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This study aimed to gain mechanistic insights into the nature of the pore pathway of fully hydrated human stratum corneum from lag time data obtained using a model polar permeant, urea. Lag times were deduced from transport experiments with human epidermal membranes and with human epidermal membranes after ethanol or chloroform–methanol treatment. A tortuous pore pathway transport model and a `bottleneck' transport model were employed for data analysis, and their appropriateness for the observed data was examined. Important outcomes from the present study with intact and with delipidized stratum corneum were as follows. Long lag times (around 60–800 min) for the transport of urea in human epidermal membranes were generally observed. These results were consistent with an extremely tortuous pore pathway as would be expected if it is associated with the polar/aqueous region of the stratum corneum intercellular lipids (i.e. the bilayers in the intercellular region). The permeability of the stratum corneum increased after ethanol treatment, and, at the same time, the tortuosity decreased but remained relatively high. Chloroform–methanol treatment further increased the permeability and further decreased the tortuosity. Since delipidization by ethanol and chloroform–methanol treatments decreased the tortuosity of the pore pathway, these results suggest that the effectively highly tortuous pathway for polar permeants in stratum corneum may be associated with the polar regions of the intercellular lipids. Untreated skin samples that had high electrical resistance were observed to have longer lag times than those with low resistance; this is consistent with the hypothesis that skin samples of high resistance have less appendage routes or less damage and transport polar permeants predominantly via the tortuous pathways involving the intercellular lipid regions of the stratum corneum. Neither the tortuous pathway transport model alone nor the `bottleneck' transport model alone seems to perfectly represent the experimental data, and a modified model (a hybrid of the two models) has been proposed to be more consistent with the lag time data and the morphology of fully hydrated stratum corneum. The present study has demonstrated the usefulness of lag times obtained with a polar permeant in better understanding the transport mechanisms involved with the pore pathway.

In many cases it is instructive to use a single human epidermal membrane (HEM) sample to perform successive in vitro permeability experiments under varied experimental conditions. This study focused upon the feasibility of such successive permeability experiments in side-by-side, two-chamber diffusion cells. It was shown that for permeability experimental protocols which involved performing one permeability experiment per day and extensive washing between permeability experiments, the barrier properties of HEM samples were altered significantly within the first 72 h of the protocol. However, if the HEM is supported in the diffusion cell with a porous synthetic membrane, a single HEM sample remains essentially unaltered with respect to mannitol permeability and electrical resistance for up to 5 days. This suggests that protecting the HEM from physical stress is an essential element in performing successive permeability experiments.

An experimental protocol, using an initial 1 min direct current (DC) applied potential of 4 V followed by alternating current (AC), was established to: (a) increase conductance and permeability and decrease lag time for human epidermal membrane (HEM) relative to unaltered HEM and; (b) maintain constant conductance and permeability during flux studies. The protocol allowed specific permeation parameters of the membrane to be characterized under electrically enhanced, constant flux conditions. The permeability, lag time, and effective membrane thickness were determined using a nonionic polar permeant, urea, while the enhanced conductance was maintained at a constant level with AC. A tortuous pore pathway model was employed to analyze the data. The AC protocol increased membrane permeability, and decreased lag time and effective membrane thickness relative to similar parameters obtained in previous studies from unaltered HEM. Lag times ranged from 32.0 to 105.5 min, and permeability coefficients calculated from steady state fluxes ranged from 1.68 to 6.03x10(-7) cm/s for HEM samples with electrical resistance values during transport of 2.3-8.0 kOmega x cm2. Effective membrane thicknesses were calculated to range from 0.34 to 0.61 cm during AC iontophoresis. Significant additional results were obtained when the protocol was applied for two consecutive runs using the same HEM sample, with time for the HEM sample to recover between runs. During the second run, the applied potential was adjusted to reproduce the conductance obtained on the first run. Under these conditions, the consecutive runs yielded essentially the same lag time, permeability and effective membrane thickness values. These results suggest that constant fluxes can be achieved by keeping HEM electrical conductance constant during AC iontophoresis.