Charge-density safety limit (Shannon criterion)

The charge-density and charge-per-phase safety limit is the empirical boundary separating stimulation parameters that are tissue-safe from those that cause damage. Shannon's widely used formulation plots the logarithm of charge density against the logarithm of charge per phase and finds tissue damage above a line log(D) = k - log(Q), where D is charge density per phase (microcoulombs per square centimeter), Q is charge per phase (microcoulombs), and k is an empirical constant fit to histological damage data (originally from cortical macroelectrodes).

Both quantities matter because damage depends jointly on how much charge is delivered (charge per phase, related to the volume of tissue activated) and how densely it is concentrated (charge density, related to the peak electrochemical stress at the interface). The Shannon boundary is a guideline, not a guarantee: it was derived for particular electrode sizes and pulse regimes, may be conservative or optimistic for microelectrodes and chronic protocols, and does not capture all mechanisms (for example mass-action neuronal hyperactivity or cumulative effects), so device design also respects the electrode's own charge injection capacity and material limits.