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Nanoscale Ears based on Artificial Stereocilia

Flavio Noca -
Michael Hoenk, Brian Hunt, Dan Choy, Bob Kowalczyk
Microdevices Laboratory
Jet Propulsion Laboratory (JPL)
Pasadena, CA 91109

Jimmy Xu
Division of Engineering
Brown University, Providence, RI 02192

Petros Koumoutsakos, Thomas Werder, Jens Walther
Institute for Computational Sciences
ETH Zurich, Switzerland

Popular version of paper 2aEA2
Presented Tuesday morning, December 5, 2000
ASA/NOISE-CON 2000 Meeting, Newport Beach, CA

The world's most advanced acoustic systems perform incredible feats of detection and targeting, unerringly tracking tiny moving objects from fast-moving ships or other vessels despite severe background noise and clutter. They can detect objects buried in several feet of mud, navigate perfectly in complete darkness, and communicate over long distances in the ocean. While man-made acoustic technologies are better than ever, nature's acoustic systems remain unsurpassed.

At the most basic level, nature's acoustic systems are designed differently from state-of-the-art technology. We rely on ultra-sensitive membrane devices (thin films similar to the skin on a drum) and high-speed computers to detect and analyze sound in a "serial" fashion, or one bit of information at a time. On the other hand, nature uses thousands of tiny rod-like structures (called stereocilia, Figure 1) to detect sound and converts it to nerve impulses for processing in a "parallel" fashion, simultaneously analyzing numerous bits of data. Considering the advantages of stereocilia and neural processing begins to explain why technology has been unable to outperform nature. Moreover, with the help of recent advances in nanotechnology, we hope to create artificial stereocilia to begin learning how we can benefit from nature's example to make better acoustic sensors and systems.

We are presently developing a unique technology that will enable a new class of innovative microsensors and microactuators for real-world applications in gas or liquid environments. The primary product of this research will be the demonstration of acoustic sensing with artificial stereocilia arrays (Figure 2). The miniaturization and directional sensitivity intrinsic to this approach could ultimately lead to revolutionary advances in acoustic detection and signal processing.

Figure 1: Anatomy of the ear. (Top Left) Sketch of the human ear (Alec N. Salt, Washington University). (Top right) Cross-section through the cochlear canal (R. Eckert et al., "Animal Physiology," 1988). (Bottom Right) Organ of Corti (R. Eckert et al., "Animal Physiology," 1988). (Bottom Left) Stereocilia on inner hair cells. Scale bar: 1 micrometer (J. O. Pickles, "An introduction to the physiology of hearing," 1988).

Stereocilia are rod-like structures (Figure 1) found in the cochlea (inner ear) of all hearing animals. Sound shakes the eardrum, and in turn causes a sliding movement of the endolymphatic fluid (the inner-ear fluid which immerses the cochlea's inner hair cells) and motion of the tectorial membrane, the gel-like covering over a part of the inner ear known as the organs of Corti. This fluid and/or membrane displacement causes the deflection of stereocilia, which is then translated into a nerve signal. The sound of rustling leaves sets the stereocilia in a swinging motion whose amplitude barely exceeds a few atomic diameters! The conversion of acoustic signals into electrical nerve signals in the cochlea of biological organisms relies on the high sensitivity and small scale of stereocilia arrays, which is difficult to duplicate using conventional membrane-based technology for converting between acoustic and electrical signals.

Stereocilia are also present in animals' vestibular systems, which help them maintain balance and orientation. For example, lobsters contain a vestibular organ known as the statocyst which contains stereocilia. Fluid in the statocyst moves the stereocilia relative to the organ's statolith, compacted sand grains that are assimilated by the lobster. Similarly, stereocilia populate fishes' lateral line system, a mechanical-based sensor system for detecting differences in water pressure. They help fish tailor water flows along their body and, presumably, also aid them in spatial identification of preys and predators. Finally, even in non-hearing organisms (hydra, jellyfish, sea anemones), stereocilia may be present as mechanical-based receptors for detecting swimming prey.

The widespread use of stereocilia as acoustic and micro-flow transducers in the biological world represents a powerful incentive to develop acoustic sensor technology based on artificial stereocilia.

A majority of existing acoustic sensors are based on membrane deflection to detect sound. In nature, membranes are generally present as devices for exchanging energy between the acoustic environment and the actual zone where such conversion takes place, typically the cochlea, where the stereocilia are the ultimate performers of this task. Stereocilia are orders of magnitude smaller than membranes, and biological systems use membranes mostly at moderate scales, whereas at scales of microns (millionths of a meter), stereocilia predominate.

The technological difficulty of making nanometer-scale stereocilia geometries has precluded the use of artificial stereocilia in acoustic sensors. Figure 2 shows that biological stereocilia have diameters of the order of tens of nanometers (billionths of a meter) and a large aspect ratio, that is, a large length compared to their small diameters. Conventional MEMS (Micro Electro-Mechanical Systems) fabrication technologies cannot produce objects with diameters this small, and nanometer-scale MEMS is in its infancy. Our approach to this problem is to apply recently developed techniques for fabricating highly uniform arrays of carbon nanotubes that have diameters and aspect ratios comparable to biological stereocilia. The most recently discovered form of pure carbon, carbon nanotubes are rolled-up sheets of carbon atoms arranged in a hexagon pattern.

Figure 2: (Top Left) Highly ordered arrays of parallel carbon nanotubes. The nanotubes are characterized by a narrow size distribution, repeating patterns on a large scale and high densities. Ordered nanotubes with diameters from 10 nanometers to several hundred nanometers and lengths up to 100 micrometers can be produced. Scale bar: 200 nanometers (J. Li, C. Papadopoulos, J.M. Xu, and M. Moskovits, Appl. Phys. Lett. Vol 75, 367-369, 1999). (Bottom Left) Bundle of stereocilia protruding from an inner hair cell of the guinea-pig cochlea. Scale bar: 500 nanometers (J. O. Pickles, "An introduction to the physiology of hearing," 1988). (Right) Image of a human hair magnified 1000 times (Images of Nature, D. Adams, University of the Western Cape, South Africa, and J. Mayer, Arizona State University). The inset images illustrate the relative size of carbon nanotube arrays, stereocilia, and human hair when placed at the same scale.

This new nano-fabrication approach places stereocilia-based transducers within reach for the first time. Several unique properties of stereocilia and carbon nanotubes provide motivation for this choice:

  • The nanometer-scale diameter of stereocilia provides extreme sensitivity to small signals. Natural stereocilia have the capability of sensing signals below the noise levels due to "Brownian motion," the fluctuations due to heat.
  • Detecting acoustic signals from the air should be possible with artificial stereocilia made from carbon nanotube arrays (in contrast to natural stereocilia, which are always found in enclosed, liquid-filled environments). This unique property is possible because of the high strength of carbon nanotubes and their extreme sensitivity to small strains.
  • The directional sensitivity of artificial stereocilia is particularly important in applications involving the detection of localized sources in a background that is isotropic, or uniform in every direction. This technique for sound localization is common in fish. The most familiar application of directional acoustic sensors is hearing aid technology, in which directional sensitivity provides the ability to hear conversations in a crowded room.
  • Relative to membrane-based acoustic sensors, stereocilia effectively increase the capture of flow energy through a mechanical lever advantage while preserving the size of the projected area (attachment surface), a property that can be exploited to enable a high degree of miniaturization without loss of sensitivity.
  • The miniaturization enabled by stereocilia provides the basis for eventually fabricating an artificial cochlea, with signal processing using biologically inspired delay-line circuits.
  • While the present bio-inspired technology will benefit the development of a miniature air- or water-coupled acoustic sensor with directional sensitivity, several other applications are envisioned:

  • A "nanostethoscope" to probe nano/micro-scale biological activity by "listening to the music of life." In particular, the device can be used to test water quality (by detecting microflows generated by swimming microbial life) or monitor the health of living cells (by measuring the activity level in the intracellular medium, including metabolic events and biochemical reactions). For instance, the nanostethoscope may one day detect, monitor, and characterize the "sounds" of cancerous cells, which are known to have a more intense intracellular activity than healthy cells. This instrument can also serve one of the goals of NASA Astrobiology initiative to search for signatures of extant life on other planets.
  • "Smart skin" based on the fish lateral line system. The instrument can be used for the detection, localization, and identification of "preys" and "predators" in an ocean environment. It can also be integrated on a robotic fish for navigation and control. Finally, it can provide potential benefits to the aero/hydro-dynamics community as a hairy (passive or active) skin for the control of important fluid flows known as boundary layer flows.
  • An artificial "stridulator" for producing sounds. Insects such as crickets possess stridulators, body parts that rub together to produce sound important for their communication and other tasks. Artificial stereocilia (functioning as actuators instead of sensors) will be capable of generating acoustic signals for applications in active acoustic instruments such as sonar and sodar (sonic detection and ranging) arrays, acoustic systems that measure turbulence and wind speed in the atmosphere.
  • Figure 3: The "nanostethoscope" will one day be able to tap the "sounds of life" by sensing the microflows generated by biological activity (insets illustrate relative size at the same scale). ( Left) Microtubule network emanating from the nucleus of a mouse-cell (from G. Karp "Cell and Molecular Biology," 1999). The microtubules act like a "freeway network" for the transport of organelles and material across the cell. The nanostethoscope is envisioned to listen to the buzzing intracellular activity, and distinguish healthy cells from malignant ones. (Right) Spermatozoa jiggling around a human ovum (The Learning Company, 1997). Being able to sense swimming micro-organisms and biochemical events will help improve water quality and support NASA efforts in the search for signatures of life on other planets.

    The artificial stereocilia work is supported at the Jet Propulsion Laboratory (JPL) by the NASA Cross Enterprise Technology Development Program (CETDP) Breakthrough Sensors and Instrument Component Technology (BSICT), the Deep Space Systems Program Center for Integrated Space Microsystems (CISM), and the Director's Research and Development Fund (DRDF). JPL is a division of the California Institute of Technology. The carbon nanotube array fabrication technology was developed by Professor Jimmy Xu's group at Brown University and is supported by the Office of Naval Research (ONR), the Air Force Office of Scientific Research (AFOSR), and Motorola, Inc. Computational work is sponsored by ETH Zurich.

    Additional information about the BSICT and CISM programs is available at:

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