Electrical cochlear implants (eCIs) are effective at restoring hearing when used in a quiet environment. However, eCIs do not work as well in noisy surroundings and they are unable to fully capture the experience of listening to music for individuals with impaired hearing. Optical cochlear implants (oCIs) may be able to overcome some of the limitations of eCIs. Light can be controlled and directed with greater precision than electricity. By using optogenetics to stimulate auditory nerve cells, oCIs are expected to improve the resolution of the auditory frequency. To advance the development of oCIs, the Fraunhofer Institute for Photonic Microsystems (Fraunhofer IPMS) is collaborating with the Max Planck Institute for Multidisciplinary Natural Sciences (MPI-NAT) and University Medical Center Göttingen (UMG) to evaluate its OLED-on-silicon technology for use in optical cochlear implants. The IPMS team, which originally developed the technology for microdisplays, has created miniature OLED-on-silicon probes to use as optical stimulators in oCIs. OLED-on-silicon technology for optogenetics. Courtesy of Fraunhofer IPMS. “With OLED-on-silicon technology, we can bring tiny, locally controllable light pixels onto a chip,” Uwe Vogel, head of Microdisplays and Sensors at Fraunhofer IPMS, said. “This chip can be flexibly designed to reach the desired locations even in curved structures like the cochlea. This allows light to be used precisely, where electrical stimulation alone is insufficient.” In a conventional eCI, each electrode contact also stimulates distant nerve cells that code different (nonauditory) frequencies. The broad current spread of the eCI reduces the frequency resolution of the electrical sound coding in the implant. Optical implants use a different approach. In an oCI, dozens of independent, microscale light emitters can be placed along the frequency axis of the cochlea to project light onto this small area. The emitters can spatially target and optically stimulate light-sensitive auditory nerve cells in the cochlea to increase the number of independent audio frequency bands. The IPMS team drew on its experience with OLED-on-silicon technology for microdisplays to create CMOS-integrated light sources for a pixelated OLED microsensor that can control spatially distributed light channels on an individual basis. The channels can be assigned to corresponding audio frequencies in the cochlea through a serial interface. The light sources for the OLED microsensor provide high pixel density and brightness with low power consumption. Electrical versus optical cochlea stimulation. (Top) The current from each of the 12 electrode contacts of a conventional electrical cochlear implant (eCI) spreads widely. (Below) The light from nearly one hundred independent microLEDs in an optical cochlear implant (oCI) can be projected onto a small area. Courtesy of MPI-NAT. “The development of optical cochlear implants promises better hearing for the severely hearing impaired,” MPI-NAT professor Tobias Moser said. “MPI-NAT and UMG are working closely with partners such as Fraunhofer IPMS on the technological solutions required for this.” Optogenetics, a technique that introduces light-sensitive proteins into cells to control them, is commonly used to study nerve cells and activate or inhibit specific neuronal populations. To deliver light stimulation to cells in a precise way, small, locally selective light sources, like the OLED-on-silicon probes, are required. So far, the OLED-on-silicon technology has shown that it can provide the brightness needed for an oCI and that it can be integrated into a cochlear implant. However, the team still needs to verify the flexibility and biocompatibility of the OLED-on-silicon probe. According to the team, these properties can usually be achieved with the silicon microtechnology used for the OLED-on-silicon project. Based on the results of the project to date, the use of OLED-on-silicon technology for optogenetics shows promise. The researchers will continue to work to refine the technology for use in optical cochlear implants and potentially in other optogenetic applications. “Intelligent, implantable stimulators based on optical stimulation could also be used for other medical therapies, such as laryngeal pacemakers, cardiac pacemakers, pain relief, retinal implants, or deep brain stimulation,” Moser said.
