Faradaic vs capacitive charge injection

Electrodes move charge across the tissue interface by two mechanisms. Capacitive (non-faradaic) injection charges and discharges the electrical double layer with no electron transfer or chemical change, so it is inherently reversible and safe but limited in charge. Faradaic injection transfers electrons via a redox reaction; if the reaction is fast and reversible (surface-bound, like iridium oxide's Ir3+/Ir4+), it delivers far more charge safely, but if it drives irreversible reactions (water electrolysis, metal dissolution, gas evolution) it damages tissue and electrode alike.

Practical materials sit on a spectrum: titanium nitride and porous carbons are largely capacitive; iridium oxide and PEDOT are pseudocapacitive or reversibly faradaic; smooth platinum mixes a little reversible faradaic chemistry with double-layer charging. Safe stimulation design is fundamentally about staying in the reversible regime, using charge-balanced biphasic pulses and staying inside the water window so no net irreversible reaction accumulates.