Laser tweezers make polymers shrink

17 Jun 2002

A novel laser technique for shrinking polymers could deliver new medical and communications applications.

Japanese scientists have discovered a unique way to produce reversible volume changes in polymer gels that could lead to new gel-based systems for light-sensitive actuators, sensors, slide valves and artificial muscles. Previous methods have relied on the heat from a laser to trigger volume changes. However Hiroaki Misawa and colleagues at the University of Tokushima, Japan, have developed an alternative technique that uses the radiation force of the beam.

Misawa discovered that focusing a laser beam sharply on a poly(N-isopropylacrylamide) gel, a technique called "laser-trapping", caused localized shrinking of up to 20 µm. "Our research shows that sharp focusing, also known as the laser-tweezer set-up, allows a volume phase transition to take place," explained Misawa. "This is without light absorption and heating, which is the most common mechanism." The gel resumes its original volume when the laser is removed.

The laser-trapping mechanism works by modifying the weak attraction forces in the gels. Focusing a laser beam on the gel's surface generates a localized radiation force. This alters the hydrophobic segment-segment interactions inside the polymer coils of the gel and a sudden expulsion of water from the gel network leads to the "shrinking" effect.

"We have been involved in this type of research since 1989. We used laser tweezers to trap small dielectric particles and gather them up into the focal point of the beam," Misawa said. "Applying this technique to gels was a logical extension of the research."

The gels showed responses of between 0.1 and 1 second, but Misawa wants to improve the accuracy of these times. "We want to control polymer properties to make response times more accurate," he said. "Characterizing chain entanglement, cross-linkage of the polymer network and solvent polymer friction effects could give us this option."