A cochlear implant helps many people with hearing loss hear again. This device sends sound signals to the cochlear nerve. However, some people have damaged cochlear nerves, so cochlear implants don’t work for them. For these individuals, doctors use an auditory brainstem implant (ABI). An ABI stimulates the brainstem to restore hearing.

Current ABIs are rigid, which causes problems. They don’t fit well against the brainstem’s curved surface. This poor contact leads to side effects like dizziness or facial twitching. Doctors often turn off many electrodes to reduce these issues. As a result, most ABI users hear only vague sounds and struggle to understand speech.

At EPFL’s Laboratory for Soft Bioelectronic Interfaces, researchers created a new soft ABI. This device uses tiny platinum electrodes embedded in thin silicone. The flexible array is only a fraction of a millimeter thick. It bends easily to match the brainstem’s shape, improving contact with tissue. Better contact reduces unwanted nerve stimulation and side effects.

The researchers published their work in Nature Biomedical Engineering. They tested the device in macaques with normal hearing to measure its effectiveness. The animals performed a task where they pressed a lever to show if tones were the same or different. Researchers mixed ABI stimulation with normal sounds to help the animals adjust. The macaques eventually distinguished ABI signals almost like real sounds.

Why a soft implant matters

The soft ABI’s flexibility is key to its success. Rigid ABIs create air gaps, causing excessive current spread and nerve issues. The soft design follows the brainstem’s curves, allowing more electrodes to stay active. This improves sound clarity. The device’s microfabrication also offers design freedom. Researchers can adjust electrode layouts or increase their number for better sound tuning. In tests, the macaques showed no discomfort or side effects, unlike human ABI users. The animals even triggered the device themselves, suggesting the stimulation felt natural.

Moving to human use requires more steps. The implant’s materials must meet medical standards and prove long-term reliability. Researchers suggest testing the device during human ABI surgeries to confirm its benefits. In macaques, the implant stayed stable for months without shifting, a major improvement over rigid ABIs. These findings offer hope for clearer, more comfortable hearing restoration.