Implantable cardiac pacemakers have been around since the late 1950s. More recently, the same basic techniques have been applied to stimulate the vagus nerve for the control of epilepsy, to stimulate the sacral roots to control the bladder and correct erectile dysfunction, and to stimulate nerves in the spine for the control of pain and angina. In addition, interest in functional electrical stimulation (FES) has grown rapidly during recent years, due primarily to progress made in miniaturized hardware that makes multichannel stimulators possible. New surgical techniques enable the use of chronically implanted stimulators to stimulate specific nerves and brain sections directly within the body, making it possible to restore function lost due to disease or trauma. Advances are being made rapidly in the development of implants for restoring limbs, sight (e.g., through artificial retinas or by direct stimulation of the visual cortex), and hearing (e.g., through cochlear implants) [Loeb, 1989].
As shown in Table 7.1, and with the exception of cardiac defibrillation, all other applications in which the electrodes are placed in close contact with target tissue require the delivery of relatively narrow pulses (<2ms) of low voltage (<12 V) at low current (<35mA). These can easily be produced with miniature circuits that use standard bipolar or MOSFET transistors (discrete transistors or as part of an IC), tantalum capacitors, and implantable-grade lithium batteries. Implantable stimulators typically use either a constant-current source or a capacitor discharge circuit as output stages to generate stimulation pulses.
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