THERMOELECTRICAL ANALYSIS OF THE HUMAN SKIN BARRIER

The influence of temperature on the resistive and capacitative propert ies of human stratum corneum in vitro was studied to determine where w ithin substructures of stratum corneum, the electrical resistance R an d capacitance C components reside. Heating-cooling cycles were designe d 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 a nalysis were carried out simultaneously in pH 7.4 phosphate buffered s aline, starting with prehydrated stratum corneum (70% w/w) of pH 7.4. (B) Heat treatment was performed before electrical analysis, using dri ed stratum corneum (< 10% w/w), followed by prehydration and measureme nt of the electrical properties in phosphate buffered saline at 20-deg rees-C. Square-wave alternating current pulses of 13 muA cm-2 were app lied every 60 s. Analysis of the resulting voltage waveform across str atum 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 ener gy of 5.4 +/- 0.7 kcal mol-1 was measured, which was close to the acti vation energy of K+ diffusion in a fluid aqueous medium. The total res istance 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 c orneum suggest the presence of highly conductive pathways through the membrane. Between 60 and 75-degrees-C an abrupt decline of the resista nces R1 and R2 and a rapid rise of the capacitances C1 and C2 was obse rved. This temperature interval corresponded to the temperature interv al of the second thermal transition observed in human stratum corneum, which is a lipid phase transition. Beyond 75-degrees-C, the resistanc es were fairly constant, while the capacitances continued to increase. The changes in the resistances and capacitances brought about by heat ing to 75 and 95-degrees-C were completely irreversible. This is in ag reement with X-ray diffraction studies, which