Because laser light can be precisely targeted, very specific shape changes can be introduced. Gel/laser combinations could find applications ranging from actuators (which convert energy into movement; an electric motor, for instance) to sensors.

Applications of this laser technique are still some way off, but light seems one of the most promising ways of triggering movement in non-living materials. Changes in temperature, acidity and chemical conditions can provoke size and shape changes in polymer gels, but these responses are often too slow to be truly useful.

Now, Hiroaki Misawa and colleagues at the University of Tokushima, Japan, say that they can make polymer gels shrink and swell in a split-second. Targeting laser light at the centre of a cylinder made of a polymer called N-isopropylacrylamide pinches together the tube's edges to form a dumb-bell shape. The original cylinder shape returns when the laser is switched off. The team reports its results in Nature1.

This movement is possible because of odd behaviour often seen in polymer gels. The attractive forces holding neighbouring molecules together and the repulsive forces trying to separate them are finely balanced. Small chemical and physical changes can disrupt this balance, making the whole polymer violently expand or collapse. This 'phase transition' is similar to what happens when heated liquid vaporizes at its critical point.

Misawa's team now shows that radiation forces from focused laser light disturb this delicate push-me, pull-me equilibrium -- and induce phase transition. The change is reversible and does not alter the polymer's shape over time.

"Repeated cycling did not change the thresholds of shrinkage and expansion," the group says. And crucially, they prove that the shrinking is not caused by temperature increases accompanying the laser radiation.

Crucial to the team's discovery is their use of 'heavy water', D₂O, says Zhibing Hu, a polymer gels researcher at the University of North Texas, Denton. In heavy water, the hydrogen found in normal water has been replaced by its chemical cousin deuterium. Heavy water looks and flows like normal water, but has different chemical properties.

In a gel, D₂O does not absorb the laser light's energy, and so stays cool where normal water would heat up. Heating the surrounding gel would prevent delicate control of the induced movement.

The technique could have medical applications, Hu notes. "Because gels that contain a large amount of water are very similar to tissues, in principal this method could be used to collapse tissue without heating effects," he says. "This may be useful for some cancer treatments." And it could be used as a new way of triggering controlled drug release from a gel.