In Vivo and In Vitro Comparison of the Charge Injection Capacity of Platinum Macroelectrodes.

Platinum (Pt) is the most commonly used metal for stimulating electrodes. This study aims to determine the amount of charge that can be delivered without causing irreversible electrochemical reactions (charge injection capacity, Q inj) of Pt macroelectrodes (geometric surface area >0.001 cm2) in vitro and in vivo using voltage transient measurements. Pt macroelectrodes were stimulated with biphasic charge-balanced cathodic-first constant-current pulses in phosphate buffered saline. Potential excursions were measured (versus Ag/AgCl electrode) and used to determine Qinj. The in vitro Qinj were compared to those measured in vivo following: acute and chronic implantation close to the retina; chronic intracochlear implantation; and acute subdural implantation, in the cat. Qinj increased with pulsewidth from 35 to 54 μC/cm2 for respective pulse widths of 100 to 3200 μs per phase in vitro. Qinj was significantly less in vivo. There was no significant difference in Qinj between acutely (3.84 to 16.6 μC/cm2 with pulsewidths of 100 to 3200 μs) and chronically (6.99 to 15.8 μC/cm2 with pulsewidths of 200 to 3200 μs) implanted suprachoroidal electrodes. Intracochlear Qinj was not different to suprachoroidal Qinj, while subdural Qinj was significantly less than the suprachoroidal Q inj (p < 0.05). These results have important implications in providing guidelines on Qinj for the safe use of Pt stimulating macroelectrodes and question the relevance of measuring Qinj in vivo using voltage transients. [ABSTRACT FROM PUBLISHER]