Accelerated testing of electrode degradation for validation of new implantable neural interfaces

Neural prostheses, such as cochlear implants or deep brain stimulators, can modulate neural activity and restore lost physiological function by performing electrical stimulation and neural recordings. However, prolonged stimulation can degrade electrodes and adversely affect their performance over long-term implantation. Therefore, integrating the electrodes’ health monitoring system is required for new implantable neural interface designs. However, validating the electrode degradation monitoring system with in-vivo experiment is slow and highly challenging. Furthermore, artificially generating the degradation of electrodes in in-vitro analysis is also time-consuming. This paper proposes an experimental setup for accelerated electrode degradation by elevating temperature and electrical stimulation. In order to demonstrate feasibility, a previous generation electrode material (tungsten) was used, and Electrochemical Impedance Spectroscopy (EIS) was measured every hour to analyse the electrochemical properties. As a result, optical microscopy images, before and after testing, show the morphology changes of the tungsten wire electrodes. The minimum accelerated testing to create electrode failure was 6 hours. Following prolonged stimulation, the results show electrode erosion possibly exacerbated by the evolution of hydrogen gas, while the EIS plots illustrate the slight increase of impedance over time in certain frequency bands, likely due to the progressive decline of the electrode surface area.

I. Introduction

Neuroprosthetics are medical devices employed to modulate neural activity and typically restore or regain some lost function due to a dysfunctional pathway in the nervous system [1], [2]. For example, deep brain stimulation uses current pulses to modulate network dynamics and treat conditions such as Parkinson’s disease, essential tremor and epilepsy [3]–[5]. Another example is the cochlear implant, which directly stimulates the auditory nerve to regain hearing ability for the profoundly deaf [6]. It is essential such implantable neuroprosthetic devices function reliably and safely over many years (i.e. decades or a lifetime) in patients.