Skin Impedance Taking into Consideration the Multi-layer Configuration of Epidermal Stratum Corneum

Abstract

It is well known that the epidermal stratum corneum has very large influence on the skin impedance. In the present paper, the formative mechanism of the skin impedance and the skin resistivity, and dielectric constant are discussed, with the structure of this part taken into consideration. The essential points of this paper consist of following items :
(1) The mathematical expression of the skin impedance Z taking into account the so-called multi-layer configuration of the stratum corneum is obtained by the following equation :
Z=∫^d₀2ρ (x) /1-jωε (x) ε₀ρ (x) ·dx/S
where, d : thickness of stratum corneum
x : distance measured from skin surface to inner tissue
ε (x), ρ (x) : specific dielectric constant ε (x) and resistivity ρ (x) of stratum corneum at point x
ε₀ : dielectric constant of vacuum
S : area of an electrode
(2) The formative mechanism of the frequency dispersion appearing on the average dielectric constant and the resistivity of the stratum corneum is analyzed with the above equation. In this case the existence of two dispersions, that is, the structural dispersion based on the multi-layer configuration and the dispersion of the another type, is made clear. The latter dispersion arises on the basis of the infinitesimal inhomogeneous structure, especially of the cell membranes, at the distance x and is like the so-called α, β dispersions appearing on the live tissues. The frequency characteristics of ε (x), ρ (x) are determined by this dispersion and the large specific dielectric constant appears.
(3) Specific dielectric constants and resistivities at every point in the stratum corneum of a living, human skin are experimentally determined and the multi-layer configuration characteristics of these values are made clear. Resistivities decrease exponentially with the distance from the skin surface, and specific dielectric constants decrease slightly in the beginning and then increase exponentially with the distance from the surface.