Capacitive versus Faradaic Microelectrodes for Extracellular Stimulation: A Fully Coupled FEM–Hodgkin–Huxley Study of Thresholds and Current Redistribution

ABSTRACT Extracellular microstimulation depends on neuronal excitability and on how current enters the electrolyte through the electrode–electrolyte interface and the cell–electrode cleft. Here we compare two ideal interface limits—capacitive (polarizable) and Faradaic (non‐polarizable)—using a fully coupled finite‐element model linked to a Hodgkin–Huxley neuron with an explicit axon initial segment. Using square current pulses, we quantify activation thresholds as the charge delivered per unit electrode area. We consider 10 µm (AIS‐aligned) and 100 µm (soma‐aligned) disk electrodes and vary the cleft gap from 100 nm to 2 µm, spanning typical adherent multielectrode‐array conditions. Across all geometries, capacitive interfaces require less charge density to trigger spikes than Faradaic interfaces, with the advantage increasing in tighter clefts. Time‐resolved maps show that both interfaces initially exhibit edge crowding; however, capacitive charging locally increases interfacial impedance and redistributes current toward regions beneath the cell, whereas Faradaic contacts remain near‐equipotential and sustain an edge‐dominated pattern. All operating points fall below conservative Shannon safety limits. These results clarify when capacitive microelectrodes can outperform Faradaic ones under current control and provide guidance for MEA design.