Charge-injection limit (water window / Shannon criterion)

The charge-injection limit is the maximum charge per phase an electrode can deliver in a real stimulation pulse before its potential excursion leaves the safe water window (roughly -0.6 to +0.8 V versus Ag/AgCl, beyond which water is reduced or oxidized) and irreversible reactions begin. Unlike CSC it is pulse-specific: it depends on pulse width, waveform, interphase gap, and any DC bias.

For tissue safety two coupled quantities matter, charge per phase and charge density, captured empirically by the Shannon equation, log(D) = k - log(Q), where a lower k marks a more conservative (safer) limit. The practical consequence: shrinking an electrode raises charge density fast, so small high-selectivity contacts reach the safety limit at very little total charge, which is precisely why high-charge materials (iridium oxide, PEDOT, nanostructured coatings) are needed to stimulate at micro scale.