Impedance Properties of Metal Electrodes for Chronic Recording from Mammalian Nerves

Over the last few years methods have been developed for recording chronically from mammalian nerves with implanted electrodes contained in silastic cuffs. The impedance of the electrodes and the changes that take place over time were studied. Platinum-iridium electrodes remain stable over long periods of time, whereas the impedance of silver wire electrodes increases and eventually the silver fractures. The impedance at low frequencies (e.g., 10 Hz) provides a measure of the capacitance of the metal electrodes, whereas the impedance at higher frequencies (1 kHz) mainly measures the resistance of the tissue filling the cuff. Increases in resistance due to growth of connective tissue around intact nerves have been studied as well as changes in resistance after cutting a nerve. Impedance measurements provide a useful way to follow the properties of electrodes and nerves in basic neuroscientific research and in future clinical applications of these chronic recording methods.