A method of determining absolute rates of diffusion and electroosmotic convective flow through individual pores in porous ion-selective membranes is described. The method is based on positioning a scanning electrochemical microscope (SECM) tip directly above a membrane pore and detecting electroactive molecules as they emerge from the pore. Absolute diffusive and electroosmotic fluxes, electroosmotic drag coefficient, convective velocity, and pore radius can be evaluated in a single experiment by measuring the faradaic current at the SECM tip as a function of the iontophoretic current passed across the membrane. Electroosmotic transport of hydroquinone through a permselective polymer (Nafion), contained within ∼50-µm-radius pores of a 200-µm-thick mica membrane, is used as a model system to demonstrate the analytical method. Analysis of electroosmotic transport parameters obtained by SECM suggests that the average electroosmotic velocities of solvent (H 2 O) and solute (hydroquinone) in the Nafion are significantly different, a consequence of the differences in their chemical interactions with the current-carrying mobile cations (Na + ).Iontophoresis is the transport of molecular species under the influence of an electrical potential gradient. 1 The pharmaceutical and medical communities are actively researching the iontophoretic transport of ions and molecules through skin as an alternative method of drug administration for humans. 2 In this application, a small electrical current is driven between two electrodes that are placed in contact with the outer surface of the skin. The molecular species of interestsi.e., the drugsis dissolved in a thin layer of solution between one electrode and the skin and is transported across the skin at a continuously controlled rate that is determined by the applied current. The drug molecules traverse the skin and are transported throughout the body by the circulatory system.

Physiological structures associated with iontophoretic paths in hairless mouse skin and two cultured skin models ("EpiDerm" by Mattek, Corp., and "SKIN2" by Advanced Tissue Sciences, Inc.) are reported. Visualization of ionic paths at current densities between 20 and 100 microA/ cm2 is accomplished by the counterdirectional transport of Fe(CN)6(4-) and Fe3+, resulting in the controlled precipitation of colloidal Prussian blue, Fe4[Fe(CN)6]3, at sites of high ionic conductivity. Examination of the Fe4[Fe(CN)6]3-stained tissues using optical microscopy allows unequivocal assignment of iontophoretic paths to physiological structures in the stratum corneum. Deposition of Fe4[Fe(CN)6]3 occurs exclusively at hair follicles in hairless mouse skin, indicating that these appendages provide highly conductive porous paths during iontophoresis. In contrast, the counterdirectional transport of Fe(CN)6(4-) and Fe3+ across cultured skin models, which lack appendages, results in the deposition of Fe4-[Fe(CN)6]3 along the boundaries of corneocytes. This observation suggests that paracellular iontophoretic transport through lipid bilayer regions is the predominant transport path in the absence of low-resistance pores.