexural strength, impact strength, hardness, water absorption and desorption, linear thermal expansion, polymerization shrinkage and glass transition temperature were determined. The materials studied showed high elastic modulus, hardness and glass transition temperature. A relatively low linear thermal expansion was obtained but poor impact strength and low flexural strength, indicating brittleness. Acceptable values were obtained for water absorption.

Acrylic Resins↗
PubMed1992

Thermal expansion of glassy polymers.

The thermal expansion of a number of glassy polymers of interest in dentistry has been studied using a quartz dilatometer. In some cases, the expansion was linear and therefore the coefficient of thermal expansion readily determined. Other polymers exhibited non-linear behaviour and values appropriate to different temperature ranges are quoted. The linear coefficient of thermal expansion was, to a first approximation, a function of both the molar volume and van der Waal's volume of the repeating unit.

Biocompatible Materials↗
PubMed1991

Study of polymeric systems based on 2,2 bis-4(2-hydroxy-3-methacryloyl-oxypropoxy) phenyl propane.

2,2 bis-4(2-hydroxy-3-methacryloyloxypropoxy) phenyl propane is the basic monomer for a large number of proprietary dental composite filling materials. In this paper, studies have been made of its copolymers with various comonomers. 2,2 bis-4(2-hydroxy-3-methacryloyloxypropoxy) phenyl propane--tetrahydrofurfuryl methacrylate copolymers, at about the 95/5% v/v level, showed an enhanced modulus over that of the test material itself, to give a Young's modulus of -4.8 GPa. This reflected an enhanced activation energy of the glass transition temperature, indicating a very specific free volume effect of the diluent monomer. Further studies 2,2 bis-4(2-hydroxy-3-methacryloyloxypropoxy) phenyl propane-tetrahydrofurfuryl methacrylate room temperature polymerized copolymers showed that enhanced modulus could be achieved by including in the system small amounts of inhibitor; 0.3% v/v 2,6 di-tertiary butyl phenol elevated the Young's modulus of a benzoyl peroxide--NN-dihydroxy ethyl p-toluidine cured system from 3.1 to 3.6 GPa. Such resins may be useful in improved dental fissure sealants and composite filling materials.

Benzoyl Peroxide↗
PubMed1991

Residual monomer in acrylic polymers.

Three proprietary dental resins were found to include acrylates (as distinct from methacrylates). As acrylates are well known to have adverse skin reactions, the nature and content of the acrylates were obtained. One resin contained methyl acrylate, and the other two ethyl acrylate. Also, the extractable free monomer from one proprietary and one experimental bone cement was determined as a function of time. This was carried out: (a) on unpolymerized material as soon as it formed a coherent dough; and (b) on already polymerized material. The extraction of n-butyl methacrylate from the experimental bone cement was very much less than that of methyl methacrylate from the proprietary bone cement. In both cases, whilst the kinetics of loss for short times appeared to conform to diffusion theory, subsequent loss did not; this was attributable to both continued polymerization of the monomer and to the concentration dependence of the diffusion coefficient.

Acrylates↗
PubMed1991

Invalidity for Pseudomonas aeruginosa of an accepted model of bacterial permeability to beta-lactam antibiotics.

The accepted model for the penetration of beta-lactam antibiotics into gram-negative bacteria is that proposed by Zimmermann and Rosselet (Antimicrob. Agents Chemother. 12:368-372, 1977). The model assumes (i) that diffusion of the antibiotic molecules across the outer membrane obeys Fick's law and can be characterized by a permeability constant for any given combination of organism and drug, (ii) that drug hydrolysis within the periplasm obeys Michaelis-Menten kinetics, and (iii) that a steady state is rapidly attained between drug uptake and hydrolysis. The model has allowed accurate prediction of antibiotic MICs for Escherichia coli strains from a knowledge of their beta-lactamase production and permeability characteristics. It has been suggested that the model is inappropriate for Pseudomonas aeruginosa, but attempts to confirm this have been bedevilled by experimental difficulties in estimating permeability coefficients for this species. In the present study, we tested a prediction of the model that the overall resistance of P. aeruginosa transconjugants containing a plasmid-encoded beta-lactamase should continue to depend partly on permeability. Transconjugants with PSE-4 beta-lactamase were constructed in host strains with widely different levels of intrinsic, presumably impermeability-determined resistance. Contrary to the prediction of the model, all the transconjugants developed identical overall levels of resistance to substrate beta-lactams, such as azlocillin and cefoperazone, irrespective of the initial levels of intrinsic resistance of the recipient strains. We conclude that the model is inappropriate for P. aeruginosa, and possible explanations for the organism's behavior are discussed.

Anti-Bacterial Agents↗
PubMed1989

Reaction products of chloramphenicol acetyltransferases from enterobacteria and Haemophilus influenzae.

The reaction products of Types I, II and III chloramphenicol acetyltransferases (CATs) from reference strains of Escherichia coli and Type II CATs from clinical isolates of Haemophilus influenzae were examined by HPLC. Types I and III CATs produced 1, 3 diacetylchloramphenicol at pH 6.8 and 7.8 whereas Type II enzymes failed to produce this diester at either pH. All the CATs produced monoacetylchloramphenicol at both pH values. Neither mono- nor di-acetylchloramphenicol was stable. Breakdown of enzymically-produced mono- and di-esters proceeded more rapidly at pH 7.8 than at pH 6.8. Similar breakdown behaviour was observed with synthetically manufactured 1, 3 diacetylchloramphenicol.

Acetylation↗
PubMed1987

The mechanical properties of elastomeric poly(alkyl methacrylate)s.

A range of poly(alkyl methacrylate)s in the range C5 to C13 with varying degrees of crosslinking, have been studied with respect to stress-strain behaviour. Where the extensions to break were sufficiently high, stress-strain properties conformed well to the statistical theory of rubber elasticity, the Mooney/Rivlin C2 term being sensibly zero. All materials studied were very elastic, exhibiting extremely little permanent set. The energy to break decreases very rapidly as the homologous series is ascended, and 0.5% crosslinking agent is perfectly adequate to give elastic properties. Hence either n-pentyl or hexyl methacrylates are to be preferred in soft prosthesis formulations on mechanical grounds.

Acrylates↗
PubMed1987

Polymerization shrinkage of methacrylate esters.

The polymerization shrinkage of a range of poly(n-alkyl methacrylates) in the range C1 to C16, some of their isomers and the polymers of cyclic and heterocyclic methacrylates have been measured using densitometry. The percentage volume shrinkage decreases with the size of the substituent side group. This proved to reflect the fact that the change in molar volume on polymerizing a methacrylate ester is reasonably constant at 22 cc/mol irrespective of the geometry of the substituent group. However, the glass transition temperature of the polymer depends very critically on the geometry of the side group. Hence one method for the development of low shrinkage glassy polymers is to investigate methacrylate esters of large molar volume, but with side group geometry that results in a high glass transition temperature. Polymerization shrinkage can be used to determine the degree of conversion of a polymer, using the value of 22 cc/mol as the change in molar volume.

Acrylates↗
PubMed1986

Water absorption characteristics of some unfilled resins.

The water absorption characteristics of resins, upon which many composite filling materials are based, have been studied. In general, the equilibrium uptake is less than that predicted from the water absorption of composite filling materials, if it is assumed that the polymer alone absorbs the water. Also, whereas there are very large differences between sorption and desorption diffusion coefficients in composites, there is very little difference in the case of unfilled resins. Furthermore, sorption in the case of composites is generally very much slower near equilibrium than predicted. These phenomena are all explicable because composite filling materials contain water soluble impurities which enhance uptake, which is ultimately determined by a balance between osmotic and elastic pressures. This implies the existence of internal strains in composites.

Absorption↗