Wearable FES Electrodes

Functional electrical stimulation (FES) has been used to revitalise the muscles of people suffering from various kinds of injury. However, when human skin is incorporated into electrical circuits, it must not be treated as a passive component. Skin’s electrical properties must be known when electrodes deliver electrical stimulation to the body, whether by hydrogel electrodes or by electrodes embedded in apparel. Failure to address this issue increases the risk of skin burns due to too high current through the skin/electrode interface. We have demonstrated that there is a relationship between electrode size and measured voltage. The rise of voltage with a reduction of electrode size can be explained by the diminution of the skin contact area with resulting higher skin/electrode impedances. Thus, finding an electrical skin model that represents the behaviour of human skin is important for circuit design and the product development process.


Introduction
E-textiles have been used to achieve a great variety of functional products. Developing textiles with embedded electrodes to stimulate human muscles requires a clear understanding of the interactions between skin and the electrodes (along with the type of stimulation, the size and type of electrodes, and the muscles that will receive electrical stimulation).
The experimental work reported here leads to a greater understanding of the effects of electrode size, and responses to the challenge of determining the electrical impedance of the skin/electrode interface. The aim has been to assess the viability of replacing hydrogel electrodes with screen-printed wearable electrodes. The viability of interest relates to the performance of the electrodes during stimulation treatments, but also the consistency of operation to ensure the safety of individuals experiencing the stimulation.
The experimental approach has been to use a Wheatstone Bridge electrical circuit incorporating a human arm with two attached electrodes [1]. This approach allows the determination of a balance between the skin/electrode system and an equivalent electrical circuit (the electrical skin model). This is when there is zero voltage across the Wheatstone Bridge. It is widely recognized that skin has capacitive properties as well as resistive properties, and both capacitor and resistor values are dependent of the condition of the skin. This inherent variability means that achieving a balance is non-trivial, and there were occasions in our experimental work where the balance condition was elusive.

The Commercial Stimulator
The instrument used for all the tests undertaken was the TENS portable EV906A, as illustrated in Figure 1. This unit provides asymmetrical bi-phasic square pulses, with the pulse rate adjustable from 2 to 150 Hz, and the pulse width adjustable from 50 to 300 µs, with variable pulse amplitude.

Measurement of Skin/Electrode Impedance Using a Wheatstone Bridge
The circuit, based on a publication by Lawler et al. [1] and illustrated in Figure 2, balances the impedance of a human arm fitted with two electrodes and a skin/electrode model represented by resistors and capacitors. An oscilloscope was used to measure the voltages and waveforms applied to the bridge as well as the voltages and waveforms across the bridge.

Relationship between Electrode Size and Voltage
Reducing the electrode size is of interest when preferentially targeting specific muscles rather than stimulating many. Using a gel electrode, the area of contact of the anode was progressively reduced to obtain the results shown in Figure 3. The output of the TENS device was kept constant, and the voltages across the electrodes were recorded. The rise of the voltage with the reduction of the electrode size can be explained by the diminution of the skin contact area with the reduction of the electrode area leading to higher electrical resistances.

Testing Textile Electrodes
Preliminary tests were undertaken to compare the behaviour of screen printed electrodes (as illustrated in Figure 4) to gel electrodes for different electrode sizes. These tests kept the TENS settings constant, and recorded voltages across the electrodes ( Table 1). The compression exerted by the test fabric was 5 mmHg.  The conclusion that can be drawn from such experiments is that the two types of electrodes have similar behaviours and performances.

Impedance Values for the Skin/Electrodes Assembly
Using the Wheatstone Bridge circuit, the values of the resistors and capacitors were recorded and used to calculate the impedance of the skin/electrode assembly.