The cochlear implant (CI) enables worldwide over 700,000 people with profoundly reduced hearing and deaf people to hear again. So far, the auditory nerve has been stimulated by electrical impulses. The quality of this artificial hearing differs greatly from natural hearing. Due to the extensive current propagation in the cochlea, large groups of nerve cells are activated instead of just a few nerve cells – comparable to playing a piano with boxing gloves instead of individual fingers.
Patients with CIs are able to communicate in 1-on-1 conversations, but are often dependent on lip-reading in environments with background noise and multiple speakers. Further, music appreciation is limited. A fundamental improvement is promised by the targeted stimulation of the auditory nerve with light.
Optical cochlear implant in the cochlea of a rat.
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After 12 years of research on fundamental questions of hearing with light, the hear-ing research on the Göttingen Campus by Prof. Tobias Moser, Director of the Institute for Auditory Neuroscience at the University Medical Center Göttingen (UMG), is on the way to develop an optical cochlear implant towards clinical ap-plicability. In cooperation with a team of engineers from the Department of Microsystems Enigineering (IMTEK) at the University of Freiburg, led by Dr. Patrick Ruther, an optical cochlear implant system with integrated micro light-emitting diodes (µLEDs) was developed, which is suitable for long-term studies.
This system optically stimulates the auditory nerve in the animal model of human hearing loss, which has been previously made light-sensitive by genetic manipulation. The system is much smaller and lighter than the clinically used CI, and can therefore also be used in rodents. The scientists have now gone an important step forward: in behavioral experiments they could show that the multichannel optical CI enables deaf rodents to hear again – and this over a period of weeks.
For the first time, the functionality of the newly developed optical cochlear implants comprising up to 10 µLEDs with an edge length of only 0.25 millimeters could be tested in animal models over a period of more than one month: Prior to the experiments, the rodents were injected with a harmless virus to make their auditory nerve light-sensitive via molecular light switches (channelrhodopsins). With the help of acoustic stimuli, the animals were trained on behavioural task, then deafened by means of an ototoxic drug, and provided with an optical CI. "Surprisingly, some deaf animals immediately transferred the light signal over the sound stimulus used for training prior to deafening. This could indicate that the optical stimulation comes close to the natural hearing impression," says Dr. Daniel Keppeler, one of the first authors of the publication and staff member at the Institute for Auditory Neuroscience, UMG.
To achieve this milestone of the optical CI development, the LED probes had to be well encapsulated, which is the only way to protect the sensitive electronic components from the saline solution in the cochlea. "The greatest challenge for us is the encapsulation of the implants. This is crucial for their long-term stability in the animal model," said Dr. Michael Schwärzle, first author and former employee at the Department of Microsystems Engineering at the University of Freiburg.
The development of a mobile processor at the Institute for Auditory Neuroscience in Göttingen has been equally important for the success of behavioural experiments running over several weeks. The small computing unit converts ambient sound into electrical signals via an integrated microphone and transmits them to the optical CI. The miniaturization of the optical cochlear implant for testing in rodents was another challenge for the interdisciplinary team of scientists. To be worn by the animals without any problems in behavioural experiments, the system should be as light as possible. The entire CI just weighs 15 grams, which corre-sponds to about a tablespoon of sugar.
COMPAMED-tradefair.com; Source: University Medical Center Göttingen