Shaped by the sun

A solar cell mounted on the light actuated material can move and track a light source without wires, gears or motors. Credit: Fio Omenetto, Tufts University

A team of American engineers have developed a material that can make precise movements and form complex three dimensional shapes without wires or energy sources, instead responding to light.

The researchers say that the substance could be useful for developing smart light-driven systems, like solar cells that can move to follow the sun over the course of the day, or soft robots that move with light signals.

The material is a film made of two layers. The first layer is silk fibroin with gold nanoparticles integrated into it, which makes crystals that bend and reflect light in an interesting way – similar to the iridescent shimmer that comes from opals, butterfly wings and some feathers. This substance contracts when heated up and expands when cooled.

The second layer is a polymer called polydimethylsiloxane, which expands when heated. When light hits the substance, the top layer contracts and the lower layer expands, causing the substance to bend. Shining different wavelengths of light during production allows the material to be “programmed” to respond in specific ways. Different light at different angles can then make the material form 3D shapes

A paper published in Nature Communications describes the material.

“With our approach, we can pattern these opal-like films at multiple scales to design the way they absorb and reflect light,” says Fiorenzo Omenetto, an author on the study and a professor of engineering at Tufts University.

“When the light moves and the quantity of energy that’s absorbed changes, the material folds and moves differently as a function of its relative position to that light.”

While other light-sensitive materials have been created, they require fairly complicated chemistry to make, and are also relatively energy intensive. The advantage of this one is it’s relatively simple to create and tune to certain wavelengths of light.

The paper demonstrates a ‘photonic sunflower’ developed by the authors: solar cells combined with the film. The moving film keeps sunlight coming in at a constant angle, maximising the efficiency of the solar cells.

“We are able to achieve exquisite control of light-energy conversion and generate ‘macro motion’ of these materials without the need for any electricity or wires,” says Omenetto.