Scientists at Tufts University School of Engineering have created light-activated composite devices. They are capable of making precise, visible movements and creating complex three-dimensional shapes without the need for wires, other activating materials, or energy sources. The design combines programmable photonic crystals with an elastomeric composite that can be created at macro and nano scale to react to light.

The study opens up new possibilities for the development of intelligent systems that will work in response to light. For example, highly efficient, self-aligning solar cells. Their peculiarity is that they automatically follow the direction and angle of the sun’s light. Also, the prospect opens up for engineers to create light-driven microfluidic valves or soft robots. Details on creating a “photonic sunflower” whose petals bend in response to light, tracing its path and angle of refraction are described in an article for the journal Nature Communications.

Color arises from the absorption and reflection of light. This process consists of a series of complex interactions. Objects absorb light of certain frequencies and reflect others. The angle at which light meets a surface affects which wavelengths are absorbed as well as the heat generated by that absorbed energy.

The photonic material, developed by the Tufts University team, combines two layers. The first is an opal-like silk fibroin film doped with gold nanoparticles (AuNP) that form photonic crystals. The second is a substrate made of polydimethylsiloxane (PDMS), a silicon-based polymer. Besides its remarkable flexibility, durability and optical properties, silk fibroin is unusual in that it has a negative coefficient of thermal expansion (CTE). This means that it shrinks when 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. Thus, the material bends as one side expands and the other contracts or expands more slowly.

Most opto-mechanical devices that convert light into motion require complex and energy-intensive fabrication or customization. With this in mind, scientists have achieved precise control over the conversion of light energy and the generation of macro motion of these materials without the need for electricity or wires.