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Shark skin: the next high-sensitivity op amp?

The August 2007 print edition of Scientific American has a fascinating article on how sharks sense electric fields to help them locate nearby prey (“The Shark's Electric Sense,” available online only to paid subscribers). They have this ability due to a structure called the ampullae of Lorenzini, named after the 17th century anatomist who first described them, although their function was not fully understood or measured until the 1960s. The shark's snout has hundreds of these ampullae, forming a focused receptor array.

These sensors are also very sensitive to changes in temperature, and can detect changes as small as 0.2 degree C. Even more interesting, the ampullae communicate the data to the shark's brain by firing nerve pulses. As the electric field intensity and polarity changes, the firing rate of each ampula increases. We techies call it pulse-frequency modulation (PFM).

Are the shark's ampullae sensitive? Absolutely! Careful research data shows they can typically sense a field as low as one-millionth of a volt (1 microvolt) across one centimeter, and perhaps as low as 15 billionths of a volt (15 nanovolts). To make this point more tangible to the reader, the article used a very dramatic analogy: it said that the 1 microvolt sensitivity is equivalent to sensing a field across one centimeter due to a 1.5 V battery with one terminal in the water in Long Island Sound, NY, and the other at Jacksonville, FL, about 850 miles (1370 km).

My first reaction to the analogy was “no way that's possible,” so I did a basic calculation; I worked out a field strength of about 10 nV/cm for 1.5 V across that distance, which is less, by a factor of 100, than the stated typical sensitivity data. My second reaction was “no matter, those sharks are still darn good.” And my third reaction was “and so are our op amps,” since a good op amp in properly designed circuit can certainly discern microvolt and nanovolts signals.

We read a lot about “mechatronics”, the seamless blending of electronics, mechanics, sensors, microdevices, and smart embedded control. Mechatronics is happening now and will be coming on stronger, absolutely. But I suspect we'll also see an expanded version which I'll call “biomechatronics” which combines mechatronics with biological structures, such as the ear. Let's be humble: living organisms can do things which our technology cannot even begin to approach.

I also realized one other thing from this article. It's a good idea to use a dramatic, tangible analogy to make an important point “real” to the reader. It certainly brought the point home for me–but it's a good idea to try to be a little more accurate, too.

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