Implantable electrodes fall into several subcate-gories. Electrodes that travel across the skin to insert into muscle are percutaneous intramuscular electrodes. They are usually inserted using a hypodermic needle, and have a barbed end to ensure stability once inside the muscle. Percutaneous electrodes have been used primarily in developmental research protocols or temporary applications. Their long-term survival has not been as great as that for implanted electrodes (Memberg et al., 1993). There are also implantable intramuscular electrodes that are more robust than the percutaneous versions, mainly because of their thicker lead wires. Epimysial electrodes are surgically sewn onto the epimysial surface of muscles. They typically have a conductive metallic alloy core or disc and a surrounding sili-cone elastomer skirt. Nerve cuff electrodes directly stimulate nerves by surrounding them circumferen-tially. Implanted cables link the stimulator and the stimulating electrodes and are complex in design to
withstand bending stresses and the body's saline environment. Photographs of several implanted electrodes are found in Fig. 9.1.
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