NANOCI-Nanotechnology Based Cochlear Implant With Gapless Interface to Auditory Neurons

Abstract

: Cochlear implants (CI) restore functional hearing in the majority of deaf patients. Despite the tremendous success of these devices, some limitations remain. The bottleneck for optimal electrical stimulation with CI is caused by the anatomical gap between the electrode array and the auditory neurons in the inner ear. As a consequence, current devices are limited through 1) low frequency resolution, hence sub-optimal sound quality and 2), large stimulation currents, hence high energy consumption (responsible for significant battery costs and for impeding the development of fully implantable systems). A recently completed, multinational and interdisciplinary project called NANOCI aimed at overcoming current limitations by creating a gapless interface between auditory nerve fibers and the cochlear implant electrode array. This ambitious goal was achieved in vivo by neurotrophin-induced attraction of neurites through an intracochlear gel-nanomatrix onto a modified nanoCI electrode array located in the scala tympani of deafened guinea pigs. Functionally, the gapless interface led to lower stimulation thresholds and a larger dynamic range in vivo, and to reduced stimulation energy requirement (up to fivefold) in an in vitro model using auditory neurons cultured on multi-electrode arrays. In conclusion, the NANOCI project yielded proof of concept that a gapless interface between auditory neurons and cochlear implant electrode arrays is feasible. These findings may be of relevance for the development of future CI systems with better sound quality and performance and lower energy consumption. The present overview/review paper summarizes the NANOCI project history and highlights achievements of the individual work packages.

FIG. 1

Schematic concept of the NANOCI project. Overview (A) and mid-modiolar cross-section through the cochlea with a CI electrode array inserted into the scala tympani (pink, B) at distance from the auditory neuron (green, B–D). By providing structural support, guidance, and attraction cues from an injected, functionalized nanomatrix (blue shaded fill, B–D) and from the modified electrode array surface (white coating of pink electrode array, C and D), a guided growth of the neurite onto the electrode surface is induced where a gapless nerve-CI interface is formed. CI indicates cochlear implants.

FIG. 2

Prototype of the NANOCI electrode array featuring 36 electrodes on table (A) and after insertion into a cadaveric human temporal bone, as visualized through cone-beam computed tomography (B and C). This prototype yields proof of concept, that channel numbers can be tripled with today's manufacturing methods. No intracochlear damage and an intact basilar membrane were observed (D). However, due to increased stiffness of the device, a full insertion was not possible.

FIG. 3

Specifications (A) and photographs of the prototype on table (B) and inserted into the cochlea (C) of the guinea pig cochlear implant with four electrodes (arrows in B). The length of the intracochlear array is 4 mm, as indicated with a stopper (blue oval) located between two dark markers at distance 3 and 5 mm from the tip for better estimate of achieved insertion depth. Catheters for intracochlear injection of the nanomatrix in fluidic form (D). The catheters were manufactured in two variants with different diameters of the silicone tubing (0.3 mm with yellow standard syringe needle and 0.6 mm with white standard syringe needle). The black dot indicates the 3-mm insertion depth as a reference point (arrows in D). CI indicates cochlear implants.

FIG. 4

Proof of concept of the gapless interface between auditory neurons (green) and the CI-electrode array in scheme (A) and in the deafened guinea pig inner ear in vivo (B). Auditory neurons are stained with a neuron-specific marker for β-III tubulin (green, TUJ), cell nuclei are stained in blue (DAPI nuclear staining), photograph provided by Marcus Müller and Hubert Löwenheim. CI indicates cochlear implants.