Bio-inspired Material Can Follow the Sun

A team from Harvard University has invented flexible materials that can respond autonomously to light and other stimuli, paving the way for solar panels that follow the sun.

Researchers have developed new, bio-inspired materials that can move in response to different stimuli. The materials—called liquid-crystal elastomers (LCEs)—could be used to develop solar panels that can automatically rotate to follow the sun, as well as be applied to robotics, adhesives, and other next-generation applications.

Key to the research of scientists at Harvard’s Wyss Institute for Biologically Inspired Engineering and John A. Paulson School of Engineering and Applied Sciences are magnetic fields, which they used to control the molecular structure of LCEs. In this way, they created microscopic, three-dimensional polymer shapes that can be programmed to move in any direction in response to multiple types of stimuli, according to a Wyss news release.

“What’s critical about this project is that we are able to control the molecular structure by aligning liquid crystals in an arbitrary direction in 3D space, allowing us to program nearly any shape into the geometry of the material itself,” said Yuxing Yao, a graduate student who worked on the research in the lab of Wyss professor Joanna Aizenberg.

Liquid crystal elastomers deform in response to heat. The shape they take depends on the alignment of their internal crystalline elements, which can be determined by exposing them to different magnetic fields during formation. (Image source: Wyss Institute at Harvard University)

Rubbery Polymer Materials

LCEs are rubbery polymers that contain liquid crystalline compounds; these control the directions in which the materials can move and stretch. They are bio-inspired by natural examples, such as the pads of gecko feet. Gecko footpads are sticky and covered with setae—microscopic, hairlike structures that—because of their high flexibility as well as their chemical and physical composition—are the reason why the lizards can scale and grip walls and ceilings so easily.

Researchers ultimately aim to develop LCEs that are this flexible and nimble. However, to date, researchers have managed to develop synthetic LCEs that can deform in only one or two dimensions, which limits their ability to move throughout space and take on different shapes.

The Harvard team has now overcome this challenge with microstructures comprised of LCEs cast into shapes that can deform in response to heat, light, and humidity. Moreover, their specific reconfiguration is controlled by their own chemical and material properties, researchers said.

Controlling the Shape

They achieved their result by exposing the LCE to a magnetic field while they were being synthesized. This made all of the liquid-crystalline elements inside the LCEs line up along that magnetic field, keeping this structure even after the polymer solidified.

By varying the direction of the magnetic field during this process, the scientists could control how the resulting LCE shapes would deform when heated to a temperature that disrupted the orientation of their structures, researchers said. Moreover, when the shapes returned to ambient temperature, they resumed their initial, inherent shape.

The team also programmed their LCE shapes to reconfigure themselves in response to light by integrating light-sensitive, cross-linking molecules into the structure during polymerization.

This resulted in a structure that—once illuminated from a certain direction—would contract on its side that faced the light, causing the entire shape to bend toward the light. This ultimately allowed the LCEs to respond to their environment by continuously re-orienting themselves to autonomously follow the light. 

Programming the described shape changes in LCEs can be used for a number of applications, researchers said. Perhaps most interestingly, these materials can be used to design solar panels that turn to follow the sun—like a sunflower does naturally—for improved energy capture.

Other uses include the creation of encrypted messages that are only revealed when heated to a specific temperature, actuators for tiny soft robots, or adhesive materials—the stickiness of which can be switched on and off, Yao said. The technology could also form the basis of autonomous source-following radios, multilevel encryption, sensors, and smart buildings.

Researchers published a paper on their work in the journal PNAS.

Elizabeth Montalbano is a freelance writer who has written about technology and culture for 20 years. She has lived and worked as a professional journalist in Phoenix, San Francisco, and New York City. In her free time, she enjoys surfing, traveling, music, yoga, and cooking. She currently resides in a village on the southwest coast of Portugal.

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