Active middle-ear implant based on piezoelectric thin-film cantilevers

In a typical cochlear implant, flexible lead with stimulating electrodes is inserted in the scala tympani, a fluid-filled cavity in the cochlea. When the electrical stimulation is applied, it propagates through fluid in the scala tympani and across the basilar membrane, separating the scala tympani and the scala media, an adjacent compartment of the cochlea containing the hair cells. Such rather remote operation of existing cochlear implants does not allow fine localized targeting of the hair cells, limiting their pitch resolution. Cochlear implants have not undergone significant changes in their design or function since 1985, when the first multi-channel cochlear implant was developed by Cochlear and approved by FDA. Since then, FDA approved similarly-designed cochlear implants by two other companies, one by Advanced Bionics in 1996 and another by MedEl in 2001. An apparent lack of innovation in cochlear implant is partially due to the fact that, despite their limited pitch resolution, they provide rather faithful reproduction of human speech. The remaining “holly grail” of the cochlear implant industry is a device with sufficient pitch resolution for listening to music. So far, that goal remains outside the reach, at least for the devices based on electrical stimulation of cochlea. As a welcome first step toward an alternative method of cochlear stimulation, a group of engineers at the Fraunhofer Institute for Manufacturing Engineering and Automation in Stuttgart, led by Dr. Kaltenbacher, developed a device that can be placed in the middle-ear to bypass the ossicles (the auditory bones) and provide direct acoustic stimulation of the fluid in the scala tympani. In theory, such a design can: 1) be less invasive, 2) be easily implanted in an outpatient procedure, and 3) potentially provide better sound quality than existing cochlear implants. The implant does require that at least some of the hair cells are still present in the cochlea (unlike the other types of cochlear implants). In order to bypass the bones in the middle ear, the sound is picked up by an externally-mounted microphone, converted to infrared light, passed through the tympanic membrane, picked up by a photo diode, and finally converted back to the sound waves with MEMS-based piezoelectric thin-film cantilevers (see the inset). So far, the engineers are testing individual components of the device, with a finished prototype tests planned for 2014.