Role Of Electroosmosis Research Articles

Electrolytic flow in partially saturated charged micro-channels: Electrocapillarity vs electro-osmosis

Wettability is the main factor controlling the fluid flow in an electrically neutral partially saturated micro-channel. If the micro-channel body carries electric charges and is fully saturated by a conductive fluid, electro-osmosis is considered the driving force for fluid movement. The flow of electrolytes in an electrically charged partially saturated micro-channel, however, needs further attention where the electrocapillary and electro-osmosis can simultaneously exist. We, thus, investigated the movement of KCl electrolytes with different concentrations (0.1, 0.5, 1, and 3 M) in a partially saturated (air-filled) and electrically charged micro-channel fabricated in a conductive substrate (aluminum) using micro-fluidics. We additionally studied the contact angle-based wettability alteration of an electrolyte/air/aluminum substrate system under an electric field. This allowed us to link the change in capillary forces due to the electricity-induced wettability alteration to micro-fluidic flow observations, i.e., a link between electro-osmosis and capillary forces. Our theoretical analysis revealed that at low concentration, the role of electro-osmosis and electrocapillarity on fluid flow in partially saturated charged micro-channel is relatively comparable. At 0.1 M KCl concentration, the change in wettability due to the applied electric field contributed to over 42% of the induced flow of the solution in the micro-channel. As the ionic concentration increases, the role of capillary pressure fades and electro-osmosis becomes the dominant process controlling the flow. At 3.0 M KCl concentration, electrocapillarity contributed only 23% to the induced flow under the applied electric field in the micro-channel. The results reveal the importance of electro-osmosis along with electrocapillary flow in partially saturated electrically charged micro-channels.

Current-Induced Concentration Polarization of Nanoporous Media: Role of Electroosmosis.

Current-induced concentration polarization of nanoporous media is explored theoretically by using approach of local thermodynamic equilibrium within nanopore cross-sections. The problem is solved in quadratures in terms of irreversible thermodynamics. The phenomenological coefficients are further specified by using capillary space-charge model for straight slit-like and cylindrical capillaries. This analysis reveals several novel features of current-induced concentration polarization related to the (electro)osmotic volume transfer. It confirms the previous finding that volume transfer can suppress the limiting-current phenomena but obtains more accurate criteria for this. In particular, it shows that the critical nanopore size and/or electrolyte concentration depend on the nanoporous-medium relative thickness. Under no-limiting-current conditions, the salt concentration at the interface between nanoporous medium and unstirred layer is a nonmonotone function of current density. This gives rise to unconventional current-voltage characteristics. Moreover, under certain conditions, the analysis predicts the existence of ranges of "prohibited" current densities where the problem does not have 1D stationary solution, which could give rise to a kind of "phase separation" with coexisting zones of different local current densities corresponding to the same voltage. Besides the advanced understanding of current-induced concentration polarization of nanoporous media, this analysis provides guidelines for the optimization of sample preconcentration systems in (bio)chemical microanalysis.

Role of Electroosmosis in the Permeation of Neutral Molecules: CymA and Cyclodextrin as an Example

To quantify the flow of small uncharged molecules into and across nanopores, one often uses ion currents. The respective ion-current fluctuations caused by the presence of the analyte make it possible to draw some conclusions about the direction and magnitude of the analyte flow. However, often this flow appears to be asymmetric with respect to the applied voltage. As a possible reason for this asymmetry, we identified the electroosmotic flow (EOF), which is the water transport associated with ions driven by the external transmembrane voltage. As an example, we quantify the contribution of the EOF through a nanopore by investigating the permeation of α-cyclodextrin through CymA, a cyclodextrin-specific channel from Klebsiella oxytoca. To understand the results from electrophysiology on a molecular level, all-atom molecular dynamics simulations are used to detail the effect of the EOF on substrate entry to and exit from a CymA channel in which the N-terminus has been deleted. The combined experimental and computational results strongly suggest that one needs to account for the significant contribution of the EOF when analyzing the penetration of cyclodextrins through the CymA pore. This example study at the same time points to the more general finding that the EOF needs to be considered in translocation studies of neutral molecules and, at least in many cases, should be able to help in discriminating between translocation and binding events.

Alteration of electroosmotic volume flow through skin by polyethylene glycols.

We have studied the effect of polyethylene glycols (PEGs) on the iontophoretic flux of acetaminophen (AAP) using conventional in vitro iontophoresis methodology. A series of PEGs with average molecular weight (MW) ranging from about 100 to 1,500 was studied. The results were analyzed to explain how PEGs affect the electroosmosis and flux through skin. As a marker molecule for the direction and magnitude of electroosmotic volume flow (EVF), AAP was used. PEG decreased both anodal and cathodal AAP flux markedly. The magnitude of this decrease in flux increased as the MW and the concentration of PEG increased. From the Helmholtz-Smoluchowski equation, it was expected that the increase in viscosity and the decrease in dielectric constant are thought to be the main reason for the decrease in EVF and the flux. The large increase in solubility of AAP in PEG solution may also play an important role, because this increase lowers the partition of AAP into the stratum corneum. When 30 % diethylene glycol solution was used, the magnitude of EVF was estimated to be about 1.5 μl/cm(2) h, and it decreased as the MW of the PEG increased. These results and discussions clearly suggest that the incorporation of organic solubilizers and penetration enhancers into the iontophoretic formulation should be carefully decided after a thorough understanding of their effect on flux. Overall, these results provide further mechanistic insights into the role of electroosmosis in flux through skin, and how they can be modulated by PEG and their MW.

Electrophoresis of a colloidal particle embedded in electrolyte saturated porous media

We consider a charged colloidal sphere undergoing steady migration under an imposed electric field through an uncharged hydrogel, modeled as a Brinkman medium. The transport of ions is governed by the Nernst–Planck equation coupled with the Brinkman equation for fluid flow and Poisson equation for electric field. The motion deterring effects due to the induced electric field and electroosmosis in a gel medium are governed by the degree of the double layer polarization and electromigration of ions. These effects are correctly accounted in the present model. Based on the effective medium (EM) approach, we have estimated the mobility of the particle through the balance of hydrodynamical and electrical forces for a wide range of electrokinetic parameters such as, charge density of the particle, hydrogel permeability and ionic concentration (Debye layer thickness). A correct measure of the hydrodynamical drag and electrical force experienced by the particle in electrophoresis is made. We have validated our computed results for mobility by comparing with the available experimental and theoretical results. The role of electroosmosis on the hydrodynamics of the particle is analyzed by comparing the drag with the corresponding uncharged case. Our results suggest that the retardation effects are strong for moderate values of the Debye–Huckel parameter and grow as the permeability of the hydrogel is increased.

Modulation of electroosmosis and flux through skin: effect of propylene glycol

The effect of propylene glycol (PG) on transdermal flux under current was investigated using conventional in vitro iontophoresis methodology. The results were evaluated to explain how PG affects the electroosmotic volume flow (EVF) and electromigrational flux through skin. As a marker molecule for the direction and magnitude of EVF, a non-charged neutral molecule, acetaminophen (AAP), was used. At pH 7.4, the direction of EVF was from anode to cathode. During anodal and cathodal current application, PG decreased AAP flux and this decrease was proportional to the concentration of PG, indicating that the presence of PG in the medium decreased the EVF. This decrease is likely due to the decrease in dielectric constant of the medium and the increases in medium viscosity by the addition of PG. The increase in AAP solubility and the viscosity of the medium by PG may also contribute to the decrease in diffusional flux. The magnitude of EVF was estimated to be about 4.2 μl/cm(2 )h. The effect of PG on the flux of a positively charged drug, donepezil hydrochloride (DH), was further investigated using pH 4.6 phosphate buffer solution. The permselectivity of skin in this solution was also investigated and revealed that the isoelectric point of hairless mouse skin is higher than pH 4.6. Anodal delivery showed much higher flux than cathodal and passive flux, indicating that electromigration is playing the major role for DH flux. As the concentration of PG increased, anodal flux of DH decreased. The main reason for this decrease in electromigration is likely due to the increase in medium viscosity. These results and discussions clearly suggest that the incorporation of frequently used organic cosolvents and penetration enhancers into the iontophoretic formulation should be carefully chosen with a thorough investigation for their effect on flux. Overall, these results provided further mechanistic insights into the role of electroosmosis and electromigration in flux across skin, and how they can be modulated by organic cosolvent, PG.

Effect of pH and penetration enhancers on electroosmotic flow and flux through skin

We have investigated the effect of pH and four penetration enhancers on the electroosmotic volume flow (EVF) and flux through skin to get more detailed understanding of this phenomenon and its effect on flux. The results were evaluated in relation EVF and the permeability change by current induced skin damage. At pH 7.4, we have confirmed that the direction of convective solvent flow is from anode to cathode. At pH 4.0, no permselectivity was observed and it seems that this pH is close to the isoelectric point of skin. At pH 3.0, the permselectivity of skin is reversed. From the difference in flux between just before (47 μg/cm2 h) and after (32 μg/cm2 h) cathodal current-off, the magnitude of EVF is estimated to be smaller than 1.5 μl/cm2 h, if we consider the recovery of skin to a lower permeability after current-off. At pH 7.4, Oleic acid (OA) and propylene glycol monolaurate (PGML) increased the passive flux markedly. Synergistic effect in flux between OA and current was observed for both anodal and cathodal current. The use of isopropyl myristate (IPM) in combination with anodal current resulted in reduced flux when compared to the flux of anodal current alone. Cathodal flux of OA or PGML treated skin increased continuously until the current was off. However, to the contrary of our expectation, flux decreased after current-off. We think this is mainly due to the recovery of damaged skin (flux decreasing effect), though the disappearance of EVF may slightly increase the flux. For IPM and propylene glycol, the combination of enhancer with cathodal current inhibited the flux, similar to that observed for anodal delivery. Overall, these results provide further information on the role of electroosmosis and the effect of penetration enhancers in combination with current on flux through skin.

Role of electro-osmosis in the impedance response of microchannel-nanochannel interfaces

The influence of net fluid flow on the low-frequency ac response of a microchannel-nanochannel interface under dc bias is studied theoretically using a simple 1D model based on the Poisson-Nernst-Planck-Stokes equations. The model describes cross-sectional averaged transport wherein the electro-osmotic flow is controlled by the magnitude of the dc bias and captures essential features of the problem related to the micro-nano interface, specifically geometric focusing effects and nanochannel control of the net fluid flow. This model predicts behavior which differs from that predicted by a purely electrodiffusive formulation. The high-frequency edge of the Warburg branch of the complex impedance plot has a slope which deviates from the π/4 Warburg value, decreasing with increasing bias, and there are corresponding changes in the overall phase as seen in the Bode plots. This can be attributed to a streaming contribution to the capacitive reactance of the device as well an increase in the conductance of the depleted region, both due to net fluid flow. The increase in conductance, corresponding to reduced interfacial depletion, also permits dc currents above the classical electrodiffusive limit.

Iontophoretic Transport of Donepezil Hydrochloride through Skin: Flux Enhancement by Chemical Enhancer and Iontophoresis

ABSTRACT − The objective of this work is to investigate the effect of chemical enhancer and current on the flux of done-pezil hydrochloride (DH) through skin. Ethanol and N-methyl pyrrolidone (NMP) were used as chemical enhancers in com-bination with iontophoresis. We also have studied the effect of pH on flux and evaluated the role of electroosmosis. In vitroflux study was performed at 33 o C, using side-by-side diffusion cell and full thickness hairless mouse skin. Passive flux ofDH without enhancer was very small. As the concentration of enhancer increased, passive flux increased. After currentapplication, flux increased markedly and the time to reach maximum decreased. Without enhancer, maximum flux was about50 fold larger than that obtained without current. These results indicate that electromigration is playing a major role for thetransport. As the enhancer concentration increased, flux also increased. NMP and ethanol increased not only the passivedelivery, but also the iontophoretic delivery. Flux results indicate that ethanol has better ability than NMP in enhancing thetransport of DH. The magnitudes of increase in flux by these enhancers indicate that there is a large synergistic effect influx enhancement. Flux results from pH study showed that electroosmotic flow is reversed at low pH and the flux is hin-dered. These results provided some information on the flux enhancing ability of ethanol and NMP in combination with ion-tophoresis. The data also provided some mechanistic insights into the role of electromigration and electroosmosis on fluxthrough skin.Key words

Levodopa의 이온토포레시스 경피전달: 올레인산 아이크로에멀젼 및 에탄올의 투과증진

In order to develop optimal formulation and iontophoresis condition for the transdermal delivery of levodopa, we have evaluated the effect of two permeation enhancers, ethanol and oleic acid in microemulsion, on transdermal delivery of levodopa. In vitro flux studies were performed at , using side-by-side diffusion cell and full thickness hairless mouse skin. Current density applied was and current was off after 6 hours application. Levodopa was analysed by HPLC at 280 nm. The o/w microemulsions of oleic acid in buffer solution (pH 2.5 & 4.5) were prepared using oleic acid, Tween 80 and ethanol. The existence of microemulsion regions were investigated in pseudo-ternary phase diagrams. Contrary to our expectation, cumulative amount of levodopa transported from microemulsion (pH 2.5) for 10 hours was similar to that from aqueous solution in all delivery methods (passive, anodal and cathodal). When pH of the micro-emulsion was pH 4.5, cumulative amount of levodopa transported for 10 hours increased about 40% (anodal) to 50% (cathodal), when compared to that from aqueous solution. Flux from pH 4.5 microemulsion showed higher value than that from pH 2.5 in all delivery methods. These results seem to indicate that electroosmosis plays more dominant role than electrorepulsion in the flux of levodopa at pH 2.5. The effect of ethanol on iontophoretic flux was studied using pH 2.5 phosphate buffer solution containing 3% or 5% (v/v) ethanol. Flux enhancement was observed in passive and anodal delivery as the concentration of the ethanol increased. Without ethanol, cathodal delivery showed higher flux than anodal delivery. Anodal delivery increased the cumulative amount of levodopa transported 1.6 fold by 5% ethanol after 10 hours. However, in cathodal delivery, no flux enhancement of levodopa was observed during current application and only marginal increase in cumulative amount transported after 10 hours was observed by 5% ethanol. These results seem to be related to the decrease in dielectric constant of the medium and the lipid extraction of the ethanol, which decrease the electroosmotic flow, and thus decrease the flux. Overall, the results provide important insights into the role of electroosmosis and electrorepulsion in the transport of levodopa through skin, and provide some useful informations for optimal formulation for levodopa.

The role of electro-osmosis in the electric-field-induced movement of charged macromolecules on the surfaces of cells

The surfaces of most cells bear a net negative charge. The imposition of an electric field parallel to the surface of the cell should produce, therefore, an electro-osmotic flow of fluid towards the cathodal side of the cell. Our analysis of a simple model of the cell surface indicates that a negatively charged mobile macromolecule will be swept by this electro-osmotic flow of fluid to the cathodal side of the cell if its zeta potential, zeta 1, is less negative than the zeta potential of the cell surface, zeta 2. Conversely, if zeta 2 is less negative than zeta 1, the negatively charged macromolecule will accumulate at the anodal side of the cell. Our experimental results demonstrate that concanavalin A (Con A) receptors on embryonic muscle cells normally accumulate at the cathodal side of the cell, but that they can be induced to accumulate at the anodal side of the cell by preincubating the myotubes either with neuraminidase, a treatment that removes negatively charged sialic acid residues, or with the lipid diI, a treatment that adds positive charges to the surface of the cell. Addition of the negatively charged lipid monosialoganglioside (GM1), on the other hand, enhances the accumulation of Con A receptors at the cathodal side of the cell.

Electroosmosis in Nitella.

The role of electroosmosis was studied directly in Nitella. The cells were mounted in a water-tight barrier between two chambers containing reversible electrodes for the application of potentials, and fitted with calibrated capillaries to measure water movement. No water movement was found when small existing bioelectric potentials were short-circuited through an external connection, nor when external potentials up to 1 or 2 volts were applied (producing currents up to 5 microa). Higher potentials (up to 10 volts) caused small movements of water, toward the negative pole. Larger and often irreversible water movements were produced by potentials up to 20 volts-sometimes persisting after current flow. A variety of evidence suggests that the effects are caused by injury at the cathodal end of the cell, allowing water to be attracted osmotically at the intact end and forced out at the injured end (transosmosis). This injury is reversible under small applied potentials, irreversible after large ones (100 to 200 times the natural bioelectric values). Such water flows persist in low salt concentrations (up to 0.09 M NaCl) but almost completely vanish in isotonic (0.26 M) mannitol. This confirms the osmotic, rather than the electroosmotic nature of the water movement. It is estimated that electroosmosis cannot account for more than 1 per cent of the water movement (or turgor) in Nitella cells. The dead cellulose walls display a small electroosmotic water flow at very high current densities (under 20 volts applied potential).