ACOUSTIC SENSORS BASED ON ARTIFICIAL STEREOCILIA

A unique, biologically inspired acoustic transducer based on artificial stereocilia is being developed. Stereocilia are rod-like structures found in the cochlea of all hearing animals. Auditory transduction results from stereocilia deflection, due to shear of the endolymphatic fluid and/or motion of the tectorial membrane.

Stereocilia are also present in the vestibular system of animals, as detectors of statolith motion or endolymphatic fluid displacement in the semicircular canals. Similarly, stereocilia populate the lateral line system of fish for spatial identification of sound sources. Finally, even in non-hearing organisms (hydra, jellyfish, sea anemones), stereocilia may be present as exquisitely sensitive mechanoreceptors for swimming prey detection (plankton). A majority of existing acoustic sensors are based on membrane deflection to detect sound. In Nature, membranes are generally present as coupling devices between the acoustic environment and the actual transduction zone, typically the cochlea, where the ultimate transducers are the stereocilia. Stereocilia are orders of magnitude smaller than membranes, and biological systems use membranes mostly at moderate scales, whereas at micron scales, stereocilia predominate.

The technological difficulty of fabricating nanometer-scale stereocilia geometries has precluded the use of artificial stereocilia in acoustic sensors. Our approach to this problem is to apply recently developed techniques for fabricating highly uniform arrays of carbon nanotubes that have diameters and lengths comparable to or smaller than biological stereocilia.

This transducer will enable directional sensitivity and miniaturization of mechanoreceptors while enhancing sensitivity, ultimately leading to revolutionary ad-vances in acoustic detection and signal processing. In particular, artificial stereocilia will be ideally suited for exploiting techniques such as stochastic resonance to detect acoustic signals below the noise floor. These unique signal processing capabilities could one day help explain the speculative activities of crawling animals and fish prior to earthquakes, and help humans detect potential precursor signs of ground tremor below ambient noise levels.