Sandia researcher George Bachand
Mimicking Fish
August / September 2009
Volume 7 Number 4
Anyone who's travelled from the desert Southwest, say, to the lush green of upstate New York knows that the colors people choose to wear change with the environment in which they find themselves. So what about—€”say, brown in the desert, green (or some matching color) in northeast farm country? Certainly any military landing force would like such a capability—€”marine blue while on the ocean, for example, and then some more fitting color as camouflage during landfall.
While that capability seems improbable, certain fish make these changes all the time, and scientists at the Center for Integrated Nanotechnologies (CINT) in Albuquerque are learning how to mimic these procedures in synthetic materials. CINT is operated by Sandia and Los Alamos national laboratories for the Department of Energy's Office of Science.
Consider a fish with skin speckles of black and red. To blend with a dark environment, the fish might bring together its black hues into significant patches, while distributing the red pigments in tiny dots all over its surface. The red would fade to a faint pink; a predator's eye would see only black.
Were the environment more reddish, the camouflaging fish could shepherd its red pigments into significant aggregations, while retiring its black pigment into tiny particles over its entire body, forming a background of entirely unnoticeable gray. Its red torso would blend with the red environment.
In the fish, this change is propelled by motor proteins—€”little moving proteins that resemble workers walking with sacks of cement on their shoulders. (Human cells have these workers as well.) They walk around the cell's skeleton, which is formed of little protein logs called microtubules. The worker proteins carry skin pigment crystals of color, rather than cement. The food for these workers is a fuel called ATP that is produced by each cell.
The Sandia work removes these components from cells and upends the motor proteins so they are upside down. By sticking their tops—€”in effect, their shoulders, or former cargo-carrying surface—€”on a laboratory slide made sticky, the legs of the little entities move back and forth above them as if they were walking upside down and getting nowhere, or like patrons at a rock concert waving their arms above their heads.
Free-ranging microtubules added to the mix are carried along by the waving arms like singers who have jumped from the stage into the audience and are passed from overhead hands to overhead hands around the auditorium.
To these microtubules, Sandia scientist George Bachand has attached quantum dots that sit like jewels on a finger ring around the tubules. Quantum dots—€”nanoscopic, light-emitting dots of cadmium selenide—€”emit light. The wavelength of the light they emit depends solely on their size, or molecular aggregation. The same material, if of different size, will emit a different color of light when stimulated.
So imagine two sets of differently sized quantum dots, each reflecting a separate color. When Bachand pushes a chemical switch, the legs of the upside-down workers bring forward free-ranging microtubules bearing a particular size of quantum dot. The process aggregates them. When Bachand hits the switch again, the dots separate into the obscurity whence they came.
The scientific work is much simpler than the more complicated biological procedures used by the camouflaging fish. But the intent is the same: to produce a surface that automatically blends with its surroundings.
In fact, the colors emitted by quantum dots are more variable, because they emit different frequencies of light than they adsorb, while the biological system merely reflects incoming wavelengths. But they perform similar coloring functions.
"Camouflage outfits that blend with a variety of environments without need of an outside power source—€”blue when at sea and then brown in a desert environment—€”is where this work could eventually lead," says Bachand. "Or the same effect could be used in fabricating chic civilian clothing that automatically changes color to fit different visual settings." Such clothing could be a reality in five to ten years, he says.
While it's difficult to fix a dollar value on an innovation so radical that industry hasn't yet even considered it, there is no doubt that the market from environmentalists looking to dress in keeping with their environment, fashionistas looking to stay one jump ahead in style competition, and military purchasing agents looking for innovative materials to protect their soldiers, would be large.
To put motor proteins in motion or switch them off, nature uses complex signaling networks. The Bachand group's method is simpler. It involves the genetic insertion of a kind of docking port in the motor protein's structure. What docks are zinc ions. Bound zinc ions turn the protein's action to "off." Stripping zinc ions out with chemical agents allows the motor protein to work again. The effect is controllable, and even reversible.
"We essentially reengineered the protein structure to introduce a switch into the motor," says Bachand. "So we can now turn our nanofluidic devices on and off."
Previous efforts at regulating motor activity have used fuel intake as a control mechanism: the less the fuel, the slower the process. The Bachand group's switch, operated independently of fuel changes, resembles the improvement in early automobile technologies when a simple ignition switch took over for more complicated rheostats.
When motor-transported microtubules collide, the microtubules stick together and twist until they resemble a desk phone cord. The twisting process ultimately forces the formation of stable rings approximately five micrometers in diameter. Their docked quantum dots (cadmium selenide) produce a range of light frequencies.
When mechanical strain in the rings causes them to rupture, the cracked segments are tugged out by the nearby motors until the ring is completely disassembled. The formation and destruction of the two states—€”free microtubules and rings—€”can be reversibly controlled. Thus the dots can be tightly packed or dispersed—€”optically, an essential ingredient in the perception of color change.
The process resembles the action of fish color changes, which require one group of motor proteins carrying pigments to be "on" all the time while a second group of motor proteins is turned on by complex biological processes at the right time. This produces a tug-of-war between motor groups that results in pigment dispersion and ultimately a color change. When the second motor is switched off, the color returns to the ground aggregate state.
"Our overall process mimics the fish," says Bachand. "We essentially go from a dispersed particle state to a concentrated one and then back again to dispersed, similar to the fish. Thus, in principle, the mechanism could produce a color change. The underlying science provides a new basis for materials scientists to begin working toward real-world applications."
Neal Singer is a science writer at Sandia National Labs.
Copyright © 2012 | Innovation America