Electrodes

Created Tuesday 25 February 2014

To record myoelectrical signals Ag-AgCl electrodes can be used. These are made of silver with a thin layer of silver-chloride on the surface. A discussion of the features of these electrodes can be found in [Webster 1992]. The electrodes used in this project are Ag-AgCl electrodes. See. Recording Electrode Characteristics

The type of electrodes used for stimulation and recording in the project has been chosen since they are easy available, inexpensive, easy to use and fulfilled the demands. The electrode types are very commonly used and could be supplied by second souce. Stimulation electrodes have different characteristics than recording electrodes

Motion Artefacts

The electrical conditions of an electrode are very complex. A simplified model can be seen below [ref webster]. Especially the half-cell potential (the potential occurring from the dissolution of metal ions into the electrolyte) is significant for the recording electrodes. This potential depends on the equilibrium of the different ions.

electrodemodel.png

When the skin or the recording electrode is stressed the electrolytic equilibrium is shifted and thus the half-cell potentials are changed [Webster 1992]. This may be the case if, for instance, the electrode is tapped, twisted or changing shear forces are applied. These actions will disturb the equilibrium states of the electrode-electrolyte, electrolyte-skin and the electrolytes in the epidermis. If the reference electrode does not experience the exact same changes, then the result will be a rapid change in the potential between the electrodes. This signal is termed motion artefacts. The shape of the motion artefacts is dependent on the mechanical action on the electrodes, the type of electrodes and the involved electrolytes. The motion artefacts will be occurring at random times depending on the conditions under which the electrodes are used. Stretching the skin can give rise to up to 10mV motion artefacts [Webster 1984], which should be compared to a typical peak value of 1mV in myoelectric signals.

Due to the stochastic nature of the myoelectrical signal it is believed that it is not easy to filter out the motion artefacts. For this reason the electrodes must be protected from mechanical influences and stretching of the skin must be avoided.