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Cochlear implants (CIs) have proven to be highly effective neural prostheses, widely used to restore hearing in individuals with sensorineural hearing loss. However, following implantation, CIs can trigger an immune response that leads to cell layer formation (fibrosis) in the cochlea causing low stimulation efficiency and suboptimal clinical outcomes. Electrochemical Impedance Spectroscopy (EIS) offers a powerful method for assessing the functionality of CI both during and after surgery. However, it‘s full potential, particularly in analyzing the electrical properties of CIs, detecting the cell layer by distinguishing between different cell types, remains unutilized. This study aims to develop and validate an equivalent electrical circuit (EEC) model that accurately represents the electrical properties of CI arrays and can reliably detect formation of cell layer on CI electrode arrays.

The study involves four CI electrode arrays from different manufacturers (MED-EL, Advanced Bionics, Oticon, and Cochlear). Impedance measurements were conducted using an HP4192A impedance analyzer in a frequency range from 5 Hz to 13 MHz. The electrical equivalent circuit (EEC) of the CI electrodes was modelled, involving linear elements, as well as the electrode-electrolyte interface, using two non-linear bilayer models (Cole-Cole and Schwan-Faraday). A layer of human mesenchymal stromal cells (MSCs) was developed on the CI electrodes and impedance measurements were conducted to analyze changes in the impedance and its sensitivity and specificity.

A general nonlinear electric element circuit model applicable to all types of CIs was derived that allows the determination of local impedances between neighboring CI array electrodes with an accuracy of < 10%. Our cell layer experiments demonstrated a clear increase in impedance across the frequency range when a layer of cells was present on the CI electrode. This increase was reversed after enzymatic cleaning, confirming that the observed impedance changes were due to the cell layer. These findings validate the use of impedance spectroscopy for detecting biological layers on cochlear implant electrodes.

In conclusion, our study developed and validated an EEC model to describe the electrical properties of CI electrode arrays along with its ability to detect presence of cell layer on CI electrodes using precision impedance spectroscopy. Our findings underscore the potential of modelling and identifying the coverage of electrodes with biological material using impedance spectroscopy.

References

[1] Sehlmeyer M, Bhavsar MB, Zimmermann S, Maier H. A simple electrical circuit model for impedance spectroscopy with cochlear implant electrodes. Hear Res. 2024 Nov;453:109125. DOI: 10.1016/j.heares.2024.109125
[2] Sehlmeyer M, Makarenko M, Schoerner N, Bhavsar MB, Blank T, Maier HJ, Kral A, Maier H, Zimmermann S. Electrical equivalent circuit for analyzing the effect of signal shape on power distribution in cochlear implant electrodes and surrounding tissue. Sci Rep. 2025 Jun 20;15(1):20136. DOI: 10.1038/s41598-025-04840-5