Biomedical Engineering Reference
In-Depth Information
The mechanism of electrical conductivity in the body involves ions as charge car-
riers. Therefore, picking up bioelectric signals involves converting the ionic currents
into electric currents that will flow through wires. This conversion process is carried
out by electrodes that consist of electrical conductors in contact with the aqueous ionic
(electrolyte) solutions in the body. At the interface between the electrode and the
electrolyte solution, an electrochemical reaction needs to take place for a charge to be
transferred.
When no current is flowing between the electrode and the organism, a potential, known
as the half-cell potential, exists across the boundary. If current flows, then the potential
may drop, an effect known as polarization, and the sensitivity of the electrode may be
reduced.
Polarizable electrodes pass a current between the electrode and the electrolytic solution
by changing the charge distribution in the solution near the electrode. No actual current
crosses the electrode-electrolyte interface. Nonpolarized electrodes allow current to pass
freely across the interface without changing the charge distribution in the electrolytic
solution.
Electrodes made from the noble metals such as platinum are highly polarizable, and
the charge distribution in the electrolyte adjacent the electrode is different from that
of the bulk. This limits the ability of such electrodes to measure DC or low-frequency
signals. In addition, if the electrode moves with respect to the electrolyte solution, the
charge distribution in the solution adjacent to the electrode surface will change, which
will generate a voltage change that will appear as a motion artifact in the measure-
ment. For these reasons, nonpolarizable electrodes are preferred for most biomedical
measurements.
The silver-silver chloride electrode is a good example of a nonpolarizable electrode
that is suitable for many biomechanical applications. Such electrodes have a silver base
with a silver-silver chloride matrix on the surface. They do not polarize and thus are more
capable of conducting low-frequency and DC signals and are also less prone to generating
motion artifacts.
A simplified equivalent circuit of an electrode is a parallel resistor and capacitor
with the resistor describing the DC and low-frequency impedance of the electrode and the
capacitor describing the higher-frequency AC component. Typical surface electrodes have
impedances of between 2 and 10 k
, with larger electrodes having lower impedances and
needle or microelectrodes having much higher values (Cromwell, Weibell et al., 1973).
2.5.1 Body-Surface Biopotential Electrodes
This category of electrodes includes those that can be placed on the body surface for
recording bioelectric signals. The integrity of the skin is not compromised, and they
can be used for short- or long-duration applications. The earliest bioelectric potential
measurements relied on immersion electrodes that were simply buckets of saline solution
into which the patient placed a hand and a foot, as shown in Figure 2-82. As expected,
this type of electrode presented many difficulties in regard to restrictions on the patient
position and movement as well as the possibility of spillage.
Plate electrodes, first introduced in 1917, were a great improvement on immersion
electrodes. They were originally separated from the skin by cotton pads soaked in saline
to emulate the immersion electrode mechanism. Later, an electrolytic paste was used in
place of the pad with the metal in contact with the skin.
