Photonic material in the shape of a flower can move in response to light, closely tracking the angle of maximum exposure. Image: Fio Omenetto, Tufts University.
Photonic material in the shape of a flower can move in response to light, closely tracking the angle of maximum exposure. Image: Fio Omenetto, Tufts University.

Researchers at Tufts University School of Engineering have created a light-activated composite material able to execute precise, visible movements and form complex three-dimensional shapes without the need for wires or other actuating materials or energy sources. The material combines programmable photonic crystals with an elastomeric composite that can be engineered at the macro- and nanoscale to respond to illumination.

This research provides new avenues for the development of smart light-driven systems, such as high-efficiency, self-aligning solar cells that automatically follow the Sun's direction and angle of light, light-actuated microfluidic valves and soft robots that move with light on demand. The researchers report their work in a paper in Nature Communications.

Color results from the absorption and reflection of light. Behind every flash of an iridescent butterfly wing or opal gemstone lie complex interactions in which natural photonic crystals embedded in the wing or stone absorb light of specific frequencies and reflect others. The angle at which the light meets the crystalline surface can affect which wavelengths are absorbed and the heat that is generated from that absorbed energy.

The photonic material designed by the Tufts team combines two layers: an opal-like film made of silk fibroin doped with gold nanoparticles (AuNPs), forming photonic crystals, and an underlying substrate of polydimethylsiloxane (PDMS), a silicon-based polymer. In addition to remarkable flexibility, durability and optical properties, silk fibroin is unusual in having a negative coefficient of thermal expansion (CTE), meaning that it contracts when heated and expands when cooled. PDMS, in contrast, has a high CTE and expands rapidly when heated. As a result, when the novel material is exposed to light, one layer heats up much more rapidly than the other, causing the material to bend as one side expands and the other contracts, or expands more slowly.

"With our approach, we can pattern these opal-like films at multiple scales to design the way they absorb and reflect light," said Fiorenzo Omenetto, professor of engineering at Tufts and corresponding author of the paper. "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."

Most optomechanical devices that convert light to movement require complex and energy-intensive fabrication or setups, but that's not the case with this novel material. "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," Omenetto said.

The researchers programmed the photonic crystal films by applying stencils and then exposing them to water vapor to generate specific patterns. The pattern of surface water altered the wavelengths of light absorbed and reflected from the film, thus causing it to bend, fold and twist in different ways, depending on the geometry of the pattern, when exposed to laser light.

Using this material, the researchers developed a 'photonic sunflower', containing integrated solar cells in the bilayer film that could track a light source. The photonic sunflower kept the angle between the solar cells and a laser beam nearly constant, maximizing the cells' efficiency as the light moved.

The system would work as well with white light as it did with laser light. Such wireless, light-responsive, heliotropic (Sun-following) systems could potentially enhance light-to-energy conversion efficiency for the solar power industry. The team's demonstrations of the material also included a butterfly whose wings opened and closed in response to light, and a self-folding box.

This story is adapted from material from Tufts University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.