Electrodes

Wet Gel Or Paste With Electrolytes

The mechanical or viscous properties of the contact medium are important, and often the electrolyte is thickened by a gel substance or contained in a sponge or soft clothing. Commercial electrocardiogram (ECG) electrodes are often delivered as pregelled devices for single use, and the medium may contain preservatives to increase storage life, or quartz particles for abrading purposes on the skin.

Generally, the ionic mobility and therefore the conductivity in a high viscosity paste are lower than in a liquid. Wet electrolytes of high concentration (>1%) penetrate the skin actively, with a time constant often quoted to be of the order of 10 min (Tregear, 1966; Almasi et al., 1970; McAdams et al., 1991b). However, actually the process is not exponential (as diffusion processes are not), and may go on for hours and days (Grimnes, 1983a) (see Figure 4.20). The penetration is stronger the higher the electrolyte concentration, but also more skin irritating. NaCl is better tolerated by human skin at high concentration than most other electrolytes. Figure 7.5 shows the electrolyte penetration into the skin the first 4 h after electrode onset to the skin. Impedance at 1 Hz is dominated by stratum corneum electrolyte content, with less than 1% contribution from the electrode’s own small-signal polarizing impedance. If sweat ducts are filled or have recently been filled, the conductance of the ducts shunts the high impedance of the dry stratum corneum.

The conductivity σ of some often used contact creams/pastas are: Redux creme (Hewlett Packard) 10.6 S/m, Electrode creme (Grass) 3.3 S/m, Beckman-Offner paste 17 S/m, NASA Flight paste 7.7 S/m, and NASA electrode cream 1.2 S/m. NASA Flight paste contains 9% NaCl, 3% potassium chloride (KCl), and 3% calcium chloride (CaCl), in total 15% (by weight) of electrolytes. Thick electroencephalogram (EEG) paste may contain as much as 45% KCl.

In comparison, 0.9% NaCl (by weight) physiological saline solution has a conductivity of 1.4 S/m; most gels are therefore strong electrolytes. Seawater contains about 3.5% salts, and the Dead Sea contains >25% salts with a composition of 50% MgCl2, 30% NaCl, 14% CaCl2, and 6% KCl. That is quite different from seawater salt (NaCl 97% of total salt content). The Dead Sea is called “dead” because its high salinity prevents plants and fish from living there.

Experience has shown that the stronger the gel, the quicker the penetration into the skin and sweat ducts. However, skin reactions such as skin irritation and reddening are also faster. For a quick ECG examination, stronger gels can be used; for monitoring during days, the contact gel must be weak. Most people appreciate hours of bathing in seawater, so a salt content of 3.5% should in many cases be acceptable.

For electrodermal activity (Chapter 10.3), a contact wet gel must have a low salt content to assure rapid emptying of the ducts.