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PubMed1977

Release of drugs from ointment bases II: In vitro release of benzocaine from suspension-type aqueous gels.

The in vitro release of benzocaine, suspended in an aqueous gel, through silicone rubber membranes was studied to test an extension of existing mathematical models. The theoretical treatment proposed is intended for experimental systems involving release, through a non-porous membrane, of a drug whose concentration is a few times (larger than or equal to 3) greater than its solubility in the vehicle. For either micronized (2micrometer) or macrosize (125 micrometer) drug, the Q (amount released) versus t1/2 (time1/2) plots were not linear until substantial time had elapsed. Excellent agreement was found between the experimental points and theoretical plots, generated by a computer fit to experimental data of an equation derived from a reported vehicle-boundary diffusion layer model. The values of the solubility and of the diffusion layer model. The values of the solubility and of the diffusion coefficient of benzocaine in the gel, calculated by the present mathematical treatment from release data, were in agreement with literature data. The particle size of released benzocaine did not influence the release pattern, thus confirming release in the present conditions to be diffusion rather than dissolution controlled. The present method is applicable for determining the solubility and diffusion coefficient of drugs in vehicles in cases not contemplated in current release theories.

Benzocaine↗
PubMed1987

Morphological and chiral symmetry breaking in reaction-diffusion systems.

Morphological and chiral symmetry breaking in reaction-diffusion systems is considered on the basis of the theory of imperfect codimension-two bifurcations. A new type of pattern selection with two triggers is elucidated: (1) morphologically asymmetric structures displaying optical activity can probably be originated from initially racemic and homogeneous conditions when chiral interaction, having the characteristic strength delta (such as electroweak interaction and circularly polarized light) as well as external field, having the characteristic strength eta (such as gravitational field and electrostatic field) are considered; (2) the selective sensitivity of molecular chirality and morphological asymmetry is omicron(delta 1/3) and omicron(eta 1/3), respectively; the sensitivity of mode-mode interaction between chiral polarization and concentration vector is omicron(delta 2/3) or omicron(eta 2/3), respectively. The relation of these conclusions to the life problem is discussed briefly.

Amino Acids↗
PubMed2005

Initial stages of Ti growth on diamond (100) surfaces: from single adatom diffusion to quantum wire formation.

Using first-principles total energy calculations within density functional theory, we investigate the energetics, kinetics, and transport properties of Ti on clean and hydrogen-terminated diamond (100)-2x1 surfaces at increasing Ti coverages. On a clean surface, an isolated Ti adatom prefers to adsorb on top of a C-C dimer row, and also diffuses faster along the dimer row direction. As the Ti coverage increases, the preferred adsorption site converts from an atop site to a site located between the dimer rows. Passivation of the surface at the monohydride coverage not only greatly enhances the Ti mobility, but also weakens the diffusion isotropy. Based on these energetic and kinetic characteristics, we propose a viable approach to fabricate ideal Ti quantum wires on hydrogen-terminated diamond substrates.

Journal Article↗
PubMed2004

Physicochemical parameters associated with nanoparticle formation in the salting-out, emulsification-diffusion, and nanoprecipitation methods.

PURPOSE: The aim of this work was to relate the physicochemical properties of the aqueous and organic phases used for nanoparticle (NP) preparation to the formation of NP produced by salting-out, emulsification-diffusion, and nanoprecipitation. METHODS: Methacrylic acid copolymer and poly(vinyl alcohol) (PVAL) were selected as NP polymer and emulsifying agent, respectively. Salting-out and emulsification-diffusion NP batches were prepared modifying the PVAL content in the aqueous phase. For nanoprecipitation, NP were produced with variation of the polymer content and type of solvent in the organic phase. RESULTS: For salting-out and emulsification-diffusion, NP formation was discussed in terms of the emulsification theory. The nanoemulsion obtained during NP preparation was visualized by scanning electron microscopy. Aqueous and organic phases used for NP preparation were characterized by their viscosity and surface tension. NP characteristics such as particle mean size, residual surfactant, suspendability in water after freeze-drying, and morphology were explained in terms of these properties. For nanoprecipitation, NP formation was analyzed considering the diffusion-stranding phenomenon. CONCLUSIONS: NP formation by salting-out and emulsification-diffusion was related to PVAL chain interactions at the droplet interface (e.g., reduction in the interfacial tension, mechanical stabilization, and steric stabilization) and in the bulk solution (hydrodynamic stabilization). For nanoprecipitation, chi(solvent-water) and delta(delta solvent-water) of the organic phase solvents were well related to the NP characteristics.

Diffusion↗
PubMed2004

Intrinsic scaling complexity in animal dispersion and abundance.

Ecological theory related to animal distribution and abundance is at present incomplete and to some extent naive. We suggest that this may partly be due to a long tradition in the field of model development for choosing mathematical and statistical tools for convenience rather than applicability. Real population dynamics are influenced by nonlinear interactions, nonequilibrium conditions, and scaling complexity from system openness. Thus, a coherent theory for individual-, population-, and community-level processes should rest on mathematical and statistical methods that explicitly confront these issues in a manner that satisfies principles from statistical mechanics for complex systems. Instead, ecological theory is traditionally based on premises from simpler statistical mechanical theory for memory-free, scale-specific, random-walk, and diffusion processes, while animals from many taxa generally express strategic homing, site fidelity, and conspecific attraction in direct violation of primary model assumptions. Thus, the main challenge is to generalize the theory for memory-free physical, many-body systems to include a more realistic memory-influenced framework that better satisfies ecological realism. We describe, simulate, and discuss three testable aspects of a model for multiscaled habitat use at the individual level: (1) scale-free distribution of movement steps under influence of self-reinforcing site fidelity, (2) fractal spatial dispersion of intra-home range relocations, and (3) nonasymptotic expansion of observed intra-home range patch use with increasing set of relocations. Examples of literature data apparently supporting the conjecture that multiscaled, strategic space use is widespread among many animal taxa are also described. We suggest that the present approach, which provides a protocol to test for influence from scale-free, memory-dependent habitat use at the individual level, may also point toward a guideline for development of a generalized theoretical framework for complex population kinetics and spatiotemporal population dynamics.

Animal Migration↗
PubMed2001

Accuracy of Kubelka-Munk reflectance theory applied to human dentin and enamel.

The Kubelka-Munk (K-M) theory provides a reflectance model for translucent materials placed on backings of different colors. We hypothesize that Kubelka-Munk (K-M) theoretical diffuse reflectance spectra of dentin and enamel are in good agreement with observed diffuse reflectance. The aim of this study was to measure the reflectance of enamel and dentin specimens and to compare the measured values of reflectance with K-M theoretical values. Disc-shaped specimens of enamel, dentin, and enamel/dentin were prepared from extracted teeth. Diffuse reflectance spectra were measured on three backings by means of a reflectance spectrophotometer over every wavelength (lambda) from 400 to 700 nm at three thicknesses. The measured reflectance values were fit by non-linear regression to corrected K-M theory. The low value of the reported errors associated with the application of K-M theory illustrated that the theoretical diffuse reflectance spectra of dentin and enamel are in good agreement with the observed diffuse reflectance.

Absorption↗
PubMed2005

Diffusion and home range parameters from rodent population measurements in Panama.

Simple random walk considerations are used to interpret rodent population data collected in Hantavirus-related investigations in Panama regarding the short-tailed cane mouse, Zygodontomys brevicauda. The diffusion constant of mice is evaluated to be of the order of (and larger than) 200 meters squared per day. The investigation also shows that the rodent mean square displacement saturates in time, indicating the existence of a spatial scale which could, in principle, be the home range of the rodents. This home range is concluded to be of the order of 70 meters. Theoretical analysis is provided for interpreting animal movement data in terms of an interplay of the home ranges, the diffusion constant, and the size of the grid used to monitor the movement. The study gives impetus to a substantial modification of existing theory of the spread of the Hantavirus epidemic which has been based on simple diffusive motion of the rodents, and additionally emphasizes the importance for developing more accurate techniques for the measurement of rodent movement.

Animals↗
PubMed2004

NMR measurement of self-diffusion coefficients by slice selection.

Most of the time, so-called inversion-recovery experiments concern longitudinal nuclear magnetization of the whole sample, the region of interest being limited by the transmitting-receiving coil. Here we address the question of what occurs if the region of interest is purposely limited to a thin slice selected by means of procedures employing magnetic field gradients. Gradients of both magnetic fields (B(0), the static magnetic field, and B(1), the radio-frequency magnetic field) can be used. In this study we resorted essentially to B(1) gradients and novel procedures, based on the natural inhomogeneity of the B(1) field delivered by a saddle coil, are described. It is obvious that molecules leaving and entering the slice during the evolution (recovery) period should influence the magnetization recovery. Molecular self-diffusion is responsible for such effects, experimentally visible and accounted for by an appropriate theory which has been approximated for by permitting an easy physical assessment. This approach should lead to alternative methods for measuring self-diffusion coefficients.

Journal Article↗
PubMed1990

Ammon's horn sclerosis: its pathogenesis and clinical significance.

Sclerosis of the cornu Ammonis or Ammon's horn sclerosis (AHS) is an "often-described, yet hitherto enigmatic phenomena" as Spielmyer put it in 1927. It has been found in cases with ischemia, anoxia or hypoglycemia and in more than half of the epileptic brains examined at autopsy. Various theories about its pathogenesis have been propounded. Among them, the "Pathoklise" theory of the Vogts and the vascular theory of Spielmeyer and his associates were prevailing until recently. In 1953, two articles were published to contribute to the pathogenesis of ictal automatism (a type of complex partial or temporal lobe seizures). One is the incisural sclerosis theory by Penfield and his associates and the other is the Ammon's horn sclerosis theory by Sano and Malamud. The former authors described a diffuse sclerosis of the infero-mesial temporal structures without, however, specifically relating it to AHS. They considered it was the result of localized anoxia of that portion of the brain caused by incisural herniation occurring during parturition. Sano and Malamud maintained that AHS is a result of convulsions, a distinct scar adjacent to which epileptogenic foci may develop in the course of time to cause ictal automatism. The latter theory was corroborated by Sano, Falconer and others. Falconer expanded the theory to the assertion that not only ictal automatism but other types of intractable epilepsy may be due to "mesial temporal (Ammon's horn) sclerosis". The most recent development in the pathogenesis of AHS is the excitotoxicity theory. Namely, AHS is caused by excessive excitation of neurons, probably by putative excitatory neurotransmitters, especially, glutamate. For this theory, there is a significant body of evidence. The problem of AHS, an old research subject and a matter of long-lasting controversy, has now been updated and become one of the newest topics in the field of experimental neurobiology.

Animals↗
PubMed2005

Molecular simulation of the shear viscosity and the self-diffusion coefficient of mercury along the vapor-liquid coexistence curve.

In earlier work [G. Raabe and R. J. Sadus, J. Chem. Phys. 119, 6691 (2003)] we reported that the combination of an accurate two-body ab initio potential with an empirically determined multibody contribution enables the prediction of the phase coexistence properties, the heats of vaporization, and the pair distribution functions of mercury with reasonable accuracy. In this work we present molecular dynamics simulation results for the shear viscosity and self-diffusion coefficient of mercury along the vapor-liquid coexistence curve using our empirical effective potential. The comparison with experiment and calculations based on a modified Enskog theory shows that our multibody contribution yields reliable predictions of the self-diffusion coefficient at all densities. Good results are also obtained for the shear viscosity of mercury at low to moderate densities. Increasing deviations between the simulation and experimental viscosity data at high densities suggest that not only a temperature-dependent but also a density-dependent multibody contribution is necessary to account for the effect of intermolecular interactions in liquid metals. An analysis of our simulation data near the critical point yields a critical exponent of beta = 0.39, which is identical to the value obtained from the analysis of the experimental saturation densities.

Journal Article↗
PubMed1973

Kinetics of diffusion and convection in 3.2-A pores. Exact solution by computer simulation.

The kinetics of transport in pores the size postulated for cell membranes has been investigated by direct computer simulation (molecular dynamics). The simulated pore is 11 A long and 3.2 A in radius, and the water molecules are modeled by hard, smooth spheres, 1 A in radius. The balls are given an initial set of positions and velocities (with an average temperature of 313 degrees K) and the computer then calculates their exact paths through the pore. Two different conditions were used at the ends of the pore. In one, the ends are closed and the balls are completely isolated. In the other, the ball density in each end region is fixed so that a pressure difference can be established and a net convective flow produced. The following values were directly measured in the simulated experiments: net and diffusive (oneway) flux; pressure, temperature, and diffusion coefficients in the pore; area available for diffusion; probability distribution of ball positions in the pore; and the interaction between diffusion and convection. The density, viscosity, and diffusion coefficients in the bulk fluid were determined from the theory of hard sphere dense gases. From these values, the "equivalent" pore radius (determined by the same procedure that is used for cell membranes) was computed and compared with the physical pore radius of the simulated pore.

Cell Membrane Permeability↗
PubMed1995

Diffusion and perfusion imaging techniques.

Diffusion imaging techniques including the Stejskal-Tanner and the stimulated emission of amplitude echoes (STEAM) pulse sequences are discussed. The calculation of apparent diffusion coefficient (ADC) maps is reviewed. The perfusion imaging techniques of blood oxygen level dependent (BOLD) and first pass bolus perfusion techniques as well as the newer technique of echo-planar imaging with signal targeting and altering radiofrequency (EPISTAR) are also discussed. Finally, the theory of intravoxel incoherent motion (IVIM) and its relationship to both diffusion and perfusion phenomena is examined.

Brain Diseases↗
PubMed2006

Monte Carlo-based inverse model for calculating tissue optical properties. Part I: Theory and validation on synthetic phantoms.

A flexible and fast Monte Carlo-based model of diffuse reflectance has been developed for the extraction of the absorption and scattering properties of turbid media, such as human tissues. This method is valid for a wide range of optical properties and is easily adaptable to existing probe geometries, provided a single phantom calibration measurement is made. A condensed Monte Carlo method was used to speed up the forward simulations. This model was validated by use of two sets of liquid-tissue phantoms containing Nigrosin or hemoglobin as absorbers and polystyrene spheres as scatterers. The phantoms had a wide range of absorption (0-20 cm(-1)) and reduced scattering coefficients (7-33 cm(-1)). Mie theory and a spectrophotometer were used to determine the absorption and reduced scattering coefficients of the phantoms. The diffuse reflectance spectra of the phantoms were measured over a wavelength range of 350-850 nm. It was found that optical properties could be extracted from the experimentally measured diffuse reflectance spectra with an average error of 3% or less for phantoms containing hemoglobin and 12% or less for phantoms containing Nigrosin.

Algorithms↗
PubMedDate not supplied

The neuroendocrine phenotype, cellular plasticity, and the search for genetic switches: redefining the diffuse neuroendocrine system.

The term neuroendocrine has been used to define cells that secrete their products in a regulated manner, in response to a specific stimulus. The neuroendocrine system includes neurons and endocrine cells sharing a common phenotypic program characterized by the expression of markers such as neuropeptides, chromogranins, neuropeptide processing enzymes SPC2 and SPC3 (subtilase-like pro-protein convertases) or dense core secretory granules. Various theories such as the APUD (amine precursor uptake decarboxylation) concept, the diffuse neuroendocrine system (DNES) or the paraneuron concept have been put forth to classify neuroendocrine cells as a cohesive group. Neuroendocrine characteristics have been used as evidence of a common embryological origin for normal and neoplastic cells. However, it is now recognized that neuroendocrine characteristics can be observed in various cell types, such as immunocytes, that are not of a common embryological origin with either neurons or endocrine cells. We propose to redefine previous "neuroendocrine" concepts to include the notion that activation of specific genetic switches can lead to the expression of a partial or full neuroendocrine phenotype in a variety of cell types, including immune cells.

Animals↗
PubMed2004

Reaction rates of heavy metal ions at goethite: relaxation experiments and modeling.

In the present paper we extend our theory that calculates the fastest reaction step observable in suspensions containing charged microcrystals and heavy metal cations. The calculation requires the solution of the nonlinear Poisson-Boltzmann equation for nonsymmetric electrolytes plus the Nernst-Planck equation for transport of ions in electric fields. We find that the diffusional transport of ions to and from the surface is the rate-limiting process for our experimentally observed maximum rates. At low pH and low metal ion concentration the diffusion of metal ions is the rate-limiting step, whereas for high pH and high metal ion concentration the diffusion of the solvated protons controls the overall relaxation rate. The validity of this theory is checked for the reactions of Pb2+ and Cd2+ with goethite by means of pressure jump relaxation experiments over a wide range of temperature and pH. In all cases we observe fast processes (relaxation in the range of 10(3) s(-1)) in quantitative agreement with the theory, followed by slower processes, most probably caused by diffusion into the interior of the porous microcrystals.

Journal Article↗
PubMed2005

Molecular dynamics simulation study on the transient response of solvation structure during the translational diffusion of solute.

The transient response function of the density profile of the solvent around a solute during the translational diffusion of the solute is formulated based on the generalized Langevin formalism. The resultant theory is applied to both neat Lennard-Jones fluids and cations in liquid water, and the response functions are obtained from the analysis of the molecular dynamics simulations. In the case of the self-diffusion of Lennard-Jones fluids, the responses of the solvation structures are in harmony with conventional pictures based on the mode-coupling theory, that is, the binary collision in the low-density fluids, the backflow effect from medium to high density fluids, and the backscatter effect in the liquids near the triple point. In the case of cations in water, the qualitative behavior is strongly dependent on the size of cations. The pictures similar to simple dense liquids are obtained for the large ion and the neutral molecule, while the solvent waters within the first solvation shell of small ions show an oscillatory response in the short-time region. In particular, the oscillation is remarkably underdumped for lithium ion. The origin of the oscillation is discussed in relation to the theoretical treatment of the translational diffusion of ions in water.

Journal Article↗
PubMed1996

Some new observations on pulse sequence dependent diffusion related edge enhancement in MR microscopy.

Self-diffusion of nuclear spins has been suggested to cause edge enhancement in images especially on a microscopic scale. According to previously published work, theory suggests that edge enhancement is caused by motional narrowing due to the boundaries and spin self-diffusion during the data acquisition period. More careful examination reveals that edge enhancement due to motional narrowing develops only under a few specific conditions. This lack of generality of motional narrowing theory, as well as experimental observations, indicate that edge enhancement due to effects other than motional narrowing alone can exist. It is found that edge enhancement depends greatly on the data acquisition mode; therefore, the images obtained are different depending on the pulse sequence employed. For example, excessive attenuation of DC components due to diffusion can result in edge enhancement in the spin echo signal. However, in the case of FID-like signals, DC components are preserved while positive high frequency parts are attenuated, thereby degrading resolution. The new phenomenon observed has been termed selective spectral suppression since the observed edge enhancement results from the selective attenuation of certain frequency components in the nuclear signals due to diffusion-dependent signal attenuation for a given pulse sequence.

Diffusion↗