JANUARY 2, 2001
The Sound of One Cell Growing
Researchers think they can build a "nanomicrophone"
based on tiny hairs -- rather than a membrane -- that
could hear such a thing
Edited by Douglas Harbrecht
The search for a new method of detecting life on
distant planets may have led to the invention of the
smallest and most sensitive microphone ever devised.
Indeed, the tiny sensor may permit researchers to
listen to the sound of a swimming bacterium or hear the
gurgling of fluids inside living cells. "There's a
whole world buzzing down there," observes Flavio Noca,
who heads the research effort at Jet Propulsion
Laboratory (JPL) in Pasadena, Calif. "Movement is one
of the signatures for life."
The design of the new sensor, developed in conjuction
with NASA, borrows from biology. Unlike all acoustical
microphones in use today, the device doesn't rely on a
drum-like membrane to capture sound waves. Instead, it
mimics the field of tiny hairs, called stereocilia,
that line the inner ear and transmit sound to the
brain. Noca's artificial ear is constructed from arrays
of nanoscale tubes of carbon, so small they're measured
on the scale of billionths of a meter. Like cilia, the
tiny filaments bend in response to the slightest change
Alan Hall, science and technology correspondent for
Business Week Online, recently asked researcher Noca
about the invention and its eventual uses. Here are
edited excerpts of their conversation:
Q: How did the idea of artificial stereocilia come
A: At the beginning of last year, JPL set for itself
the grand challenge of determining whether it would be
possible to detect signatures of life on other planets,
and a solicitation for proposals was released
internally. The requirements on the device were
stringent because it had to be placed on a spacecraft
-- power, size, and weight are strong limitations.
A colleague, Michael Hoenk, and I thought that a
universal signature of life is movement -- even at the
molecular level -- and being able to sense such
activity could provide some answers. By working out the
numbers, I found that flat acoustic membranes would
never do the job of sensing nanoscale movement. That's
how I came up with the idea of using protruding rods
for detecting movement in mid-May, 1999. I approached
some professors at Caltech studying swimming micro-
organisms, and they agreed that this was a "cute" idea.
Q: How did you arrive at carbon nanotubes?
A: Quite by accident. Our group supervisor, Brian Hunt,
was about to start a nanotechnology effort at JPL and
had been in contact with Professor Jimmy Xu, who's now
at Brown University. Xu told him that he had just been
able to manufacture perfectly ordered nanotube arrays.
These nanoscale rods protruding from a surface were
just what we needed for our sensors.
It was only in the next few days, by early June, 1999,
that I realized that these nanorod arrays already
existed in nature in the form of stereocilia. I was
even more surprised to find out that cilia are to be
found almost everywhere. Until then, human technology
was just not capable of reproducing such small gadgets.
With these nanotube arrays, the artificial stereocilia
were about to become reality.
Q: Membranes seem pretty common in nature -- our ear
drums, for example. Why not just make them smaller?
A: Most of our competitors are trying to do just that.
Several research groups are attempting to make arrays
of micromembranes and assemble them on a single silicon
wafer. The U.S. Navy wants such a chip so that it can
make acoustic cameras for divers to detect mines in
turbid water and at night. A membrane-based approach to
directional sensing of sound waves is being developed
by Lucent Technologies. This group recently
demonstrated a tent-shaped sensor with a membrane on
each of the four facets.
Q: Then what makes artificial stereocilia superior?
A: The miniaturization of conventional acoustic sensors
is limited by the increasing stiffness of membranes as
the size is reduced. In nature, membranes are present
only as coupling devices between the acoustic
environment and the zone, typically the cochlea, where
the signal is picked up by stereocilia. Nature has
evolved toward this solution, probably because of the
unique properties of stereocilia at very small scales.
This is consistent with the absence of microscale
tympanic membranes in living systems.
Our project is motivated by the observation that
conventional approaches to acoustics cannot duplicate
the unique properties of biological hearing organs.
Moreover, the nanotube arrays are simple and cheap to
make, and can be assembled into large and dense arrays.
Q: You said stereocilia were "found almost everywhere"
in nature. Where, for example?
A: Stereocilia are found in the cochlea of all hearing
animals. They're also present in the vestibular -- or
balance -- system of animals, from our inner ear to
lobsters. Stereocilia populate the lateral-line system
of fish for the measurement of water flows along the
animal body and, presumably, also for identifying the
direction of sound sources. Even nonhearing organisms,
such as hydra, jellyfish, and sea anemones, rely on
stereocilia to detect swimming prey.
Q: Then why are scientists just now looking at
stereocilia as a model for acoustic sensors?
A: The idea of membranes was also taken from nature,
and today they're universally used as acoustic
transducers in microphones. But it is stereocilia that
do the real work. They weren't considered because the
technological basis for making ordered arrays of
nanometer-scale carbon nanotubes did not exist until
recently. Our proposal represents the first viable
technological alternative to membranes.
Q: What makes stereocilia unique?
A: The nanometer-scale diameter of stereocilia provides
extreme sensitivity to small signals. Natural
stereocilia have the capability of sensing signals
below the natural movement of molecules. Unlike current
microphones, stereocilia are directional -- the tubes
always bend away from the source of sound. Also, among
our surprising results is a calculation showing the
feasibility of using artificial stereocilia in an air
environment in contrast to biological stereocilia,
which are always found in liquids.
Q: Where do you foresee early applications?
A: It's likely that nanotube-based acoustic sensors
will first find their way into hearing-aid technology,
especially because of their directional sensitivity.
Humans rely on two separated ears to detect the
direction of a sound source. Directional sensitivity is
important for being able to pick up conversations in a
crowded room. You want to hear the person in front of
you and not all the chat going on around you. The
military obviously has a keen interest in this
Q: Are there other military possibilities?
A: One is imitating fish's lateral lines, which are
long canals coated with stereocilia on the side of
animal. We think that fish use these for prey
detection, localization, and identification, and also
as flow-control devices. In the past few years, there
has been an incentive to develop autonomous swimming
robots, so sensors for flow control would be very
useful. It's still not well understood how fish detect
prey with lateral lines, but the Navy would be very
interested in having a similar device to detect foreign
objects in the ocean in a passive way.
Q: What about medicine?
A: A nanoscale "acoustic" sensor will capture the
frequencies of chemical and metabolic events occurring
within a cell. Some time in the future, we can envision
these "nanostethoscopes" floating in the bloodstream
and body fluids and listening passively to biochemical
events occurring in the body.
For instance, it's known that the intracellular
activity of cancerous cells tends to be much higher
than for healthy cells. A nanoexplorer loaded with a
nanostethoscope could probably "hear" such cells. It
may one day be possible to detect tumors when only a
few cells are cancerous.
Q: Have you considered founding a startup company?
A: Many venture capitalists have shown interest.
Because of JPL ethics regulations, we're not allowed to
own a startup while being a member of the JPL
community. The license is owned by the California
Institute of Technology since we're Caltech employees.
However, Caltech does offer priorities to the Caltech
inventors when giving out licenses. We're waiting for
the project to become more mature before thinking of
venturing in the real world.
Q. What's your immediate goal?
A: Develop an actual working device and prove that it
satisfies all expectations.
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