Manmade “Chameleons”

By TANG YUANKAI

Professor Peng Huisheng giving “Magical Chemistry” class to middle school students.

FINDING chameleons is no easy task. As most school children could tell you, they have a peculiar knack for changing color. This unique ability is the product of specialized chromatophore cells – pigment-containing and light-reflecting cells found in many classes of animals, but not mammals.

The crafty chameleon boasts chromatophore cells in three overlapping layers beneath a transparent outer skin. The location and movement of pigment within these layers, controlled by impulses from the brain, dictates color at any given time.

Changeable skin color serves two purposes: to adapt to immediate environment in order to remain undetected by predators such as birds, and also to provide a kind of “social signaling” to other chameleons. In the case of males, color change is also an important draw card when courting the opposite sex.

For scientists, simulating chameleons’ color changing abilities is both a dream and a riddle.

Researchers from Shanghai Fudan University’s Laboratory of Advanced Materials at the Department of Macromolecular Science recently invented a color-changing electrochromic smart material. Their breakthrough came from forming a composite fiber from polydiacetylene and carbon nanotubes for the first time.

The key point with regard to the color-changing mechanism of chameleons is its “sensitivity” – the autonomic nervous system controls the spread of melanocyte, and hence, colors. The distribution of pigment in the cells also changes according to light, temperature or other environmental factors.

Polydiacetylene (PDA) is a “sensitive” polymer. Its color changes rapidly from blue under the room temperature, to red when heated with an electrical current, and to orange when encountered with higher temperatures. It also changes color on coming into contact with different stimuli, such as chemical reagents, solutions of varying acidities, mechanical friction, or other biomolecules.

PDA is not commonly used in industry due to two inherent flaws. Firstly, low conductivity means that color change requires a large, costly electric current. Secondly, color change is irreversible, i.e., once a change takes place, there is no going back to the “home” color.

Researchers at Fudan University have solved this second problem. In their laboratories, once the current passing through PDA is switched off, it reverts to its “home” color. Whereas chameleons need 20 seconds to fully change their color, this new and improved material does it in two seconds. In the future, changing the color of various household products could be as easy as flicking a switch.

The miracle makers in Fudan are all very young. Doctoral tutor Peng Hui-sheng, who heads up the research team, has just entered his thirties.

Peng and his colleagues were at first committed to the research of carbon nanotubes, and started looking into this futuristic material in 2008.

Carbon Nanotubes, made in essence of graphite, have a distinctive chemical make-up that boasts extremely high electrical conductivity at room temperature. Since it was first created in 1991, it has become a key focus of research in the field of materials science. Peng says his team’s chameleon fiber was actually an accidental discovery while looking into certain properties of polydiacetylene-carbon nanotube composites.

Peng recalls that in one experiment his team connected a polymer to a carbon nanotube and witnessed a beautiful, if unexpected, color change. “We turned up the electric current, and it changed to a brilliant red. Over a number of subsequent sessions we changed the structure of the composite to create a variety other shades, such as yellow, orange and brown. But the best thing was that when we turned the electric current off, the material returned to its original color!” As the electric conductivity of carbon nanotubes is close to that of certain metals, such as copper and iron at room temperature, Peng’s team came up with the idea of “weaving the tubes in” with PDA to make the latter more conductive and hence color variable.

After many more experiments and trial attempts at manufacture, his team finally produced a composite fiber exhibiting the unique properties of both materials. It has high electrical conductivity and highly controllable color change abilities. What’s more, its breaking strength is beyond one megapascal. The team has applied for many patents both at home and abroad for their wonder material.

The width of carbon nanotube is incomprehensibly small – 100,000th of one hair. To the touch, the composite fiber feels like a regular chemical fiber. Peng sees a “colorful” future in which lampshades, quilts and clothes could all be made from his composite.

But the usefulness of carbon nanotubes extends far beyond the household and fashion. “The Boeing 787 is constructed from a strong and light carbon fiber composite. Carbon nanotube fiber, however, is one 10th as dense as carbon fiber, considerably lighter and immensely stronger,” Peng said, adding that for the auto industry, especially for the manufacture of high-end sports cars, this new material is a godsend. It reduces weight, hence decreasing fuel consumption, in addition to being stronger, more durable and easily moldable.

Professor Peng and his team are now working on using carbon nanotube fiber in solar cells to increase their efficiency.

It is believed that Professor Peng’s invention will eventually distinguish itself in aerospace and military roles, and in photoelectric devices. A super strong material with the color change abilities of a chameleon may sound like the future, but the technology is here today.