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4 Measurement of Biosignals and Analog Signal Processing

the body and the measurement electronics. Thus, electrodes have an important role

in the electrical measurement circuit, which is why a basic understanding of the elec-

trochemical processes is required. A more detailed description can be found in [4]. At

the electrode the change of the charge carrier type takes place; from dissolved ions

(electrolyte) to free electrons in the metallic contact surface of the electrode. First the

situation in the thermodynamic equilibrium is considered, i.e. without measuring cur-

rent. The transition between electrolyte and metal represents a electrochemical phase

boundary. The two phases usually have a different chemical potential, which is bal-

anced by charge exchange through the phase boundary. In a metal-electrolyte trans-

ition, the atoms of the metal surface donate electrons to the electrolyte. Positively

charged atoms then remain on the metallic side of the interface. As a result, polarized

water molecules accumulate on the electrolyte side, which, due to their spatial charge

distribution, have an electric dipole moment and are therefore polar. The alignment

of the water molecules causes, that dissolved ions with hydrate shells¹ can attach to

the ion. This arrangement is called Helmholtz layer, an extension of this model Gouy-

Chapman layer. The extent of the boundary layer is only one atomic monolayer on the

metallic side, i.e. approximately 0.1 nm, and on the electrolyte side a few nm up to

several hundred nm at very low electrolyte concentration.

The measurement of electrical biosignals requires charge carrier exchange at the

phase boundary. However, direct contact of the electrode metal with the skin can lead

to problems with electrical contact for several reasons. For example, wetting of the

metal surface with the superficial body fluid is often uneven and highly dependent

on patient movement, or body hair prevents direct contact between metal and skin.

Therefore, an additional electrolytic electrode gel or spray is used as a contact agent.

In disposable adhesive electrodes, the gel is in a cup between the internal electrode

metal and the skin contact area in the center of the adhesive ring (see Figure 4.3, left).

For reusable electrodes such as suction electrodes (cf. Figure 4.3, right), either gel is

first applied to the metal surface or spray is applied to the skin before use. Silver is very

often used as the electrode metal. With external voltage, chemical reactions begin that

allow current to flow across this phase boundary. In one current direction, electrons

from the silver enter the silver chloride phase and dissociate AgCl to form Ag⁺ and Cl⁻

(cathodic reaction), in the other current direction, Ag⁺ and Cl⁻ions associate to form

AgCl (anodic reaction). The current is thus carried in the electrolyte by Cl⁻ions. The

electrode gel or spray has a correspondingly high Cl⁻- concentration.

The electrodes have another metrological significance. The charge exchange at

the phase boundary to establish thermodynamic equilibrium causes charging of the

electrode. The electrode potential depends on the electron affinity of the metal and the

chemical potential of the electrolyte. In principle, both measuring electrodes have the

1 The hydrate shell is the attachment of water molecules around an ion. Due to the polarity of the

water molecules, they arrange themselves directionally around the ion in the form of a shell.