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At least 415 records · Page 23Linked to original sources
PubMed2006

Perturbation theory for the diffusion equation by use of the moments of the generalized temporal point-spread function. II. Continuous-wave results.

We investigate the performance of the method proposed in Part I of this paper in several situations of interest in diffuse optical imaging of biological tissues. Monte Carlo simulations were extensively used to validate the approximate scaling relationship between higher-order and first-order self moments of the generalized temporal point-spread function in semi-infinite and slab geometry. More specifically we found that in a wide range of cases the scaling parameters c1, c2, c3 [see Eq. (36) of Part I] lie in the intervals (1.48, 1.58), (3.1, 3.7), and (8.5, 11.5), respectively. The scaling relationships between higher-order and first-order self moments are useful for the calculation of the perturbation of a single defect in a straightforward way. Although these relationships are more accurate for inclusions of linear size less than approximately 6mm, their performance is also studied for larger inclusions. A good agreement, to within approximately 10%, was found between the perturbations of single and multiple defects calculated with the proposed method and those obtained by Monte Carlo simulations. We also provide formulas for the calculation of the moments up to the fourth order for which it is clear how lower-order moments can be used for the calculation of higher-order moments.

Algorithms↗
PubMed1977

Solid state theory of competitive diffusion of associated Na+ and K+ in cells by free cation and vacancy (hole) mechanisms, with application to nerve.

If, as recent evidence indicates, most cell potassium is associated with macromolecular fixed charge, then diffusion of potassium ions in cells might occur by (1) diffusion of the small fraction of free potassium in cell water (analogous to electrons in the conduction band of a semiconductor) or by (2) diffusion of vacancies on association sites (analogous to holes in a semiconductor). Derivations of the Fick first law of diffusion predict that partial substitution of sodium for potassium in the cell produces opposite effects on the effective diffusion constant of potassium for those mechanisms. Application of that substitution to nerve data suggests that rubidium ions diffuse by a free cation result when the nerve is clamped at its resting potential, but by a vacancy mechanism when the nerve is clamped at zero voltage.

Cations, Monovalent↗
PubMed1999

Diffusion in cell-free and cell immobilising kappa-carrageenan gel beads with and without chemical reaction

Diffusion into and from kappa-carrageenan gel beads was studied, both in the absence and presence of bacterial cells, both with and without biochemical reaction. The solutes were indole, L-serine, and L-tryptophan. The reaction was that of indole and L-serine to give L-tryptophan. Established theory concerning diffusion of a single solute in cell-free gels was found to describe well the effect of the gel on diffusivity. Simultaneous diffusion of the three solutes resulted in lower diffusivities than those for individual solutes, suggesting the need to use multicomponent diffusion theory. The effect of cells on diffusion could only be accounted for by models assuming permeable cells. Diffusion with chemical reaction was reasonably well described by an effectiveness factor calculated using an effective diffusivity estimated from diffusion data without reaction. Copyright 1999 John Wiley & Sons, Inc.

Journal Article↗
PubMed1981

Concerning the oxygenation of tissues and cells.

The hypothesis that our tissues are oxygenated by 'pure' diffusion is rejected because diffusion is never 'pure' in living tissues-it is everywhere contaminated by the 'impurity' of convective or agitative 'flow'. The suggested alternative hypothesis is that tissue cells breathe by 'convection + diffusion' like all other aquatic organisms and that all aquatic respiration is identical. This change alters the interrelationship between Cell Theory, Membrane Theory, and Diffusion Theory: it also invalidates the premises of Krogh's tissue oxygenation model (Krogh's Cylinder), and his conclusions. The presumption that cell respiration is active (rather than passive, as is implicit in classical theory) has wideranging consequences: the following are only a few: -cells must work to procure oxygen, and - the ratio of work done to oxygen procured must depend on the oxygen content of the ECF. -since pO2 is only one of many factors determining oxygen content, it cannot be more than one of many factors determining the adequacy or efficiency of cell oxygenation: etc.

Cells↗
PubMed2001

Kinetic theory of point vortices: diffusion coefficient and systematic drift.

We develop a kinetic theory for point vortices in two-dimensional hydrodynamics. Using standard projection operator techniques, we derive a Fokker-Planck equation describing the relaxation of a "test" vortex in a bath of "field" vortices at statistical equilibrium. The relaxation is due to the combined effect of a diffusion and a drift. The drift is shown to be responsible for the organization of point vortices at negative temperatures. A description that goes beyond the thermal bath approximation is attempted. A new kinetic equation is obtained which respects all conservation laws of the point vortex system and satisfies a H theorem. Close to equilibrium, this equation reduces to the ordinary Fokker-Planck equation.

Journal Article↗
PubMed2002

Susceptibility-induced T(2)-shortening and unrestricted diffusion.

In this work, discrepancies between recent theories for susceptibility-induced T(2)-shortening and classical results, namely outer sphere theory and diffusion through local fields with random gradients, are considered. These discrepancies are assigned to the use of unrestricted diffusion in the new theories.

Diffusion↗
PubMed2003

A theory of inhaled anesthetic action by disruption of ligand diffusion chreodes.

A theory is proposed for the clinical actions and behavior of volatile anesthetic agents. Evidence supports the nonspecific and ubiquitous effects of these drugs in which many ligand-receptor systems may be involved. The volatile anesthetic drugs interfere with the diffusion of ligands to receptors across the hydrodynamic landscape on the protein surface. The hydropathic states of the amino acid side chains on the landscape of an active site influence the water to form evanescent paths, called chreodes as defined by Waddington and offered as an appropriate term by Kier. These chreodes are altered by the presence of volatile anesthetic drugs, leading to a reduction in the diffusion of many ligands to their active sites. This effect is sufficient to decrease the responses of these receptors, leading to an anesthetic state.

Anesthetics, Inhalation↗
PubMed2001

Field theory for reaction-diffusion processes with hard-core particles.

We show how to build up a systematic bosonic field theory for a general reaction-diffusion process involving hard-core particles in arbitrary dimension. We discuss a recent approach proposed by Park, Kim, and Park [Phys. Rev. E 62, 7642 (2000)]. As a test bench for our method, we show how to recover the equivalence between asymmetric diffusion of excluding particles and the noisy Burgers equation.

Journal Article↗
PubMed1982

The sustained release of antimicrobial drugs from bone cement. An appraisal of laboratory investigations and their significance.

The release of gentamicin sulphate, sodium fusidate and diethanolamine fusidate from Palacos and CMW cements was studied using elution and serial plate transfer tests. Further tests were made to assay the drug remaining in the cement after antibacterial activity could no longer be detected by the above methods, to detect the sustained slow release of the residual drug, and to ascertain the mechanism of release. The results confirmed that the release of gentamicin sulphate could be detected for longer from Palacos cement than from CMW cement, but the opposite was true for sodium fusidate. Little difference was found in the case of diethanolamine fusidate. Comparison of elution and serial plate transfer tests, and of results of elution in buffers of different pH, demonstrated that the test method employed had a significant effect on the results, and the omission of details of methodology from some publications made comparison and evaluation of results difficult. Varying quantities of residual drug were found in cement from which antibacterial activity could no longer be demonstrated; further tests for sustained, slow release showed that the antibiotic did not remain fixed in the cement but was released at a rate too slow to be detected in the elution and serial plate transfer tests. It is concluded that antibiotics are released from the cement by a process of diffusion, but tests to determine the mechanism of diffusion were unhelpful. The theory of diffusion of drugs through solid matrices, and the clinical implications of the experimental findings, are discussed.

Acrylic Resins↗
PubMed2003

Reaction-diffusion dynamics: confrontation between theory and experiment in a microfluidic reactor.

We confront, quantitatively, the theoretical description of the reaction-diffusion process of a second-order reaction to experiment. The reaction at work is Ca(2+)/CaGreen, a fluorescent tracer for calcium. The reactor is a T-shaped microchannel, 10 microm deep, 200 microm wide, and 2 cm long. The experimental measurements are compared with the two-dimensional numerical simulation of the reaction-diffusion equations. We find good agreement between theory and experiment. From this study, one may propose a method of measurement of various quantities, such as the kinetic rate of the reaction, in conditions yet inaccessible to conventional methods.

Biophysics↗
PubMed2006

Excluded volume in the generic van der Waals equation of state and the self-diffusion coefficient of the Lennard-Jones fluid.

In the previous papers applying the generic van der Waals equation of state the mean excluded volume was defined with the contact diameter of particles at which the potential energy is equal to zero-the size parameter in the case of the Lennard-Jones potential. This parameter appears as the upper limit of the integral for the generic van der Waals parameter B (mean excluded volume divided by the density) in the generic van der Waals equation of state. Since the choice is not unique, in this paper we reexamine the manner of defining the upper limit and propose another choice for the upper limit. We also propose an interpretation of the free volume overlap factor alpha appearing in the free volume theory of diffusion and a method of estimating it in terms of the intermolecular potential energy only. It is shown that with the so-estimated free volume overlap factor and the new choice of the upper limit of the integral for B the self-diffusion coefficient in the modified free volume theory of diffusion not only acquires a better accuracy than before, but also becomes calculable in terms of only the intermolecular interaction potential without an adjustable parameter. We also assess some of effective diameters of molecules proposed in the literature for their ability to predict the self-diffusion coefficient within the framework of the modified free volume theory of diffusion.

Journal Article↗
PubMed2000

Diffusion in inhomogeneous media: theory and simulations applied to whole cell photobleach recovery.

A continuum description for diffusion in a simple model for an inhomogeneous but isotropic media is derived and implemented numerically. The locally averaged density of diffusible marker is input from experiment to define the sample. Then a single additional parameter, the effective diffusion constant, permits the quantitative simulation of diffusive relaxation from any initial condition. Using this simulation, it is possible to model the recovery of a fluorescently tagged protein in the endoplasmic reticulum (ER) after photobleaching a substantial region of a live cell, and fit an effective diffusion constant which is a property both of the geometry of the ER and the marker. Such quantitative measurements permit inferences about the topology and internal organization of this organelle.

Algorithms↗
PubMed2004

Experimental verification of the Smoluchowski theory for a bimolecular diffusion-controlled reaction in liquid phase.

We report experimental verification of the Smoluchowski theory for diffusion-controlled reactions in solution at the steady-state limit. We have determined both the diffusion coefficients and the self-termination reaction rates of the diphenylmethyl radical simultaneously. Smoluchowski theory is insufficient to discuss the reaction rate for the self-termination reaction of the diphenylmethyl radical, so the reaction rate of an encounter complex based on the Collins-Kimball treatment is estimated.

Binding Sites↗
PubMed2002

Effect of noise and occupancy on optimal reverberation times for speech intelligibility in classrooms.

The question of what is the optimal reverberation time for speech intelligibility in an occupied classroom has been studied recently in two different ways, with contradictory results. Experiments have been performed under various conditions of speech-signal to background-noise level difference and reverberation time, finding an optimal reverberation time of zero. Theoretical predictions of appropriate speech-intelligibility metrics, based on diffuse-field theory, found nonzero optimal reverberation times. These two contradictory results are explained by the different ways in which the two methods account for background noise, both of which are unrealistic. To obtain more realistic and accurate predictions, noise sources inside the classroom are considered. A more realistic treatment of noise is incorporated into diffuse-field theory by considering both speech and noise sources and the effects of reverberation on their steady-state levels. The model shows that the optimal reverberation time is zero when the speech source is closer to the listener than the noise source, and nonzero when the noise source is closer than the speech source. Diffuse-field theory is used to determine optimal reverberation times in unoccupied classrooms given optimal values for the occupied classroom. Resulting times can be as high as several seconds in large classrooms; in some cases, optimal values are unachievable, because the occupants contribute too much absorption.

Acoustics↗