Soft implant, clearer sound

While cochlear implants have revolutionised the lives of many individuals with severe hearing loss, some patients with severely damaged auditory nerves have been left behind. A new soft brainstem implant developed at EPFL may offer a safer, more effective solution — and bring sound back to those for whom silence was once a permanent condition.

Hearing is unique among our senses: it can be restored through technology. Cochlear implants have made it possible for over a million people with profound hearing loss to experience the sensation of sound again. These devices bypass damaged hair cells in the inner ear and stimulate the auditory nerve directly. Just a few decades ago, this would have been unthinkable.

But for individuals whose auditory nerve is too damaged or absent, cochlear implants are not an option. Instead, these patients rely on auditory brainstem implants (ABIs) — devices that bypass both the cochlea and auditory nerve and stimulate the cochlear nucleus in the brainstem directly.

Unfortunately, current ABI technology has significant drawbacks. The cochlear nucleus is located in a densely packed region of the brainstem, surrounded by sensitive nerves that control facial muscles, swallowing and more. Because of this, electrical signals from the implant often spread to unintended areas, causing unpleasant side effects like facial twitching, dizziness or pain.

To minimise these effects, surgeons often disable several electrodes — which in turn reduces the implant’s effectiveness. As a result, most ABI users gain only limited hearing, often just enough to assist with lip reading, but not enough to understand fluent speech.

Now, researchers at the Laboratory for Soft Bioelectronic Interfaces at EPFL have developed a new kind of auditory brainstem implant. The device uses micrometre-scale platinum electrodes embedded in a thin, flexible silicone film, forming an array that is just a fraction of a millimetre thick. This ultra-soft implant can mould itself precisely to the brainstem’s curved surface, ensuring closer and more stable contact with the target tissue.

By minimising air gaps between the implant and brain tissue, the EPFL team hopes to reduce the spread of electrical signals and thereby avoid off-target stimulation — one of the major limitations of conventional ABIs.

To evaluate their device, the researchers conducted behavioural experiments on macaques with normal hearing. Unlike previous methods that relied mostly on surgical tests, these experiments allowed the team to observe how well the animals could distinguish between different electrical stimulation patterns — a key indicator of auditory perception.

Crucially, the animals showed no signs of pain, discomfort or facial muscle activity, even at stimulation levels typically used in humans. This suggests that soft ABIs deliver more targeted and safer stimulation than current devices.

The study, recently published in 'Nature Biomedical Engineering', represents a compelling step forward in the field of neuroprosthetics. The new implant houses 11 soft electrodes and could open the door to better, more natural hearing for patients who currently have few effective options.

The researchers are optimistic, but emphasise that additional studies and regulatory processes are needed before the soft ABI reaches patients.