Controlled only by light, smart new materials twist, bend and move

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

The research offers new avenues for the development of intelligent light-driven systems, such as high-efficiency, self-aligning solar cells that automatically follow the direction and angle of sunlight, microfluidic valves actuated by the light. light or flexible robots that move with light on demand. . A “photonic sunflower,” whose petals curl in and out of lighting and follow the path and angle of light, demonstrates the technology in a March 12, 2021 article in Nature communications.

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

The photonic material designed by the Tufts team brings together 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 that it has a negative coefficient of thermal expansion (CTE), which means that it contracts when it is is heated and expands when cooled. PDMS, on the other hand, has a high CTE and expands rapidly when heated. As a result, when the new material is exposed to light, one layer heats up much faster than the other, so the material bends when one side expands and the other contracts or expands more slowly.

“With our approach, we can model these opal-like films at multiple scales to design how they absorb and reflect light. As light moves and the amount of energy absorbed changes, the material bends and moves differently depending on its position relative to that light, ”said Fiorenzo Omenetto, corresponding author of the study and Professor Frank C. Doble engineering at Tufts.

While most optomechanical devices that convert light into motion involve complex and energy-intensive fabrication or setups, “we are able to achieve exquisite control over the conversion of light energy and generate a ‘macro’ ‘movement of these materials without the need for electricity or wires, “says Omenetto.

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

The authors demonstrated in their study a “photonic sunflower”, with solar cells embedded in the bilayer film so that the cells follow the light source. The photonic sunflower kept the angle between the solar cells and the laser beam almost constant, maximizing the efficiency of the cells as the light moved. The system would work with both white light and laser light. Such wireless, light-sensitive heliotropic (sun tracking) systems could potentially improve the efficiency of light-to-energy conversion 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.