The twisting and bending capabilities of the human muscle system enable a dynamic range of motion, from walking and running to reaching and grasping. Replicating in a robot something as seemingly simple as waving a hand, however, requires a complex series of motors, pumps, actuators, and algorithms. Researchers have designed a polymer, known as a liquid crystal elastomer (LCE), that can be “programmed” to twist and bend in the presence of light.
The 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, forming both right-handed and left-handed structures. This approach could change the properties of a given system on-the-fly.
The researchers uncovered this dynamic behavior through computer modeling of a microscopic LCE post anchored to a surface in air. Molecules (the mesogens) that extend from the LCE backbone are all aligned at 45 degrees (with respect to the surface) by a magnetic field; in addition, the LCEs are cross-linked with a light-sensitive material. When they simulated shining a light in one direction, the LCE molecules would become disorganized and the entire LCE post twists to the left; shining light in the opposite direction caused it to twist to the right.
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 are aligned in another direction in the bottom half. With the application of light, these chimera structures can simultaneously bend and twist, mimicking the complex motion enabled by the human muscular system. This enables researchers to create dynamic and reversible movements through coupling chemical, optical, and mechanical energy.