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

Objective. Our mission is twofold: (1) find a way to safely inject more charge through platinum electrodes than the Shannon limit (k = 1.75) permits and (2) nurture an interest in the neural stimulation community to understand the electron transfer process occurring on neural stimulating electrodes. Approach. We report here on measurements of the electrode potential, performed on platinum neural stimulating electrodes in the subcutaneous space of an anesthetized rat under neural stimulation conditions. Main results. The results for six platinum electrodes with areas ranging from 0.2 mm2 to 12.7 mm2 were similar to prior results in sulfuric acid, except that the measured potentials were shifted negative 0.36 V because of the pH difference between the two media. The anodic 'end' potential, measured at t = 20 ms after the onset of the biphasic current pulse, was the primary focus of the data collected because previous results had shown that as charge injection crosses the Shannon limit (k = 1.75), this potential moves into a range where platinum surface oxidation and dissolution is likely to occur. The behavior of Ve(t = 20 ms) over a range of electrode surface areas studied was consistent with our sulfuric acid study. Implicit, but little noticed, in Shannon's formulation is that small and large platinum electrodes behave the same in terms of k value; our data supports this idea. Significance. We hypothesize that the k = 1.75 Shannon limit for safe stimulation designates a charge-injection boundary above which platinum toxicity becomes a relevant consideration for living cells around an electrode, a possibility that can be directly tested, and is a vital step forward in mission (1).