Polymer Shrinks And Swells

THE OSCILLATING Belousov-Zhabotinsky (BZ) reaction, famous for producing dazzling color-changing patterns, is now also finding use as a novel way to move matter.

Shingo Maeda of Waseda University, in Tokyo, and colleagues designed a polymer gel that undulates with a peristaltic motion similar to the way cilia or intestinal muscles pulse with rhythmical, directional contractions. As the gel undergoes the BZ reaction, a ruthenium catalyst periodically changes its oxidation state back and forth, generating chemical waves that propagate outward, causing the gel to shrink and swell (Angew. Chem. Int. Ed. 2008, 47, 6690).

The authors write that they envision use of the phenomenon to develop lifelike systems—"biomimetic devices such as autonomous matter transporters or self-beating micropumps that could function without external control."

The new gel, composed of a cross-linked polymer and ruthenium, is activated by dunking it in a solution of citric acid, which sets off the BZ reaction. Eventually, the reaction medium gets spent, but with the addition of more citric acid, the process starts over again, Maeda explains.

The discovery comes on the heels of work from Kenichi Koshigawa at the University of Kyoto and colleagues, who also studied the BZ reaction's potential to move matter ( J. Phys. Chem. C 2008, 112, 3032). Rather than the swelling and shrinking of gels, their system harnesses a solution that undergoes the BZ reaction. They were able to use that to propel a small piece of paper along the chemical wave fronts.

Koshigawa notes that the transport speed of the Waseda group's gel is still slow (about 1 mm per minute) compared with that of their solution (about 10 mm per minute). "The remaining problem is how to speed up the deformation rate," Koshigawa says referring to the polymer system.

Maeda points out that the composition of the gels actually changes during the reaction, making possible a wider range of applications than Koshigawa's strategy.