Robots may soon flex their muscles

Researchers in Japan have discovered that laser light can quickly and accurately flex potential polymer muscles, a development that may give life to flexible robotic arms.

Polymer gels have been touted as a potential source of power for robot arms because they expand and contract when stimulated by heat or certain chemicals. Because laser light can be precisely targeted, very specific shape changes can be introduced.

Hiroaki Misawa and colleagues at the University of Tokushima targeted laser light at the centre of a cylinder made of N-isopropylacrylamide.

The light pinches together the tube’s edges to form a dumb-bell shape but resumes its original shape when the laser is switched off. Movement is made possible due to manipulation of attractive forces holding neighbouring molecules together and the repulsive forces trying to separate them.

Small chemical and physical changes can disrupt this balance, making the polymer violently expand or collapse.

This ‘phase transition’ is similar to what happens when heated liquid vaporises at its critical point but Misawa’s team has shown that radiation forces from focused laser light disturb this push and pull 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,’ said researchers.

Crucial to the team’s discovery is their use of ‘heavy water’, D2O, said 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In heavy water, the hydrogen found in normal water has been replaced by deuterium.

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 said. ‘This may be useful for some cancer treatments.’