Fig. 9

Composite of the electrical equivalent model of electrode–skin-underlying tissue. a Electrode–skin-underlying tissue composite. b Equivalent model of electrode–skin-underlying tissue. The electrode–conductive gel interface is represented by the resistor \( R_{\text{eg}} \) in series with a parallel combination of a constant phase element (CPE) \( Y_{\text{ei}} \) and a resistor \( R_{\text{ei}} \). The stratum corneum is represented by a resistor \( R_{\text{sc}} \) in series with a parallel combination of a CPE \( Y_{\text{sd}} \) and a resistor \( R_{\text{sd}} \). The underlying (living) tissue is modeled by a resistor \( R_{\text{t}} \). c The simplified Cole system electrical equivalent model of electrode–skin interface because the impedance of the electrode–conductive gel interface is much smaller than that of the stratum corneum [20]. This model is represented by a resistor \( R_{s} \) in series with a parallel combination of a CPE and a resistor \( R_{d} \). \( R_{s} \) is associated with the resistance of the wire, conductive gel, and sweat. CPE and \( R_{d} \) are determined by the property of the stratum corneum between the metal electrode and the underlying tissue. CPE is the capacitance of the stratum corneum denoted by \( Y_{cpe} = Y_{0} (j\omega )^{n} \), where \( \omega \) is the angular velocity, \( Y_{0} \) is the magnitude of CPE at \( \omega = 1 \), n is a constant, j is the imaginary unit \( \sqrt { - 1} \), and \( R_{d} \) is the resistance of the stratum corneum