THERMOELECTRICAL ANALYSIS OF THE HUMAN SKIN BARRIER

The influence of temperature on the resistive and capacitative properties of human stratum corneum in vitro was studied to determine where within substructures of stratum corneum, the electrical resistance R and capacitance C components reside. Heating-cooling cycles were designed in accordance with earlier calorimetric and spectroscopic studies of thermal transitions of human stratum corneum lipids and/or proteins. Two different protocols were used. (A) Heat treatment and electrical analysis were carried out simultaneously in pH 7.4 phosphate buffered saline, starting with prehydrated stratum corneum (70% w/w) of pH 7.4. (B) Heat treatment was performed before electrical analysis, using dried stratum corneum (< 10% w/w), followed by prehydration and measurement of the electrical properties in phosphate buffered saline at 20-degrees-C. Square-wave alternating current pulses of 13 muA cm-2 were applied every 60 s. Analysis of the resulting voltage waveform across stratum corneum yielded an equivalent electrical model of stratum corneum composed of a series connection of two RC circuits (R1 parallel-to C1 and R2 parallel-to C2). Below 60-degrees-C a constant activation energy of 5.4 +/- 0.7 kcal mol-1 was measured, which was close to the activation energy of K+ diffusion in a fluid aqueous medium. The total resistance of stratum corneum was less than 100 kOMEGA cm2, which is very low compared to the resistance of black lipid membranes (1-10 MOMEGA cm2). Both the low activation energy and resistance of human stratum corneum suggest the presence of highly conductive pathways through the membrane. Between 60 and 75-degrees-C an abrupt decline of the resistances R1 and R2 and a rapid rise of the capacitances C1 and C2 was observed. This temperature interval corresponded to the temperature interval of the second thermal transition observed in human stratum corneum, which is a lipid phase transition. Beyond 75-degrees-C, the resistances were fairly constant, while the capacitances continued to increase. The changes in the resistances and capacitances brought about by heating to 75 and 95-degrees-C were completely irreversible. This is in agreement with X-ray diffraction studies, which