Biphasic pulses are often used to electrically stimulate nerve tissue because of safety considerations, because they are charge balanced.
Monophasic pulses are generally more effective than biphasic ones to stimulate nerve tissue. Yet, monophasic pulses lead to a build up of charge that can eventually lead to tissue damage, e.g., to the delicate inner ear structures in case of cochlear implants. Biphasic pulses have the advantage that charge doesn't build up, because the net current injection per pulse is zero. However, often one phase is excitatory, and the other not or less so, making biphasic pulses less effective than monophasic ones. Therefore, in cochlear implant research, people are playing around with asymmetric pulses (pseudomonophasic pulses) where one pulse is short and of high amplitude, and the other phase long and of low amplitude) and/or with biphasic pulses with an interphase gap to make the first phase more effective. Another way is to play around with triphasic pulses where one large excitatory phase is used, and 2 adjacent shorter phases of opposite polarity (Fig. 1).
Fig. 1. Different pulse shapes used in cochlear implants. SYM: symmetric; C: cathodic first; A: anodic-first; RPS: reversed pseudomonophasic; IPG: interphase gap; TRI: tripolar. source: (Machery & Cazals, 2016)
- Macherey & Cazals (2016). In: Van Dijk et al. (eds). Physiology, Psychoacoustics and Cognition in Normal and Impaired Hearing. Advances in Experimental Medicine and Biology (2016); 894. Springer, Cham