Using an off-the-shelf camera flash, researchers at Jilin University, China, turned an ordinary sheet of graphene oxide into a material that bends when exposed to moisture. They then used this material to make a spider-like crawler and claw robot that move in response to changing humidity, without the need for any external power.
Graphene oxide sheets treated with a brief exposure to bright light in the form of a camera flash exhibited reversible bending at angles from 0 to 85 degrees in response to switching the relative humidity between 33% and 86%. Researchers also demonstrated that their method is repeatable, and the simple robots they created have good stability. Graphene-based materials were used because they are incredibly thin and have unique properties such as flexibility, conductivity, mechanical strength, and biocompatibility.
The material can be made moisture responsive through a chemical reaction called reduction, which removes oxygen from molecules. The Jilin University research team experimented with using a camera flash as a simple and effective way to create moisture-responsive graphene. The flash allowed the researchers to remove oxygen from — or reduce — just one side of a sheet of graphene oxide. When moisture is present, the reduced side of the graphene oxide absorbs fewer water molecules, causing the non-reduced side to expand and the sheet to bend toward the reduced side. If the material is then exposed to dry air, it flattens out.
To make a moisture-driven crawler, the researchers cut flash-treated graphene oxide into an insect shape with four legs. The free-standing crawler was about 1 centimeter wide and moved forward 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.
The researchers also made a claw shape by sticking together eight 5-by-1 millimeter ribbons of flash-treated graphene oxide in a star shape. When moisture was present, the claw closed within 12 seconds. It returned to an open position after 56 seconds of exposure to dry air.
The team is now working on ways to improve the control of the material’s bending, and are experimenting with ways to gain more complex performance from robots made of moisture-responsive graphene oxide.