Cochlear implants: Using light to improve hearing

Cochlear implants are devices that partially restore hearing in wearers. Unfortunately, the signal transmission from the implant to the auditory nerve is still rather basic, thus limiting the sound quality. Future implants could be more accurate in this setting by using light versus electrical pulses to stimulate nerve cells in the ear.

In this MEDICA-tradefair.com interview, Prof. Tobias Moser explains how the "OptoHear" project designs an optical cochlear implant that could overcome the limitations of today’s implants and reveals what lies ahead for the researchers.

Prof. Moser, you are developing an optical cochlear implant as part of the "OptoHear" project. Why is there a need for this type of implant?

Prof. Tobias Moser: Half a year to a year of auditory training helps cochlear implant wearers to recognize and understand speech in quiet environments without visual cues. This becomes more difficult if there is background noise or other ambient sounds.

People with normal hearing ability can resolve around 2000 different tone pitches. Today's cochlear implants only have 12 to 24 frequency channels, the stimulation current of which spreads extensively within the saline solution of the cochlea. This means, many nerve fibers are being stimulated at once versus individual fibers. Implant wearers thus can find it difficult to distinguish between pitches, which means they have a harder time understanding spoken language, and this often also affects their enjoyment of music.

How does an optical cochlear implant solve this problem?

Moser: Since light can be more easily focused and spatially confined, certain regions in the cochlea can be stimulated better with light versus electrical pulses. By nature, nerve cells are not light-sensitive, which means they need to be adapted via gene therapy. To do this, we use non-pathogenic viruses to introduce new genetic information into the cells. These form a channel in the cell membrane that opens when light hits. This excites the nerve fiber and sends an impulse to the brain.

Instead of electrical conductors, we would use waveguides - optical fibers - to guide light into the cochlea. Nerve fibers near the lower end of the cochlea resonate to higher frequency sound. Those near the upper end respond to lower frequencies. When a high tone is received, we would activate an optical fiber that only stimulates the nerve cells for the higher frequency sound.

What are the challenges in this setting?

Moser: One challenge is that we are changing a medical device. We can take some components of conventional cochlear implants that are based on electrical stimulation, including the externally worn sound processor, or the connecting unit to the implant. However, we must develop new components to integrate the light sources in the implant, for example, and get the optical fibers to emit light in a controlled manner and guide it to the right place.

As far as the gene therapy aspect is concerned, there is currently only one study related to ear gene therapy. It pursues a different goal, which means we still need some pioneering work as it pertains to genetically modifying the nerve cells in people's ears. This is also the first evidence we must provide in this setting: We must proof that gene therapy with non-pathogenic viruses is safe and has no side effects.

When will optical cochlear implants be available on the market?

Moser: We have only been optimizing and validating our research findings via animal models at this stage. We plan to launch a clinical trial in 2026 to evaluate the concept's usability and safety for humans. A follow-up study must then show advantages over the electrical cochlear implant. If successful, I would assume approval can be obtained by the end of the 2020s. We still have a long way to go but considering the various aspects of development it is also a very ambitious goal.