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A soft, silicone-based brainstem implant from EPFL shows major promise in restoring hearing for patients with severe nerve damage, outperforming traditional ABIs in comfort and sound precision. Credit: SciTechDaily.com
A new soft auditory brainstem implant (ABI) developed by researchers at EPFL may revolutionize hearing restoration for people who can’t benefit from cochlear implants.
Unlike traditional rigid ABIs, this flexible device conforms to the brainstem, reducing side effects and improving sound perception. Tested in macaques, the implant allowed animals to distinguish artificial stimulation patterns nearly as well as natural sounds — an exciting sign that richer, more precise prosthetic hearing could soon reach human patients.
Breakthrough for Patients Beyond Cochlear Implants
Over the past few decades, cochlear implants have helped many people regain hearing. These devices are considered one of the most successful applications of neurotechnology. However, they don’t work for everyone — especially for individuals whose cochlear nerve is too damaged to transmit sound signals to the brain. For these patients, an alternative option is an auditory brainstem implant (ABI), which bypasses the ear entirely and stimulates the brainstem directly.
Current ABIs, however, have limitations. They are made with rigid materials that don’t conform well to the brainstem’s curved surface. This poor contact can cause unintended nerve activation, leading to side effects like dizziness or facial twitching. As a result, many of the implant’s electrodes are switched off, and most users only perceive vague sounds with limited speech recognition.
A Flexible New Approach to Hearing Restoration
Now, researchers at EPFL’s Laboratory for Soft Bioelectronic Interfaces have developed a new kind of ABI made from soft, flexible materials. Their thin-film device uses micrometer-scale platinum electrodes embedded in silicone, forming a highly pliable array just a fraction of a millimeter thick. This design improves contact with the brainstem and may reduce unwanted side effects by limiting stimulation to the intended nerves.
“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.
The soft auditory brainstem implant (ABI) developed at EPFL is designed to gently conform to brain tissue, enhancing signal precision and patient comfort. Credit: © 2025 EPFL/Alain Herzog – CC-BY-SA 4.0
Testing Hearing Perception in Real-Time
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.”
Rethinking Implant Design with Soft Interfaces
“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.”
Comfort-Driven Results in Animal Studies
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.”
Steps Toward Human Use
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.”
Reference: “High-resolution prosthetic hearing with a soft auditory brainstem implant in macaques” 18 April 2025, Nature Biomedical Engineering.
DOI: 10.1038/s41551-025-01378-9
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