A simple electrical circuit model for impedance spectroscopy with cochlear implant electrodes

Although cochlear implants are an established method of restoring hearing, they can have limitations such as increasing current spread and decreasing frequency resolution due to tissue growth around the electrode array. Impedance measurements in cochlear implants have become a versatile tool for intra- and post-operative diagnosis of cochlear implant state. However, most clinical devices use current pulse stimulation already available in the implants and analyze the voltage response in the time-domain and spread along the cochlea. To use the full potential of impedance spectroscopy in differentiating cell types, measurement over an extended frequency range is required. This study presents a simple electrical equivalent circuit for impedance spectroscopy with cochlear implants in a 2-pole configuration. The electrical equivalent circuit describes the electrical properties of the cochlear implant electrode and its electrochemical behavior at the electrode-electrolyte interface by comparing two non-linear bilayer models, Cole-Cole and Schwan-Faraday. The model is validated for four cochlear implant electrodes from four different manufacturers (MED-EL FlexSoft, AB HiFocus SlimJ, Oticon EVO, Cochlear Nucleus CI622) characterized by impedance spectroscopy between 5 Hz and 13 MHz. In the future, this electrical equivalent circuit may help to extract parameters for differentiating cell types around the cochlear implant electrode from an impedance spectroscopic measurement.