Swiss researchers have developed a flexible auditory brainstem implant (ABI) which they claim perfectly fits the curved surface of the brainstem and may enable prosthetic hearing in people who can’t have cochlear implants.
They said they had successfully demonstrated in macaques the implant’s potential to restore high-resolution “prosthetic hearing”.
“The results support the concept that a soft multichannel ABI can efficiently stimulate the cochlear nucleus and produce high-resolution auditory percepts without secondary effects,” the researchers concluded in Nature Biomedical Engineering on 18 April 2025.
“Soft multichannel ABIs may aid the rehabilitation of individuals with profound hearing loss who are ineligible for cochlear implants.”
The team, from the Laboratory for Soft Bioelectronic Interfaces at the science and technology institution, Ecole Polytechnique Fédéralé de Lausanne (EPFL) developed a soft, thin-film ABI.
The device uses micrometre-scale platinum electrodes embedded in silicone, forming a pliable array which is a fraction of a millimetre thick.
The researchers said the novel approach enabled better tissue contact, potentially preventing off-target nerve activation and reducing side effects
“Designing a soft implant that truly conforms to the brainstem environment is a critical milestone in restoring hearing for patients who can’t use cochlear implants,” said Professor Stéphanie Lacour, head of the laboratory.
“Our success in macaques shows real promise for translating this technology to the clinic and delivering richer, more precise hearing.”
ABIs are for those whose cochlear nerve is too damaged for a standard cochlear implant.
Current ABIs are rigid implants that do not allow for good tissue contact, the researchers said, causing doctors to commonly switch off most of the electrodes due to unwanted side effects such as dizziness or facial twitching. This enables most ABI users to perceive only vague sounds, with little speech intelligibility.
“Most users of ABIs experience sound awareness, which aids in lip reading, yet not speech intelligibility,” the researchers wrote.
They said the existing clinical ABI did not conform to the curvature of the human cochlear nucleus and this “lack of conformability may be a contributing factor to the limited efficacy of the ABI in humans”.
Dual site implant
“We implemented a soft neurotechnology prosthesis onto the central nervous system to produce specific auditory percepts,” they wrote.
“We engineered a dual-site (brainstem and cortex) implantable system, scaled to macaque anatomy, for the analysis of auditory perception evoked by electrical stimulation of the cochlear nucleus.
“A soft multichannel ABI, fabricated using thin-film processing, provided high-resolution auditory percepts, with spatially distinct stimulation sites eliciting cortical responses akin to frequency-specific tuning.”
Behavioural responses collected over several months were found to be sufficiently precise to distinguish stimulations from adjacent channels.
Efficacy was assessed through intra-operative, electrophysiological and behavioural characterisation, all of which led to a comprehensive benchmark for auditory neuroprostheses.
The implant offered “a unique opportunity to develop and optimise central auditory neuroprostheses” and represented “a series of notable advances in the field of auditory neuroprostheses”, they added.
Why a soft array?
“Our main idea was to leverage soft, bioelectronic interfaces to improve electrode-tissue match,” said Dr Alix Trouillet, a former postdoctoral researcher at EPFL and co-first author of the study.
“If the array naturally follows the brainstem’s curved anatomy, we can lower stimulation thresholds and maintain more active electrodes for high-resolution hearing.”
Conventional ABIs rest on the dorsal surface of the cochlear nucleus, which has a 3mm radius and a complex shape, she said.
Rigid electrodes leave air gaps, leading to excessive current spread and undesired nerve stimulation, Dr Trouillet said. By contrast, the EPFL team’s ultra-thin silicone design easily bends around the tissue.
The flexible microfabrication could be reconfigured for different anatomies. “The design freedom of microlithography is enormous,” she said.
“We can envision higher electrode counts or new layouts that further refine frequency-specific tuning. Our current version houses 11 electrodes – future iterations may substantially increase this number.”
