Smart Crystals

Organic crystals change shape and then go back to their original shape when exposed to light of different colors.

Researchers in Japan have discovered organic crystals that change shape when illuminated with ultraviolet light and then return to their original form under visible light. In an April 11 Nature paper, they show that the crystals can go back and forth between the two different shapes up to 100 times before cracking. The crystals could lead to tiny machines that are driven by light, similar to today's microelectromechanical systems used in microfluidics chips and optical communications systems, which are driven by electricity.

Nano power: A thin whisker of crystal bends when exposed to ultraviolet light, pushing a gold particle (seen as a black spot) 90 times its weight 30 micrometers. Shining visible light on the crystal straightens it up. Credit: Masahiro Irie

The researchers, led by Masahiro Irie, a chemistry professor at Kyushu University, in Fukuoka, alternately shine ultraviolet and visible light on a rectangular crystal to make it contract and then extend back to its original length. They have also created a rod-shaped crystal that they were able to bend and then straighten up as many as 80 times. When it bends, the rod can push a gold microparticle, which is 90 times heavier than the crystal, through a distance of 30 micrometers.

Previously, Irie invented a type of organic molecule that changes color in response to light; he first described it in the mid-1980s. While studying single crystals of these molecules, his research team found that the crystals also change shape. "We were [very] excited to see the photo-induced shape change for the first time," Irie says.

Certain polymers and glasses are known to change shape when exposed to light. But this is the first time researchers have seen the effect in crystals, says Tomiki Ikeda, professor of polymer chemistry at the Tokyo Institute of Technology. Their regular structure could make shape-changing crystals useful in various applications. "This means that one can induce a precise change in shape," Ikeda says. "Every time, you will have the same change in shape upon exposure to the same light with the same intensity ... which is very important for precise control of mechanical work in micrometer-sized devices."

For example, the crystals could be incorporated into microfluidics chips to push tiny volumes of liquid through narrow channels; fluids are currently moved with voltage or pressure from external pumps. "It's much nicer if you could do it with light rather than connect electrodes or apply heat to it, something that would perhaps interfere with the process you're trying to control," says Mark Warner, a theoretical physicist at Cambridge University who studies shape-changing materials.

Shape-changing crystals could also be used in next-generation switches in fiber-optic communication systems. In these systems, packets of data traveling from one point to the other go through multiple hubs where they are switched to the correct route. Currently, light pulses are converted into electrical signals for switching, but fast next-generation switches might use microscopic mirrors to reflect the light from one optical fiber onto another. The crystals could be used to move the mirrors.

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The shape-changing crystals have some other advantages over polymers and glasses for use in micro-mechanical applications because they respond to light relatively quickly and with significant changes. They display "a much more dramatic effect than is available in other materials," says J. Michael McBride, an organic-chemistry professor at Yale University, in New Haven, CT. The crystals respond to light faster than polymers do, transforming in 25 microseconds while polymers take a few seconds. And they undergo more deformation than glasses do. The rectangular crystal, for instance, contracts by 5 to 7 percent of its length, whereas glassy molecules change by about 2 percent. But Warner points out that even though polymers are slower to respond, certain rubbery polymers show more than 100 percent deformation, so they might be more useful. "It depends on the application you have in mind."

Irie says that his research team is now trying to prepare various types of crystals that could handle shape changes many more than 100 times without cracking. The team is also designing new compounds that show different types of shape changes, such as going from a rod shape to a plate shape.