Inspired by origami, researchers at Donghua University in Shanghai, China, have built self-folding paper from extremely thin sheets of graphene oxide. The new paper bends in response to light or heat, and can be made to “walk” on a surface and even turn corners. The material could be used in a range of applications, including sensing, artificial muscles and robotics.
Origami is the ancient Japanese art of paper folding, and can transform a lightweight flat material into a strong and flexible 3D object. Its principles have inspired engineers to design a host of structures including vehicle airbags, satellite components and artificial muscles.
Self-folding structures are especially useful when they can be programmed to fold and unfold by exposing them to an external stimulant such as light. Such structures contain an active material or materials that respond to these stimuli. These active materials are usually polymer-based, which means that they respond well to changes in temperature, solvent, humidity, electricity and light. However, such materials can also be unstable and difficult to fabricate.
Strong and flexible
Now, researchers led by Hongzhi Wang and Meifang Zhu at the College of Materials Science and Engineering at Donghua have used extremely thin graphene-oxide nanosheets as the building blocks for a self-folding paper. The paper is flexible and easy to manipulate, and has a high tensile strength.
The sheets are made of several layers of reduced graphene oxide (rGO), which is very stable and does not change shape in response to external stimuli. Self-folding is achieved by coating some parts of the rGO sheet with several layers of graphene oxide that contains the polymer polydopamine (GO-PDA).
Unlike rGO, the GO-PDA layers contain water molecules that have been absorbed from the surrounding air. When the material is heated using an infrared laser, some of the water is driven out of the GO-PDA layers, which causes them to shrink. The rGO layers do not shrink, however, and the result is a sharp bending of the sheet in places where GO-PDA is present. When the light is switched off, the GO-PDA reabsorbs water from the air as it cools and the sheet flattens out.
By carefully patterning sheets of rGO with GO-PDA, the team made simple paper robots that can move forwards and backwards (see video above). They could even turn corners, which is a first for such a walking structure. Self-folding was also used to make a “hand” that could grasp and hold objects five times heavier than its own weight.
“We can programme how this paper bends so we can make it walk and turn around, as well as fold into pre-designed shapes simply by applying light or heat to it,” Wang explains. “We believe our work will help in the development of next-generation industrial mechanical actuators that could be used in applications like wireless remotely controlled microrobots, microfluidic chemical analysis, tissue engineering and artificial muscles, to name but a few.”
Wang and Zhu are now busy trying to make smaller versions of their paper. “As the device scales down in size, especially to the nanoscale, its folding properties will change significantly,” says Wang. “We are therefore interested in developing a nanosized all-graphene origami structure.”
The material is described in Science Advances.
- This article first appeared on nanotechweb.org