Human Skin and Keratinized Tissue

Epithelia are cells organized as layers, and skin is an example. Cells in epitheUa do form gap junctions. In particularly tight membranes, these junctions are special tight junctions. The transmembrane admittance is dependent on the type of cell junctions and to what extent the epithelium is shunted by channels or specialized organs (e.g., sweat ducts in the skin). 

The impedance of the skin is dominated by the SC at low frequencies. It has generally been stated that skin impedance is determined mainly by the SC at frequencies below 10 kHz and by the viable skin at higher frequencies (Ackmann and Seitz, 1984). This will of course be dependent on factors such as skin hydration, electrode size and geometry etc., but it may nevertheless serve as a rough guideline. A finite element simulation on a concentric two-electrode system used by Yamamoto et al. (1986) showed that the SC accounted for approximately 50% of the measured skin impedance at 10 kHz but only approximately 10% at 100 kHz (Martinsen et al., 1999). 

An open question is whether the conductance in SC in addition to the ionic component also has an electronic component (e.g., as a semiconductor). 

Yamamoto and Yamamoto (1976) measured skin impedance on the ventral side of the forearm with a two-electrode system and an AC bridge. They used Beckman silver/silver chloride electrodes filled with gel and measured 30 min after the electrodes had been applied. The skin was stripped with cellulose tape 15 times, after which the entire SC was believed to have been removed. Impedance measurements were also performed between each stripping so that the impedance of the removed layers could be calculated. The thickness of the SC was found to be 40 pm, which is more than the common average values found elsewhere in the literature. For example, Therkildsen et al. (1998) found a mean thickness of 13.3 pm (minimum 8 pm/maximum 22 pm) when analyzing 57 samples from nonfriction skin sites on Caucasian volunteers. However, the moisture increase caused by electrode occlusion and electrode gel has most certainly significantly increased the SC thickness. 

The calculated data from Yamamoto and Yamamoto (1976) are presented in Figiure 4.16. 

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