Biomedical Engineering Reference
In-Depth Information
skin with respect to the electrode is then buffered by several millimeters of electrolyte,
with the result that motion artifacts are reduced.
The advantages of this recessed electrolyte design can be realized in a much simpler
design that lends itself to mass production. The electrodes consist of a conductive metal
electrode bonded to a layer of open-cell foam that is impregnated with a high-viscosity
electrolyte gel. Frequently these electrodes are attached to a press stud through an insulated
adhesive disk that holds the electrode in place against the skin.
A modification of this design replaces the electrode with a thin film of foil that can
deform for a better fit on a curved body. The sponge can then be replaced with a thinner
hydrogel film saturated with an electrolyte gel. This electrolyte-hydrogel mix is very
sticky, so no further adhesive is required.
Very thin electrode films can be used in place of the foil electrodes if they are backed
by a strong polymer film. The advantage of these is that if the metal film is thin enough
(typically 1
m or less) they are x-ray transparent. This is particularly useful for monitoring
premature babies with sensitive skins, as the repeated application and removal of electrodes
can be a severe skin irritant (Bronzino, 2006). Some of these electrode types are shown in
Figure 2-84.
Special problems encountered in monitoring the ECGs of astronauts during long
durations in space and under conditions of perspiration and considerable movement led
to the development of spray-on electrodes. In these, a small spot of conductive material
is sprayed or painted directly on the skin, which has previously been treated with an
electrolyte coating (Cromwell, Weibell et al., 1973).
Electrodes that do not use externally applied electrolyte gels are known as
dry
. They
can easily be applied and held in place using a rubber band or tape. They are made from
graphite or metal-powder-impregnated silicone rubber to produce a flexible conductive
material similar to those discussed in the section on tactile sensors. These are applied
directly to the skin, and the electrolyte layer is formed by a film of sweat that collects
under the contact area.
Dry electrodes are used in home medical monitoring devices and on consumer goods
like exercise bikes and treadmills to pick up an ECG signal. The signals picked up by
these electrodes are generally much noisier than those obtained from the wet types. This
μ
FIGURE 2-84
Electrodes for
chronic patient
monitoring.
[Adapted from
(Bronzino 2006).]
