Passive Electrical Properties of Human Stratum Corneum during Application of Electric Fields

Abstract

The stratum corneum (SC) is the outermost, essentially dead, layer of the skin and is known as the most effective barrier to small ions, i.e. it exhibits a very large electrical resistance compared to the underlying tissue^1,2. The SC is composite of corneocytes surrounded. by regions of lipids, with the space between adjacent corneocytes containing 2 to 6 lipid bilayers. Inside a cross linked structure of keratin gives mechanical stability³. Since the SC is hydrated the corneocytes are water rich and swollen, with conducting interior electrolytes. The SC is interrupted by appendages such as hair follicles and sweat ducts which provide direct aqueous pathways through the skin^1. Electrically the skin behaves in first approximation like a capacitor shunted by a resistor. More information about the skin impedance is given in^4–6. Below about 0.8V the skin resistance behaves linearly with respect to the voltage. It is widely known, that the transdermal resistance decreases with application of higher voltage, for instance for iontophoresis^7,8 or or electroporation^2,9–11. The decrease in resistance due to the application of low voltages (U_skin< 5V) takes place within millisecond to several minutes, depending on the applied voltage^7,12. This speeds up by orders of magnitude, if the voltage reaches a range above 30V^2,11,13. Two different explanations are favored for the both regions of voltages:

• (1) low voltage (< 5V): Preexisting pathways are increased by field induced water inflow and possibly change from conduction within the aqueous pathways to convection by electroosmosis^8,14. These preexisting pathway may include appendages^15,16 or tortuous pathways within the stratum corneum structure¹³. When the voltage is turned off, the pathways shrink to their original size and the resistance of the skin recovers.

• (2) high voltage (>30V): If the voltage across a single bilayer reaches conditions where water can be electrically introduced into the bilayer structure (200mV - IV per bilayer) electroporation may occur. Our recent understanding is that aqueous pores are created within the lipid environment which can expand while the electric field is large (> 1kV/cm). If the pulse is short (order of µs) no significant heating may occur. The decrease of the resistance due to electroporation is very rapid^11,17 on the order of less then 10µs. Subsequently the skin resistance will recover by shrinking and finally closing of electrically induced pores.