A study of intra-cochlear electrodes and tissue interface by electrochemical impedance methods in vivo

This paper presents methods, results and analysis for measurements of the electrochemical impedance of platinum electrodes (approximately 0.43 mm2) over a 6-month implantation in the cat cochlea. The study aimed to improve our understanding of the effects of tissue response on impedance behaviour. An increase in impedance in the post-operative period was evident with a rise of the distorted arc at high frequencies in the complex plane, correlating to anomalous charge transport at the electrode-tissue interface. The impedance at low frequencies generally showed a capacitive dispersion modelled as a constant phase element, indicating a blocking characteristic of the electrodes. The study suggests that a reduction and changes in composition of perilymph or extracellular fluid adjacent to the electrodes, as a consequence of tissue response, causes the elevated "contact impedance". This affects the efficiency and quality of neural stimulating electrodes and neural recording electrodes. The finding of the crucial role of perilymph or extracellular fluid thin layer provides a new strategy for surface materials of neural electrodes, which is discussed in the paper. The interface characteristics must be considered during interpretation of studies undertaken in vitro or in acute experiments in vivo, where physiological fluid is abundant.