Soft Auditory Brainstem Implant Shows Promise for Restoring Hearing Without Side Effects

[kevinaggner]

Over the past few decades, cochlear implants have helped many regain hearing, making them one of the most successful neurotechnology devices to date. However, for individuals with severe damage to the cochlear nerve—rendering traditional implants ineffective—a potential alternative is the auditory brainstem implant (ABI).

Current ABIs, though, are rigid and often fail to conform to the brainstem’s complex shape. This poor contact leads to unintended stimulation of nearby nerves, causing side effects such as dizziness or facial twitching. As a result, many electrodes are disabled during programming, leaving users with minimal sound perception and limited speech understanding.

A team at EPFL’s Laboratory for Soft Bioelectronic Interfaces has now developed a breakthrough soft, thin-film ABI that could change that. The new implant uses platinum microelectrodes embedded in silicone to form a flexible array just a fraction of a millimeter thick. This innovation, detailed in Nature Biomedical Engineering, improves electrode-tissue contact and may significantly reduce unwanted nerve activation.

“Creating an implant that conforms naturally to the brainstem’s shape is a critical step toward restoring hearing for patients who can't benefit from cochlear implants,” said Professor Stéphanie P. Lacour, head of the lab. “Our success in non-human primate models is a promising step toward clinical translation.”

To test the functionality of the soft ABI, researchers conducted behavioral experiments in macaques with normal hearing. Instead of relying solely on surgical data, they trained the animals to respond to auditory discrimination tasks, helping them assess how well the subjects could distinguish between different patterns of electrical stimulation.

“Developing the implant is just half the challenge—the other half is teaching the animal to communicate what it hears,” explained co-first author Emilie Revol, who led the training phase during her doctoral research.

The macaques were taught to press or release a lever based on whether they heard two tones that were the same or different. As the animals mastered the task, the researchers gradually replaced natural tones with signals from the soft ABI, eventually testing whether the monkeys could distinguish subtle changes in stimulation from different electrode pairs alone.

“Our results suggest the brain treated the soft ABI signals similarly to natural sound,” said Revol.

Co-first author Alix Trouillet, a former postdoctoral researcher at EPFL, emphasized the implant’s design advantages. “Matching the brainstem’s curved surface with a soft interface reduces the required current and keeps more electrodes active, which is key to higher-resolution hearing.”

Unlike rigid ABIs that sit awkwardly on the 3 mm-radius cochlear nucleus, EPFL’s silicone-based design wraps snugly around the tissue, minimizing gaps and limiting current spread. Its flexible microfabrication also allows for tailored designs, potentially increasing the number of electrodes and improving sound frequency mapping.

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The macaque study revealed another critical finding: the absence of side effects commonly seen in human ABI users. “We didn’t observe any facial twitches or signs of discomfort, even at full stimulation levels,” said Revol. “In fact, the monkey repeatedly triggered the stimulation itself, which strongly suggests the input wasn’t unpleasant.”

While the results are highly encouraging, further steps are needed before the soft ABI reaches clinical use. According to Lacour, the next phase could involve testing the device during human surgeries, with EPFL’s clinical partners in Boston potentially inserting the soft array briefly before implanting the standard ABI to assess its effectiveness in reducing off-target nerve stimulation.

All materials used in human implants must meet stringent medical standards and demonstrate long-term stability, so additional development and validation are required. Still, the research points to a promising future where softer, smarter implants may restore clearer, more natural hearing to those who currently have few options.