The team at Harvard’s Wyss Institute for Biologically Inspired Engineering and John A. Paulson School of Engineering and Applied Sciences (SEAS) have used magnetic fields to control the molecular structure of liquid crystal elastomers (LCEs) to create microscopic three-dimensional polymer shapes that can be programmed to move in any direction in response to multiple types of stimuli. The work could be used for solar panels that turn to follow the sun for improved energy capture, actuators in soft robots or sticky surfaces where the stickiness can be switched on or off.
“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 in the lab of Prof Joanna Aizenberg.
The microstructures created by Yao and Aizenberg’s team are made of LCEs cast into arbitrary shapes that can deform in response to heat, light, and humidity, and whose specific reconfiguration is controlled by their own chemical and material properties. By exposing the LCE precursors to a magnetic field while they were being synthesized, all the liquid crystalline elements inside the LCEs lined up along the magnetic field and retained this molecular alignment after the polymer solidified. By varying the direction of the magnetic field during this process, the scientists could dictate how the resulting LCE shapes would deform when heated to a temperature that disrupted the orientation of their liquid crystalline structures. When returned to ambient temperature, the deformed structures resumed their initial, internally oriented shape.
Such programmed shape changes could be used to create encrypted messages that are only revealed when heated to a specific temperature, actuators for tiny soft robots, or adhesive materials whose stickiness can be switched on and off. The system can also cause shapes to autonomously bend in directions that would usually require the input of some energy to achieve. Unique motions could be achieved by exposing different regions of an LCE structure to multiple magnetic fields during polymerization, which then deformed in different directions when heated.
The team was also able to program their LCE shapes to reconfigure themselves in response to light by incorporating light-sensitive cross-linking molecules into the structure during polymerization. Then, when the structure was illuminated from a certain direction, the side facing the light contracted, causing the entire shape to bend toward the light. It is this self-regulated motion that allows the crystals to continuously reorient themselves to autonomously follow the light.
Solar panels covered with such microstructures that turn to follow the sun as it moves across the sky like a sunflower would result in more efficient light capture. The technology could also form the basis of autonomous source-following radios, multilevel encryption, sensors, and smart buildings.
“Our lab currently has several ongoing projects in which we’re working on controlling the chemistry of these LCEs to enable unique, previously unseen deformation behaviors, as we believe these dynamic bioinspired structures have the potential to find use in a number of fields,” said Aizenberg.
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