For years, chemists have tried to make materials that change shape in directed ways, but, for the most part, they have succeeded only in creating materials that move in a disorderly fashion. If materials are too rigid, they may break easily, and if they are too soft, they may be just an amorphous blob. Now scientists from Kyushu University in Fukuoka and from Osaka City University, both in Japan, have produced crystals that can repeatedly and reversibly change shape when irradiated with certain wavelengths of light.
Principal investigator Masahiro Irie, now at Rikkyo University in Tokyo, said that the crystals could function as micromechanical devices that move biological cells or control fluid flow in microreactors. He said that these actuators would not require wires or other direct connections because they are controlled by light.
To build the appropriate crystals, the researchers had to use the right chemicals. They chose to use diarylethenes because they undergo reversible isomerization — even in rigid crystals — when irradiated with light. However, previous experiments with diarylethenes have shown the change to be irreversible, so the researchers had to alter the chemicals to make the process reversible. Irie said that they believe the presence of fluorine atoms may explain why their crystals can change shape reversibly, while other chemicals cannot.
To induce the shape change, the researchers illuminated the crystals with 365-nm (UV) light using an LED from Keyence Corp. of Osaka, Japan. To reverse the shape change, they shined >500-nm light on the crystals using a bandpass filter and a xenon-mercury lamp from Moritex Corp. of Tokyo. They took photographs of the crystals using a Nikon digital camera connected to either a Leica or a Nikon microscope. The crystals ranged in size from 10 to 100 μm.
When exposed to UV light, a square crystal morphed into a “lozenge,” and a rectangular crystal contracted by as much as 7 percent. Visible light reversed these changes. The researchers carried out up to 20 cycles of shape changes without seeing any evidence of structural strain.
In their second experiment, they prepared rodlike crystals. Focusing a laser on the rods caused them to bend rapidly. The rods became straight upon irradiation with visible light. The scientists employed a pulsed 355-nm Nd:YAG laser from Spectra-Physics of Mountain View, Calif. They did not need much power — only 60 mJ per pulse. Because the shape change happened fast, they needed a high-speed camera, which they obtained from Vision Research Inc. of Wayne, N.J. They also used a Hamamatsu image intensifier.
The rods underwent 80 cycles of bending and straightening without any noticeable structural fatigue, and the bending rate was only ∼25 μs. The researchers demonstrated how these rods can be used to manipulate objects by placing a gold or silica microsphere by a rod and irradiating the rod with the pulsed LED. Like a catapult, the rod hurled the gold microsphere 30 μm away, no small feat considering that the microsphere weighed 90 times more than the rod (see figure).
“Our important achievement is that the isomerization at the molecular scale is amplified to bulk mechanical works of crystals,” Irie said, adding that the group wants to reveal the detailed amplification mechanism of shape changes in the future.
The work is detailed in the April 12 issue of Nature.