Electron transfer processes occurring on platinum neural stimulating electrodes: pulsing experiments for cathodic-first, charge-imbalanced, biphasic pulses for 0.566 ⩽ k ⩽ 2.3 in rat subcutaneous tissues

Abstract

Objective. Charge injection through platinum neural stimulation electrodes is often constrained by the Shannon limit (Shannon 1992 IEEE Trans. Biomed. Eng. 39 424–6) of k = 1.75. By leveraging the tools of electrochemistry to better understand the reactions at electrode-tissue interface, we endeavor to find a way to safely inject more charge than allowed if the traditional Shannon limit were followed. Approach. In previous studies on platinum electrodes using charge-balanced, cathodic-first, biphasic pulses, we noted that during the secondary anodic phase, the electrode potential moves into a range where platinum dissolution is possible when charge injection is greater than k = 1.75. Platinum dissolution products are known to be toxic to brain tissues. We hypothesize that by injecting less charge in the anodic phase than the cathodic phase, the anodic potential excursions will decrease, thereby avoiding potentials where platinum dissolution is more likely. Main results. Our findings show that using these charge-imbalanced pulses decreases the anodic potential excursions to a level where platinum oxidation and dissolution are less likely, and aligns the anodic potentials with those observed with charge-balanced stimulation at k < 1.75—a range widely accepted as safe for stimulation with platinum. Significance. From these results, we further hypothesize that charge-imbalanced biphasic stimulation would permit more charge to be safely injected through platinum electrodes than would be permitted if the dogma of charge-balanced biphasic stimuli were followed. Testing this hypothesis in cat brain in the same manner as the experiments that formed the basis for the Shannon plot could open the door for safe charge injection through platinum electrodes at levels greater than k = 1.75.