JPL Develops Artificial Cochlea

Business Week Online recently published an interview with Flavio Noca of The Jet Propulsion Laboratory (JPL) in Pasadena, CA. He invented an artificial cochlea to assist in the search for life on other planets. The interview is summarized in this article; you can read the interview in its entirety at http://biz.yahoo.com/bizwk/010102/bh.html.

If you take apart a microphone, you will find a stretched membrane that vibrates in response to acoustic energy (much like an eardrum). The motion of the membrane reflects the shape of the sound wave in the air, and that motion is transformed to an electrical signal that can be transmitted using standard electrical techniques. The design has been around since the first microphone, and is basically unchanged since that time.

Until now!

In an effort to produce a more sensitive microphone, a team at JPL chose to mimic nature's methods; they have developed what is essentially an artificial cochlea. Their device uses artificial hair cells to detect sound in the same manner as an ear. The hair cells consist of arrays of tiny carbon tubes, which bend in response to acoustic energy, just as their natural counterparts do. The hair cell motion is transformed to electrical signals, just as the motion of the stretched membrane in traditional microphones is transformed. The resulting electrical signal is much like the signal from a traditional microphone, except that it contains information about much fainter sounds.

The key to the artificial cochlea is the array of carbon nanotubes, a new technology recently developed by Jimmy Xu at Brown University. The nanotubes are able to detect acoustic vibrations that are far quieter than those that can be detected by traditional (membrane) approaches. The problem with membranes is that they become stiffer as they become smaller. If a membrane is small enough that a very quiet sound wave might be able to move it, the membrane is so stiff that it won't bend when the sound wave hits it. Nanotubes are perfectly adapted to this application, because their stiffness doesn't necessarily increase as they become smaller.

Nanotubes have huge potential for improving hearing aids. In addition to their astounding sensitivity, two other properties intrigue investigators.

One is the fact that nanotubes are naturally directional - they always bend away from the source of the sound. Directionality in normal hearing is derived from the use of two ears, but nanotube technology can potentially provide directional information from a single sensor.

The second intriguing characteristic of carbon nanotubes is their potential ability to operate in an air environment. This is in contrast to ALL natural hair cells, which operate in a liquid. The ability to operate in air is attractive, because it eliminates the complex apparatus that performs the air to liquid interface in natural systems. This apparatus, which includes the eardrum and the hearing bones in our auditory system, reduces the sensitivity of our hearing mechanism.

Nanotubes also have potential military and medical applications, all of which involve the ability to detect very faint sounds. Military applications might include new sensors for detecting submarines, while a promising medical application is highly sensitive stethoscopes, including tiny ones that float in the bloodstream and detect biochemical events that are currently undetectable.