Impedance changes in chronically implanted and stimulated cochlear implant electrodes

Abstract

Objectives

Electrode impedance increases following implantation and undergoes transitory reduction with onset of electrical stimulation. The studies in this paper measured the changes in access resistance and polarization impedance in vivo before and following electrical stimulation, and recorded the time course of these changes.

Design

Impedance measures recorded in (a) four cats following 6 months of cochlear implant use, and (b) three cochlear implant recipients with 1.5–5 years cochlear implant experience.

Results

Both the experimental and clinical data exhibited a reduction in electrode impedance, 20 and 5% respectively, within 15–30 minutes of stimulation onset. The majority of these changes occurred through reduction in polarization impedance. Cessation of stimulation was followed by an equivalent rise in impedance measures within 6–12 hours.

Conclusions

Stimulus-induced reductions in impedance exhibit a rapid onset and are evident in both chronic in vivo models tested, even several years after implantation. Given the impedance changes were dominated by the polarization component, these findings suggest that the electrical stimulation altered the electrode surface rather than the bulk tissue and fluid in the cochlea.

Keywords:

• Electrode impedance
• Access resistance
• Polarization impedance
• Cochlear implant
• Electrical stimulation

Acknowledgements

This work was funded by the Cooperative Research Centre (CRC) for Cochlear Implant and Hearing Aid Innovation and the HEARing CRC, established and supported under the Australian Government's Cooperative Research Centres Program. The electrodes were made in-house by Ms Helen Feng. The data for the cat study were collected in conjunction with the Bionics Institute's study, supported by the National Institute of Health (NIDCD contract number NIH-N01-DC-3-1005). We gratefully acknowledge the time and effort taken by our three cochlear implant recipients. Dr Jin Xu, Ms Anne Coco, Ms Stephanie Epp, Ms Lauren Donley, Mr David Perry, and Dr James Fallon were responsible for the surgery, some impedance measures, and the day-to-day maintenance of the cats. The Bionics Institute acknowledges funding from the Victorian State Government's infrastructure grant scheme.