Moisture-Driven ‘Robots’ Crawl with No External Power Source

“The development of smart materials such as moisture-responsive graphene oxide is of great importance to automation and robotics,” said Yong-Lai Zhang of Jilin University, China, and leader of the research team. “Our very simple method for making typical graphene oxides smart is also extremely efficient. A sheet can be prepared within one second.”

The researchers used flash-treated graphene oxide to create a crawler that moved when humidity was increased. Switching the humidity off and on several times induced the crawler to move 3.5 millimeters in 12 seconds, with no external energy supply.
Credit: Yong-Lai Zhang of Jilin University

In the journal Optical Materials Express, from The Optical Society (OSA), the researchers reported that graphene oxide sheets treated with brief exposure to bright light in the form of a camera flash exhibited reversible bending at angles from zero to 85 degrees in response to switching the relative humidity between 33 and 86 percent. They also demonstrated that their method is repeatable and the simple robots they created have good stability.

Although other materials can change shape in response to moisture, the researchers experimented with graphene-based materials because they are incredibly thin and have unique properties such as flexibility, conductivity, mechanical strength and biocompatibility. These properties make graphene ideal for broad applications in various fields. For example, the material’s excellent biocompatibility could allow moisture-responsive graphene oxide to be used in organ-on-a-chip systems that simulate the mechanics and physiological response of entire organs and are used for drug discovery and other biomedical research.

Making a moisture-responsive material
Other groups have shown that graphene oxide can be made moisture responsive through a chemical reaction called reduction, which removes oxygen from molecules. In fact, the researchers previously demonstrated that both sunlight and UV light can induce the effect. However, these approaches were hard to precisely control and not very efficient.