Light-loving polymer acts like a sunflower

Many living things exhibit phototropism – the ability to track a light source and align to it. The best-known examples are plants that self-orient to face the Sun throughout the day: most of us immediately think, appropriately enough, of sunflowers.

Creating an artificial material that can do this has proved challenging.

There are artificial “smart” materials that can move in response to a stimulus without direction – this is known as nastic behaviour – but until now no synthetic material has been able to exhibit tropistic behaviour.

Now, in a paper published in the journal Nature Nanotechnology, US researchers have revealed a nanostructured polymer material that, formed into small, cylindrical, stem-like shapes, is able to follow a beam of light – much like sunflowers do.

Xiaoshi Qian, Yusen Zhao and Yousif Alsaid, from the University of California Los Angeles, and their colleagues 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 then shaped the resulting 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 light-harvesting materials, as the cylinders bend autonomously to expose the tip to the maximum amount of light.

The system is termed a sunflower-like biomimetic omnidirectional tracker (SunBOT).

The research team’s results show that an array of SunBOTs can, in principle, be used in solar vapour generation devices – equipment that can extract fresh water from sea water or sewage.

If light is striking them at oblique angles, the SunBOTs can achieve up to a 400% solar-energy-harvesting improvement over nastic artificial materials.

The authors are understandably bullish about the possible applications for SunBOTs.

They conclude their report by stating: “The nearly infinite degree of freedom in adaptive locomotion may lead to self-sustained, untethered soft robots, autonomously capable of real-time learning and performing complex tasks in various environments.

“This work may be useful for enhanced solar harvesters, adaptive signal receivers, smart windows, self-contained robotics, solar sails for spaceships, guided surgery, self-regulating optical devices and intelligent energy generation (for example, solar cells and biofuels), as well as energetic emission detection and tracking with telescopes, radars and hydrophones.”

Related reading: Glowing, colour-changing polymer

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