Light-loving polymer acts like a sunflower

New smart material could be a game-changer for solar energy.

Many living things exhibit phototropism – the ability to track a light source and align to it. Plants, for example – such as the appropriately named sunflower – selforient to face the Sun throughout the day.

Creating an artificial material that can do this has proved challenging, however. Some “smart” materials can move in response to a stimulus without direction – known as nastic behaviour – but no synthetic material has been able to exhibit tropistic behaviour.

Now, in a paper published in Nature Nanotechnology, researchers from the University of California Los Angeles have revealed a nanostructured polymer material that, formed into small, cylindrical, stem-like shapes, is able to follow a beam of light.

Xiaoshi Qian, Yusen Zhao and

Yousif Alsaid used a photo-responsive nanomaterial that efficiently absorbs light and transforms it into heat, combining it with a thermo-responsive polymer that contracts when heated. They shaped this material into small cylinders.

When light hits the cylinders, they absorb it and become hotter, but only on the side facing the light source. As the material contracts on the illuminated side, the cylinder bends towards the light beam. Once the top of the cylinder aligns with the beam, the underside of the shaft, now in the shadow of the light, cools down, expands and stops the motion of the cylinder.

The cylinders can follow a light beam continuously in a wide range of directions, a trait the authors suggest could be used to improve the efficiency of lightharvesting materials, as the cylinders bend autonomously to expose the tip to the maximum amount of light.

The system is called SUNBOT – a sunflower-like biomimetic omnidirectional tracker.