The Sound of One Cell Growing
Copyright 2000-2001
Source: Business Week Online
January 2, 2001
Researchers think they can build a "nanomicrophone"
based on tiny hairs -- rather than a membrane -- that could hear such
a thing
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 in pressure.
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
about?
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
technology.
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.
Edited by Douglas Harbrecht
The McGraw-Hill Companies Inc.
All rights reserved.
http://www.businessweek.com/bwdaily/dnflash/jan2001/nf2001012_818.htm