Dynamic Impedance Model of the Skin-Electrode Interface for Transcutaneous Electrical Stimulation

Abstract

Transcutaneous electrical stimulation can depolarize nerve or muscle cells applying impulses through electrodes attached on the skin. For these applications, the electrode-skin impedance is an important factor which influences effectiveness. Various models describe the interface using constant or current-depending resistive-capacitive equivalent circuit. Here, we develop a dynamic impedance model valid for a wide range stimulation intensities. The model considers electroporation and charge-dependent effects to describe the impedance variation, which allows to describe high-charge pulses. The parameters were adjusted based on rectangular, biphasic stimulation pulses generated by a stimulator, providing optionally current or voltage-controlled impulses, and applied through electrodes of different sizes. Both control methods deliver a different electrical field to the tissue, which is constant throughout the impulse duration for current-controlled mode or have a very current peak for voltage-controlled. The results show a predominant dependence in the current intensity in the case of both stimulation techniques that allows to keep a simple model. A verification simulation using the proposed dynamic model shows coefficient of determination of around 0.99 in both stimulation types. The presented method for fitting electrode-skin impedance can be simple extended to other stimulation waveforms and electrode configuration. Therefore, it can be embedded in optimization algorithms for designing electrical stimulation applications even for pulses with high charges and high current spikes.

Sub-threshold depolarizing pre-pulses can enhance the efficiency of biphasic stimuli in transcutaneous neuromuscular electrical stimulation

“…In addition, computer simulations suggest that the spatial position of the current source in relation to the Ranvier nodes strongly affects the local transient changes in excitability by DPPs [ 5 ]. Finally, transcutaneous electrical stimulation is associated with a high voltage drop across the cutaneous and subcutaneous tissue, which has the highest electrical impedance [ 25 ]. On the other hand, on deeper nerve fibers like tibial nerve, the electrical field appears strongly attenuated or, in another word, “weaker.” In most studies that report a reduction of excitability by DPPs, the stimulation electrodes have been placed close to the targeted fibers, e.g., cuff electrodes encircling the epineurium, or transcutaneous stimulation over the cutaneous sensory nerves, which induce a local “strong” electrical field.…”

Nerve excitation using an amplitude-modulated signal with kilohertz-frequency carrier and non-zero offset

“…In their animal studies, Shen et al [ 4 ] used voltage-controlled TES, and therefore voltage- versus current-controlled stimulation may at least partially explain the purported benefits of TAMS for bladder control by transcutaneous pudendal nerve stimulation. For voltage-controlled stimulation, the current amplitude that activates the nerve fibers depends on the frequency of stimulation because the impedance of the electrode-skin interface, which also depends on time, determines the voltage drop across the interface [ 21 ]. In contrast, for current-controlled stimulation, the amplitude of the current delivered to the tissue does not depend on frequency.…”

Optimization of Interphase Intervals to Enhance the Evoked Muscular Responses of Transcutaneous Neuromuscular Electrical Stimulation

“…In this sense, the introduction of an IPI may allow the fibers to come back to the rest condition and, therefore, recover their normal excitability. Additionally, previous papers show that the injection of charge during the anodic phase reduces the skin-electrode impedance, which may enhance the neuromuscular response to the cathodic phase (20,21).…”