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At least 793 records · Page 44Linked to original sources
PubMed2004

Curing depths of a universal hybrid and a flowable resin composite cured with quartz tungsten halogen and light-emitting diode units.

This in vitro study evaluated curing depths of a universal hybrid resin composite with two viscosities (Tetric Ceram and Tetric Flow) after curing with 6 different quartz tungsten halogen and light-emitting diode curing units. Irradiance (light intensity) of the curing units varied between 200 and 700 mW/cm2. The curing units were used for standard, soft-start, or pulse curing. Curing times were 20 and 40 s for standard curing, 3 + 10 s and 3 + 30 s for pulse curing, and 40 s for soft-start. Resin composite specimens, 5 in each group, with a diameter of 4 mm and a height of 6 mm, were made in brass molds and cured from one side at a distance of 6 mm. After 2 weeks, the specimens were ground longitudinally half through the specimen. Curing depth was then determined by measurement of Wallace hardness for each half millimeter starting at 0.5 mm from the top surface. For all curing units and for both resin composites an increased curing time led to statistically significantly higher depth of cure (P < 0.0005). Tetric Flow showed a statistically significantly higher depth of cure than Tetric Ceram (P < 0.0005). All curing units cured more than 2.0 mm of both composites from a distance of 6 mm at 20 s curing time. The value for 40 s was 3.0 mm. The composite closer to the surface than the depth of cure value was equally well cured with all curing units investigated. There was a significant linear correlation between the determined irradiance of the curing units and the depths of cure obtained (20s: r = 0.89, P < 0.025; 40 s: r = 0.91, P < 0.01).

Aluminum Compounds↗
PubMed2005

Detection of apoptosis and drug resistance of human breast cancer cells to taxane treatments using quartz crystal microbalance biosensor technology.

Taxanes are used for the treatment of many human cancers, as first- and second-line chemotherapeutics. In the course of treatment many patients develop resistance or hypersensitivity to one form of taxane and require a different taxane to rescue the therapeutic benefit of the drug. There is currently no method to reliably predict tumor responses to taxanes prior to therapy or when resistance or hypersensitivity develops. We adapted the quartz crystal microbalance (QCM) biosensor technique to study responses of human mammary epithelial tumor cells to taxanes. Studies indicate that stable frequency and resistance levels are reached at 24 h. Cells in the QCM can then be treated with taxanes and responses monitored in real time via frequency and resistance changes reflecting alterations of cell mass distribution and viscoelastic properties. Distinct shifts in frequency and resistance accurately predicted apoptosis or resistance to treatment, as determined in parallel convention assays. QCM analysis accurately predicted docetaxel was more effective than paclitaxel and MCF-7 cells were more resistant to taxanes compared to MDA-MB-231 cells. These studies suggest "signature" patterns for taxane responsivity could be compared to those of patient biopsy samples to predict therapy outcome prior to treatment for initial therapy or to rescue therapy efficacy.

Antineoplastic Agents↗
PubMed2004

An in vivo study to compare a plasma arc light and a conventional quartz halogen curing light in orthodontic bonding.

The purpose of this study was to compare the effectiveness of a plasma arc lamp with a conventional tungsten quartz halogen lamp in orthodontic bonding. Twenty consecutive patients had their brackets bonded either with Transbond XT (n = 10) or Fuji Ortho LC (n = 10). In total, 352 teeth were bonded, 176 in each group. Using a randomized cross-mouth control study design, where diagonally opposite quadrants were assigned a particular treatment, the bonds were then either cured with the control light, namely a halogen lamp, or a plasma arc lamp. The halogen light was used for 20 seconds per tooth and the plasma arc lamp for 3 seconds per tooth. The measurement parameter used was bond failure and the patients were monitored for a period of 6 months following initial bond placement. In the Transbond XT group, the proportion of bond failures was 3.41 per cent for both the halogen and the plasma arc lamp. For the Fuji Ortho LC group, the proportions were 11.4 and 10.2 per cent, respectively. No difference was observed with respect to in-service bond failure proportions between bonds cured with the plasma arc or the conventional halogen lamp, irrespective of the bonding agent. Use of the plasma arc lamp could therefore lead to considerable savings in clinical time. However, this must be weighed against the increased purchase price of the plasma arc lamp.

Adhesives↗
PubMed2005

A comparison of tungsten-quartz-halogen, plasma arc and light-emitting diode light sources for the polymerization of an orthodontic adhesive.

This study investigated whether there were differences between the debond stress and adhesive remnant index (ARI) of an adhesive cured with three different orthodontic light sources. Sixty sound premolar teeth were divided into three groups of 20. A standard pre-adjusted edgewise premolar bracket (Victory Series) was bonded to each tooth using a light-cured orthodontic adhesive, Transbond X. Group 1 (control) specimens were cured with an Ortholux XT (tungsten-quartz-halogen bulb) light for 20 seconds, group 2 with an Ortho lite (plasma arc) for 6 seconds and group 3 with an Ortholux LED light-emitting diode for 10 seconds. The specimens were debonded 24 hours later using a universal mechanical testing machine, operating at a crosshead speed of 0.5 mm minute(-1). The Weibull modulus and a Logrank test showed no statistically significant differences between the three groups for debond stress. The ARI was assessed at x10 magnification. The ARI scores for group 2 were significantly different (P < 0.01) from those of groups 1 and 3 (between which there was no significant difference). For group 2 there was a greater tendency for failure to occur at the adhesive/tooth interface than for the other two groups. There appears to be no reason why any of the three types of light source cannot be used in orthodontics. Polymerization, as effective as that produced by conventional bulb light sources, was obtained with the short exposure times recommended for the plasma arc or light-emitting diode sources.

Bicuspid↗
PubMed1993

Induction of micronuclei in cultured human lymphocytes exposed to quartz halogen lamps and its prevention by glass covers.

The light emitted by an uncovered tungsten halogen lamp (12 V, 50 W), equipped with a dichroic mirror, induced time-related and distance-related increases in the frequency of micronucleated lymphocytes following in vitro exposure. Peripheral blood lymphocytes from three healthy donors showed the same degree of sensitivity. This clastogenic effect could be ascribed to the emission of far-UV wavelengths (UVB and UVC) and was prevented by interposing a common glass cover. These results support the findings of our genotoxicity studies in repair-deficient bacteria and carcinogenicity studies in hairless mice, thereby suggesting possible health hazards associated with illumination with halogen quartz bulbs, to which an enormous number of individuals are nowadays exposed. These hazards can be simply avoided by using glass covers.

Adult↗
PubMed1999

In situ monitoring of peptide-bound dsDNA selection on a GCN4-bZIP-immobilized quartz-crystal microbalance.

A highly sensitive 27 MHz quartz-crystal microbalance (QCM) was applied as a device of dsDNA in vitro selection. When GCN4-bZIP peptides were immobilized on the small Au electrode of the QCM, dsDNAs having TGACTCA sequences could be mainly selected from DNA library having a 15-bp random region, which was consistent with the sequence observed by X-ray crystallography.

Amino Acid Sequence↗
PubMed1980

The use of the argon laser-assisted quartz scalpel for burn wound excision to fascia.

We have developed a new method for coupling the hemostatic capabilities of argon laser light with the mechanical advantages of a sharpened quartz blade using fiberoptics. Using the new laser-assisted scalpel, a series of symmetrical skin excisions was done in 20- to 30-kg pigs on one side and with electrosurgery on the other. The laser scalpel was superior in its hemostatic properties (p less than 0.01) with no statistically significant difference in surgical speed. Take of split-thickness skin grafts was 90% in beds excised with either the laser or the electrosurgical device. Histologic sections showed less tissue damage with the laser-assisted scalpel than with electrocautery.

Animals↗
PubMed1999

Disorder effects in low concentration dispersions of small quartz spheres in cyanobiphenyl liquid crystals.

A photopyroelectric study of the specific heat and the thermal conductivity of small quartz sphere (aerosil) dispersions in heptylcyanobiphenyl (7CB) and octylcyanobiphenyl (8CB) liquid crystals with a concentration rho(s)=0.005 g cm(-3) is reported. The thermal conductivity data show that, even at this low concentration, the elastic strain introduced in the liquid crystal by the aerosil particle is not completely annealed at the nematic-isotropic (NI) phase transition. In 8CB, annealing has been found at the smectic-nematic phase transition, while in the case of 7CB it occurs in the nematic phase also. Moreover, the depression of the NI transition temperature has been found to be much smaller than the one predicted by a random field model in which quenched disorder simply affects the average order of the sample and no effects of the elastic strain are considered.

Journal Article↗
PubMed2001

Adsorbed layer structure of cationic surfactants on quartz.

Recent atomic force microscopy (AFM) surface images of surfactant adsorbed at solid and solution interfaces have shown apparent micellar aggregates familiar from bulk self-assembly. This contradicts the classical picture of laterally unstructured bilayers within which neutron reflectometry (NR) measurements have previously been analyzed. Applying both techniques to surfactant adsorption on quartz, we show that film thickness and coverage parameters derived from NR results are generally consistent with those from AFM and bulk self-assembly. NR by itself allows us to distinguish between actual bilayer and probable aggregate adsorption, which will be of particular importance when a solution's rheology makes AFM imaging impractical.

Journal Article↗
PubMed2001

Small quartz silica spheres induced disorder in octylcyanobiphenyl (8CB) liquid crystals: a thermal study.

A photopyroelectric technique has been applied to the study of specific heat and thermal conductivity of homeotropically aligned mixtures of small quartz spheres (aerosil) and octylcyanobiphenyl (8CB) with concentration 0</=rho(s)</=0.04 g/cm(3). Thermal conductivity data show that, even at these very low concentrations, an annealing of the disorder introduced by the aerosil takes place on cooling at the smectic-A-nematic (Sm-A-N) phase transition and not only at the nematic-isotropic (N-I) one. This means that there is some elastic strain in the nematic phase of the sample which is not quenched. Accordingly the suppression of the N-I transition temperature as a function of rho(s) does not fit a random field with a random dilution model that accounts for random quenched disorder only. High resolution specific heat measurements at the A-N and N-I transition show the effect of the aerosil is not the same. While in the first case its peak is suppressed with increasing concentration, in the second case there are some indications that outside the two-phase coexistence region it is enhanced. The effect of surface-induced alignment is also discussed to explain some discrepancies between our data and the ones reported in literature. It is concluded that the amount of disorder in the sample does not depend on rho(s) only, but also on other variables such as external fields. Finally, a relaxation phenomenon in the aerosil network that partially compensate the disordering effect of the particles is suggested to explain the concentration dependence of the transition temperatures.

Journal Article↗
PubMed2002

High-frequency measurements of interfacial friction using quartz crystal resonators integrated into a surface forces apparatus.

A quartz crystal resonator covered with a thin sheet of mica was integrated into a surface forces apparatus. The shifts in resonance frequency and bandwidth were monitored as the mica surface came into contact with a spherical lens approached from above. We compare experiments where the lens was either coated with a second mica sheet or just had a silver layer evaporated onto its surface. For the contact with the silver surface, strong maxima in bandwidth occurred during the formation and the disruption of the contact. No such maxima were seen when approaching and separating two mica surfaces. We attribute this increased dissipation to sliding and rolling friction, involving plastic deformation of the metal surface under oscillatory load.

Journal Article↗
PubMed2000

Excitons and optical properties of alpha-quartz

We present an ab initio study of the optical properties of alpha-quartz. The absorption spectrum is calculated by solving the Bethe-Salpeter equation for the interacting electron-hole system and found to be in excellent agreement with the measured spectrum up to 10 eV above the absorption threshold. We find that excitonic effects are crucial in understanding the sharp features in the absorption spectrum in this energy range. They are also crucial in the ab initio computation of the static dielectric constant, significantly enhancing its value.

Journal Article↗
PubMed2001

Crystalline post-quartz phase in silica at high pressure.

alpha-quartz, which has been reported to undergo pressure-induced amorphization, was found to transform to a monoclinic, crystalline phase when compressed to 45 GPa at room temperature in a close to hydrostatic, helium pressure medium. The x-ray powder diffraction data obtained could be indexed based on a monoclinic cell, and the intensities are in agreement with a P2(1)/c model structure built up of 3x2 zigzag chains of SiO6 octahedra. This new polymorph of silica, which is metastable under ambient conditions, has been isolated for the first time and is one of several possible competing dense forms containing octahedrally coordinated silicon.

Journal Article↗
PubMed2002

First-principles theory of the EPR g tensor in solids: defects in quartz.

A theory for the reliable prediction of the EPR g tensor for paramagnetic defects in solids is presented. It is based on density functional theory and on the gauge including projector augmented wave approach to the calculation of all-electron magnetic response. The method is validated by comparison with existing quantum chemical and experimental data for a selection of diatomic radicals. We then perform the first prediction of EPR g tensors in the solid state and find the results to be in excellent agreement with experiment for the E(1)(prime) and substitutional phosphorus defect centers in quartz.

Journal Article↗
PubMed2003

High speed microtribology with quartz crystal resonators.

Quartz resonators in contact with metal-covered spheres have been used to study the tribology of micron-sized metal-metal contacts at high speed (1 m/sec and high frequency (12 MHz), while maintaining a shear amplitude in the nanometer range. The data aquisition is based on ring-down experiments, where the electrical excitation is periodically interrupted and the free decay of the oscillation is analyzed. At contact, an amplitude-dependent frequency and decay rate are found, indicative of an underlying nonlinear equation of motion. Using perturbation theory and the two-timing approximation, a nonlinear spring constant kappa(1)(x) and a nonlinear drag coefficient xi(1)(x) are explicitly derived. We find a local slip-to-stick-transition at a shear amplitude of 0.5 nm. Coating the gold sufaces with a self-assembled thiol monolayer removes the stick.

Journal Article↗
PubMed2003

E' centers in alpha quartz in the absence of oxygen vacancies: a first-principles molecular-dynamics study.

The displacement of an oxygen atom in pure alpha quartz is studied via first-principles molecular dynamics. The simulations show that when an O atom in a Si-O-Si bridge is moved away from its original equilibrium position, a new stable energy minimum can be reached. Depending on the spin state and charge Q of the system, this minimum can give rise to either a threefold oxygen (singlet ground state and Q=+1) or to an unsaturated Si atom carrying a dangling bond (triplet state). In the latter case, the hyperfine parameters associated with the 29Si dangling bond are in rather good agreement with electron paramagnetic resonance/electron nuclear double resonance experiments.

Journal Article↗
PubMed2004

Bridging the gap between macro- and nanotribology: a quartz crystal microbalance study of tricresylphosphate uptake on metal and oxide surfaces.

We report a quartz crystal microbalance study of the nanodynamical properties of tricresylphosphate (TCP) reaction films formed on high purity Fe, Cr, Fe oxide, and Cr oxide surfaces at elevated temperatures. The data reveal trace levels of interfacial slippage, potentially in conjunction with viscoelastic effects, for reaction films characterized by very low macroscopic friction coefficients. In contrast, rigidly attached TCP reaction films are observed in systems characterized by high macroscopic friction coefficients.

Journal Article↗
PubMed2005

Origin of the opalescence at the alpha-beta transition of quartz: role of the incommensurate phase studied by synchrotron radiation.

The origin of the light scattering observed at the alpha-beta transition of quartz is still a subject of controversy. We present structural studies performed during the coexistence of the alpha and the intermediate incommensurate (inc) phases using simultaneously synchrotron x-ray diffraction and optical techniques. The small and large angle light scatterings are due, respectively, to the orientation domains of the 3q inc phase and to the alpha phase twins revealed by diffuse x-ray scattering. In the vicinity of the interphase boundary, the two light scattering regions, both with perturbed properties, form a complex multiscale structure.

Journal Article↗