materials containing microscopic liquid crystals that move and stretch in any direction in response to their environment, such as solar panels whose microstructure follows the sun
Inspired by natural microstructures whose movement gives materials and expected properties, such as the highly flexible hair-like features which make gecko's feet sticky, researchers at the Wyss Institute for Biologically Inspired Engineering and the John A Paulson School of Engineering and Applied Sciences at Harvard investigated liquid-crystal elastomers (LCEs), materials whose molecular structure contains flexible, stretchy polymers with internal crystalline elements that dictate the directions in which the elastomeric sections can move and stretch. Until now, synthetic LCEs have mostly only been able to deform in one or two directions.
The team, directed by Joanna Aizenberg and Yuxing Yao, experimented with synthesising LCEs in a magnetic field which could be manipulated as the materials formed. The field influenced the orientation of the liquid crystal elements, and this orientation was maintained once the polymer had solidified. This orientation dictated how the material would deform when heated to a temperature that disrupted the structure of liquid-crystal elements. When cooled, however, the material would return to its original shape, which was dictated by casting the forming polymer.
“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 Yao, who is first author on a paper describing the work in Proceedings of the National Academy of Sciences.
Among the possibilities for these structures are encrypted messages that can only be read when the material is heated to a specific temperature, such as with actuators for soft robots, or adhesive materials whose stickiness can be switched on and off. Manipulation of the magnetic fields during the polymer formation could programme in unique motions, for example by exposing different regions of the forming polymer to multiple magnetic fields: this could result in a structure that can be bent in different directions along its dimensions when heated.
One particularly intriguing property was achieved by incorporating light-sensitive cross-linking molecules into the polymer during polymerisation. When the solidified polymer was exposed to light, its shape bent towards the light, and if the light moved, it moved in response and tracked the movement. Additionally, polymers could be created that responded to both heat and light, creating a single-material object capable of multiple forms of movement and response mechanisms.
The team suggests that such material could be used to create a solar panel covered in microstructures that will follow the sun as it moves across the sky during the day, maximising the amount of solar energy the panel captures. Similarly, depending on the properties of the cross-linker, it could form a material that would autonomously track the source of radio signals; it could also form the basis of multilevel encryption, components for sensors, and materials for smart buildings.
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