Required matching accuracy of biphasic current pulse in multi-channel current mode bipolar stimulation for safety

In neural stimulation, a current mode stimulation is preferred to a voltage mode stimulation, as it has more control over injecting charge into tissue. A matched biphasic current pulse is often employed in current mode stimulation. For safe neural stimulation, in other words, to ensure zero-net charge transfer (charge balance) into tissue, it is required to utilise a precisely matched biphasic current pulse. Mismatch in the biphasic current pulse causes residual charge on stimulating electrodes during stimulation, which will induce DC current flowing into tissue, possibly leading to tissue damage. In this paper, we derive mathematical expressions of the required matching accuracy on the biphasic current pulse under 4 different situations to ensure a safe neural stimulation; 1) single channel stimulation without shorting, 2) single channel stimulation with shorting, 3) multi-channel stimulation without shorting and 4) multi-channel stimulation with shorting.