Micro-machined mechanical cochlea

Scientists at the University of Michigan have developed the first micro-machined, life-sized, mechanical cochlea, the tiny organ responsible for converting acoustic vibrations into electrical signals for the brain to interpret.

Scientists at the University of Michigan (UM) have developed the first micro-machined, life-sized, mechanical cochlea, the tiny organ responsible for converting acoustic vibrations into electrical signals for the brain to “read” and interpret as different sounds.

Most people with hearing loss have lost the ability to translate acoustic sound waves into electrical signals for the brain, so developing a device capable of simulating this function is an important step in the effort to help at least some of the estimated 560 million people who will experience hearing loss by this year.

While the UM system is not yet ready for use as an implant, the 3-centimetre device could potentially be used as part of a cochlear implant. More immediate applications include a low-power sensor for military or commercial applications, said College of Engineering associate professor Karl Grosh.

The three advantages of the mechanical cochlea built at UM are its life-sized dimensions, its suitability for mass production, and its use of a unique low-power mechanical method to do acoustic signal processing, Grosh said.

The human cochlea is a snail-shaped organ measuring about a cubic centimetre in the inner ear. If you unwind the spiral, it would equal the length of the UM mechanical cochlea.

The mechanical cochlea works in the same way as its biological counterpart. In the biological cochlea, the basilar membrane, which winds along the cochlear spiral, is stiffer at the base and becomes softer as it approaches the centre. In the engineered cochlea developed by Grosh and doctoral student Robert White, a fluid-filled duct etched onto a chip acts as the cochlear spiral.

When sound waves enter the mechanical cochlea’s input membrane, a wave is created, which travels down the duct, interacting with a tapered micro-machined membrane, analogous to the basilar membrane. This process allows the device to separate different frequency tones.

In the biological cochlea, sensory hair cells in the spiral detect the sound waves travelling through the fluid, and translate the sound waves into electrical signals, which the auditory nerve carries to the brain. The ear hears different sounds depending on where the wave vibrates in the cochlea.

The goal is to use the mechanical cochlea as a sensitive microphone, perhaps in tandem with a cochlear implant, Grosh said, the same way an external microphone, a microprocessor and an antenna work together in present implants.

Cochlear implants work by sending signals for different frequencies to electrodes implanted in the cochlear spiral. The auditory nerves then transport these signals to the brain. Researchers are adding arrays of sensors to the mechanical cochlea, which would make it possible to use the new device to drive the electrodes in a cochlear implant.

The work is primarily funded by the National Science Foundation and the Office of Naval Research.