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.
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.”
