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Biomedical subjects

W J O'Brien

Publications and source records attributed to W J O'Brien.

At least 127 records · Page 7Linked to original sources
PubMed1978

Surface finish of dental composite restorative materials.

Five composite filling materials were finished using six finishing procedures currently in clinical use. The average roughness produced by each procedure was measured by means of a Surfanalyzer with four replications for each procedure on each of the five materials. Statistical analysis indicated that finishing with silicon carbide disks produced the smoothest surfaces. Acceptable surfaces were obtained with stones or with burs. Use of an alumina paste after a green stone-white Arkansas sequence did not produce a significant improvement. Two processes of filler particle abrasion were distinguished by means of electron microscopy: wear and shear. Fracture striations indicated particle shear. The roughest surfaces were produced by the use of a diamond instrument which is contraindicated. Acceptable finishing procedures for the composite materials tested include silicon carbide disks for accessible areas or 12 fluted finishing burs for more inaccessible areas.

Composite Resins↗
PubMed1978

Effect of hydrostatic tensile stress on the growth of Escherichia coli and Bacillus cereus.

The specific growth rates of Escherichia coli and Bacillus cereus were measured for growth media in a flask, a lens-plate arrangement simulating an isolated capillary space, and a lens-plate arrangement under hydrostatic tensile stress. The specific growth rates of the bacteria were the same for the flask and lens-plate arrangement without hydrostatic tensile stress, but were enhanced when the growth media were subjected to hydrostatic tensile stress. The enhanced specific growth rates reached steady values at a tensile stress of 40 pascals. The effect was observed up to tensile stresses of around 100 pascals. The maximum increase in specific growth rate was 25% for E. coli and 22% for B. cereus.

Bacillus cereus↗
PubMed1977

Evidence for a complex of three beta-oxidation enzymes in Escherichia coli: induction and localization.

The enzymes for beta-oxidation of fatty acids in inducible and constitutive strains of Escherichia coli were assayed in soluble and membrane fractions of disrupted cells by using fatty acid and acyl-coenzyme A (CoA) substrates containing either 4 or 16 carbon atoms in the acyl moieties. Cell fractionation was monitored, using succinic dehydrogenase as a membrane marker and glucose 6-phosphate dehydrogenase as a soluble marker. Acyl-CoA synthetase activity was detected exclusively in the membrane fraction, whereas acyl-CoA dehydrogenase, 3-hydroxyacyl-CoA dehydrogenase, enoyl-CoA hydratase, and 3-ketoacyl-CoA thiolase activities that utilized both C4 and C16 acyl-CoA substrates were isolated from the soluble fraction. 3-Hydroxyacyl-CoA dehydrogenase, enoyl-CoA hydratase, and 3-ketoacyl-CoA thiolase activities assayed with both C4 and C16 acyl-CoA substrates co-chromatographed on gel filtration and ion-exchange columns and cosedimented in glycerol gradients. The data show that these three enzyme activities of the fad regulon can be isolated as a multienzyme complex. This complex dissociates in very dilute preparations; however, in those preparations where the three activities are separated, the fractionated species retain activity with both C4 and C16 acyl-CoA substrates.

3-Hydroxyacyl CoA Dehydrogenases↗
PubMed1977

Colored emission of rare earth ions in a potassium feldspar glass.

The viability of rare earth ions as fluorescent coloring additives for dental porcelain was investigated. The data presented allow the formulation of porcelain with specific color properties under illumination by ultraviolet light. Attention is drawn to the possibility of energy transfer occurring with the result that prediction of color may not be done by application of the additive color-mixing scheme.

Aluminum Silicates↗
PubMed1977

Surface properties of dental amalgam: roughness produced by setting reaction.

During the setting of dental amalgam, silver-mercury (gamma1) and tin mercury (gamma2) intermetallic compounds (phases) are formed. Since these compounds have distinct crystalline forms, they may be discerned on the surface of a setting sample. Visual identification of these phases on polished and etched samples is often difficult and, hence, surface morphology studies have an advantage in following the stages of the setting reaction. The purpose of this investigation was to use high-resolution light microscopy to follow the course of the setting reaction. Amalgam specimens were triturated and condensed against glass plates under pressure. The sample surfaces were then photographed under a high-resolution metallographic microscope using oblique illumination. The roughness of four samples for each alloy type was determined with a surface profile analyzer at times between ten minutes and 23 hours after trituration. An electron microprobe was used to identify the gamma1 and gamma2 phases. The gamma1 phase had a polyhedramorphology, and the gamma2 crystals formed intermeshed smooth areas. During the setting reaction, the average times for the roughness to reach 95% of the final value were 189, 227, 96, 65, and 116 minutes after trituration for the various amalgam types studied. All alloys tested required polishing several hours after trituration.

Crystallography↗
PubMed1977

Wetting of amalgam alloys by mercury.

The degree of wetting of amalgam alloy by mercury during trituration is an important variable in determining reaction rates and the degree of amalgamation. The purpose of this study was to measure the contact angle made by mercury on various phases present in amalgam alloy in air and on commercial alloys. During amalgamation, mercury is in contact with the oxides present on the amalgam alloy as well as the metallic amalgam phase. To measure the degree of wetting of the metallic phases, silver, tin, and copper were melted in such proportions as to give specimens of silver, tin, the alpha, beta, and gamma silver-tin phases, the eutectic in the silver-copper system. These were cast in the form of cylinders, sectioned, and the surfaces prepared by sandblasting. Oxide specimens of AgO, Ag2O, SnO, and SnO2 were prepared by compaction in a mold. Four commercial amalgam alloys also were tested. Mercury drops were then placed on the surfaces and the contact angles measured at different time intervals. The initial contact angles of mercury on the alpha, beta, gamma, and eutectic phases were found to be 90degrees, 112 degrees, 145 degrees, and 138 degrees, respectively. Mercury reacted with silver and tin, resulting in a change of contact angle with time. Contact angles measured on the commerical alloys were also high, on the order of 145 degrees. The gamma phase had contact angles closest to those of commercial alloys. Surface oxide phases are most likely responsible for this poor wettablility by mercury.

Copper↗
PubMed1977

The corneal penetration of trifluorothymidine, adenine arabinoside, and idoxuridine: a comparative study.

Trifluorothymidine (F3TdR) and idoxuridine (IDU) were observed to penetrate through the cornea from the epithelial side at a greater rate than adenine arabinoside (ARA-A) during in vitro corneal perfusions. Removal of the epithelium increased the rate of penetration of F3TdR and IDU by about twofold and the rate of ARA-A penetration by fivefold. The kinetics of antiviral penetration did not display saturation points at high antiviral concentrations, thus indicating that these three antiviral drugs penetrate the cornea by nonfacilitated diffusion. The sole breakdown product detected following F3TdR penetration in vitro, in situ, and in controls was 5 carboxy-2'-deoxyuridine (5-COOH-2'-dUd). The sole breakdown product isolated during ARA-A penetration experiments was hypoxanthine arabinoside (ARA-HX), and control experiments indicated that ARA-A was stable at pH 7.6. IDU was degraded to 2'-deoxyuridine (dUd) in control experiments, but during corneal penetration experiments IDU was degraded to a mixture of dUd and iodouracil (IU).

Animals↗
PubMed1972

Limiting factors in phytoplankton algae: their meaning and measurement.

There are two common responses of plants to changes in concentration of limiting factors: change in the final yield (type I response) or change in the growth rate (type II response). Type II is typical of phytoplankton algae in nature, yet some experiments have failed to show growth rate changes because of inappropriate experimental design.

Ecology↗