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Introduction: Cochlear implants (CIs) were developed to enable deaf patients to hear by directly stimulating their auditory nerve electrically. Recent findings from our labs have shown that standard CI electrodes are not only suitable for intracochlear stimulation but also for monitoring, and can be converted into in vivo sensors using electrochemical methods [1]. This novel CI function allows the monitoring of the intracochlear microenvironment and the condition of chronically implanted CI electrodes. In previous acute experiments on anesthetized rats, the CI was used to monitor intracochlear oxygen concentrations. In this study, we are testing the influence of various anesthetics on the electrochemical in vivo sensing and investigate whether electrochemical measurements are also possible in the awake state of CI users.

Methods: Adult deafened Wistar rats were bilaterally implanted with CI arrays connected to specially designed head connectors. All CI animals underwent several months of sound lateralization training [2], [3], [4], during which they demonstrated excellent hearing performance. In addition, electrochemical measurements were performed at least once a week using the unmodified CI electrodes. These included electrochemical characterization of the electrodes and the surrounding microenvironment using reciprocal derivative chronopotentiometry and cyclic voltammetry, as well as measurements of intracochlear oxygen concentration using combined chronoamperometric and potentiometric methods. The experiments were performed under three different conditions: isoflurane anesthesia, ketamine/xylazine anesthesia, or in a waking state.

Results: Electrochemical CI sensing showed good and comparable electrode characterization in ketamine/xylazine anaesthetized rats and in awake rats, while sensing characteristics massively changed under isoflurane anesthesia. Electrochemical CI sensing in awake rats did not result in behavioral changes during the measurements. Under all conditions, stable and reproducible measurements of the oxygen concentration at the electrode surface could be performed over months.

Conclusions: Our results show that electrochemical CI sensing is possible not only under controlled conditions in anesthetized animals, but also in awake and behaving animals. This opens up the possibility of continuous monitoring of the direct intracochlear microenvironment and electrode conditions by the implant itself, thus offering a novel CI function in addition to the clinically established impedance monitoring in CI users.

References

[1] Weltin A, Kieninger J, Urban GA, Buchholz S, Arndt S, Rosskothen-Kuhl N. Standard cochlear implants as electrochemical sensors: Intracochlear oxygen measurements in vivo. Biosens Bioelectron. 2022 Mar 1;199:113859. DOI: 10.1016/j.bios.2021.113859
[2] Rosskothen-Kuhl N, Buck AN, Li K, Schnupp JW. Microsecond interaural time difference discrimination restored by cochlear implants after neonatal deafness. Elife. 2021 Jan 11;10:e59300. DOI: 10.7554/eLife.59300
[3] Buchholz S, Schnupp JWH, Arndt S, Rosskothen-Kuhl N. Interaural level difference sensitivity in neonatally deafened rats fitted with bilateral cochlear implants. Sci Rep. 2024 Dec 16;14(1):30515. DOI: 10.1038/s41598-024-82978-4
[4] Buchholz S, Arndt S, Schnupp JWH, Rosskothen-Kuhl N. Interactions of Interaural Time and Level Differences in Spatial Hearing with Cochlear Implants. Adv Sci (Weinh). 2026 Jan;13(6):e00918. DOI: 10.1002/advs.202500918