Biomedical subjects

M Braden

Publications and source records attributed to M Braden.

At least 37 records · Page 2Linked 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↗

PubMed2001

Nuclear magnetic imaging of an osmotic water uptake and delivery process.

The water absorption of NH4F-doped silicone cylinders was studied by spin-echo Fourier nuclear magnetic resonance imaging, in conjunction with gravimetric (mass uptake) and ionic concentration (release of NH4F) measurements. The addition of NH4F greatly increases the water uptake as the inclusions are surrounded by droplets of water when wetted by water diffusing in the polymer. These droplets then expand due to the osmotic pressure difference between the internal droplet and external solution; this expansion is opposed by an elastic restraining force. The rate of ingress into the polymer will depend on the relative chemical potential of water associated with the polymer and the droplet, as well as the diffusion coefficient. Later stages of the absorption process are dominated by the droplets. The dilution of the droplets (from water diffusing into the droplets) slowly reduces the chemical potential driving the process. Hence, the later stages occur without any notable concentration gradient but a slight chemical potential gradient. If the droplet expansion exceeds that which can be restrained by the material, failure around the droplet occurs resulting in the release of the salt and the solution within the droplet.

Ammonium Compounds↗

PubMed2001

The effect of two hydrophilic monomers on the water uptake of a heterocyclic methacrylate system.

The room temperature polymerising system poly(ethyl methacrylate) (PEM)/tetrahydrofurfuryl methcrylate (THFM) has been modified by replacing some of the THFM by hydroxyethyl methacrylate (HEMA) and hydroxypropyl methacrylate (HPM), respectively. In both cases, the equilibrium uptake of the parent system is reduced substantially, in spite of the hydrophilic nature of these monomers. The effect is less with HPM. Corresponding to these decreases in uptake are substantial increases in the diffusion coefficients. This points to changes from a cluster-dominated process, to a more continuum-based process in the dual monomer systems. Addition of chicken serum albumin to these systems increases water uptake. At higher levels of HEMA addition, there is a substantial increase in polymerisation exotherm.

Absorption↗

PubMed2001

A polymeric system for the intra-oral delivery of an anti-fungal agent.

Oral candidal infections are often persistent and intractable and thus the aim of this study was to develop a polymeric sustained release device to improve the topical treatment of these infections. A self curing system based on poly(ethyl methacrylate) and tetrahydrofurfuryl methacrylate (PEM/THFM) was used with chlorhexidine diacetate (CX) added at levels between 0 and 12% w/w. Water uptake by the device was assessed gravimetrically and CX release measured by UV spectrometry. Anti candidal activity was established by culturing azole sensitive and resistant strains of Candida albicans in the presence of the polymeric delivery device with and without CX. Candidal growth was measured by turbidimetry or surviving colony-forming unit (CFU) formation. There was an initial high release of CX over 24 h followed by a slow diffusion up to 7 days. CX inhibited candidal growth and survival markedly in vitro, with the test samples showing less than 0.5 x 10(-7) CFU/ml compared to controls (3-4 x 10(-7) CFU/ml). These results indicate the potential of a chlorhexidine containing PEM/THFM polymeric system in the treatment of persistent candidal infections.

Administration, Oral↗

PubMed2001

The effect of particle size on the gelation of tissue conditioners.

Tissue conditioner materials comprise a methacrylate polymer powder, usually poly(ethyl methacrylate) (PEM), and a plasticiser that also contains ethanol. The aim of this study was to investigate the effect of ball milling the PEM on gelation time. PEM was used as received and after ball milling for 2, 4, 8 and 16 h. Particle size was measured in each case and gelation time at 37 degrees C when mixed with butyl phthalyl butyl glycollate (BPBG). Gelation time reduced with increasing milling time of the powder, however it was found that increase in the fines initially had a more significant affect than overall reduction in average particle size. The gelation time of unmilled and 16 h milled PEM were also measured when mixed with BPBG and various levels of ethanol. Unmilled PEM with 4% ethanol had a gelation time of 13.5 +/- 2.0 min compared to 5.5 +/- 1.0 min for the 16h milled PEM which was similar to that for the commercial material, Viscogel (6.0 +/- 1.0 min). Ball milling the PEM reduces the level of ethanol required to produce a clinically acceptable gelation time.

Biocompatible Materials↗

PubMed2001

Release of recombinant human bone morphogenetic protein-2 from heterocyclic methacrylate polymer systems.

The release of recombinant human bone morphogenetic protein-2 (rhBMP-2) from three room temperature polymerising methacrylate systems has been studied. These all contained poly(ethyl methacrylate) powder, but the monomer liquids comprised, respectively, tetrahydrofurfuryl methacrylate (THFM), 90/10 THFM/hydroxyethyl methacrylate (HEMA), and 70/30 THFM/ HEMA. In all cases, rhBMP-2 was released, but the addition of 10% HEMA accelerated release (a nine-fold increase in diffusion coefficient); a further increase to 30% HEMA had no additional effect. For most of the release process, a diffusion process operated, although the early stages were not well defined. At the end of the 15 day period, the release, respectively, for the PEM/THFM, PEM:90/10 THFM/HEMA and PEM:70/30 THFM/HEMA systems was 596, 878 and 923 ng (i.e. up to 92% of the rhBMP-2 added).

Biocompatible Materials↗

PubMed2001

Blends of isoprene-styrene/methacrylate monomer systems as denture soft lining material.

This work further develops the concept of using an elastomer gelled with methacrylate monomers to produce a methacrylate-based soft lining material without the use of a plasticizer. An isoprene-styrene (SIS) block copolymer was mixed with methyl methacrylate (MMA) and 1,6-hexandiol dimethacrylate (HDMA). The HDMA was used as a cross-linking agent. The elastomer/monomer ratios were maintained at 50/50 whereas the monomers ranged from 0 to 100%) HDMA. Mechanical properties and water absorption/desorption characteristics were used to assess the effect of varying the monomer compositions. The results indicated that phase separation took place, in particular at high HDMA content. This significantly increased the Young's modulus and decreased the elongation to break. Generally, the water uptake tended to decrease with increasing HDMA content, reflecting the effect of modulus. Second absorption cycles gave higher uptake values compared to the first. Formulations with a high amount of HDMA gave materials with modulus values too high for soft lining applications. This suggests that the optimum formulation requires a compromise between modulus and water uptake.

Absorption↗

PubMed2001

Isoprene-styrene copolymer elastomer and tetrahydrofurfuryl methacrylate mixtures for soft prosthetic applications.

Novel elastomer/methacrylate systems have been developed for potential soft prosthetic applications. Mixtures of varying compositions of an isoprene-styrene copolymer elastomer and tetrahydrofurfuryl methacrylate (SIS/THFMA) formed one-gel systems and were heat cured with a peroxide initiator. The blends were characterised in terms of sorption in deionised water and simulated body fluids (SBF), tensile properties and viscoelastic parameters of storage modulus and tan delta, as well as glass transition temperatures using dynamic mechanical analysis (DMA). DMA data gave two distinct peaks in tan delta, a lower temperature transition due to the isoprene phase in SIS and one at high temperature thought to be a combination of THFMA and the styrene phase in SIS. The tensile data showed a clear phase inversion within the mid range compositions changing from plastic to elastomeric behaviour. The sorption studies in deionised water showed a two stage uptake with an initial Fickian region that was linear to t 1/2 followed by a droplet growth/clustering system. The slope of the linear region was dependent on the composition ratio. The extent of overall uptake was osmotically dependent as all materials equilibrated at a much lower uptake in SBF. The diffusion coefficients were found to be concentration dependent.

Absorption↗

PubMed2001

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.

Journal Article↗

PubMed2000

Phonon dispersion of bcc iron to 10 GPa.

The phonon dispersion of bcc iron under high pressure to 10 GPa was measured at 300 K by inelastic neutron scattering. Its pressure dependence is surprisingly uniform. Contrary to the behavior found in other bcc elements, there is a lack of any significant pretransitional behavior close to the martensitic bcc-hcp transition which could be related to the Burgers mechanism. This finding confirms predictions by spin-polarized total energy calculations that explain the transition by the effect of pressure on the magnetism of iron. The high pressure frequencies were used to develop a lattice dynamical model from which thermodynamic quantities can be determined at any pressure to 10 GPa.

Journal Article↗

PubMed2000

Photocurable epoxy-polyol matrices for use in dental composites I.

There is a need for low shrinkage visible-light cure resins to replace the dimethacrylate polymers used in dental composites. Cycloaliphatic epoxy resins formulated with polyol network extenders have been considered as potential candidates to address this need. This paper presents basic materials characterisation to underpin the selection of epoxy-polyol matrices cured with cationic onium/camphorquinone initiators. The study investigates the effect of choice of polyol and the ratio of epoxy groups to polyol groups in the matrix (R). The limitations of some analytical methods are also discussed. Epoxy-polyols are shown to have significant advantages over dimethacrylates for dental as they exhibit: lower polymer shrinkage, no oxygen inhibition layer, higher strength, equivalent hardness and acceptable glass transition temperatures (by DMA). However, the mass increase of the epoxy-polyols materials in water at 37 degrees C is almost double that of the conventional dimethacrylate matrices and cracking was observed in some samples during hydration. It is demonstrated that this can be avoided by careful selection of the formulation parameters. The paper concludes that R should be selected to lie between the limits of 4 and 8 to ensure the matrix has a balance of acceptable properties. The reduction of the water-uptake of epoxy-polyol matrices remains an important target and will be addressed through further research.

Composite Resins↗

PubMed2000

Chlorhexidine release from room temperature polymerising methacrylate systems.

A series of different methacrylate monomers (with either 1 or 2.5% dimethyl-p-toluidine, DMPT) was gelled with poly(ethyl methacrylate) powder (containing benzoyl peroxide) thus forming a room temperature curing system. When doped with 5.625% chlorhexidine diacetate the release from the tetrahydrofurfuryl methacrylate-based samples was considerably greater than that from other methacrylate monomers. This seems to be due to the formation of channels in the polymer. Nuclear magnetic resonance spectroscopy, of solutions that the samples were immersed in, showed chlorhexidine was indeed being released from the polymer. It also showed that doping the polymer with chlorhexidine hindered the polymerisation, resulting in a higher level of residual monomer and low molecular weight components being leached from the polymer. The DMPT also affected the polymerisation with greater leaching from the 2.5% DMPT sample.

Anti-Infective Agents, Local↗

PubMed2000

Mechanical properties of hydroxyapatite reinforced poly(ethylmethacrylate) bone cement after immersion in a physiological solution: influence of a silane coupling agent.

PEMA-based bone cement has previously been shown to possess many advantages over traditional PMMA cements. One of these is the option of adding up to 40 wt % HA without a decrease in static mechanical strength, thus providing the potential for enhanced bioactivity. Bone cement, in vivo, is subjected to an aqueous environment and therefore, it is important to understand the influence of this upon the mechanical integrity of experimental cements. In this current investigation the static and dynamic properties of PEMA cement, with and without 30 wt % untreated and silanated HA, were examined after periods of immersion in Ringer's solution. A commercial PMMA cement was also tested in a similar manner. Relatively small changes in static mechanical properties were observed after 12 weeks storage for the PEMA cements, the largest change being for the PEMA cement reinforced with silanated HA. The PMMA cement exhibited the largest change in static strength with a decrease of 16.6%. In contrast to these results, the fatigue properties of the PEMA cements were found to decrease significantly after storage in Ringer's solution, again with the largest changes to the PEMA cement reinforced with silanated HA. This effect was attributed to the reduction in efficiency of the silane coupling agent in the presence of water. The fatigue resistance of the PMMA cement was not reduced after immersion in a saline environment.

Journal Article↗

PubMed1999

3He ion-beam analysis of water uptake and drug delivery.

3He ion-beam analysis utilises a micronuclear reaction analysis (of deuterium and carbon) and microparticle induced X-ray emission (in this case silicon and chlorine), to determine an elemental map of the surface of a sample. This study used D(2)O to model the behaviour of water in poly(tetrahydrofurfuryl methacrylate-polyethyl methacrylate) and chlorhexidine diacetate doped silicone elastomers. The poly(tetrahydrofurfuryl methacrylate-polyethyl methacrylate) systems demonstrated an initial Fickian absorption process (diffusion coefficient 1.1 x 10(-11) m(2)s(-1)) which indicated the 2-stage nature of the polymer's absorption kinetics. The doped silicone samples demonstrated an osmotic mechanism for the controlled release of drugs, with correlation between the D(2)O and the chlorhexidine diacetate inclusions increasing during the experiment. The technique proved valuable in the analysis of delivery polymers and will undoubtedly have further applications in the development of drug delivery systems.

Chlorhexidine↗

PubMed1999

Soft acrylic resin materials containing a polymerisable plasticiser II: water absorption characteristics.

The main disadvantage of soft acrylic resin denture lining materials is the loss of plasticiser leading to gradual hardening. One possible solution to this problem is the use of a polymerisable plasticiser. A material based on such a plasticiser failed in clinical trials due to a combination of prolonged water uptake and low strength. Similar materials with improved strength have been developed and it is the aim of this study to investigate their water uptake characteristics. Water uptake from distilled water, an artificial saliva and 0.9 M saline has been determined for three experimental materials and 'Supersoft', a commercial plasticised acrylic resin denture lining material. Solubilities were determined after 6 months and diffusion coefficients calculated where appropriate. All the experimental materials had a lower uptake and solubility than 'Supersoft' in all solutions. For each material uptake was lower from solution than from distilled water. All diffusion coefficients were in the range expected for this type of material. The water uptake of the experimental materials was influenced by the relative hydrophilicity of the monomer used.

Absorption↗

PubMed1999

The water uptake of poly(tetrahydrofurfuryl methacrylate).

Poly(tetrahydrofurfuryl methacrylate) possesses some unique characteristics with respect to its biocompatibility and behaviour in water. The water uptake is high (>70%) and very slow (over 3 yr), but the material remains rigid throughout the process. The mechanism behind the uptake is in two stages; an initial Fickian stage, then as the matrix approaches saturation (about 3 wt%) a second clustering mechanism takes over. The rate of uptake of the second stage of the uptake is controlled by creep (or stress relaxation), and the chemical potential driving the uptake from clustering of the furfuryl rings of the polymer. If clustering or the creep is prevented (by appropriate co-polymerisation) the polymer behaves in an ideal, Fickian manner.

Absorption↗

PubMed1999

Viscoelastic properties of some soft lining materials. I--Effect of temperature.

The time dependent, viscoelastic characteristics of a number of dental soft liners (occasionally called resilient liners) were investigated using a dynamic mechanical analyser (DMA). The materials consisted of eight commercial products including methacrylate, silicone and phosphazine-based soft polymers as well as two experimental formulations. The DMA used was of the forced vibrational variety. The test conditions, such as frequency and temperature, were chosen to mimic in use (masticatory) conditions. A wide variety of behaviour was observed and accounted for. This included a seven fold range in the storage modulus (2.79-19.7 MPa) and a 50-fold range in the loss tangent (0.029-1.52) at 37 degrees C and 1 Hz. The materials could generally be divided into either rubbery behaviour with low modulus and loss tangent, i.e. the silicones, or leathery behaviour with both higher modulus and loss tangent, i.e. the methacrylates. A search of the literature did not find a universally preferred set of properties, although a possible link with residual ridge resorption is indicated.

Aziridines↗

PubMed1999

Viscoelastic properties of some soft lining materials. II--Ageing characteristics.

A non-resonant forced vibration, dynamic mechanical analyser was employed to measure the viscoelastic characteristics of soft lining materials at 1 Hz, after storage in distilled water at mouth temperature for periods up to and including one year. The six commercial products included methacrylate, silicone, and phosphazine based polymers and the one experimental material was a methacrylate. Water sorption of the soft liners, recorded by change in sample mass, ranged from -4.39 to +48.57% and their solubilities from 0.13 to 13.58%, after one year. The heat-cured silicone was the most stable polymer in water in contrast to its autopolymerised counterpart. The excessive water uptake of this latter material resulted in a massive reduction in modulus. At the other extreme one methacrylate with a high plasticiser content hardened substantially after ageing (modulus changed from 5.87 to 72.3 MPa). Changes in loss tangent data were relatively small for all the polymers tested, even for materials with high water uptake. Reduced leaching and/or plasticiser content have led to a more stable generation of soft lining materials.

Aziridines↗