Controlled by light alone, new smart materials turn, 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 drive materials or energy sources. The design combines programmable photonic crystals with an elastomeric composite that can be designed on a macro and nano scale to respond to illumination.

The research offers new avenues for the development of smart light-controlled systems, such as high-efficiency, self-aligning solar cells that automatically follow the direction and angle of the sun, light-powered microfluidic valves or soft robots that move with light on demand. . A ‘photonic sunflower’, whose petals curl towards and away from the lighting and follow the path and angle of the light, demonstrates the technology in a paper to be released March 12, 2021 in Nature Communications

Color is created by the absorption and reflection of light. Behind every flash of an iridescent butterfly wing or opal gem are complex interactions where 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 generated by that absorbed energy.

The photonic material, designed by the Tufts team, joins two layers: an opalescent film made of silk fibroin doped with gold nanoparticles (AuNPs), which forms 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 it shrinks when heated and expands when cooled. PDMS, on the other hand, has a high CTE and expands quickly when heated. As a result, when the new material is exposed to light, one layer heats up much faster than the other, so that the material bends when one side expands and the other side contracts or expands more slowly.

“Our approach allows us to pattern these opalescent films on multiple scales to design the way they absorb and reflect light. When the light moves and the amount of energy absorbed changes, the material folds and moves differently as a function of relative position relative to that light, ”said Fiorenzo Omenetto, the study’s corresponding author and Frank C. Doble Professor of Engineering at Tufts.

While most optomechanical devices that convert light into motion involve complex and energy intensive fabrication or setups, “we are able to gain excellent control over the conversion of light energy and generate” macro motion “of these materials without affecting electricity or wires are needed, ‘said 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 absorbed and reflected light from the film, causing the material to bend, fold and twist in different ways when exposed to laser light, depending on the geometry of the pattern.

The authors showed in their study a “photonic sunflower”, with solar cells integrated into the double-layer film so that the cells followed the light source. The photonic sunflower kept the angle between the solar cells and the laser beam nearly constant, maximizing the efficiency of the cells while moving the light. The system would work just as well with white light as it would with laser light. Such wireless light-responsive heliotropic (sun tracking) systems could potentially improve light-to-energy conversion efficiency for the solar industry. Demonstrations of the team’s equipment also included a butterfly with wings opening and closing in response to light and a folding box.