Image: H. FINKELMANN et al./Univ. of Freiburg

Scientists Use Light to Make Polymers Change Shape and Size

For some time scientists have added photosensitive molecules to polymers to manipulate how the materials bend light. But now researchers at the University of Freiburg in Germany and at the University of Cambridge have managed the reverse: in this week's issue of Physical Review Letters, they describe how, for the first time ever, they used light to actually change the shape and size of a polymermaking it shrink or expand with the flip of a switch.

The scientists placed photosensitive molecules in the cross-linking bridges that hold long linear chains of polymers together to form larger network of molecules. When they exposed the resulting polymer to light, it contracted by 22 percent in length while expanding slightly in width and thickness. When they turned the light off, the polymer returned to its original size within hours. By absorbing a photon of light, photosensitive molecules can reposition chemical bonds and thus create a "kink" in the polymer chain. According to the new research, these kinks control the polymer's contraction.

Scientists had previously changed the shape of such polymers, known as nematic elastomers, using heat. The image on the right shows a heated strip of nematic elastomer contracting to lift a 30-gram weight. When the strips cools, the weight is lowered. The Freiburg group proposes that light can also induce shape changes anywhere from 10 to 400 percent. "These are reversible," they write, "and offer interesting possibilities as actuators and other devices which can generate mechanical work."