Photonic material in the shape of a flower can move in response to light, closely tracking the angle of maximum exposure. Credit: Fio Omenetto, Tufts University.
Schematic showing the mechanism of heliotropic movement.
Patterned butterfly-like structure that opens and closes its wings in response to light.Like a flower turning its petals to the sun, researchers from Tufts University have designed a photonic composite that twists and bends in response to light [Wang et al., Nature Communications (2021) 12:1651, https://doi.org/10.1038/s41467-021-21764-6].
Many natural systems use heliotropic behaviors to harvest light to drive structural transformations, producing color changes for camouflage, signaling, communication, regulating temperature, or photosynthesis. In artificial systems, this ability could be used to maximize the efficiency of light-harvesting devices or enable biomedical and soft robotic devices to convert light energy into mechanical movement.
The team from Tufts, together with colleagues from Northwestern University and Università degli Studi di Pavia in Italy, were inspired by natural heliotropic behaviors to create a photo-responsive composite composed of layers of a porous biopolymer, silk fibroin, doped with gold nanoparticles on an elastomeric polydimethylsiloxane (PDMS) substrate. The nanoparticle-doped biopolymer layer forms an iridescent photonic crystal known as silk inverse opal (SIO).
“Using photonic crystals and their iridescence allows us to control the motion of the material, the way it folds and bends in response to the way light illuminates it,” explains Fiorenzo Omenetto, Frank C. Doble Professor of Engineering at Tufts, who led the work.
The researchers’ novel approach relies on the difference in coefficient of thermal expansion between SIO and PDMS. SIO has a negative coefficient of thermal expansion so it contracts when heated and expands when cooled. PDMS substrate, which has a high coefficient of thermal expansion, behaves in the opposite fashion, expanding when heated and contracting when cooled. When exposed to light, photothermal heating produces a disparity in the physical response of the two materials, resulting in a bending movement as one material contracts while the other expands. The composite, therefore, behaves as an optomechanical actuator moving in response to light.
“[Our] new concept of precisely and dynamically manipulating light-energy conversion within an optomechanical system by coupling photonic function and elastomeric materials offers a structure-based approach that can be easily implemented,” says Omenetto.
This approach enables the possibility of programming a response by constructing SIOs with different nanostructures, different numbers of layers or different lattice constants. Complex three-dimensional structures can be readily constructed from layered materials to predetermine the spectral response of the photonic structure.
As an example, Omenetto and his team fabricated a sunflower-like structure mounted with solar cells that bends and twists to follow a light source. A butterfly-like construction, with patterned wings to absorb and reflect light differently, can be made to flap in response to illumination.
“We are able to achieve control of light-energy conversion at the microscale and generate motion of these materials at the macroscale without the need for electricity or wires,” point out Omenetto. “We can achieve programmed folding, anisotropic bending, actuation of different or specific portions of the material, and light-tracking.”
This wireless light-responsive approach could boost light-to-energy efficiency in photosensitive systems or artificial photosynthesis devices. Carbon nanotubes could replace gold nanoparticles and other flexible materials could be used such as hydrogels, liquid crystal elastomers, and shape memory polymers to expand the possibilities of the approach.
“The material could be practical with appropriate scale-up of the composite, which requires some additional work and engineering,” says Omenetto. “We will also continue to optimize the spectral response and the configuration of the composite so that it responds to white light to achieve true ‘heliotropic’ motion.”
Ximin He, CIFAR Azrieli Global Scholar and Assistant Professor of Materials Science and Engineering at the University of California, Los Angeles, believes the work could open up new and broad-reaching opportunities for autonomous and multifunctional optodevices.
“The design uniquely incorporates a stop-band of photonic lattice that matches the absorption of photo-absorbers in a biomorph film. This creates a high contrast between the photothermal effect on one side of the film and the other, leading to the controllability and programmability of photo-actuation,” she points out. “This enables complex three-dimensional programmable reconfiguration and phototropic motion, as well as a proof-of-concept ‘light-tracking’ solar panel.”
The design principle could also be expanded to other materials systems, such as hydrogels, liquid crystal elastomers, and shape memory polymers, suggests He.
This article was first published in Nano Today 38 (2021) 101166.