Studies in the measurement, form and interpretation of some electrical properties of normal and pathological human skin in vivo
During the past five years a study has been made of the electrical properties of human skin in vivo both experimentally and theoretically, seeking to make a coherent whole out of the very diverse phenomenology reported in the very scattered literature. A Fourier transform method was used, employing novel electronic techniques and four non-invasive electrodes on the subject to separately determine the properties of the stratum corneum and the deeper tissues. Both linear and non-linear, A.C. and D.C. properties were investigated in this way, and the wide perspective obtained has made it possible to re-interpret the findings of previous workers and so assemble a self-consistent model of nearly all the known electrical properties of human skin. The main conclusions of this work are that: (a) The small-signal A.C. impedance properties of skin are extremely well fitted by a minor arc of a circle on the complex impedance plane, and the depression of the centre of the circle below the equivalent series resistance axis is very significantly greater in psoriatic skin and in hyper-hydrated normal skin. (b) This behaviour cannot be adequately interpreted by any extant physical model of skin, but can be adequately interpreted by considering the stratum corneum to be an inhomogeneous or amorphous semiconductor. (c) The semiconductivity of human skin has been demonstrated both in vitro and in vivo, and was found to have activation enthalpies averaging 0.46 eV (10.63 kcal/mole). (d) These insights into the mechanisms of electrical charge transport through skin may suggest new ways of characterizing the properties of skin using methods developed for the semiconductor industry. (e) The results in the literature of "phoreography" should be re-interpreted, with much greater emphasis being placed on the properties of the skin appendages.