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At least 1,081 records · Page 60Linked to original sources
PubMed1994

Modeling of intraluminal heating of biological tissue: implications for treatment of benign prostatic hyperplasia.

A computer model for predicting the thermal response of a biological tissue to different intraluminal heating modalities is presented. A practical application of the model is to calculate the temperature distributions during thermal coagulation of prostate by contact heating and radiative heating. The model uses a two-dimensional axisymmetric diffusion approximation method to calculate the light distribution during radiative heating. The traditional Pennes' bio-heat equation is used to calculate the temperatures in the presence of blood flow. An implicit finite difference scheme with nonuniform grid spacings is used to solve the diffusion equation for light distribution and the bio-heat equation. Model results indicate that the radiative heating of prostate by Nd:YAG (1064 mm) and diode (810 mm) lasers can be a more effective and efficient means of coagulating a large volume of prostate, as compared to contact heating of the tissue. Blood perfusion is shown to provide a considerable heat sink as the laser exposure time is increased. Surface cooling by irrigation during the laser irradiation of tissue is shown to be an effective method for delaying tissue explosion and obtaining a large volume of coagulated tissue. The model also shows that the volume of the coagulated tissue is appreciably altered by a change in the rate of energy deposition.

Animals↗
PubMedDate not supplied

Validity of the lagged normal density function as a model for pulmonary indicator dispersion.

The aim of this study was to investigate whether the lagged normal density function is a useful model for the dispersion of intravascular and diffusible indicators in the lungs. In 18 mongrel dogs anesthetized with N2O-piritramide, 221 sets of thermal-indocyanine green dye kinetics were recorded in the pulmonary artery and in the aorta after central venous indicator injection. A model-free deconvolution technique was used to compute the pulmonary transport functions for dye and heart from the measured indicator kinetics (reference method). The lagged normal density function was used to model pulmonary indicator transport. Its parameters were computed by a nonlinear least-squares procedure by iterative convolution. After baseline measurements in nine dogs, pulmonary edema was induced by central venous application of oleic acid. In nine other dogs, measurements were performed before and after postural changes from the horizontal to the vertical position. The mean transit times derived from the lagged normal density function were in good agreement with those obtained after model-free deconvolution. Although the shape (relative dispersion, skewness) of the transport function is less well described by the model, the authors conclude that the lagged normal density function is useful to determine indicator volumes of distribution that require only the correct mean transit times.

Animals↗
PubMed1988

Principles underlying the prediction of temperature in plants, with special reference to desert succulents.

The thermal motion of molecules responsible for the property we know as temperature influences essentially every aspect of plant biology. Emphasis in this review is on principles of wide biological applicability, with specific examples being chosen from research on agaves and cacti. Plant temperatures can be predicted using energy budgets incorporating shortwave and longwave radiation, heat conduction and convection, latent heat, and heat storage. Energy budgets are used here to show the effect of plant size and shortwave absorptance on tissue temperature. The importance of air temperature or transcription rate for tissue temperature is calculated quantitatively. The influences of surface appendages, such as apical pubescence and spines, on minimal temperatures near the apical meristem of cacti are predicted, such minimal temperatures influencing the geographical distribution of various species. The Boltzmann energy distribution and Arrhenius plots are presented and used to analyse thermal responses in terms of energy barriers and activation energies. The many ways that temperature can influence CO2 diffusion into a leaf are also considered. Tolerances to low and high temperatures, such as the ability of agaves and cacti to tolerate extremely high tissue temperatures of 70 degrees C, are discussed from both cellular and ecological perspectives. Although the influences of temperature on plants are multitudinous, many can be predicted, or at least analysed, based on well-established physical principles.

Adaptation, Physiological↗
PubMed2005

Influence of surface morphology on D2 desorption kinetics from amorphous solid water.

The influence of surface morphology/porosity on the desorption kinetics of weakly bound species was investigated by depositing D2 on amorphous solid water (ASW) films grown by low temperature vapor deposition under various conditions and with differing thermal histories. A broad distribution of binding energies of the D2 monolayer on nonporous and porous ASW was measured experimentally and correlated by theoretical calculations to differences in the degree of coordination of the adsorbed H2 (D2) to H2O molecules in the ASW depending on the nature of the adsorption site, i.e., surface valleys vs surface peaks in a nanoscale rough film surface. For porous films, the effect of porosity on the desorption kinetics was observed to be a reduction in the desorption rate with film thickness and a change in peak shape. This can be partly explained by fast diffusion into the ASW pore structure via a simple one-dimensional diffusion model and by a change in binding energy statistics with increasing total effective surface area. Furthermore, the D2 desorption kinetics on thermally annealed ASW films were investigated. The main effect was seen to be a reduction in porosity and in the number of highly coordinated binding sites with anneal temperature due to ASW restructuring and pore collapse. These results contribute to the understanding of desorption from porous materials and to the development of correct models for desorption from and catalytic processes on dust grain surfaces in the interstellar medium.

Journal Article↗
PubMed2007

Modification of the crystal habit of celecoxib for improved processability.

Crystallization is often used in the pharmaceutical industry for purification and isolation of drugs, and also as a means of generating polymorphs or isomorphs. The aim of this study was to investigate the role of extrinsic crystallization parameters on the crystallized product, with special emphasis on improving the mechanical properties of acicular celecoxib. Celecoxib isomorphs were prepared using different techniques (solvent crystallization and vapour diffusion) and crystallization conditions (solvents, stirring, degree of supersaturation, crystallization temperature and seeding). Powder X-ray diffractometry, spectroscopic and thermal methods were used to investigate physical characteristics of crystals. Growth kinetics and aggregation dynamics of crystallization in polar and non-polar solvents were simulated using a dynamic light scattering method. The quick appearance of broad peaks over the range of 10-8000 nm in chloroform during crystallization simulation studies indicated faster aggregation in non-polar solvents. Aspect ratio, flow, compressibility and surface area of recrystallized products were also determined. Surface topography was determined by atomic force microscopy and the lath-shaped crystals (aspect ratio of 2-4) exhibited a roughness index of 1.79 in comparison with 2.92 for needles. Overall, the lath-shaped isomorphs exhibited improved flow and better compressibility.

Calorimetry, Differential Scanning↗
PubMed2005

Competing thermal activation mechanisms in the meltinglike transition of Na(N) (N = 135-147) clusters.

The meltinglike transition in unsupported icosahedral Na(N)() clusters, with N = 135-147, has been studied by isokinetic molecular dynamics simulations based on an orbital-free version of density functional theory. A maximum in the melting temperature, T(m), is obtained for Na141, while the latent heat, deltaE, and entropy of melting, deltaS, are maximal for Na147. These observations are in close agreement with calorimetric experiments on N clusters. The size evolution of deltaS is rationalized by the emergence of important premelting effects associated with the diffusive motion of atomic vacancies at the cluster surface. The precise location of the maximum in T(m) is explained in terms of two different thermally activated structural instability mechanisms which trigger the meltinglike transition in the size ranges N = 135-141 and N = 141-147, respectively.

Letter↗
PubMed2004

Modelling of cluster emission from metal surfaces under ion impact.

Using the molecular-dynamics technique, cluster emission for 5 keV Ar bombardment of a Cu (111) surface has been investigated using a many-body (tight binding) potential for the Cu-Cu interaction. The calculations allow us to analyse the basic processes underlying cluster emission. It is found that two distinct processes can be distinguished which lead to cluster emission under energetic ion bombardment. The first process causes the emission of small clusters, which are emitted by a collective motion during the development of the collision cascade within the first picosecond after impact. Thus, emission times of such clusters agree with the emission times of atoms in sputtering. Such a process can be envisioned if, for example, a few layers below the surface, an energetic recoil causes the development of a subcascade. Energy transferred by this event to the surface is strongly directional and can lead to the simultaneous emission of a group of neighbouring surface atoms, which in some cases will remain bounded and form a cluster after emission. Typically, clusters emitted by this mechanism consist of atoms, which are neighbouring in the target and are almost exclusively surface atoms, similar to all sputtered atoms. Emission of large clusters (cluster sizes of 10 or more atoms), as observed experimentally, is a puzzling phenomenon. From our calculations we conclude that the emission of such large clusters does not occur during the collisional phase of sputtering, but happens much later (5-10 ps after ion impact). Emission can occur for spike events, where all the energy of the impinging ion is deposited locally in a small volume near to the surface, and the sputtering yield is 3-5 times the average yield. Such events are rare, but we have found a few cases in our calculations where stable clusters consisting of more than 20 atoms were emitted. Melting of the spike volume occurs, and the high temperatures and pressures produced can cause emission of large fragments during the thermal phase. The composition of such large clusters is quite different from that of small clusters. They consist of atoms from different layers and the constituents are also generally not next-neighbour atoms. This change in origin of the cluster atoms reflects the mixing and diffusion processes occurring in the melted zone before emission. The calculations indicate that hydrodynamical phenomena might play a role in the emission of large fragments. Additional calculations, where the energy was distributed 'thermally' in a three-dimensional volume under the surface for 500 fs, give very similar results, even in such cases where the kinetic phase of the collision-cascade development was absent.

Journal Article↗
PubMed1997

Capillary electrophoresis of DNA for molecular diagnostics.

A number of applications of capillary zone electrophoresis (CZE) in sieving liquid polymers (notably linear polyacrylamides and cellulose) for the analysis of polymerase chain reaction (PCR) products of clinically relevant, diagnostic DNA, are reviewed. The fields covered are: human genetics, quantitative gene dosage, microbiology and virology, forensic medicine and therapeutic DNA (notably, antisense nucleotides). Some unique, novel developments are highlighted, such as: (i) nonisocratic CZE, i.e., temperature-programmed CZE for detection of DNA point mutations; (ii) the synthesis of novel N-substituted acrylamides, offering extreme resistance to alkaline hydrolysis coupled to high hydrophilicity. In the field of denaturing gradient gel electrophoresis (DGGE), as routinely performed in gel slabs, a novel methodology is described in CZE: double-gradient DGGE. In this technique, two gradients are simultaneously applied along the migration direction: a chemical (or thermal) denaturing gradient, for partially unwinding homo- and hetero-duplexes of DNA, and a porosity gradient, for recompacting diffuse bands melting over a broader range of denaturing conditions. It is thus demonstrated that chemical gradients, in addition to temperature gradients, can be easily implemented even in a capillary format.

Bacterial Infections↗
PubMed1990

Preparation and characterization of pancreatic lipase immobilized in Eudragit-matrix.

Pancreatic lipase (EC 3.1.1.3) was immobilized by entrapping in a commercial preparation of acrylic/methacrylic acid ester-based copolymer (Eudragit E 30 D). The activity of the immobilized lipase beads with a diameter of 1.5-2.0 mm was found to be lower than that of the free lipase. The optimum pH was shifted to the alkaline region and the thermal stability increased, whereas the optimum temperature level remained unchanged. The most important reason for the decreased activity was diffusion limitations. The diffusion of the substrate and products became more pronounced, and lipolytic activity increased upon addition of n-hexane into the reaction medium. The storage and operational stabilities of the immobilized lipase were investigated, and both characteristics were found to be increased when compared to the free enzyme. Furthermore, mechanical or magnetic stirring during the operation were found to have no influence on the carrier-matrix as determined by nephelometric measurements.

Acrylates↗
PubMed1986

Spinal cord metabolic response to noxious radiant heat stimulation of the cat hind footpad.

The 2-deoxy[14C]glucose (2-DG) method was used to study glucose utilization in the cat lumbar spinal cord during noxious thermal stimulation to the hind footpad. Spinal cord glucose utilization in stimulated cats was twice that of control animals. Diffusely increased metabolic activity was seen in the ipsilateral dorsal horn. The highest levels of 2-DG were seen in the ipsilateral ventral horn, while a moderate increase was seen in the contralateral ventral horn.

Animals↗
PubMed2000

Diffusion as a mixing mechanism in granular materials.

We present several numerical results on granular mixtures. In particular, we examine the efficiency of diffusion as a mixing mechanism in these systems. The collisions are inelastic and to compensate the energy loss, we thermalize the grains by adding a random force. Starting with a segregated system, we show that uniform agitation (heating) leads to a uniform mixture of grains of different sizes. We define a characteristic mixing time tau(mix), and study theoretically and numerically its dependence on other parameters like the density. We examine a model for bidisperse systems for which we can calculate some physical quantities. We also examine the effect of a temperature gradient and demonstrate the appearance of an expected segregation.

Journal Article↗
PubMed1992

Polychlorinated biphenyls: an overview of metabolic toxicologic and health consequences.

Polychlorinated biphenyls (PCBs) are complex mixtures of chlorinated aromatic hydrocarbons which are classified as lipophilic xenobiotics in vivo. Because of their unusual thermal and insulating properties, widespread use in industry was commonplace. They eventually gained entry into the biota, and their diffuse distribution represents both a potential and actual risk to humans and animals due to their bioconcentration in the food chain and toxicity secondary to accidental occupational or environmental exposure. Experimental poisoning in various animal models demonstrates that PCBs are potent inducers of hepatic microsomal enzyme systems, transmitted transplacentally and produce reproductive dysfunction. They alter immune function by decreasing the cell-mediated response and, in addition to producing myriad manifestations, are tumorigenic. Human exposure to PCBs has been associated with similar abnormalities, but because of limited data their chronic toxicity has not been well established. This is the key problem to evaluating possible untoward health effects in humans.

Animals↗
PubMed2006

Insect eggs at a transition between diffusion and reaction limitation: temperature, oxygen, and water. art.woods@mso.umt.edu.

In diverse animal taxa, eggs and embryos are incapable of transporting oxygen by convection. In such cases, internal oxygen distributions are determined jointly by rates of oxygen consumption and diffusive transport. Here we develop a mathematical model of oxygen consumption and transport in insect eggs, with the goal of understanding-for eggs in variable-temperature environments-the interactive effects of the two processes on development. We fit the model to previously published data on development time of eggs of a sphingid moth, Manduca sexta. The fitted coefficients suggest that eggs develop at a transition point between reaction- and diffusion-limitation. We test then this conclusion with independent data on development times of eggs distributed across a set of temperatures generated by a thermal gradient bar. Finally, we develop an extension of the model that considers tradeoffs between oxygen transfer to eggs versus water loss from them. The model results provide both a rationale for why development is often mass-transfer limited and a set of new predictions about oxygen-water tradeoffs.

Animals↗
PubMed1989

Decontamination of chemical protective clothing.

This study explored decontamination procedures for removing some organic solvents from protective clothing. The permeation experiments were performed on new and decontaminated specimens in seven polymer/chemical pairs. The decontamination methods investigated were thermal decontamination and air drying at room temperatures followed by detergent washing. Breakthrough time and steady-state permeation rate were determined by two different methods for new and decontaminated materials. The results showed that unless the contamination is limited to the outside surface of an elastomer (a material found in most barrier fabrics used in chemical protective clothing) or the chemical has a very large diffusion coefficient in the material, aeration and washing with detergent may not be an effective decontamination procedure for the type of solvent studied. On the other hand, thermal decontamination was shown to be effective in removing the contaminant from the matrix of the elastomers, and the decontaminated materials had permeation parameters similar to the new materials.

Decontamination↗
PubMed2002

Quantum decoherence, Zeno process, and time symmetry breaking.

The complex spectral representation of the Liouville-von Neumann operator outside Hilbert space is applied to the decoherence problem in quantum Brownian motion. In contrast to the path-integral method, often used in the context of quantum decoherence for the case where the environment surrounding the Brownian particle (subsystem) is in thermal equilibrium, our spectral representation is applicable to systems far from equilibrium, including a pure state for the surrounding bath. Starting with this pure initial condition, the subsystem evolves in time obeying a diffusion-type kinetic equation. Hence, the collapse of wave functions is a dynamical phenomenon occurring outside Hilbert space, and is not simply a contamination of the subsystem, a popular view accepted in the so-called "environmental" approach, by the mixed nature of the thermal bath. The essential element in the understanding of quantum decoherence is the "extensivity" of quantities characterizing the thermodynamic limit. Quantum Zeno time is shown to be a lower bound of the decoherence time.

Journal Article↗
PubMed1997

Pulsed photoacoustic spectroscopy applied to the diffusion of sunscreen chromophores in human skin: the weakly absorbent regime.

Pulsed photoacoustic spectroscopy was used to study the penetration of sunscreen chromophores into human skin. This study focuses on basic solutions containing single typical filter molecules, as used in current sunscreens, dissolved in mineral oil. The pulsed form of the photoacoustic technique was preferred because it provides more detailed information on the filter distribution within the different layers of human skin. A new methodology provides better insight into the diffusion process through signal analysis in the time and frequency domains, allowing for global and depth-related characterization. The penetration of the chromophore influences the response signal by inducing changes in the optical and thermal properties at different depths within the medium. The light scattering effect of titanium dioxide was demonstrated by the same technique.

Absorption↗
PubMed1998

Computer simulation of heat and mass transfer in tissue during high-intensity long-range laser irradiation.

Three-dimensional transient finite difference numerical model of the biological tissue irradiated by powerful laser beam is developed. It is used to simulate the thermal behavior of tissue assuming that radiation wavelength is chosen to give rise for volumetric heat sources. A three-dimensional seven-flow model is used to calculate radiation propagation. Evaporation and burn-out of tissue resulting in a through hole along the axis of the beam are taken into account. Besides the water boiling and corresponding changes of thermal and optical tissue properties the model takes into account one of the heat steam transfer mechanisms. Estimates are carried out for the effects of diffusion transfer and vaporization of water from the tissue surface. Kinetics of protein denaturation process are calculated by Arrenius equation. The problem is solved numerically using discrete grid technique and adaptive time-step control algorithm.

Body Temperature↗
PubMed2001

Analysis of luminescence spectra of leucite (KAISiO4).

High-sensitivity radioluminescence (RL) and thermoluminescence (TL) measurements were carried out on samples of natural leucite (with 2.95% of Na2O) from Campania Vessa (Italy). Samples were annealed to modify the charge compensation through alkali metal self-diffusion and to produce luminescence centres. High-temperature powder diffraction, scanning electron microscopy and chemical analyses were performed to monitor the tetragonal-cubic phase transitions and the thermal drainage of alkali metal ions from the aluminosilicate lattice. The emission spectral bands of leucite (300, 380, 430, 480, 560 and 680 nm) match those of other Na/K-aluminosilicates (alkali metal feldspars) and could be attributed to similar defects. TL glow curves of increasingly pre-irradiated natural leucite (range 0-20 Gy) showed no changes above 300 degrees C, whereas some changes were observed with annealed samples (1000 degrees C for 12 h). This temperature is the starting point of Na self-diffusion within the aluminosilicate lattices. The areas of the TL glow curves of both natural and annealed beta-irradiated leucites can be fitted with a linear function with high correlation. These results are in agreement with high alkali metal loss (K2O approximately 12% and Na2O approximately 18%) during thermal pre-treatment, high cell volume expansion (from 2350 to 2500 A) and the cubic la3d reversible tetragonal I4l/a phase transition. This promotes the egress of alkali metal ions and the production of [AlO4/M+]o, [AlO4/H+]o and [AlO4]o luminescence centres.

Journal Article↗