at the University of Michigan are developing a mechanical
cochlea, a device that functions much like its human
counterpart in the ear. Yet, because it is composed
of micromachined parts and integrated circuits, the
apparatus should be inexpensive to manufacture and
could potentially capture a range of frequencies
well beyond those of human hearing.
While designed primarily as a highly efficient sensor
to detect sound waves underwater, the machined cochlea
could one day substitute for the microphone and much
of the electronics in cochlear implants at a much
Under development by National Science Foundation
(NSF) student-fellow Robert White and NSF CAREER
awardee Karl Grosh, the "microengineered hydromechanical
cochlear model," was first reported in the Jan. 21,
2005, edition of the Proceedings of the National
Academy of Sciences. The researchers described both
a mathematical model and the device they had built--a
system capable of detecting specific frequencies
across a wide range.
"The machined cochlea Grosh and White developed
fills a critical need for efficient acoustic sensing,
as well as a need of the hearing-impaired. It could
potentially offer a less-expensive substitute for
some hardware in cochlear implants," says Ken Chong,
interim director of NSF's Civil and Mechanical Systems
The division funded Grosh's CAREER award, which
recently went to create a new program in nano-bio
mechanics to promote such research.
The hydromechanical cochlea is a microelectromechanical
system, or MEMS, device, meaning that it is manufactured--and
functions--at a scale of a few millionths of a meter.
While it does not yet generate electrical signals,
it accurately collects sound data at frequencies
between 4,200 hertz and 35,000 hertz, overlapping
much of the range for the human ear (20 hertz to
"This is a critical step on the way to an engineered
cochlea," says Grosh. "With controlled and repeatable
methods, we've created a fluid chamber and membrane
that together mimic the functions of the basilar
membrane and fluid-filled chambers of the human cochlea.
We expect this type of device, once perfected, to
find uses in all kinds of sound-sensing applications
where low power is needed."
The cochlea, located in the inner ear of all mammals,
is a spiral-shaped, tubular, fluid-filled organ that
receives sound waves from the bones of the middle
ear and generates electrical signals for the brain
The new device, while not the first of its kind,
has three main benefits over existing artificial
cochlea: the methods behind its construction are
ideal for mass production; its 3-centimeter length
is comparable to the unwound human cochlea, which
is important for potential hearing aid applications;
and because there are no moving parts, the sensor
is incredibly efficient--a critical property for
potential use on autonomous underwater vehicles such
as unmanned military craft that rely on battery power.
"When someone builds a microphone, they don't do
it the same way the ear does," says White. "And yet,
the ear is an extremely successful design. We were
interested in seeing whether we could duplicate that
Just like optical chips based in part on the human
retina, or walking robots that mimic human motion,
the new design benefits from researchers' growing
understanding of the human body. White and Grosh
are not attempting to decipher the inner workings
of the cochlea. Rather, they are trying to engineer
knowledge of the cochlea into new devices. Still,
through their efforts to design an analog, the researchers
are learning more about how the biological cochlea
works, and in some cases determining that structures
in the ear, such as the outer hair cells, may have
a greater influence on hearing than previously thought.
"Many researchers have attempted to emulate the
function of the cochlea by using physical models," says
Grosh. "Our effort is novel as it takes advantage
of the tremendous control available through micromachining
With these processes, the researchers may be able
to craft large batches of inexpensive devices that
each incorporates tiny, precise features.
In its simplest form, the new device consists of
a rigid, micromachined Pyrex glass channel filled
with silicone oil and topped by a thin, tapered-width
membrane of silicon nitride. The membrane is sensitive
to higher frequency vibrations at its skinniest end
and gradually lower-frequency vibrations further
along the widening structure.
A small, separate membrane of the same material,
roughly 1 millimeter by 2 millimeters, provides another "window" to
the fluid-filled chamber. This small piece of silicon
nitride receives the initial sound waves and transmits
them into the main chamber much like the stapes in
the ear transmits sounds to a human cochlea.
If one generates a sound, the device resonates in
specific locations in response to the vibrations
produced. Each part of the membrane resonates with
a specific frequency, so when a sound wave strikes
the device, the membrane vibrates most excitedly
at the location that corresponds to the incoming
wave. That is the site where the sound wave "crests," says
While the component can detect sounds, it is not
yet configured to do anything with the information.
The next step is to affix to the membrane sensors
that can convert the vibration energy into electrical
impulses a processor can recognize.
"Microelectronics and microfabrication help us complete
the next step, adding many channels of electronic
output and developing active control like we see
in the biological cochlea," says Grosh.
Grosh and White hope to demonstrate functioning
electronics and sensing materials for a 32-channel
device in the near future. Adding active control
to the structure will be more challenging, but they
expect dramatic improvements in device performance.
University of Michigan Ann Arbor
University of Michigan Ann Arbor
University of Michigan, Ann Arbor, Michigan
Engineering Research Centers
Faculty Early Career Development (CAREER) Program
National Nanotechnology Infrastructure Network (NNIN)
#9876130 CAREER: Cochlear Analogues for Engineering Acoustics
#9986866 An Engineering Research Center In Wireless Integrated Microsystems