Stimulation-stability and preclinical bench testing of a new auditory nerve interface

OBJECTIVE: Cochlear implants are the flagship sensory neural prosthetic, enabling improved hearing and speech perception in >1 million patients with hearing loss. Despite over three decades of active work and innovations in cochlear implants, hearing performance for complex tasks such as speech-in-noise and music appreciation are still quite limited, putatively due to the challenge of selectively stimulating the auditory nerve with intracochlear electrodes through the bony cochlear wall. Our approach is to more selectively evoke hearing percepts through the development of an Auditory Nerve Implant (ANI) device that uses penetrating microelectrodes placed directly in the nerve for stimulation, and which will be evaluated through psychophysical testing as part of a first-in-human study. Stability and integrity of ANI electrodes during passive soaking and stimulation are needed to support initiation of the clinical trial, which we evaluated through thermal accelerated saline soak testing and stimulation stability testing. METHODS: The ANI electrode stability in saline was assessed by accelerated aging at ≥ 67 [Formula: see text]C with a 1.5-year equivalent endpoint using electrode impedance and microscopy to characterize outcomes. Stimulation stability testing evaluated electrode integrity after 6.3 billion pulses at 100 μA, which is the maximum clinical stimulation intensity, for a one-year equivalent use, and 1200 μA, for a high-dose protocol. Electrode impedance, longitudinal stimulation voltage, and microscopy outcomes were used to the quantify results. RESULTS: All electrodes had stable impedances around 2 kΩ at the end of the accelerated soak test, except for three with higher impedances but still below the 70 kΩ failure criteria. All but one electrode survived stimulation stability testing. Optical and electron microscopy largely showed little to no change during testing, indicating no observable tip degradation, with the exception of one soak-test device. CONCLUSIONS: The outcomes from the studies indicate that ANI electrodes exhibit a high degree of integrity and impedance stability. Microscopy characterization proved effective for evaluating electrodes, providing complementary information to standard Backscattered Scanning Electron Microscopy (BSEM). This study suggests the ANI device tolerate a 1.5 year in-dwelling period for a 1 year perceptual study of stimulation evoked percepts, paving the way for clinical translation.