Biomedical Engineering Reference
In-Depth Information
FIGURE 2-85
Internal electrodes.
[Adapted from
(Bronzino 2006).]
requires that more consideration is taken with the signal processing to compensate for
these shortcomings.
2.5.1.3 Electrodes for Internal Use
Internal electrodes are generally smaller than their surface counterparts and don't require a
gel layer because they are in intimate contact with the organism's own electrolyte solution.
These are either needle electrodes introduced through the skin or cavity electrodes that can
be implanted surgically or through a catheter as illustrated in Figure 2-85. Good examples
of the latter are the electrodes for a cardiac pacemaker, which are introduced into the heart
through a vein and are barbed to ensure that good contact is maintained even after millions
of contractions.
A needle electrode consists of a solid stainless steel needle with a sharp point. An
insulating layer covers most of the needle, exposing only a millimeter or so of the point.
When this device is inserted into skeletal muscle, electrical signals are picked up by the
exposed tip. Needle electrodes can also be made that are inserted through the hollow shaft
of a standard hypodermic needle. These fine electrodes can then remain in place for long
periods of time. Fine coiled-wire electrodes have also remained in skeletal muscles for a
number of years without adverse effects.
Arrays of surface or needle electrodes are used in a number of biomedical prostheses,
including those that are embedded in the retina or directly in the visual cortex as the final
output of visual prostheses or within the cochlea to bypass damaged sections of the ear.
Some of the most impressive of these arrays are those developed at the University of Utah
and shown in Figure 2-86.
Recent advances in electrode design allow complete rice-sized sensors to be implanted
into muscles where they communicate wirelessly with an external receiver. A magnetic
coil surrounding the implant region (usually the stump of an arm) powers the devices
(Weir, Troyk et al., 2009).
