New Polymer Material Goes for a Walk When Illuminated

A new polymer material has been developed that can undulate and propel itself forward when exposed to light.

Scientists at Eindhoven University of Technology and Kent State University clamped a strip of the polymer material in a rectangular frame and illuminated it with one fixed light source; then they sat back and watched as it “walked” on its own.

Eindhoven University of Technology scientist Anne Hélène Gélébart is showing the walking device. Courtesy of Bart van Overbeeke.

The maximum speed is about half a centimeter per second — equivalent to that of a caterpillar. The researchers think it can be used to transport small items in hard-to-reach places or to keep the surface of solar cells clean. They placed grains of sand on the strip and these were removed by the undulating movement. The strip can even transport an object uphill that is much bigger and heavier than the device itself.

The motion of the new material is due to the fact that one side contracts in reaction to light, and the other one expands, causing it to bulge when illuminated. That deformation disappears instantaneously once the light is gone. The material looks transparent to the human eye, but it fully absorbs the violet light the researchers used, creating a shadow behind it.

This is a timelapse image of the walking device. Courtesy of Bart van Overbeeke.

The research team was able to create a continual undulating movement, using this self-shadowing effect. They attached a strip of the material in a frame shorter than the strip itself, causing it to bulge. Then they shone a concentrated LED light on it, from in front. The part of the strip that was in the light, started to bulge downward, creating a dent in the strip. As a consequence, the next part of the strip that was in the light started to deform. The dent moved backwards, creating a continual undulating movement. This set the device in motion, walking away from the light. When the device was placed upside down, the wave traveled in the opposite direction, causing it to walk towards the light.

The research has been published in the journal Nature (doi: 10.1038/nature22987).