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New Self-Powered Polymers Can Move Without an External Power Source – They Only Need Light

When developing miniaturized soft robots, one of the obstacles is the need for an internal power source that meets the power to weight ratio requirement for efficient movement. In an article titled “Photomotility of Polymers” published in the journal Nature Communications, an international group involving the Air Force Research Laboratory, University of Pittsburgh and Inha University, describe new materials that convert ultraviolet light into motion directly without the need for electronics or other traditional methods.

The research was conducted at the Air Force Research Laboratory’s (AFRL) Materials & Manufacturing Directorate at Wright-Patterson Air Force Base, Ohio, under the leadership of Timothy J. White. Other members of the team include lead author Jeong Jae Wie, an assistant professor of polymer science and engineering at Inha University, South Korea and co-author M. Ravi Shankar, a professor of industrial engineering at Pitt’s Swanson School of Engineering.

Previous research teams attempting to solve this problem tried to avoid adding structures to induce locomotion by proposing the use of ambient energy resources such as acoustics, magnetic fields, heat and other temperature variations. Dr. Shankar however, feels that light is more appealing because of its temporal control, speed and the ability to target the mechanical response effectively. The group homed in on monolithic polymer films prepared from a type of liquid crystalline polymer for the material.

An animated gif of an azobenzene-functionalized liquid crystalline polymer moving when exposed to broadband ultraviolet-visible light. Image credit: Jeong Jae Wie, Inha University/AFRL

Shankar adds that preliminary research indicated that different forms of light could be used to trigger these flexible polymers  to move. A robot or similar device is however not effective unless you can control its motions tightly. Because of the work done by Dr. White and his team at AFRL, the team was able to demonstrate climbing motions, as well as directional control.

Dr. Wie pointed out that the spontaneous formation into spirals when exposed to UV light results in the “photomotility” of these specific polymers. A corresponding motion is achieved by controlling the exposure, without external power sources having to be attached directly to the polymer.

Wie added that complex robotic designs require additional weight in the form of batteries, wheels, or limb-like structures. This is incompatible with the concept of a soft or squishy robot. In the new design, there is no need for any additional mechanisms or moving parts that would result in an increase in weight and thereby limit effectiveness and motility, as the material itself is the machine.

White and his team demonstrated the polymers engaging in simple forward movement, as well as climbing a glass slide at a 15-degree angle. The flat polymer strips can move at several millimeters per second propelled by light even though they are small – approximately 15mm long and 1.25mm wide. As long as the material remains illuminated, the movement can be perpetual.

Shankar believes that the ability of these flexible polymers to move when exposed to light opens up a new dimension in the quest for soft robots. In space exploration or other extreme environments where excess weight and size is a negative, the elimination of the additional mass of batteries, moving parts and other cumbersome devices, can lead to the creation of a robot that would be beneficial in these fields.

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