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Biomedical subjects

R L Clarke

Publications and source records attributed to R L Clarke.

16 recordsLinked to original sources
PubMed1992

Modified Arrhenius equation for the frequency dependence of the transition temperature of polymers--a note on some further observations.

Activation energies obtained from differential mechanical thermal analysis work on polymers have been observed to relate to similar behaviour of diffusion coefficients of gases in polymers. Comparison of the two techniques showed the additional energy terms to be virtually identical and the pre-exponential factors to relate to segmental motion of a five-unit carbon atom.

Biocompatible Materials↗
PubMed1989

Dynamic mechanical thermal analysis of dental polymers. II. Bis-phenol A-related resins.

Dynamic mechanical analysis of bis-phenol A-related resins for dental usage provided loss tangent maxima at their transitions and an Arrhenius-type dependence of transition temperature and frequency. A constant maximum in the loss tangent curves, over the frequency range for the unfilled resins below their glass transition temperature, was thought to be caused by their high degree of unsaturation. The calculated activation energies showed the polycarbonate denture base resin required considerably greater energy for segmental motion than the other tested materials. In contrast, the ethylene imine-based temporary crown and bridge material exhibited the lowest activation energy.

Aziridines↗
PubMed1989

Modified Arrhenius equation for the frequency dependence of the transition temperatures of polymers.

A linear relationship was established for several polymers between the pre-exponential factor and the activation energy of a modified form of the standard Arrhenius equation. In addition, the energy term itself was modified such that the alpha and beta absorptions were related to the number of carbon atoms available for segmental motion. At large activation energies the modified Arrhenius relationship suggested upper limits of 495.5K and 321K for the alpha and beta transition temperatures, respectively.

Biocompatible Materials↗
PubMed1989

Dynamic mechanical thermal analysis of dental polymers. I. Heat-cured poly(methyl methacrylate)-based materials.

The visco-elastic characteristics of homogeneous, heterogeneous and fibre-reinforced poly(methyl methacrylate) for use as denture base resins were measured by a dynamic mechanical thermal analyser in the frequency range 0.033-90 Hz. Dynamic Young's modulus of both polyethylene woven yarn and the polyaramid plain fabric-reinforced acrylics showed considerable improvement over the conventional denture base, whereas the carbon fibre-reinforced acrylic produced results lower than expected. From the loss tangent curves of the homogeneous and heterogeneous methacrylates their transition temperatures were recorded and the reciprocals of both the alpha and beta absorption temperatures were plotted against frequency to obtain their respective activation energies.

Dental Materials↗
PubMed1989

Dynamic mechanical thermal analysis of dental polymers. III. Heterocyclic methacrylates.

Four heterocyclic methacrylates were investigated with respect to their loss tangent and activation energy using a dynamic mechanical thermal analyser over a limited frequency range of 0.033 to 90 Hz. The linear regression lines obtained showed an Arrhenius-type relationship between transition temperature and frequency. Their low activation energies in comparison to poly(methyl methacrylate) and bis-phenol A based methacrylates was thought to be a direct consequence of their low molecular weights.

Acrylates↗
PubMed1978

(2-exo-3-endo)-2-Aryltropane-3-carboxylic esters, a new class of narcotic antagonists.

Tropanes 4 bearing an exo aromatic group on carbon-2, an endo-carbomethoxy group on carbon-3, and either a methyl group or a hydrogen on the nitrogen were found to be narcotic antagonists which were devoid of demonstrable analgesic activity. The activity resided in the 1S enantiomer. Compound 4 (R = m-hydroxyphenyl) showed an AD50 of 0.37 mg/kg sc and 1.8 mg/kg po (rats) as an antagonist in the Harris-Pierson modification of the D'Amour-Smith test. The tropane esters for this study were prepared by a Grignard reaction which gave essentially complete 1,4-addition in the absence of copper salts. Nearly equal quantities of esters epimeric at carbon-3 were formed.

Acetylcholine↗
PubMed1978

(exo, exo)-2-Aryltropane-3-carboxylic esters, hypoglycemic agents with accompanying analgesic activity.

(exo, exo)-2-Aryltropane-3-carboxylic esters of types 6, 7, and 10 lower circulating blood glucose levels by 60--80%. This activity is accompanied by an analgesic activity roughly equal to that of codeine. Both of these activities reside in the 1R enantiomer and extensive structure-activity studies failed to separate them. The specific opioid antagonist nalorphine blocks the analgesic activity but does not diminish the hypoglycemic action. Conformational integrity afforded by the ethylene bridge is neccessary for the observed activities.

Administration, Oral↗
PubMed1975

Perianal Paget's disease: report of two cases.

A patient with extensive perianal extra-mammary Paget's disease was treated by wide local excision of the perianal skin. No underlying associated adenocarcinoma was identified, and the patient is clinically free of disease five years after treatment. A second patient had a small focus of Paget's disease confined to a hemorrhoid. The necessity for histologic evaluation of all anal lesions is emphasized.

Adult↗
PubMed1975

Beta-cocaine.

Explore the source record for details and available documents.

Anesthetics, Local↗
PubMed1975

Analgesic activity of the epimeric tropane analogs of meperidine. A physical and pharmacological study.

Condensation of cis-N-benzyl-2,5-bis(chloromethyl)pyrrolidine (6) and phenylacetonitrile afforded a mixture of epimers 7 and 8. Compound 8 was readily converted to the meperidine analog 1 prepared earlier by Bell and Archer. Compound 7 was converted to a new tropane analog of meperidine, compound 3. The ED50 of 1 and 3 in the D'Amour-Smith "tail flick" test for narcotic type analgesia, which differs by a factor of only 3 or 4 potency, supports the accumulated data that suggest that the analgesic activity of the meperidine type is not very sensitive to the conformation of the phenyl group in 4-phenylpiperidines. A proton and 13C magnetic resonance spectral comparison of 1 and 3, as well as a reevaluation of the conformational requirements of 17-19, leads to the conclusion that the differences in conformation of 1,3,17, and 18 are due to the varying degrees of flattening of piperidine ring. The 1H NMR and 13C NMR data are not consistent with the boat conformation suggested earlier for compound 17.

Analgesics↗
PubMedDate not supplied

The use of Compton scattered gamma rays for tomography.

A method of tomographic imaging of soft tissues by the use of Compton scattering of gamma rays is described. A highly collimated source (800 Ci of 60Co) and collimated detectors are used. Spatial resolutions of the order of 0.5 cm or better are achieved, with a density resoution of 2.0% to 4%. Experimental tests with dogs have shown internal organs in sagittal, coronal and transverse section. No contrast or radioactive material is required. Tests have shown that the regional dose for humans will be about 0.25 to 1 rad for one scan.

Animals↗
PubMedDate not supplied

Diffusion of water in composite filling materials.

The uptake of water by composite filling materials seems to be a diffusion-controlled process; the magnitudes of the diffusion coefficients obtained are consistent with the uptake base through the resin matrix. The diffusion coefficients measured show the diffusion coefficient to decrease with increasing water concentration, which accounts for the protracted nature of the attainment of equilibrium uptake. Diffusion coefficients are generally lower in these composites based on difunctional methacrylates, compared with methyl methacrylate, presumably because of the highly cross-linked nature of the former.

Absorption↗