Science Magazine
09-07-06
BIOENGINEERING:
Artificial Arrays Could Help Submarines Make Like a Fish
Briahna Gray
An interdisciplinary team has developed nanostructures that mimic how marine
animals hunt, evade predators, and stay in the swim of things
Listen. As you read, tiny hair cells in your inner ear amplify and convert sound
waves into electrical signals that can alert you to the output of your iPod or the
approach of a subway train. Similar structures on other animals, such as seal
whiskers and the hairs on spider legs, help those organisms to track prey and
evade predators. Now, engineers and biologists have developed the world's first
functional artificial hair cell to mimic one of nature's most widespread and
versatile data-collecting systems: the lateral lines of fish.
In a paper published in an August issue of EURASIP Journal on Applied Signal
Processing, engineer Chang Liu of the University of Illinois, Urbana-Champaign,
describes how biologically inspired microstructures enable a model fish to locate
and track a dipole source. Real fish use a linear swatch of hair cells on their
sides, known as the lateral line, to coordinate group movements, avoid predators,
and otherwise navigate. "I'm thrilled to see this," says Jeannette Yen, director of
the Center for Biologically Inspired Design at the Georgia Institute of Technology
in Atlanta. "It shows that we do understand the biological system well enough to
make a mimic that works in a similar way."
Morley Stone, a former program manager at the U.S. Defense Advanced
Research Projects Agency (DARPA), which funds Liu under a project called
BioSenSE (Biological Sensory Structure Emulation), hopes that artificial hair cells
might someday be used to navigate crewless underwater vehicles too small to be
equipped with cameras. The hair cells would greatly expand underwater imaging
capacities beyond those now generated by sonar or cameras, he notes. "When
you look through a soda straw, it's hard to get an idea of what your world looks
like," says Stone.
Like their analogs in real fish, Liu's hair cells work by measuring the movement of
nearby water. Most commercial flow sensors measure the change in electrical
resistance when flowing water cools a heated metal wire. Although Liu has also
developed lateral-line arrays using more conventional "hot wire" technology, his
hair sensors, by contrast, are activated by force. These are made using a
standard microfabrication technique called photolithography to carve polymers
into long, flexible, narrow strands about 500 to 700 micrometers long and 80
micrometers in diameter. The strands are rooted in a silicon base called a pedal,
creating a minuscule lever. When the hairs are bent, the strain on the pedal
causes a change in electrical potential that correlates to flow velocity.
Liu tested his lateral-line array by installing it in an artificial fish. The model was
attached via a rod to an agile motion stage whose positioning was directed by
signals received by the fish in response to a wriggling dipole source. Although
Liu's array used only 16 hairs rather than the 100 usually found on real fish, the
artificial fish was able to target and track the moving source.
The BioSenSE team includes biologists, neurologists, engineers, and
mathematical modelers, all working to reverse-engineer nature's blueprint. "This
is one of the largest international groups we've been able to pull together," says
Stone. For example, Sheryl Coombs, a neurobiologist at Bowling Green State
University in Ohio, has collected data on the spatial distribution of pressure along
the lateral line of real fish to develop algorithms sensitive enough to process the
wealth of information gleaned by Liu's sensors. That information was then
validated by numerical simulations carried out by biologist-engineer Joseph
Humphrey of the University of Virginia, Charlottesville, and applied to the
programming efforts of Douglas Jones, an engineer at the University of Illinois,
Urbana-Champaign. "It illustrates the best of this new set of collaborations
between biologists and engineers," says Steven Vogel of Duke University in
Durham, North Carolina, who studies biomechanics.
Coombs's experiments show that even blinded fish still orient themselves toward
movement via a "map of touch" created by their sensory system. Abroad,
zoologists Horst Bleckmann of the University of Bonn in Germany and Friedrich
Barth of the University of Vienna in Austria are studying seals and spiders,
respectively, for potential applications in both underwater tracking and airborne
drones.
At Iowa State University in Ames, engineer Vladimir Tsukruk and his team
used a synthetic hydrogel to mimic the soft cupula tissues surrounding fish hair
cells that help relay information. The gel both protects the hairs against corrosion
and makes them 10 times more sensitive. Liu's hair sensors can detect flows
slower than 1 millimeter per second, half the rate of conventional sensors.
However, increasing the sensitivity of the sensor is a double-edged sword, says
Liu, because of the added burden of filtering out unwanted noise. Scientists are
using fish biology as a guide to tackle that problem as well, managing to mimic
their hair cells' structural alignment that allows fish to weed out background
noise.
Although the sensors were developed primarily to help guide small, robotic
vehicles, Liu suggests that they could also assist submarines. For example,
submarines now employ passive sonar to avoid giving away their position. But
because that technology reads signals generated by noise, it cannot detect a
stationary submarine or the subtle vortexes shed by large rocks. In addition,
active sonar requires the emitted "ping" to travel away from the ship so that the
feedback can be analyzed. That constraint creates a blind zone around the craft
that makes subs vulnerable to sabotage by bomb-carrying divers, says Liu.
Liu says that his array can eliminate that problem by detecting movement within
a radius of about three to four times the length of the vessel, 200 meters or less
for a full-sized submarine. Liu's hair cells are sensitive enough to detect both
divers and large, unmoving bodies such as rock faces that are normally invisible
in dark or murky conditions. Hair-cell sensors also have shown the potential to
track other submarines based on wakes created minutes before, just as seals
use their whiskers to track their prey. To turn those applications into reality,
however, the artificial hair cells must be robust enough to withstand a marine
environment.
Scientists can also imagine nonmilitary applications for the sensors. Changing
the shape of the hair, Liu speculates, could yield vibration or tactile sensors in
addition to flow sensors. Scaling up production could lower the cost of
semiconductor sensors from $12 to $1 per unit, opening up markets as diverse
as sneakers, MP3 players, and stress gauges in buildings in earthquake-prone
areas.
Despite the many challenges, Stone predicts that DARPA will pick up the project
for a second term beginning this fall. And if all goes well, someday hair cells
might alert your iPod as well as your ear to the rumbling of an approaching
subway train.