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.