Electroporation of human skin: simultaneous measurement of changes in the transport of two fluorescent molecules and in the passive electrical properties

The stratum corneum (SC) of mammalian skin is a formidable barrier to the transport of both small ions and charged molecules, but large, very rapid increases in transport can be created by “high-voltage” pulses. Here a series of exponential pulses (√pulse = 1.1 ms) was used in vitro with human skin preparations. A flow-through apparatus provided simultaneous, continuous measurements for the transport of two fluorescent molecules (calcein, 623 Da, charge of zcal = −4; sulforhodamine, 607 Da, charge of zsr = −1) and the skin's passive electrical properties, with emphasis on the transdermal conductive behavior, which includes both the d.c. conductance, Gskin, and the non-linear dynamic conductance, Gdy. “High-voltage” pulsing was found to cause large and very rapid changes in the molecular flux of both molecules, and also in Gskin and Gdy. In the case of molecular transport, the relative contribution of local diffusion and local electric field-driven transport depends significantly on the molecular charge, zs. The field-driven transport during a pulse allowed estimates of the maximum fractional aquerous area Fw,s, of the skin that was transiently available during a pulse for small ions (Fw, ions = 6 ± 3 × 10−4), calcein (Fw, cal = 5 ± 3 × 10−5), and sulforhodamine (Fw,sr = 7 ± 4 × 10−5). Comparison of the post-pulse recovery of Gskin and the decrease of the molecular transport showed that the recovery of the skin barrier and molecular flux decay are not identical. These results are interpreted as being due to electrical creation of new aqueous pathways (“pores”) across the lipid regions of the skin's stratun corneum, and support the hypothesis that elecroporation is responsible for the rapid and large changes observed.