Design, Fabrication, and Evaluation of a Parylene Thin-Film Electrode Array for Cochlear Implants.

Abstract

To improve the existing manually assembled cochlear implant electrode arrays, a thin-film electrode array (TFEA) was microfabricated having a maximum electrode density of 15 sites along an 8-mm length, with each site having a 75 μm × 1.8 μm (diameter × height) disk electrode. The microfabrication method adopted photoresist transferring, lift-off, two-step oxygen plasma etching, and fuming nitric acid release to reduce lift-off complexity, protect the metal layer, and increase the release efficiency. Systematic in vitro characterization showed that the TFEA's bending stiffness was 6.40 × 10^-10 N·m² near the base and 1.26 × 10^-10 N·m² near the apex. The TFEA electrode produced an average impedance of 16 kΩ and a maximum current limit of 800 μA, measured with 1-kHz sinusoidal current using monopolar stimulation in saline. A TFEA prototype was implanted in a cat cochlea to obtain in vivo measurements of electrically evoked auditory brainstem and inferior colliculus responses to monopolar stimulation with 41-μs/phase biphasic pulses. Both physiological responses produced a threshold of ∼300 μA and a dynamic range of 5-8 dB above the threshold. Compared with existing arrays, the present TFEA had 10⁴ times less bending stiffness, 97% less electrode area, and comparable physiological thresholds. Using a simplified structure and stable fabrication method, the present TEFA produced physical and physiological performance comparable to existing commercial devices. The present TFEA represents a step closer toward an automated process replacing the labor-intensive and expensive manual assembly of the cochlear implant electrode arrays.