20/20 lenses coat body of sea creature
Charlotte Schubert
Look closely enough at the arms of the brittlestar, a
starfish relative, and you'd see that those arms are
looking right back at you. Each one is coated with
perfect lenses that focus light onto a nerve bundle,
researchers report in the Aug. 23 Nature. Made of
skeletal material, these lens structures rival recent
engineering advances in microlens arrays.
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A brittlestar.
Aizenberg |
"To find them [microlens arrays] in nature is
absolutely astonishing," says physicist Roy Sambles of
the University of Exeter in England.
Joanna Aizenberg of Lucent Technologies' Bell Labs in
Murray Hill, N.J., and her colleagues discovered the
lenses while studying the architecture of the
transparent calcite skeletons that protect brittlestars.
In some species, the team found "an incredible array of
spherical structures" on the animals' skeleton, says
Aizenberg. The researchers began to suspect that these
arrays might be lenses when they realized the structures
occur only in those species of brittlestars that respond
to light. Such brittlestars skitter out of the way of
predators and zoom into crevices that they spy from a
distance. Brittlestar species that don't sense light
don't have the arrays, the researchers found.
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The crystalline skeleton of a
brittlestar forms an array of lenses.
Aizenberg |
To prove that the structures behave as lenses,
Aizenberg shined a light through the skeleton of the
brittlestar Ophiocoma wendtii. She found that
each spherical structure indeed focused light to a point
less than 3 micrometers across.
"When we measured the sizes of the focused regions,
we realized that these lenses were almost optically
perfect," says Aizenberg. Normally, calcite has the
optical property known as birefringence, in which light
travels two pathways to produce a double image. But the
natural lenses have orientations and internal structures
that minimize this limitation and other optical
shortcomings of calcite. For example, light shining into
a brittlestar's surface lens gets focused by a second
lens below the sphere.
Each focal spot impinges on a bundle of nerves that
feeds into a larger nerve cord, the researchers observed
by microscopy. The view for the brittlestar, suspects
Aizenberg, is "something like . . . compound eye
vision," which occurs in some insects.
Aizenberg says that the brittlestar's natural optical
elements may help guide engineers who design microlens
arrays. They now use such arrays for routing information
through optical fibers, but only in the past few years
have engineers made plastic microlens arrays in the
range of the brittlestar's minute scale. Says Sambles,
"It's exciting stuff."
References:
Aizenberg, J., et al. 2001.
Calcitic microlenses as part of the photoreceptor system
in brittlestars. Nature 412(Aug. 23):819.
Further Readings:
Additional information about
brittlestars can be found at http://home.att.net/~ophiuroid/home.html.
Sources:
Joanna Aizenberg
Bell
Laboratories
Lucent Technologies
Murray Hill, NJ
07974
Roy Sambles
Department of
Physics
University of Exeter
Stocker
Road
Exeter EX4 4QL
United Kingdom
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