Polymer gels are remarkable, shape-shifting materials. When exposed to small alterations in acidity or temperature, for instance, they can dramatically transform their appearance and size. The secret to this trick is that forces between molecules in the gels are delicately balanced in a constant tug-of-war. Anything that strengthens electrostatic or hydrophobic repulsion within these gels will make them expand; influences building attraction, typically mediated by hydrogen bonding or van der Waals interactions, will make them shrink. Unfortunately, inducing these changes with accuracy or speed has proved difficult. But in today's issue of Nature, researchers from the University of Tokushima in Japan describe a new, incredibly precise tool for morphing polymer gels: laser light.
Hiroaki Misawa and colleagues focused a laser beam at the center of a cylinder of N-isopropylacrylamide (NIPAM) and found that within an instant, the rod's middle shrank in diameter, turning it into a dumbbell. When they shut the laser off, the middle snapped back to its original width. To demonstrate that radiation forces from the laser, and not temperature, were at work, they also illuminated the gel for several minutes with a weak laser, having insufficient strength to shrink it. The gel didn't change, which would have been the case had the heat built up. Because the transformations in the gels are entirely reversible and the light can be so precisely targeted, a number of applications--though far off--are possible, including gel-laser-based actuators and sensors.