This shows a simulated LCE micropost with the nematic director oriented at 45° relative to the flat surface. Illuminating one side of the post induces the top of the post to twist relative to the fixed base; illuminating the opposite face of the post results in a twist in the opposite direction. Color indicates the regions of the post that are illuminated (yellow) or in shadow (blue). Image: Balazs Lab.
This shows a simulated LCE micropost with the nematic director oriented at 45° relative to the flat surface. Illuminating one side of the post induces the top of the post to twist relative to the fixed base; illuminating the opposite face of the post results in a twist in the opposite direction. Color indicates the regions of the post that are illuminated (yellow) or in shadow (blue). Image: Balazs Lab.

The twisting and bending capabilities of the human muscle system allow a varied and dynamic range of motion, from walking and running to reaching and grasping. Replicating something as seemingly simple as waving a hand in a robot, however, requires a complex series of motors, pumps, actuators and algorithms. But now researchers at the University of Pittsburgh and Harvard University have designed a polymer known as a liquid crystal elastomer (LCE) that can be ‘programmed’ to both twist and bend in the presence of light.

The novel LCEs, reported in a paper in Science Advances, were developed at the University of Pittsburgh’s Swanson School of Engineering by Anna Balazs, professor of chemical and petroleum engineering, and James Waters, postdoctoral associate and the paper's first author. Other researchers from Harvard University's Wyss Institute for Biologically Inspired Engineering and the John A. Paulson School of Engineering include Joanna Aizenberg, Michael Aizenberg, Michael Lerch, Shucong Li and Yuxing Yao.

These LCEs are achiral: the structure and its mirror image are identical. This is not true for a chiral object, such as a human hand, which is not superimposable with a mirror image of itself. In other words, the right hand cannot be spontaneously converted to a left hand. When the achiral LCE is exposed to light, however, it can controllably and reversibly twist to the right or twist to left, allowing the same LCE to form both right-handed and left-handed structures.

"The chirality of molecules and materials systems often dictates their properties," Balazs explained. "The ability to dynamically and reversibly alter chirality or drive an achiral structure into a chiral one could provide a unique approach for changing the properties of a given system on-the-fly. To date, however, achieving this level of structural mutability remains a daunting challenge. Hence, these findings are exciting because these LCEs are inherently achiral but can become chiral in the presence of ultraviolet light and revert to achiral when the light is removed."

The researchers uncovered this distinctive dynamic behavior through their computer modeling of a microscopic LCE post anchored to a surface in air. Molecules (known as mesogens) that extend from the LCE backbone are all aligned at 45° (with respect to the surface) by a magnetic field; in addition, the LCEs are cross-linked with a light-sensitive material.

"When we simulated shining a light in one direction, the LCE molecules would become disorganized and the entire LCE post twists to the left; shine it in the opposite direction and it twists to the right," said Waters. These modeling results were corroborated by experimental findings from the Harvard group.

Going a step further, the researchers used their validated computer model to design ‘chimera’ LCE posts, where the molecules in the top half of the post are aligned in one direction and the molecules in the bottom half are aligned in another direction. With the application of light, these chimera structures can simultaneously bend and twist, mimicking the complex motion produced by the human muscular system.

"This is much like how a puppeteer controls a marionette, but in this instance the light serves as the strings, and we can create dynamic and reversible movements through coupling chemical, optical and mechanical energy," Balazs said. "Being able to understand how to design artificial systems with this complex integration is fundamental to creating adaptive materials that can respond to changes in the environment. Especially in the field of soft robotics, this is essential for building devices that exhibit controllable, dynamic behavior without the need for complex electronic components."

This story is adapted from material from the University of Pittsburgh, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.