Photomotility of Polymers Could Enable Soft Robots to “Travel Light”

Novel materials that convert UV light into energy without the need for electronics could provide the basis for a lightweight internal power source that would mobilize miniaturized robots efficiently, without adding bulk.

An international research team from Inha University, University of Pittsburgh and the Air Force Research Laboratory (AFRL) demonstrated photoinduced motion in monolithic polymer films prepared from azobenzense-functionalized liquid crystalline polymer networks (azo-LCNs). The material was irradiated with a broad spectrum UV-visible light (320 to 500 nm), which transformed the films from flat sheets to spiral structures that could translate large distances when irradiated continuously.

An azobenzene-functionalized liquid crystalline polymer moving when exposed to broadband ultraviolet-visible light. Courtesy of Jeong Jae Wie, Inha University/AFRL.

"Our initial research indicated that these flexible polymers could be triggered to move by different forms of light," said professor M. Ravi Shankar. "However, a robot or similar device isn't effective unless you can tightly control its motions. Thanks to the work of Dr. White and his team at AFRL, we were able to demonstrate directional control, as well as climbing motions."

The directionality of the photomotility was programmed by the orientation of the anisotropy to the principal axes of the specimens. Motion was found to occur without modulating or multiplexing the actinic light source, and could occur on an arbitrary surface. Previous engineered constructs have required a temporally modulated stressor and anisotropic surface interaction to manifest directional motion.

According to the researchers, the photomotility is a spontaneous mechanical response of the anisotropic materials. The material by itself is the motile device and there is no requirement for a composite, multimaterial design or other special conditions. By directly transducing photons into motion, the weight penalty of articulated mechanisms, actuators or on-board power sources is eliminated.

The research team was able to make the azo-LCN film ascend a 15° incline. Courtesy of J.J. Wie, et al. (2016). Photomobilty of polymers. Nat Commun 7, 13260, doi: 10.1038/ncomms13260.
"Complex robotic designs result in additional weight in the form of batteries, limb-like structures or wheels, which are incompatible with the notion of a soft or squishy robot," said professor Jeong Jae Wie. "In our design, the material itself is the machine, without the need for any additional moving parts or mechanisms that would increase the weight and thereby limit motility and effectiveness."

In addition to simple forward movement, the team was able to make the polymers climb a glass slide at a 15-degree angle. While the flat polymer strips are small (approximately 15-mm long and 1.25-mm wide), they can move at several millimeters per second propelled by light. As long as the material remains illuminated, movement can be perpetual.

"The ability for these flexible polymers to move when exposed to light opens up a new ground game in the quest for soft robots," Shankar said. "By eliminating the additional mass of batteries, moving parts and other cumbersome devices, we can potentially create a robot that would be beneficial where excess weight and size is a negative, such as in space exploration or other extreme environments."

The research was published in Nature Communications (doi:10.1038/ncomms13260).

Video CAPTION Exposed to ultraviolet-visible light, a 15?µm thick azo-LCN samples experiences 'photomotility.' The locomotion of these materials is a direct conversion of the input light energy. Courtesy of Jeong Jae Wie, Inha University/AFRL.Exposed to ultraviolet-visible light, a 15?µm thick azo-LCN samples experiences 'photomotility.' The locomotion of these materials is a direct conversion of the input light energy. Courtesy of Jeong Jae Wie, Inha University/AFRL.

Exposed to ultraviolet-visible light, a 15-µm thick azo-LCN sample experiences 'photomotility.' The locomotion of these materials is a direct conversion of the input light energy. Courtesy of Jeong Jae Wie, Inha University/AFRL.