Soft Brainstem Implant Delivers High-resolution Hearing

Over the last couple of decades, many people have regained hearing functionality with the most successful neurotech device to date: the cochlear implant. But for those whose cochlear nerve is too damaged for a standard cochlear implant, a promising alternative is an auditory brainstem implant (ABI). Unfortunately, current ABIs are rigid implants that do not allow for good tissue contact. As a result, doctors commonly switch off a majority of the electrodes due to unwanted side effects such as dizziness or facial twitching-leading most ABI users to perceive only vague sounds, with little speech intelligibility.

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.

Now, a team at EPFL's Laboratory for Soft Bioelectronic Interfaces has developed a soft, thin-film ABI. The device uses micrometer-scale platinum electrodes embedded in silicone, forming a pliable array just a fraction of a millimeter thick. This novel approach, published in Nature Biomedical Engineering, enables 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. Our success in macaques shows real promise for translating this technology to the clinic and delivering richer, more precise hearing," says Stéphanie P. Lacour, head of Head of the Laboratory for Soft Bioelectronic (LSBI) Interfaces at EPFL.

Probing "prosthetic hearing" with a complex behavioral task

Rather than simply relying on surgical tests, the researchers ran extensive behavioral experiments in macaques with normal hearing. This allowed them to measure how well the animals could distinguish electrical stimulation patterns as they would with natural acoustic hearing.

"Half the challenge is coming up with a viable implant, the other half is teaching an animal to show us, behaviorally, what it actually hears," says Emilie Revol, co-first author on the project and a former PhD student at EPFL. She meticulously trained the animals to perform an auditory discrimination task: the monkeys learned to press and release a lever to indicate whether consecutive tones were the "same" or "different."

"We then introduced stimulation from the soft ABI step by step, blending it with normal tones at first so the monkey could bridge the gap between acoustic and prosthetic hearing," says Revol. "Ultimately, the goal was then to see if the animal could detect small shifts from one electrode pair to another when only stimulating the soft ABI. Our results suggest that the animal treated these pulses almost the same way it treated real sounds."

Why a soft array?

"Our main idea was to leverage soft, bioelectronic interfaces to improve electrode-tissue match," explains 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 3 mm radius and a complex shape. Rigid electrodes leave air gaps, leading to excessive current spread and undesired nerve stimulation. By contrast, the EPFL team's ultra-thin silicone design easily bends around the tissue.

Beyond conformability, the soft array's flexible microfabrication means it can be reconfigured for different anatomies. "The design freedom of microlithography is enormous," says Trouillet. "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."

Improved comfort and fewer side effects

A crucial outcome of the macaque study was the absence of noticeable off-target effects. The researchers report that, within the tested range of electrical currents, the animal showed no signs of discomfort or muscle twitches around the face-common complaints from human ABI users. "The monkey pressed the lever to trigger stimulation itself, time and again," explains Revol. "If the prosthetic input had been unpleasant, it probably would have stopped."

Path to clinical translation

Although these findings are promising, the path to a commercially available soft ABI will require additional research and regulatory steps. "One immediate possibility is to test the device intraoperatively in human ABI surgeries," says Lacour, noting that the team's clinical partners in Boston regularly perform ABI procedures for patients with severe cochlear nerve damage. "They could briefly insert our soft array before the standard implant to measure if we truly reduce stray nerve activation."

In addition, every material in an implant destined for human use must be fully medical grade and show robust, long-term reliability. Yet the researchers are confident, thanks to the demanding tests the device has already withstood: "Our implant remained in place in the animal for several months, with no measurable electrode migration," notes Trouillet. "That's a critical step forward given how standard ABIs often migrate over time."