Biomedical Engineering Reference
In-Depth Information
10.2.4 EEG Electrodes
The most commonly used electrodes for recording electroencephalographic signals from
the brain (EEG) are cup electrodes and subdermal needle electrodes. Cup electrodes are made
of platinum or tin approximately 5-10 mm in diameter. These cup electrodes are filled with a
conducting electrolyte gel and can be attached to the scalp with an adhesive tape.
Recording of electrical potentials from the scalp is difficult because hair and oily skin
impede good electrical contact. Therefore, clinicians sometimes prefer to use subdermal
EEG electrodes instead of metal surface electrodes for EEG recording. These are basically
fine platinum or stainless-steel needle electrodes about 10 mm long by 0.5 mm wide, which
are inserted under the skin to provide a better electrical contact.
10.2.5 Microelectrodes
Microelectrodes are biopotential electrodes with an ultrafine tapered tip that can be
inserted into individual biological cells. These electrodes serve an important role in record-
ing action potentials from single cells and are commonly used in neurophysiological studies.
The tip of these electrodes must be small with respect to the dimensions of the biological
cell to avoid cell damage and at the same time sufficiently strong to penetrate the cell wall.
Figure 10.8 illustrates the construction of three typical types of microelectrodes: glass micro-
pipettes, metal microelectrodes, and solid-state microprobes.
In Figure 10.8a, a hollow glass capillary tube, typically 1 mm in diameter, is heated and
softened in the middle inside a small furnace and then quickly pulled apart from both ends.
This process creates two similar microelectrodes with an open tip that has a diameter on the
FIGURE 10.8 Biopotential microelectrodes: (a) a capillary glass microelectrode, (b) an insulated metal micro-
electrode, and (c) a solid-state multisite recording microelectrode.
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