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 Q_inj. The in vitro Q_inj 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. Q_inj 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 Q_inj was not different to suprachoroidal Q_inj, while subdural Q_inj 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 Q_inj in vivo using voltage transients.