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Concentration Gradient (concentration + gradient)
Selected AbstractsEffect of Concentration Gradient on the Morphology Development in Polymer Solutions Undergoing Thermally Induced Phase SeparationMACROMOLECULAR THEORY AND SIMULATIONS, Issue 7 2007Bai Tao Jiang Abstract Anisotropic porous polymeric materials fabricated from the phase separation method via spinodal decomposition are used in various practical engineering applications. We studied the formation of anisotropic porous polymeric materials numerically, by imposing an initial linear concentration gradient across a model polymer solution. The initial concentration gradient is placed at three different regions of the polymer sample for comparison purposes. All the simulation results are in good agreement with published experimental observations, which are reported from the applications of porous polymeric membranes. The structure development shows that an anisotropic porous morphology forms when an initial linear concentration gradient is applied to the model polymer solution. [source] Concentration Gradient Along the Scala Tympani After Local Application of Gentamicin to the Round Window Membrane,THE LARYNGOSCOPE, Issue 7 2007Stefan K. Plontke MD Abstract Objectives: The distribution of gentamicin along the fluid spaces of the cochlea after local applications has never previously been demonstrated. Computer simulations have predicted that significant basal-apical concentration gradients might be expected, and histologic studies indicate that hair cell damage is greater at the base than at the apex after local gentamicin application. In the present study, gradients of gentamicin along the cochlea were measured. Methods: A recently developed method of sampling perilymph from the cochlear apex of guinea pigs was used in which the samples represent fluid originating from different regions along the scala tympani. Gentamicin concentration was determined in sequential apical samples that were taken after up to 3 hours of local application to the round window niche. Results: Substantial gradients of gentamicin along the length of the scala tympani were demonstrated and quantified, averaging more than 4,000 times greater concentration at the base compared with the apex at the time of sampling. Peak concentrations and gradients for gentamicin varied considerably between animals, likely resulting from variations in round window membrane permeability and rates of perilymph flow. Conclusions: The large gradients for gentamicin demonstrated here in guinea pigs account for how it is possible to suppress vestibular function in some patients with a local application of gentamicin without damaging auditory function. Variations in round window membrane permeability and in perilymph flow could account for why hearing losses are observed in some patients. [source] Controllable Soluble Protein Concentration Gradients in Hydrogel Networks,ADVANCED FUNCTIONAL MATERIALS, Issue 21 2008Brian J. Peret Abstract Here, controlled formation of sustained, soluble protein concentration gradients within hydrated polymer networks is reported. The approach involves spatially localizing proteins or biodegradable, protein-loaded microspheres within hydrogels to form a protein-releasing "depot." Soluble protein concentration gradients are then formed as the released protein diffuses away from the localized source. Control over key gradient parameters, including maximum concentration, gradient magnitude, slope, and time dynamics, is achieved by controlling the release of protein from the depot and subsequent transport through the hydrogel. Results demonstrate a direct relationship between the amount of protein released from the depot and the source concentration, gradient magnitude, and slope of the concentration gradient. In addition, an inverse relationship exists between the diffusion coefficient of protein within the hydrogel and the slope of the concentration gradient. The time dynamics of the concentration gradient profile can be directly correlated to protein release from the localized source, providing a mechanism for temporarily controlling gradient characteristics. Therefore, each key biologically relevant parameter associated with the protein concentration gradient can be controlled by defining protein release and diffusion. It is anticipated that the resulting materials may be useful in 3D cell culture systems, and in emerging tissue engineering approaches that aim to regenerate complex, functional tissues. [source] Optimization of control parameters of cadmium zinc telluride Bridgman single crystal growthCRYSTAL RESEARCH AND TECHNOLOGY, Issue 8 2007Liu Juncheng Abstract The temperature gradient within a furnace chamber and the crucible pull rate are the key control parameters for cadmium zinc telluride Bridgman single crystal growth. Their effects on the heat and mass transfer in front of the solid-liquid interface and the solute segregation in the grown crystal were investigated with numerical modeling. With an increase of the temperature gradient, the convection intensity in the melt in front of the solid-liquid interface increases almost proportionally to the temperature gradient. The interface concavity decreases rapidly at faster crucible pull rates, while it increases at slow pull rates. Moreover, the solute concentration gradient in the melt in front of the solid-liquid interface decreases significantly, as does the radial solute segregation in the grown crystal. In general, a decrease of the pull rate leads to a strong decrease of the concavity of the solid-liquid interface and of the radial solute segregation in the grown crystal, while the axial solute segregation in the grown crystal increases slightly. A combination of a low crucible pull rate with a medium temperature gradient within the furnace chamber will make the radial solute segregation of the grown crystal vanish. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Vapour growth and morphology of PbBr2 crystalsCRYSTAL RESEARCH AND TECHNOLOGY, Issue 7 2007Takamasa Kaito Abstract In this paper, the vapour transport of PbBr2 under vacuum during vacuum distillation refining and its condensation on the wall of the vessel were described. The macro- and micro morphologies of PbBr2 crystals grown in the vessel (glass tube) were studied. The crystal shape changed dramatically depending on the positions of condensation in the vessel, i.e., the crystal shape varied from an isometric polyhedron to columnar crystals with facets, and to a massive crystal without facets with a rise in the wall temperature. These results were interpreted in terms of the concentration gradient of the molecules in the vessel, surface roughening and/or surface melting of the crystals. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Morphogens and cell survival during developmentDEVELOPMENTAL NEUROBIOLOGY, Issue 4 2005Patrick Mehlen Abstract The notion of "morphogens" is an important one in developmental biology. By definition, a morphogen is a molecule that emanates from a specific set of cells that is present in a concentration gradient and that specifies the fate of each cell along this gradient. The strongest candidate morphogens are members of the transforming growth factor-, (TGF-,), Hedgehog (Hh), and Wnt families. While these morphogens have been extensively described as differentiation inducers, some reports also suggest their possible involvement in cell death and cell survival. It is frequently speculated that the cell death induction that is found associated with experimental removal of morphogens is the manifestation of abnormal differentiation signals. However, several recent reports have raised controversy about this death by default, suggesting that cell death regulation is an active process for shaping tissues and organs. In this review, we will present morphogens, with a specific emphasis on Sonic Hedgehog, a mammalian member of the Hh family, not as a positive regulators of cell differentiation but as key regulators of cell survival. © 2005 Wiley Periodicals, Inc. J Neurobiol 64: 357,366, 2005 [source] Numerical simulation of sediment-associated water quality processes for a Mississippi delta lakeECOHYDROLOGY, Issue 3 2009Xiaobo Chao Abstract Three major sediment-associated processes were presented to describe the effects of sediment on the water quality processes, including the effect of sediment on the light intensity for the growth of phytoplankton (PHYTO), the adsorption,desorption of nutrients by sediment and the release of nutrients from the bed sediment layer. A formula was generated from field measurements to calculate the light attenuation coefficient by considering the effects of concentrations of chlorophyll and suspended sediment (SS). The concentrations of adsorbed and dissolved nutrients because of adsorption,desorption were calculated using two formulas that were derived based on the Langmuir Equation. The release rates of nutrients from the bed sediment were calculated by considering the effects of the concentration gradient across the water-sediment interface, pH, temperature and dissolved oxygen (DO) concentration. Model algorithms describing the adsorption and desorption of nutrients from sediment particles as well as the release of nutrients from bed sediment were tested using experimental data. These sediment-associated water quality processes were included in a three-dimensional (3D) water quality model, CCHE3D_WQ, developed by the National Center for Computational Hydroscience and Engineering (NCCHE), to simulate the concentrations of PHYTO and nutrients in a shallow Mississippi Delta lake with special emphasis on sediment-related processes. The simulated concentration of PHYTO (as chlorophyll) and nutrients were generally in good agreement with field observations. This study shows that there are strong interactions between sediment-associated processes and water quality constituents. Copyright © 2009 John Wiley & Sons, Ltd. [source] A microfluidic device for characterizing the invasion of cancer cells in 3-D matrixELECTROPHORESIS, Issue 24 2009Tingjiao Liu Abstract A microfluidic device was developed for the study of directed invasion of cancer cells in 3-D matrix with concentration gradient. This device consists of two parallel perfusion channels connected by two cell culture chambers. To mimic extracellular matrix (ECM), gelled basement membrane extract (BME) was used to support 3-D distribution of breast cancer cells (MCF7) in cell culture chambers. A stable linear concentration gradient of epidermal growth factor (EGF) was generated across the chambers by continuous perfusion. Using the device, we investigated MCF7 cell invasion induced by different concentrations of EGF in 3-D matrix. It was found that cancer cells responded to EGF stimulation with forming cellular protrusions and migrating towards high EGF concentration. We further investigated the anti-invasion effect of GM 6001, a matrix metalloproteinase inhibitor. We identified that matrix metalloproteinase inhibition repressed both cellular protrusion formation and cell migration in 3-D matrix. These findings suggest that EGF is able to induce MCF7 cell invasion in 3-D extracellular matrix and this effect is dependent on proteolytic activity. This device is relatively simple to construct and operate. It should be a useful platform for elucidating the mechanism of cancer invasion and screening anti-invasion drugs for cancer therapy. [source] Relationship between lysosomal membrane destabilization and chemical body burden in eastern oysters (Crassostrea virginica) from Galveston Bay, Texas, USAENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 6 2002Hyun-Min Hwang Abstract Lysosomal destabilization was measured by using hemocytes of eastern oysters (Crassostrea virginica) collected along a chemical concentration gradient in Galveston Bay, Texas, USA. Results of the lysosomal response were compared to concentrations of organic compounds and trace elements in oyster tissue. Concentrations (on a dry-wt basis) ranged from 288 to 2,390 ng/g for polycyclic aromatic hydrocarbons (PAHs), 38 to 877 ng Sn/g for tri- n -butyltin (TBT), 60 to 562 ng/g for polyclorinated biphenyls (PCBs), and 7 to 71 ng/g for total DDT. Trace element concentrations (on a dry-wt basis) ranged from 1.1 to 4.0 ,g/g for Cd, 105 to 229 ,g/g for Cu, 212 to 868 ,g/g for Al, and 1,200 to 8,180 ,g/g for Zn. The percentage of destabilized lysosomes ranged from 34 to 81%. A significant positive correlation (p < 0.05) was observed between lysosomal destabilization and body burden of organic compounds (PAHs, PCBs, TBT, and chlorinated pesticides). No significant correlation was found between metal concentrations and lysosomal destabilization. Based on lysosomal destabilization, the study sites in Galveston Bay can be placed in one of three groups: healthy (Hanna Reef and Confederate Bay), moderately damaged (Offats Bayou and Todd's Dump), and highly damaged (Yacht Club and Ship Channel). Lysosomal destabilization that is consistent with toxic chemical body burdens supports previous observations that lysosomal membranes are damaged by toxic chemicals and indicates that this method can serve as an early screening tool to assess overall ecosystem health by using oysters. [source] Rapid Generation of Biologically Relevant Hydrogels Containing Long-Range Chemical GradientsADVANCED FUNCTIONAL MATERIALS, Issue 1 2010Jiankang He Abstract Many biological processes are regulated by gradients of bioactive chemicals. Thus, the generation of materials with embedded chemical gradients may be beneficial for understanding biological phenomena and generating tissue-mimetic constructs. Here a simple and versatile method to rapidly generate materials containing centimeter-long gradients of chemical properties in a microfluidic channel is described. The formation of a chemical gradient is initiated by a passive-pump-induced forward flow and further developed during an evaporation-induced backward flow. The gradient is spatially controlled by the backward flow time and the hydrogel material containing the gradient is synthesized via photopolymerization. Gradients of a cell-adhesion ligand, Arg-Gly-Asp-Ser (RGDS), are incorporated in poly(ethylene glycol)-diacrylate (PEG-DA) hydrogels to test the response of endothelial cells. The cells attach and spread along the hydrogel material in a manner consistent with the RGDS-gradient profile. A hydrogel containing a PEG-DA concentration gradient and constant RGDS concentration is also shown. The morphology of cells cultured on such hydrogel changes from round in the lower PEG-DA concentration regions to well-spread in the higher PEG-DA concentration regions. This approach is expected to be a valuable tool to investigate the cell,material interactions in a simple and high-throughput manner and to design graded biomimetic materials for tissue engineering applications. [source] Heterogeneity effects on possible salinity-driven free convection in low-permeability strataGEOFLUIDS (ELECTRONIC), Issue 4 2009J. M. SHARP JR Abstract Although studies of free convection commonly focus on highly permeable strata, but numerical analyses indicate that density-driven free convection may also occur in heterogeneous low-permeability strata. Traditional Rayleigh number criteria are overly conservative in predicting thermohaline convection in these systems; so, numerical models are used to make inferences on the process. Simulations with stochastic realizations of permeability fields show that dense plumes can take preferential pathways to sink through generally low-permeability strata; patch analysis using percolation theory shows that the threshold permeability for the onset of free convection can be as low as 10,16 m2 even with a mean permeability of 10,18 m2. Threshold permeability for the percolation pathways decreases with increasing concentration gradient, vertical correlation length and the mean and variance of the permeability. The connectedness of relatively high-permeability zones is important in initiating and controlling plume fingers of free convection in both single-layer and sand-shale sequence models. Permeable units above and below are conducive to free convection through intervening low-permeability strata if buoyancy gradients exist. This heterogeneity is on scales that are difficult to sample by drilling and too localized to be simulated in regional models but may be significant in solute transport in these systems. [source] Controllable Soluble Protein Concentration Gradients in Hydrogel Networks,ADVANCED FUNCTIONAL MATERIALS, Issue 21 2008Brian J. Peret Abstract Here, controlled formation of sustained, soluble protein concentration gradients within hydrated polymer networks is reported. The approach involves spatially localizing proteins or biodegradable, protein-loaded microspheres within hydrogels to form a protein-releasing "depot." Soluble protein concentration gradients are then formed as the released protein diffuses away from the localized source. Control over key gradient parameters, including maximum concentration, gradient magnitude, slope, and time dynamics, is achieved by controlling the release of protein from the depot and subsequent transport through the hydrogel. Results demonstrate a direct relationship between the amount of protein released from the depot and the source concentration, gradient magnitude, and slope of the concentration gradient. In addition, an inverse relationship exists between the diffusion coefficient of protein within the hydrogel and the slope of the concentration gradient. The time dynamics of the concentration gradient profile can be directly correlated to protein release from the localized source, providing a mechanism for temporarily controlling gradient characteristics. Therefore, each key biologically relevant parameter associated with the protein concentration gradient can be controlled by defining protein release and diffusion. It is anticipated that the resulting materials may be useful in 3D cell culture systems, and in emerging tissue engineering approaches that aim to regenerate complex, functional tissues. [source] Microfluidics: Surface-Treatment-Induced Three-Dimensional Capillary Morphogenesis in a Microfluidic Platform (Adv. Mater.ADVANCED MATERIALS, Issue 47 200947/2009) The cover shows confocal images of 3D sprouting into matrix material in microfluidic channels. Roger Kamm and co-workers report on p. 4863 that robust induction of realistic angiogenesis into the 3D matrix material under simultaneous imaging and a stably controlled concentration gradient of chemoattractants can be achieved. The formation of a 3D vascular network is demonstrated to be a direct consequence of surface treatment of the region of the device-containing matrix material. [source] Surface-Treatment-Induced Three-Dimensional Capillary Morphogenesis in a Microfluidic PlatformADVANCED MATERIALS, Issue 47 2009Seok Chung Robust induction of realistic angiogenesis into a 3D matrix material under simultaneous imaging and a stably controlled concentration gradient of chemoattractants is presented. The formation of a 3D vascular network is demonstrated to be a direct consequence of surface treatment of the region of the device-containing matrix material. [source] Thermal-diffusion and diffusion-thermo effects on convective heat and mass transfer in a visco-elastic fluid flow through a porous medium over a stretching sheetINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 9 2006A. M. Salem Abstract An analysis has been carried out to obtain the thermal-diffusion and diffusion-thermo effects on the mixed free forced convective and mass transfer in a visco-elastic fluid flow through a porous medium over a stretching sheet. Here, the driving force for the flow is provided by an impermeable sheet stretched with a velocity proportional to the distance from a slit and buoyancy effects due to both temperature and concentration gradient. The partial differential equations governing the problem under consideration have been transformed by a similarity transformation into a system of ordinary differential equations which are solved numerically by applying the shooting method. The effects of Soret number, Dufour number, visco-elastic parameter, Porosity parameter, Grashof number and modified Grashof number on the velocity, temperature and concentration have been discussed. Numerical results for the problem considered are given and illustrated graphically. Copyright © 2006 John Wiley & Sons, Ltd. [source] Synthesis and Dielectric Properties of Niobia Coating on BaTiO3INTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 6 2009Jia-Jia Gan Different from conventional powder mixing, this study demonstrates a method of homogeneous coating for niobia (Nb2O5) on ferroelectric barium titanate (BaTiO3) powders. The precipitation of Nb coating with pH has been determined quantitatively by the inductance-coupled plasma method. Crystalline phases were determined by X-ray diffractometry, and the formation of a "core-shell" structure with a Nb concentration gradient was observed by a transmission electron microscope with X-ray energy-dispersive spectroscopy. The dielectric constant (K) of the samples prepared by the nanocoating method showed a more stable temperature coefficient of capacitance and well satisfied the requirements of X7R. [source] Three-dimensional CFD model for a flat plate photocatalytic reactor: Degradation of TCE in a serpentine flow fieldAICHE JOURNAL, Issue 2 2009Asefeh Jarandehei Abstract Computational fluid dynamics (CFD) simulation was applied to a photocatalytic reactor with surface reaction for trichloroethylene (TCE) oxidation at various pollutant concentrations, and flow rates. First-order and Langmuir-Hinshelwood kinetics for TCE removal rate were considered. The results were compared with those from experiments of Demeestere et al. (Appl Catal B Environ. 2004;54:261,274) in a flat plate photocatalytic reactor with serpentine geometry. The flow regime was laminar. Through the CFD simulation, the velocity field and the concentration gradient of TCE in the reactor were studied in detail. At Reynolds numbers around 900, the laminar flow becomes unstable. Under such a condition, when flow passes the 180° sharp turns, due to formation of secondary flow and consequently vortices, there is a lot of cross-sectional mixing in the reactor. This kind of studies can help us to model the photocatalytic reactor as accurately as possible. © 2008 American Institute of Chemical Engineers AIChE J, 2009 [source] Lactate transport and transporters: General principles and functional roles in brain cellsJOURNAL OF NEUROSCIENCE RESEARCH, Issue 1-2 2005Leif Hertz Abstract Lactate is transported across cell membranes by diffusional, saturable cotransport with protons, mediated by monocarboxylate transporters (MCTs). This transport is bidirectional and in the absence of a transcellular H+ gradient, it can increase the intracellular concentration of lactate up to but not beyond the extracellular level (or vice versa). If extra- and intracellular pH differ, however, the equilibrium level is determined by the gradients of both lactate anions and protons. Rates of lactate uptake are determined most often by measuring uptake of labeled lactate, e.g., [U- 14C]lactate. In the case of lactate and other compounds that are metabolized, errors are introduced easily because continuing inwardly directed diffusional net transport of label can be achieved by intracellular metabolism, reducing the intracellular level of the nonmetabolized lactate and thus maintaining a concentration gradient between extra- and intracellular concentrations of the nonmetabolized compound (metabolism-driven uptake). For measurement of facilitated diffusion kinetics, it is essential that the period during which the uptake is measured is short enough that little or no metabolism-driven uptake contributes to the measured uptake (or that first-order regression analysis is carried out to obtain initial uptake rates from nonlinear traces). To achieve initial uptake rates, incubation periods well below 1 min are generally required. Lactate uptake is fast in astrocytes, which express powerful, low-affinity MCTs, i.e., MCT1 and MCT4. Due to the low affinity of these transporters, they respond to increased lactate gradients with enhanced transporter activity. The predominant MCT in neurons is the high-affinity MCT2, which can only increase its activity to a limited extent in the face of an increased lactate gradient. This is reflected by a high-affinity lactate uptake, although most investigators also have demonstrated a component of lactate uptake with lower affinity. In both neurons and astrocytes, however, facilitated diffusion is fast enough that under most conditions lactate fluxes will be determined mainly by the rate of metabolism-driven uptake, and MCT-mediated transport only will be rate-limiting after establishment of large transmembrane gradients. © 2004 Wiley-Liss, Inc. [source] Application of solid-phase concentration-dependent HSDM to the acid dye adsorption systemAICHE JOURNAL, Issue 1 2005Vinci K. C. Lee Abstract The fixed-bed adsorption of acid dyes onto granular activated carbon (Chemviron Filtrasorb 400) has been studied using a homogeneous surface diffusion model (HSDM). The model incorporates the external boundary layer mass transport and homogeneous diffusion inside the particle. A new orthogonal collocation method has been developed and used to solve the diffusion equations. This orthogonal collocation gives a faster solution method compared with the numerical Crank,Nicolson method. The surface diffusivity has been determined by an optimization procedure with minimization of sum of the error squared. The equilibrium relationship between the liquid-phase concentration and the solid-phase concentration has been described by the Redlich,Peterson isotherm. A solid-phase concentration-dependent surface diffusivity was introduced. The Darken model with the Redlich,Peterson isotherm was found to be a suitable correlation model for the adsorption of the acid dyes on carbon. The magnitude of the averaged Ds0 of each dye is in the order of AR114 > AB80 > AY117, which implies that, under the same solid-phase concentration gradient, the rate of mass transport diffusion is higher in AR114 than that in AB80 and AY117. This phenomenon may be explained by the different mobilities of the dye molecules present in the solution by the different arrangements of two sulfonic acid groups in the dye structures. © 2004 American Institute of Chemical Engineers AIChE J, 51: 323-332, 2005 [source] Protein partitioning and transport in supported cationic acrylamide-based hydrogelsAICHE JOURNAL, Issue 5 2003Shawn M. Russell The partitioning and transport of myoglobin in cationic, acrylamide-based hydrogels are studied by a microscopic visualization method. Homogeneous cationic gels are synthesized inside fused-silica capillaries with a square section, which allow a direct determination of protein concentration profiles during transient adsorption and desorption. Diffuse, self-similar profiles are observed and used to determine the equilibrium protein binding capacity and the protein diffusivity in the gel. Mass-transfer rates are found to be essentially independent of the external solution concentration, but to vary dramatically with the gel polymer concentration. A Fickian diffusion model with a flux based on the adsorbed-phase concentration gradient is consistent with the experimentally determined concentration profiles for both positive and negative protein concentration steps. The equilibrium and rate parameters determined for the capillary-supported gels also compare favorably with those obtained from macroscopic measurements using composite ion-exchange media comprising similar gels held within the pores of porous silica particles. [source] Linearized transport model for nanofiltration: Development and assessmentAICHE JOURNAL, Issue 4 2002W. Richard Bowen Finite difference linearization of pore concentration gradient in nanofiltration membranes greatly simplifies the solution of a three-parameter model (pore radius, membrane charge, and pore dielectric constant) for electrolyte rejection by removing the requirement for numerical integration of the extended Nernst,Planck equation. The validity of the linearized model is first experimentally tested by comparing with a rigorous characterization of the Desal-DK nanofiltration membrane, the linearized model closely agreeing with the numerical solution of the full model. Investigation of pore concentration profiles showed the assumption of linearity to be valid over a wide range of nanofiltration conditions. The linearized model was also successfully extended to ternary electrolyte mixtures, highlighting its main advantage over analytic solutions. Overall, the model is a powerful tool for characterization of nanofiltration membranes and subsequent prediction of separation performance. Computational demands are modest in terms of time and complexity. [source] Analysis of unstirred water layer in in vitro permeability experimentsJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 12 2009Timo Korjamo Abstract In vitro permeability experiments are used widely in drug discovery and other areas of pharmaceutical research. Much effort has been expended in developing novel epithelial models but generally much less attention has been paid to the hydrodynamic barrier in the actual experiments. The restricted liquid flow in the vicinity of solid surfaces leads to a zone where the diffusional movement of molecules exceeds the convection. This leads to formation of a concentration gradient between the bulk solution and the surface. The formed unstirred water layer (UWL) reduces the apparent permeability (Papp) of compounds that rapidly pass through the actual epithelial layer. This lowers the resolution of Papp versus fraction-absorbed assay, complicates the structure-permeability analysis and skews apparent kinetic parameters of transporter substrates. This review describes the techniques that can be used to determine the UWL thickness in permeability experiments and apparatuses described in the literature to control the in vitro hydrodynamics. © 2009 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 98:4469,4479, 2009 [source] Effect of lipid bilayer alteration on transdermal delivery of a high-molecular-weight and lipophilic drug: Studies with paclitaxelJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 9 2004Ramesh Panchagnula Abstract Skin forms an excellent barrier against drug permeation, due to the rigid lamellar structure of the stratum corneum (SC) lipids. Poor permeability of drugs can be enhanced through alteration in partition and diffusion coefficients, or concentration gradient of drug with an appropriate choice of solvent system, along with penetration enhancers. The aim of the current investigation was to assess applicability of lipid bilayer alteration by fatty acids and terpenes toward the permeation enhancement of a high-molecular-weight, lipophilic drug, paclitaxel (PCL) through rat skin. From among the fatty acids studied using ethanol/isopropyl myristate (1:1) vehicle, no significant enhancement in flux of PCL was observed (p,>,0.05). In the case of cis mono and polyunsaturated fatty acids lag time was found to be similar to control (p,>,0.05). This suggests that the permeation of a high-molecular-weight, lipophilic drug may not be enhanced by the alteration of the lipid bilayer, or the main barrier to permeation could lie in lower hydrophilic layers of skin. A significant increase in lag time was observed with trans unsaturated fatty acids unlike the cis isomers, and this was explained on the basis of conformation and preferential partitioning of fatty acids into skin. From among the terpenes, flux of PCL with cineole was significantly different from other studied terpenes and controls, and after treatment with menthol and menthone permeability was found to be reduced. Menthol and menthone cause loosening of the SC lipid bilayer due to breaking of hydrogen bonding between ceramides, resulting in penetration of water into the lipids of the SC lipid bilayer that leads to creation of new aqueous channels and is responsible for increased hydrophilicity of SC. This increased hydrophilicity of the SC bilayer might have resulted in unfavorable conditions for ethanol/isopropyl myristate (1:1) along with PCL to penetrate into skin, therefore permeability was reduced. The findings of this study suggest that the permeation of a high-molecular-weight and lipophilic drug cannot be enhanced through bilayer alteration by penetration enhancers, and alteration in partitioning of drug into skin could be a feasible mode to enhance the permeation of drug. © 2004 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 93:2177,2183, 2004 [source] Kinetic Model for Crystallization in White Ceramic GlazesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2001Agustin Escardino Theoretical equations have been developed for crystal growth rate in layers of small frit (glass) particles during firing. Throughout the process, the crystalline and the glassy phases have different compositions; therefore, the system can be considered a pseudo-two-component system consisting of a crystallizable component (structural unit) and a noncrystallizable mixture of several components. The concentration of the crystallizable component decreases in the residual glassy phase during the crystal growth process, on integrating at the surfaces of crystals having the same composition. Throughout the crystal growth process, a concentration gradient of the crystallizable component is therefore produced in the glassy phase, which results in mass transport by diffusion of this component from the bulk residual glassy phase to the surfaces of the crystals. Equations have been derived assuming that the diffusion step of the crystallizable component through the residual glassy phase is the overall crystal growth process rate-controlling step. [source] Morphology and Phase Stability of Nitrogen,Partially Stabilized Zirconia (N-PSZ)JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2001Tai-Joo Chung The surface layer of yttria-doped tetragonal zirconia materials that have been heat-treated with zirconium nitride was observed to consist of a nitrogen-rich cubic matrix with nitrogen-poor tetragonal precipitates. The precipitates had a thin, oblate-lens shape, similar to those observed in magnesia,partially stabilized zirconia. Because of the fast diffusion of N4, ions, the precipitates grew rather large, up to ,5 ,m in length, and remained stabilized in the tetragonal form at room temperature. Because the nitrided layer grew in the two-phase field, the size and distribution of the precipitates each was very irregular. The nitrogen content was observed to determine the proportion of cubic and tetragonal phases in the same way as in conventional cation-stabilized partially stabilized zirconia. A ternary phase diagram for the zirconium(yttrium),nitrogen,oxygen system was suggested to explain the concentration gradient in the cubic matrix and the phase distribution of the nitrided layer. [source] Sintering Characteristics in the BaTiO3,Nb2O5,Co3O4 Ternary System: II, Stability of So-called "Core,Shell" StructureJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2000Hirokazu Chazono The sintering characteristics and the reaction of additives with BaTiO3 (BT) were examined for two materials having Nb-rich composition (Comp.N) and Co-rich composition (Comp.C) to elucidate the relation between the stability of the core,shell microstructure and the Nb/Co ratio in the BT,Nb2O5,Co3O4 system. TEM observation revealed that the concentration gradient of Nb and Co existed in the shell region although Nb and Co macroscopically distributed homogeneously. X-ray diffraction analysis showed that the shell formation preceded the densification and completed at about 1280°C for both Comp.N and Comp.C as determined from differential scanning calorimetry. A diffusion couple experiment disclosed that Co had a larger diffusivity than Nb and that the diffusion of Co was suppressed when the sample was codoped with a sufficient amount of Nb. On the basis of these experimental results, new mechanisms of the formation and collapse of core,shell structure in the BT,Nb2O5,Co3O4 system were proposed. [source] A New Technique for Preparing a Filled Type of Polymeric Gradient MaterialMACROMOLECULAR MATERIALS & ENGINEERING, Issue 11 2006Yong-Bin Zhu Abstract Summary: So-called functionally gradient materials have received increased attention as a new type of composite whose microelements, including composition and structure, change spatially to optimize the gradient properties for a specific application. In this study, a new technique for continuously preparing a filled type of PGM was investigated through co-extrusion/gradient distribution/2-dimensional mixing with conventional polymeric material processing facilities. The processing line from co-extrusion, gradient distribution to 2-dimensional mixing was fulfilled by two extruders, a gradient distribution unit and 2-dimensional mixing units. The gradient distribution unit and 2-dimensional mixing units were designed separately in our group. As an example, a PE/GB PGM was prepared by using this new technique. The gradient variation of composition along the sample thickness direction was studied by TG and SEM. The TG results indicated that a gradient variation of the content of GB was formed along the thickness of the sample. The direct gradient distribution of GB came from SEM observation, which showed an increased stacking density of GB along the sample thickness. Experimental results indicated that the processing method with co-extrusion/gradient distribution/2-dimensional mixing can serve as a new way to produce a filled type of PGM and is worthy of further investigation. The prepared polyethylene/glass bead PGM; the graph illustrates the glass bead concentration gradient across the sample thickness. [source] Effect of Concentration Gradient on the Morphology Development in Polymer Solutions Undergoing Thermally Induced Phase SeparationMACROMOLECULAR THEORY AND SIMULATIONS, Issue 7 2007Bai Tao Jiang Abstract Anisotropic porous polymeric materials fabricated from the phase separation method via spinodal decomposition are used in various practical engineering applications. We studied the formation of anisotropic porous polymeric materials numerically, by imposing an initial linear concentration gradient across a model polymer solution. The initial concentration gradient is placed at three different regions of the polymer sample for comparison purposes. All the simulation results are in good agreement with published experimental observations, which are reported from the applications of porous polymeric membranes. The structure development shows that an anisotropic porous morphology forms when an initial linear concentration gradient is applied to the model polymer solution. [source] The Role of K+ Channels in Determining Pulmonary Vascular Tone, Oxygen Sensing, Cell Proliferation, and Apoptosis: Implications in Hypoxic Pulmonary Vasoconstriction and Pulmonary Arterial HypertensionMICROCIRCULATION, Issue 8 2006ROHIT MOUDGIL ABSTRACT Potassium channels are tetrameric, membrane-spanning proteins that selectively conduct K+ at near diffusion-limited rates. Their remarkable ionic selectivity results from a highly-conserved K+ recognition sequence in the pore. The classical function of K+ channels is regulation of membrane potential (EM) and thence vascular tone. In pulmonary artery smooth muscle cells (PASMC), tonic K+ egress, driven by a 145/5 mM intracellular/extracellular concentration gradient, contributes to a EM of about ,60 mV. It has been recently discovered that K+ channels also participate in vascular remodeling by regulating cell proliferation and apoptosis. PASMC express voltage-gated (Kv), inward rectifier (Kir), calcium-sensitive (KCa), and two-pore (K2P) channels. Certain K+ channels are subject to rapid redox regulation by reactive oxygen species (ROS) derived from the PASMC's oxygen-sensor (mitochondria and/or NADPH oxidase). Acute hypoxic inhibition of ROS production inhibits Kv1.5, which depolarizes EM, opens voltage-sensitive, L-type calcium channels, elevates cytosolic calcium, and initiates hypoxic pulmonary vasoconstriction (HPV). Hypoxia-inhibited K+ currents are not seen in systemic arterial SMCs. Kv expression is also transcriptionally regulated by HIF-1, and NFAT. Loss of PASMC Kv1.5 and Kv2.1 contributes to the pathogenesis of pulmonary arterial hypertension (PAH) by causing a sustained depolarization, which increases intracellular calcium and K+, thereby stimulating cell proliferation and inhibiting apoptosis, respectively. Restoring Kv expression (via Kv1.5 gene therapy, dichloroacetate, or anti-survivin therapy) reduces experimental PAH. Electrophysiological diversity exists within the pulmonary circulation. Resistance PASMC have a homogeneous Kv current (including an oxygen-sensitive component), whereas conduit PASMC current is a Kv/KCa mosaic. This reflects regional differences in expression of channel isoforms, heterotetramers, splice variants, and regulatory subunits as well as mitochondrial diversity. In conclusion, K+ channels regulate pulmonary vascular tone and remodeling and constitute potential therapeutic targets in the regression of PAH. [source] Towards advanced circuit board materials: adhesion of copper foil to ultra-high molecular weight polyethylene compositePOLYMERS FOR ADVANCED TECHNOLOGIES, Issue 6 2002Dmitry Abstract Polyethylene based composites are attractive materials for advanced circuit board applications because of their unique combination of properties: low dielectric constant and loss factor, light weight, high flexural modulus and low thermal expansion coefficient controlled in all spatial directions. This investigation describes a process to consolidate chopped fibers of ultra-high molecular weight polyethylene concurrently with its bonding to a copper foil. Bonding is affected by a thin sheet of low-density polyethylene, incorporating a crosslinking agent with a concentration gradient across the sheets thickness. In this single step process, the composite material is formed and bonded to the metal foil, achieving good adhesion without the use of extraneous glue. Copyright © 2002 John Wiley & Sons, Ltd. [source] | |||||||||||||||||||||||||||||||||||||||||||