Charge injection capacity (CIC)

The charge injection capacity is the maximum charge per unit electrode area that can be delivered in a single stimulation phase without driving the electrode potential outside the safe (water) window, beyond which irreversible Faradaic reactions occur. It is expressed in millicoulombs per square centimeter and depends on the material, the pulse width, the electrode geometry, and whether a bias is applied; it is measured by monitoring the electrode's polarization during a current pulse.

CIC sets the fundamental scaling problem of stimulation: because it is a charge density, smaller electrodes (desirable for spatial selectivity) can inject less total charge before reaching the limit, so high-selectivity microstimulation and high-charge therapeutic stimulation pull in opposite directions. Representative values span roughly 0.05 mC/cm^2 for smooth platinum up to a few mC/cm^2 for activated iridium oxide or PEDOT coatings, which is why coating chemistry is decisive for whether an electrode can both be small and stimulate effectively.