Thermoelectrical analysis of the human skin barrier

Abstract

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°C. Square-wave alternating current pulses of 13 μA 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 ( R 1 ∥ C 1 and R 2 ∥ C 2 ). Below 60°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 kΩ cm 2 , which is very low compared to the resistance of black lipid membranes (1–10 MΩ cm 2 ). 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°C an abrupt decline of the resistances R 1 and R 2 and a rapid rise of the capacitances C 1 and C 2 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°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°C were completely irreversible. This is in agreement with X-ray diffraction studies, which have shown that the originally predominant lamellar structure with a repeat distance of 6.4 nm does not reappear after recrystallization from 75 and 90°C. The final resistances after heating desiccated stratum corneum (⩽ 10% w/w hydration) to temperatures of 75 and 95°C and subsequent cooling were less different from the original values than those of hydrated samples, suggesting that the impact of heating depends on the level of stratum corneum hydration. After chloroform-methanol lipid extraction, stratum corneum was characterized by a single RC circuit. The resistance of extracted stratum corneum was less than 1% of the original pre-extraction value, which indicates that the resistance of stratum corneum is mainly determined by the intercellular lipids. Taken together, the correlation between the temperature dependence of the resistances and capacitances between 20 and 75°C suggests that the electrical barrier and the charge storage capacity of stratum corneum are determined by the same substructures. Most probably these structures are the intercellular lipid lamellae, because the changes of the electrical properties mainly take place within the interval 60 to 75°C, which corresponds to the phase transition of free intercellular lipids, and also because these changes are irreversible. This conclusion is also confirmed by chloroform-methanol extraction. However, the continued increases of the capacitances between 75 and 95°C, indicates that the charge storage capacity of stratum corneum also depends on lipids attached to protein. The low resistance and activation energy at temperatures between 20 and 60°C indicate that stratum corneum possesses highly conductive pathways.