THE ELECTRICAL CHARACTERISTICS OF HUMAN SKIN IN-VIVO

Purpose. The objectives of this study were to investigate the impedanc e properties of human skin in vivo and to examine the effect of iontop horesis upon them. Methods. Having established the intra- and inter-in dividual variation in basal values of skin impedance, the effect of va rying iontophoretic current density, ionic strength and counter-ion on the rate of recovery of skin impedance after iontophoresis was invest igated. Results. Passage of an iontophoretic current caused a signific ant reduction in the magnitude of the skin impedance. Increasing the c urrent density caused an even greater reduction in the value of the sk in impedance and slowed the rate of recovery. Reduction of the ionic s trength resulted in an increase in the rate of recovery following iont ophoresis. A significant increase in the rate of recovery was observed when CaCl2 replaced NaCl as the electrolyte. Although visual inspecti on revealed the presence of greater erythema when CaCl2 was used, ther e was an absence of the mild sensation experienced by volunteers when using NaCl. The last part of the study established a correlation betwe en transepidermal water loss and impedance analysis as complementary m ethods for probing skin barrier function in vivo. The data were fitted to an equivalent circuit consisting of a resistor in parallel with a constant-phase element and a mechanistic model proposed to explain the electrical properties of the skin. Conclusions. The first comprehensi ve investigation of the effect of iontophoresis on the electrical prop erties of human skin in vivo has been described. It would appear from the results, and from their interpretation, that impedance spectroscop y may be an effective method to quantify the impact of iontophoresis o n the skin, and to determine the extent to which proposed drug deliver y regimens will perturb skin barrier function.