Stimulators with a pulsatile current (PC) output typically deliver either monophasic or biphasic waveforms. Monophasic waveforms produce unidirectional current flow and thus cause a net movement of charged ions across the electrode/tissue interface. In contrast, biphasic waveforms are characterized by very rapid reversal of stimulus polarity. The absence of charged ion movement makes the symmetric biphasic waveform preferred because it minimizes skin irritation and is perceived as being more comfortable (Baker et al., 1988; Kantor et al., 1994; Laufer et al., 2001; Bennie et al., 2002). Mono-phasic stimulation is also less desirable for long-term implanted FES use, since it can cause electrode and tissue breakdown (Mortimer, 1981). Waveforms can be either constant voltage or constant current (Kantor et al., 1994; Alon et al., 1996). Constant current waveforms are used primarily (although not exclusively) in FES systems.
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