Non-linear electrical properties of skin in the low frequency range

A model for human skin impedance during surface functional neuromuscular stimulation

“…1(b) have been proposed and validated [16]. As a result the qualitative properties of each component in this linear system model have previously been identified [2], [16], [31]. However, it has long been known that the impedance of skin in response to dc current pulses displays a variety of complex nonlinear behaviors which this linear model is unable to replicate [2], [16].…”

“…While the characteristics of the skin's electrical impedance have been the subject of study under a variety of conditions [2], [15], [22], [29], the phenomena exhibited are still far from being well understood. Unlike muscular tissue resistivity, which has a linear characteristic for sinusoidal current densities below 2 mAcm 2 in the frequency range 10 Hz to 10 kHz [24], [27], [31], skin exhibits a number of amplitude and frequency dependent nonlinear effects. Many investigations have been performed to date to characterize the nonlinear dependence of skin's impedance during sinusoidal AC stimulation on both the frequency and amplitude of activation [6], [31], [32].…”

“…Unlike muscular tissue resistivity, which has a linear characteristic for sinusoidal current densities below 2 mAcm 2 in the frequency range 10 Hz to 10 kHz [24], [27], [31], skin exhibits a number of amplitude and frequency dependent nonlinear effects. Many investigations have been performed to date to characterize the nonlinear dependence of skin's impedance during sinusoidal AC stimulation on both the frequency and amplitude of activation [6], [31], [32]. As a result, models for the skin's AC impedance have been developed [15], [19], [23].…”

“…Unfortunately, while both rigorous and comprehensive, the results of these AC investigations are of limited relevance to FNS applications, where the stimulation pulses applied transcutaneously are typically of lower frequencies, higher amplitudes and rectangular in shape. This is particularly significant as it has been shown that the skin's nonlinearities are most pronounced when using high currents at low frequencies, just as is the case in transcutaneous FNS [18], [19], [31], [32].…”

“…1. The resistor primarily attempts to model the resistance of deep tissues, while the parallel combination of capacitor and linear resistor , is intended to model the skin impedance [2], [16], [31]. A series capacitance and resistance modeling the electrode-skin interface may also be included.…”