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 $Y_{\mathit{cpe}}=Y_0{(j\omega )}^n$, 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