Characteristics of phase angle, extrapolated conductance and ion relaxation time in the low-frequency admittance locus plot of human palmar skin.

The Cole-Cole equation can be used to model the electrical admittance of human skin. Using a previously published two-frequency (AC excitation) three-electrode method, the admittance locus plot (ALP) in the low-frequency region (< 1000 Hz) has been shown to be very well approximated by a straight line and can be described with frequency-independent parameters; phase angle alpha pi/2, conductance at extrapolated zero frequency G0 = 1/R0, and ion relaxation time tau. The aim of the paper is to study characteristics of alpha pi/2, G0 and tau in the low-frequency ALP of human palmar skin. The measured data are calculated off-line on a personal computer. Relative changes in alpha pi/2 are small (< 2%) even during a heavy physical-exercise period. G0 corresponds well with GDC obtained by the traditional DC method at excitations up to 200 mV (relative differences < or = 5% during baseline registration). Based on these findings, two comparatively simpler methods using only one lock-in amplifier are suggested: either measuring simultaneously with DC and at one frequency with AC excitation, or measuring at two different frequencies successively. The low-frequency part of the ALP can then be determined. G0, alpha and tau estimated by these simplified methods are in good agreement with the simultaneous two-frequency method and should therefore be suited for clinical applications.