Bilayer strategy leads to faster response in light-driven soft robots

Northwestern University researchers have advanced their quest to design life-like behaviors into robotic soft matter, reporting on new systems that move faster than previous iterations and undergo predictable 3D shape changes — all in response to light.

The work, published recently in Matter, builds upon previous CBES-supported research that showed how soft materials could be molecularly programmed to bend, rotate or even crawl on surfaces when hit with light. In those hydrogel systems, light causes the materials to contract and darkness causes them to reswell, enabling the researchers to induce specific movements by localizing the light stimulus and alternating periods of light and darkness.

In the new work, the investigators added a second layer to the materials — one that expands rather than contracts when hit with light. By coupling these layers together and leveraging their opposite responses to light, the researchers achieved a synergistic effect with bending occurring approximately five times faster than in the previously studied objects.

The research was led by CBES director Samuel Stupp and Yonggang Huang, the Jan and Marcia Achenbach Professor of Mechanical Engineering at Northwestern. Postdoctoral researcher Chuang Li of the Stupp laboratory and Yeguang Xue, a former graduate student in the Huang group, are co-first authors of the paper. Other co-authors include Liam Palmer, executive director of research at CBES, John Rogers, Louis Simpson and Kimberly Querrey Professor of Materials Science and Engineering, Biomedical Engineering and Neurological Surgery at Northwestern, and Mengdi Han, a former postdoctoral fellow in the Rogers group. 

Informed by analytical modeling, the researchers designed the macroscopic objects to undergo predictable, origami-like shape changes and one-directional walking. These life-like behaviors were also observed with different shapes of the materials. For example, linear forms of the materials displayed a gait reminiscent of an inchworm, while cross-shaped objects bent upward and then traveled with a motion similar to a four-legged octopus. 

The researchers believe these bilayer hydrogels represent a pathway to molecularly program life-like behaviors into inert soft materials for a broad range of applications in areas such as chemical production, sensing, solutions to environmental problems, and advanced medicine. 

“Constructing soft actuators from components that respond in different ways to the same stimulus offers an exciting path to systems with better performance,” Palmer said. “This work will hopefully inspire stimuli-responsive soft materials with new, more complex functionality.”

Photos of experimental materials (top row) undergoing origami-like shape changes upon exposure to light. The bottom two rows depict the folded structures predicted by the corresponding finite element simulations.