Temperature-dependent electrical and ultrastructural characterizations of porcine skin upon electroporation.

The mechanism of high-voltage pulse-induced permeabilization of the stratum corneum, the outer layer of the skin, is still not completely understood. It has been suggested that joule heating resulting from the applied pulse may play a major role in disrupting the stratum corneum. In this study, electrical and ultrastructural measurements were conducted to examine the temperature dependence of the pulse-induced permeabilization of the stratum corneum. The stratum corneum resistance was measured using a vertical diffusion holder, with the stratum corneum placed between two electrode-containing chambers. The stratum corneum resistance was reduced manyfold during the applied pulse. The extent of resistance reduction increased with pulse voltage until reaching a threshold value, above which the resistance reduction was less dependent on the pulse voltage. The stratum corneum was more susceptible to permeabilization at high temperature, the threshold voltage being lower. The stratum corneum resistance recovered within milliseconds after a single 0.3-ms pulse. High-temperature samples had a more prolonged recovery time. Using time-resolved freeze fracture electron microscopy, aggregates of lipid vesicles were observed in all samples pulsed above the threshold voltage. The sizes and fractional areas occupied by aggregates of lipid vesicles at 4 degrees C and at 25 degrees C were measured at different time points after the applied pulse. Aggregates of vesicles persisted long after the electric resistance was recovered. After pulsing at the same voltage of 80 V, samples at 4 degrees C were found to have slightly more extensive aggregate formation initially, but recovered more rapidly than those at 25 degrees C. The more rapid recovery of the 4 degrees C samples was likely due to a lower supra-threshold voltage. Viscoelastic instability propagation created by the pulse may also play a role in the recovery of the aggregates.