Iridium oxide electrode (SIROF / AIROF)

Iridium oxide is the benchmark faradaic material for charge injection. It stores and injects charge through a fast, reversible Ir3+/Ir4+ redox transition in a hydrated oxide, giving among the highest reversible charge-injection capacities of any neural material while staying within safe electrochemical bounds. Two forms dominate: activated iridium oxide film (AIROF), grown by electrochemical cycling of iridium metal, and sputtered iridium oxide film (SIROF), deposited by reactive sputtering with better mechanical robustness and manufacturability.

Because injection is faradaic (a genuine surface redox reaction) rather than purely capacitive, iridium oxide can deliver large charge per pulse at modest voltage, valuable for stimulation electrodes in visual and other prostheses. Caveats: charge-injection capacity depends strongly on pulse waveform, bias, and pulse width; AIROF can be less stable under aggressive chronic stimulation; and, as with all faradaic materials, staying reversible with no net electrolysis or dissolution is what keeps it safe.