AC impedance of the hydrogel-skin interface

Abstract

An equivalent circuit model ofthe electrode-gelskin interface impedance is described It is shown that the circuit parameters have physical sisnificance and that the study of their variations with time and electrodermal activity furnishes considerable insight into underlying physical processes. As hydrogels are hydrophillic, they adsorb excess moisture from the skin surface. Hydrogel

Volume conductor model of transcutaneous electrical stimulation with kilohertz signals

“…However, after introducing nonlinearity to the deviation factor, α, of the constant phase angle impedance, our model reproduced quite well the experimentally measured motor thresholds (figure 6). Even though we were not able to find experimental studies assessing a direct dependency of α on frequency, we varied it over the experimentally measured range for hydrogel skin electrodes (McAdams et al 1994). In estimating the parameters of the constant phase angle impedance, McAdams et al (1994) assumed that α was frequency-independent.…”

“…Even though we were not able to find experimental studies assessing a direct dependency of α on frequency, we varied it over the experimentally measured range for hydrogel skin electrodes (McAdams et al 1994). In estimating the parameters of the constant phase angle impedance, McAdams et al (1994) assumed that α was frequency-independent. Cantrell et al (2008) also assumed frequency-independent α, but since α shows a dependency on the electrode overpotential for large voltages, it can indirectly depend on frequency.…”

“…, tens of Ω) of that used by Kuhn et al (2009) for square pulses. In the simulation of the Ward and Robertson experiment (2001) (see section 2.5 Model validation), we used the “constant phase angle impedance” model of the electrode-skin interface with the parameters reported by McAdams et al (1994) for hydrogel-based electrodes. In this model, the interface is represented by a large resistance, R p , in parallel with a constant phase angle impedance, Z CPA , so that the admittance of the interface is expressed as: $$Y_{\mathit{\text{interface}}}=\frac{1}{R_p}+\frac{1}{Z_{CPA}}=\frac{1}{R_p}+\frac{{(j\omega )}^{\alpha }}{K}$$ where K is a constant with units Ohm/sec −α , α is a deviation factor from the pure capacitive behavior, and ω is the angular frequency.…”

“…First, we simulated voltage-controlled stimulation using the same 40 × 44 mm electrode of Ward and Robertson (2001), with thicknesses of 1.5 and 2.5 mm for skin and fat, respectively, to represent the forearm (Kuhn et al 2009), and applied two different waveforms: a 10 ms burst of sinusoidal stimulation and a single-cycle sine. We used the constant phase angle impedance model of the electrode-skin interface (McAdams et al 1994) as a representation of nonlinear processes that can occur at the interface. We used a frequency-dependent deviation factor, α, so that α=0.8 for f<2 kHz and α=0.9 otherwise, which is within the experimentally measured range (McAdams et al 1994).…”

“…We used the constant phase angle impedance model of the electrode-skin interface (McAdams et al 1994) as a representation of nonlinear processes that can occur at the interface. We used a frequency-dependent deviation factor, α, so that α=0.8 for f<2 kHz and α=0.9 otherwise, which is within the experimentally measured range (McAdams et al 1994). Lastly, we simulated current-controlled bipolar stimulation using 2 cm diameter disk electrodes placed 3 cm apart, and skin and fat thicknesses of 1.5 and 2.5 mm, respectively, to represent the forearm (Kuhn et al 2009).…”