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At least 253 records · Page 14Linked to original sources

PubMed2004

Thermal properties of dental materials--cavity liner and pulp capping agent.

We studied the thermal properties of cavity liners that included calcium phosphate as inorganic filler, in contrast to the conventional pulp capping agents. Therefore, thermal diffusivity, specific heat capacity, and thermal conductivity were measured. In addition, thermal conductivity results were compared with those of restorative materials and human dentin to examine thermal insulation effects. The thermal conductivity of cavity liners ranged from 0.23 to 0.28 W m(-1) K(-1), and that of pulp capping agents ranged from 0.44 to 0.48 W m(-1) K(-1). Test results indicated that the thermal conductivity of cavity liner was lower than those of human dentin, pulp capping agent, cast alloy, and composite resin for restoration, hence suggesting that cavity liner has a good thermal insulation effect.

Acrylic Resins↗

PubMed2001

Measurement of directional thermal properties of biomaterials.

This paper presents an experimental technique to measure the directional thermal conductivity and thermal diffusivity of materials. A heated thermistor heats the sample and a sensing thermistor placed about 2.5 mm away measures the temperature rise due the heating pulse at the heated thermistor. An empirical relation between the power delivered by the first thermistor and the temperature rise recorded by the sensing thermistor is used to measure the thermal conductivity of the material along the line joining the thermistors. Diffusivity of the material is determined from the delay between the power pulse in the heated thermistor and the temperature pulse at the sensing thermistor. Signal processing was done to eliminate errors in the measurement due to change of base line temperature. Uncertainty of the measurement technique was found to be 5% when tested in media of known thermal properties. The thermal conductivity and thermal diffusivity of swine left ventricle in normal and ablated conditions were measured using this technique. The thermal conductivity of the tissue dropped significantly from 0.61 to 0.50 W.m(-1).K(-1) after ablation while the diffusivity dropped from 2.1 x 10(-7) to 1.7 x 10(-7)m2.s(-1).

Analysis of Variance↗

PubMed1994

On the "door-corridor" model of gel electrophoresis. III. The gel constant and resistance, and the net charge, friction, diffusion and electrokinetic force of the migrating molecules.

The door-corridor model of gel electrophoresis enabled an estimation of the net charge of DNA molecules run in various gels. When the runs were carried out in Trisacetate-EDTA buffer having a concentration from 10 to 120 mM, the net charge in 1% agarose gel varied from 1.1 to 0.58 e per base pair. The friction between migrating molecules and gel fibers was dependent on the gel type and concentration, as well as the electric field strength and temperature during electrophoresis. In the 123 to 1,474 bp size range, the friction in 1% agarose changed from 1.91 to 378.03 x 10(-10) N.m-1.s. It was found that the friction per 123 bp DNA segment is not constant, but raises with size. The gel resistance force increases at higher electric field strengths, indicating that elastic forces govern the migration of macromolecules through gels. In the gels studied, the friction, and therefore thermal diffusion, of DNA and protein-SDS complexes scale with from 2.20 to 2.32 power of size. The ratio of thermally induced diffusion and velocity in various gels shows that there is a profound reduction of diffusion compared to velocity with increasing DNA size. This is directly linked to the high exponent relating friction and size. The high resolving power of gel electrophoresis can be correlated to the difference between the frictional coefficients of a diffusing and migrating macromolecule.

DNA↗

PubMed2000

Thermal and chemical diffusion in the rapid solidification of binary alloys

Solidification of binary alloys is characterized by the necessity to reject away from the advancing front two conserved quantities: the latent heat released at the solid-liquid interface and the solute atoms that cannot be accommodated in the solid phase. As thermal diffusion is much faster than chemical diffusion, the latter is generally assumed to be the rate limiting mechanism for the process, and the problem is addressed through the isothermal approximation. In the present paper we use the phase-field model to study the planar growth of a solid germ, nucleated in its undercooled melt. We focus on the effects of a noninstantaneous thermal relaxation. The steady growth predicted at large supersaturation in the isothermal limit is prevented. Depending on the value of the Lewis number the growth rate is limited by either mass or heat diffusion; in the latter case we observe a sharp transition between two different regimes, in which originates a nonmonotonic time dependence of the interface temperature. The effects of this transition reflect in the composition of the solidified alloy.

Journal Article↗

PubMed2006

Movement and diffusion of paramagnetic ions in a magnetic field.

The magnetic movement and thermal diffusion have been studied for Cu(2+) ions in solution. The Cu(2+) ion solution was spotted on the silica gel support and exposed to the magnetic field of 410 kOe(2) cm(-1) intensity x gradient. The Cu(2+) ions were attracted toward the filed center. The moving distance and diffusing distance were observed at various time intervals. It is shown that the Cu(2+) ions move in a large group composed of Cu(2+) ions and H(2)O molecules. The size of the group is approximately estimated to be of 4.6 mum diameter by the analysis of the drift velocity of the group and the Cu(2+) ion concentration in the group.

Journal Article↗

PubMed1996

Identification of photoacoustic transients during pulsed laser ablation of the human temporal bone: an experimental model.

Laser ablation of hard tissues during neurotologic operations has been accomplished with continuous-wave (CW) lasers in the visible and midinfrared spectrum. The mechanism of ablation at these wavelengths is secondary to photothermal-induced tissue destruction. As a result, significant thermal damage to surrounding tissue may occur. Pulsed ultraviolet (UV) lasers have been suggested as an alternative to the argon, KTP-532, and CO2 lasers currently used in clinical practice. The pulse length of Excimer lasers are considerably shorter than the thermal diffusion time of bone tissue, and as a consequence thermal injury is minimal. This makes pulsed lasers an attractive tool for tissue ablation in the ear: in essence a "cold knife." However, the short pulse width of Excimer lasers (typically 10-150 ns) can create large thermoelastic stresses in the ablation specimen. This study identifies the presence of these photoacoustic waves during the Excimer laser treatment of the cadaveric human temporal bone. A XeCl (lambda = 308 nm, tau p = 12 ns) excimer laser was used to ablate hard tissue surrounding the oval window and facial ridge with energies of 75, 45, 25, and 12 mJ/pulse. Spot size was estimated to be 0.5 mm2. Custom high-frequency polyvinyldifluoride (PVDF) piezoelectric film transducers were fabricated and attached to the promontory, round window niche, and facial ridges. The signals were amplified using a low-noise preamplifier and recorded on a digitizing oscilloscope. Photoacoustic waves were clearly identified. Notably, large acoustic waves were measured on the promontory and on both sides of the facial ridge. The implications and clinical relevance of these findings is discussed and compared to findings obtained from a model system.

Acoustics↗

PubMed2006

Periodic spatiotemporal patterns in a two-dimensional two-variable reaction-diffusion model.

Periodic spatiotemporal two-dimensional (2D) asymptotic patterns in an excitable two-variable thermochemical (reaction-diffusion) system are shown. In a one-dimensional system the traveling impulse which reflects from impermeable boundaries is a stable asymptotic solution if the diffusion coefficient of the reactant is greater than the thermal diffusivity of the system. Periodic patterns of two symmetries are presented in the 2D system: the impulse of excitation propagating along the diagonal of a square spatial domain and a structure consisting of curved impulses which propagate in the direction perpendicular to one side of a rectangular domain.

Journal Article↗

PubMed1991

Surface monitoring of cerebral cortical blood flow.

Techniques of monitoring surface blood flow in the brain allow observation of dynamic "real-time" changes in cortical blood flow (CoBF). These techniques have evolved from pial window observations that have not been quantitative and frequently are unreliable. Surface monitoring does not require the development of a clearance curve so changes in flow are seen immediately. On the other hand, the local vascular geometry may affect these techniques; therefore, large surface vessels must be avoided and the probe must be of large enough size so that some averaging effect of the cortical capillary bed will be obtained. At the present time, the two techniques available for surface monitoring are thermal diffusion flowmetry (TDF) and laser-Doppler flowmetry (LDF). Thermal methods have been available longer and more experience has been obtained in experimental, operative, and postoperative monitoring of CoBF with these techniques. LDF was used in retina, gastric mucosa, and skin, and has only recently been applied to the cerebral cortex. In the operating theater, both techniques have demonstrated increased CoBF in normal brain after arteriovenous malformation resection and have demonstrated reduced CoBF in normal brain around brain tumors. Acute changes in CoBF with vascular manipulation during aneurysm surgery have been demonstrated with TDF. Postoperative monitoring of aneurysm patients has demonstrated the development of cerebral vasospasm with TDF as well as increased flow preceding the development of malignant cerebral edema in trauma patients. Artifacts occur in TDF with irrigation, loss of surface contact, and contact with large surface vessels. LDF has artifactual changes with movement, light, if large surface vessels come in view of the probe, and changes in hematocrit. Surface monitoring shows a great deal of promise in continuous evaluation of CoBF intraoperatively and postoperatively.

Animals↗

PubMed2005

Diffuse scattering and phason fluctuations in the Zn-Mg-Sc icosahedral quasicrystal and its Zn-Sc periodic approximant.

We report on the absolute scale measurement of the x-ray diffuse scattering in the ZnMgSc icosahedral quasicrystal and its periodic approximant. Whereas the diffuse scattering in the approximant is purely accounted for by thermal diffuse scattering, an additional signal is observed in the quasicrystal. It is related to phason fluctuations as indicated by its Q(2)(per) dependence. Moreover, when compared to previous measurements carried out on the i-AlPdMn phase, we find that the amount of diffuse scattering is smaller in the i-ZnMgSc phase, in agreement with larger phason elastic constants in this phase. This is confirmed by the observation of a large number of weak Bragg peaks having a high Q(per) reciprocal space component.

Journal Article↗

PubMedDate not supplied

Thermodynamics of movable inductively heated seeds for the treatment of brain tumors.

A thermodynamic study is presented of temperature distributions created by an inductively heated 6-mm-diam Ni sphere imbedded in vivo and in vitro into porcine brain tissue. This study was performed in support of the development of a system that creates localized heat-induced lesions in deep-seated brain tumors. In this system, a magnetic "seed" will be remotely repositioned within the brain by an externally produced magnetic field. Convective effects of a hot moving seed will produce a different thermodynamic situation than that arising from an array of static implants. In this work, a study is presented of part of the expected change, in which a static sphere is heated to high temperature. Measurements were made of the temporal and spatial dependence of the temperature rise in the vicinity of the heated sphere, in vivo in four animals and in one that was euthanized immediately prior to experimentation. These results are used for parameter estimation with a theoretical model based on a point source solution to a form of the thermal diffusion equation, i.e., the "bioheat transfer equation." With this model thermal distributions from a power source of arbitrary geometry can be found using appropriate integration methods, and the method has widespread applicability. Estimates of blood flow rates, tissue thermal conductivity, and seed power absorption were found using the parameter estimation algorithm. The estimated blood perfusion exhibits a step increase following the first heating in multiple heating experiments. Thermal conductivity estimated using data from the nonperfused (in vitro) animal is 0.6 W/m degrees C. Seed power absorption is estimated correspondingly to be 0.9 W, a result confirmed independently with calorimetry. Statistical uncertainty is established for the radial decrease of the tissue temperature rise created by this method. This result allows estimation of a cell death boundary uncertainty of 0.6 mm, caused by fluctuations in power delivered to the seed, uncertainty in the temperature probe placements, and thermal properties such as blood perfusion and tissue thermal conductivity.

Animals↗

PubMed2004

Loss of chaotic trabecular structure in OPG-deficient juvenile Paget's disease patients indicates a chaogenic role for OPG in nonlinear pattern formation of trabecular bone.

The RANK-RANKL-OPG system of osteoclast regulation may play a key role in determining chaotic structure in trabecular bone. Iliac trabecular bone from juvenile Paget's disease patients deficient in functional OPG shows parallel, anisotropic structure instead of normal chaotic structure. Evidence from experimental systems suggests that RANK-RANKL-OPG controls key nonlinear "chaogenic" parameters, such as friction, forcing frequency, feedback, and boundary forcing. The RANK-RANKL-osteoprotegerin (OPG) system of osteoclast regulation may play a key role in determining chaotic structure in trabecular bone. Iliac trabecular bone from juvenile Paget's disease (JPD) patients deficient in functional OPG shows parallel, anisotropic structure instead of normal chaotic structure. Evidence from experimental systems suggests that RANK-RANKL-OPG controls key nonlinear "chaogenic" parameters, such as friction, forcing frequency, feedback, and boundary forcing. The Belousov-Zhabotinsky reaction-diffusion system, the catalytic oxidation of CO on platinum surfaces, and thermal diffusion in liquid helium allow visualization of nonlinear emergent patterns such as labyrinthine structures, turbulence, and cellular structures, all of which bear some resemblance to trabecular bone. In JPD, the gene for OPG (TNFRSF11B) is subject to an inactivating mutation, leading to increased resorption and accelerated remodeling. Histomorphometric images of iliac crest trabecular bone from teenagers suffering from JPD show a highly unusual array of parallel, regular trabecular plates, instead of the typical chaotic, fractal patterns of normal trabecular bone. Loss of OPG function is associated with a change from chaotic to regular structure, suggesting that the RANK-RANKL-OPG system is controlling key nonlinear "chaogenic" parameters. Looking at trabecular bone from the perspective of nonlinear pattern formation may help understand other phenomena, such as the marked dependence of trabecular bone's architectural and mechanical quality on remodeling rate independent of the trabecular bone mass.

Adolescent↗

PubMed1996

Simulation of transient thermal stress in gypsum-bonded investment.

The aim of this study was to clarify the effect of heating rate on the development of both transient and residual stresses in investment molds. Solid, cylindrical, gypsum-based molds were modeled and theoretical calculations made of temperature distributions with a constant heating rate. These calculations used experimental thermal diffusivity data obtained with a laser flash method. The simulations calculated transient thermal stresses during heating and at the end of heating for two surface conditions: an unrestricted surface, and a surface restricted by a casting ring. The simulation model developed nonuniform strains and stresses at casting temperature; tangential stress was compressive at the surface and tensile at the mid-point for the unrestricted surface model. The surface restricted model developed compressive tangential stresses throughout the mold at the casting temperature. This resulted in significant thermal strain differences compared to the magnitudes of expansion of the mold due to heating.

Calcium Sulfate↗

PubMed2002

Surface electromigration patterns in a confined adsorbed metal film: Ga on GaN.

The mass transport of gallium adatoms in a confined gallium bilayer on GaN(0001) is studied with photoelectron spectromicroscopy with the goal to identify the diffusing species and their lateral distribution during directional surface electromigration and/or "random" thermal diffusion. It has been found that only the gallium atoms from the second layer undergo biased diffusion involving formation of three-dimensional islands. The development of different gallium concentration patterns is described by means of a general model, considering the presence of vacancies and trapping centres for the diffusing atoms.

Journal Article↗

PubMed1980

Dynamic quantitative assessment of cortical blood flow.

Dynamic real time quantitative assessment of cortical blood flow is feasible with a thermal diffusion flow probe. The probe consists of a Peltier stack that creates a thermal gradient. The thermal gradient has been calibrated against Xe133 clearance for fast flow. Rapid changes in flow induced with pharmacologic agents, changes in perfusion pressure, or vascular manipulation are readily apparent. This technique is useful for intraoperative assessment of cortical blood flow and may prove useful in postoperative monitoring.

Animals↗

PubMed2003

The forensic analysis of thermal transfer printing.

Thermal transfer printing refers to printing processes that utilize heat to produce an image by either physical or chemical means or by a combination of both. As the technology has improved and the supplies have become less expensive, the use of thermal printing in the personal and business markets has increased significantly. Specifically, dye diffusion thermal transfer and thermal mass transfer have become predominant in the production of counterfeit credit cards, drivers' licenses, and other types of documents produced on plastic media. Chemical analysis by means of thin layer chromatography (TLC) has proven to be useful in characterizing various types of inks (e.g., writing and inkjet inks). In this study, the authors examined 81 different samples that included a total of 54 printer samples (43 photographic prints on paper and eleven plastic card samples) and 27 printer ribbons. A new TLC method was developed and tested utilizing a solvent system (80% n-hexane, 3% methyl ethyl ketone, and 17% ethyl acetate) that is capable of producing excellent resolution.

Journal Article↗

PubMed2002

Thermal and chemical modification of dentin by 9-11-microm CO2 laser pulses of 5-100-micros duration.

BACKGROUND AND OBJECTIVES: Previous studies have shown that dentin can be thermally modified by pulsed CO(2) laser irradiation to form a more highly mineralized tissue. The implications are important for the potential laser modification or removal of dentinal and root caries and the transformation of dentin to a more acid resistant mineralized tissue. STUDY DESIGN/MATERIALS AND METHODS: Time resolved radiometry measurements with TEA CO(2) laser pulses were used to determine the magnitude of the absorption coefficients of dentin at the highly absorbed CO(2) laser wavelengths and to measure the temperature excursions during lambda = 9.3, 9.6, 10.3, and 10.6 microm laser irradiation at irradiation intensities of 0.1-8 J/cm(2) per pulse. In addition, photoacoustic and transient reflectance measurements were used to monitor the loss of water and organics and to detect the thresholds for surface modification and tissue ablation. RESULTS: The absorption coefficients were measured to be 5,000; 6,500; 1,200; and 800 cm(-1) at lambda = 9.3, 9.6, 10.3, and 10.6 microm, respectively. The surface temperatures of dentin were markedly higher than those measured on enamel for similar irradiation intensities due to the lower reflectance losses of dentin and the lower thermal diffusivity of dentin at the respective wavelengths. Hence, lower fluences are required for the thermal decomposition of dentin. Ablation typically occurred with the first few laser-pulses during multiple pulse irradiation and eventually ceased after modification of dentin to a more highly mineralized enamel-like tissue. The debris ejected during the initial laser pulses shielded the surface by as much as 60% at the low fluences employed in this study. Optical and electron microscopy and IR spectroscopy indicated that incident laser pulses with incident fluence as low as 0.5 J/cm(2) at 9.3 and 9.6 microm wavelengths with a duration of 5-8-micros were sufficient to induce chemical and morphological changes in dentin. CONCLUSIONS: In this study, the laser parameters for the efficient thermal modification of dentin with minimum heat deposition at CO(2) laser wavelengths were firmly established.

Dental Caries↗

PubMed1998

Tissue thermal damage caused by bipolar forceps can be reduced with a combination of plastic and metal.

BACKGROUND: We created a new 5-mm bipolar forceps. Thermo-insulating plastic was used for the creation of the jaws, while the metal conductor is buried deep inside the plastic. METHODS: A porcine model was used in experimental testing against a standard metallic 5-mm bipolar forceps at three different wattage settings (20, 40, 60 W). Histological sections were evaluated by a pathologist who was blind to the wattage and instrument tested. RESULTS: The new instrument was shown to cause statistically significant reduced thermal damage. In addition, its lateral thermal diffusion is superior to standard bipolar forceps. CONCLUSIONS: Thermo-insulating plastics can reduce thermal damage to tissue during electrosurgery.

Animals↗

PubMed1994

Chromophore-assisted laser inactivation of proteins is mediated by the photogeneration of free radicals.

Chromophore-assisted laser inactivation (CALI) is a technique that selectively inactivates proteins of interest to elucidate their in vivo functions. This method has application to a wide array of biological questions and an understanding of its mechanism is required for its judicious application. We report here that CALI is not mediated by photoinduced thermal denaturation but by photogenerated free radicals. Thermal diffusion calculations suggest that the temperature changes resulting from CALI are too small to cause thermal denaturation, and Arrhenius plots of CALI are inconsistent with a photothermal mechanism. CALI shows an energy dose reciprocity above a threshold and can be inhibited by free-radical quenchers, thus demonstrating a photochemical mechanism of protein inactivation. The type of quenchers that are effective in inhibiting CALI indicates that the active species is a hydrogen abstractor which is not derived from molecular oxygen. We suggest that the active free-radical species is the hydroxyl radical and its very short lifetime explains the spatial specificity of CALI such that half-maximal damage is effected within 15 A from the dye moiety and no significant damage occurs at 34 A. The data are consistent with free-radical formation resulting from a sequential two-photon process.

Coloring Agents↗