Scientists at the University of Pittsburgh and Harvard University have demonstrated a new method for 'programming' liquid crystalline elastomers (LCEs) that enables them to twist and bend in the presence of light. They were able to replicate the complex twisting actions of the human muscle system in the materials, a breakthrough particularly useful for applications in soft robotics and optical devices.
In investigating the potential for replicating human motion, chirality (or the handedness of an object) becomes a major factor. Human hands, for example, are chiral as their mirror images are not identical – the right hand cannot be spontaneously converted to a left hand. Although conventional LCEs don’t exhibit complex modes of bending and twisting, these LCEs are achiral, so that their structure and mirror image are identical.
The chirality of a molecule or material often dictates its physical properties, so it is useful to dynamically tune the chirality of a system and therefore dynamically alter its properties. As reported in Science Advances [Waters et al. Sci. Adv. (2020) DOI: 10.1126/sciadv.aay5349], micron-sized “chimera” LCE posts were first anchored to a surface in air, where the applied light caused the post to bend in different ways, before the LCEs were examined to see if they could be made to controllably twist from side to side based on their computational models. The team managed to simulate the behavior of the LCE microposts and pinpoint the conditions where the posts can controllably and reversibly twist to the left or right with the application of light, and then return to their original position when the light was removed.
The study showed how to create dynamic and reversible movements through coupling chemical, optical and mechanical energy. In the past, materials were produced with desired static properties, but now they can be designed with controllable dynamic behavior, so a single material can be used for multiple applications or in different environments. Light offers a useful stimulus here, as it can be applied remotely and easily turned on and off.
As project leader Anna Balazs told Materials Today, “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.” The team is now looking to produce arrays of LCEs that can encrypt messages, and to design LCEs that dynamically alter the properties of the light when it hits the post, and hence the post is actually tailoring its own behavior, helping research in self-regulating materials.
A visual from the simulation: the red arrows in the posts indicate the orientation of the molecules (mesogens) that extend off the backbone of the polymers that make up the LCEs