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2.7. Electrochemical electrode
Biopotential measurements are made using different kinds of specialized electrochemical
electrodes. The function of electrodes is to couple the ionic potentials generated inside the body
to an electronic instrument. Biopotential electrochemical electrode is classified either as
noninvasive (e.g. skin surface) or invasive (e.g. microelectrode, wire electrode) electrodes.
When a metal is placed in an electrolyte solution, a charge distribution is created next to the
metal/electrolyte solution interface as illustrated in figure 11. The localized charge distribution
causes an electronic potential by electrochemical electrode, called half-cell potential, to be
developed across the interface between metal electrode and electrolyte solution.
The half-cell potentials of several important metals are listed in table 2. Here, a point needs to
be pointed out that hydrogen electrode is considered to be a standard electrode against which
the half-cell potentials of other metal electrodes are measured.
Figure 11. Sensing principle of indirect blood pressure measurement
Biomedical Sensor, Device and Measurement Systems
http://dx.doi.org/10.5772/59941
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Silver and zinc electrodes are immersed in an electrolyte solution. And then we may calculate
the potential drop between two electrodes. From table 2, the half-cell potentials for silver and
zinc electrodes are 0.799V and -0.763V respectively. Hence, the half-cell potentials between
two electrodes are equal to the following value:
0.799-(-0.763) =1.562V
Typically, utilizing the electrochemical electrodes composed of the same metals could measure
the half-cell potentials. Hence, the two half-cell potentials for these electrodes would be equal
in magnitude. Some common electrodes are introduced here, which is utilized as a sensor.
2.7.1. ECG electrodes
A typical flexible biopotential electrode for ECG (electrocardiogram, ECG) recording is
composed of certain polymers or elastomers which are made electrically conductive by the
addition of a fine carbon or metal powder. These electrodes as illustrated in figure 13a are
available with prepasted AgCl gel for quick easy application to the skin using a double-sided
peel-off adhesive tape. The most common type of biopotential electrode is the silver/silver
chloride electrode (Ag/AgCl), which is formed by electrochemically depositing a very thin
layer silver chloride onto the surface of silver electrode as illustrated in figure 13b. These
electrodes are recessed and imbedded in foam that has been soaked with an electrolyte paste
to provide good electrical contact with the skin. The electrolyte saturated foam is also known
to reduce motion artifacts which are produced during stress testing when the layer of the skin
moves relative to the surface of the Ag/AgCl electrode. This motion leads to the large inter‐
ference in the recorded biopotential and, in the extreme cases, could severely degrade the
measurement.
Figure 12. The charge distribution at a electrolyte/metal interface
Advances in Bioengineering
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