Iontophoretic transport across a synthetic membrane and human epidermal membrane: a study of the effects of permeant charge.

The effects of permeant charge (z) on iontophoretic-enhanced transport were investigated with synthetic Nucleopore membranes and with human epidermal membranes using a four-electrode potentiostat with side-by-side diffusion cells. The modified Nernst-Planck model (Nernst-Planck theory with an additional transport term to correct for the effect of the convective solvent flow due to electroosmosis) was first examined in a Nuclepore membrane system with model permeants calcein (z = -4), salicylate (z = -1), and a series of polystyrene sulfonates (from monomer to molecular weight of approximately 8000 with a z range of -1 to approximately -40). The flux enhancement (E) for each permeant was determined at 470 mV. Mannitol (a neutral molecule) was used as a probe to determine a correction for convective solvent flow under the same applied voltage conditions. Good agreement between the experimental results and the predictions from the modified Nernst-Planck model was found for calcein, salicylate, and polystyrene sulfonates up to molecular weight of approximately 1800 (z approximately -8). The flux enhancements for the higher molecular weight polystyrene sulfonates with greater z values were more than a factor of three lower than theoretical predictions; the electrophoretic effect and counterion binding to the permeants are proposed as possible explanations for these discrepancies between experiment and the modified Nernst-Planck theory. In the studies with human epidermal membranes, iontophoretic flux enhancements for calcein, salicylate, and taurocholate were determined at 250 and/or 470 mV. The flux enhancements were generally consistent with the results calculated from the modified Nernst-Planck model.