Origami, the ancient Japanese art of paper folding, transforms a lightweight flat material into a strong and flexible 3D object. Its principles have inspired engineers to design a host of structures, including everything from vehicle airbags to satellite components and artificial muscles. Self-folding structures are especially useful as they can be programmed to fold and unfold by exposing them to external stimuli. Such structures contain an active material or materials that respond to these stimuli.

Polymers (including gels, liquid crystalline, shape and conjugated memory polymers) have been the main type of active materials studied so far. Although these respond well to changes in temperature, solvent, humidity, electricity and light, they do suffer from the fact that they are not very stable. They are also difficult to fabricate.

Graphene oxide nanosheets as building blocks

Researchers led by Hongzhi Wang and Meifang Zhu at the College of Materials Science and Engineering at Donghua have now used graphene oxide nanosheets as the building blocks for their reduced graphene oxide paper. The paper is flexible and easy to manipulate and has a high tensile strength.

The graphene oxide can absorb water from the environment, which causes it to swell and stretch in ways that propel it forwards and backwards, and move in different directions – a first for such a walking structure. When exposed to near-infrared/laser light, the paper shrinks and thus folds. It can also grasp and hold objects five times heavier than itself and so can operate as a microrobot inside a sealed and confined space – driven only by an infrared laser.

Programmable paper

“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 tells nanotechweb.org. “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.”

The researchers, reporting their work in Science Advances DOI: 10.1126/sciadv.1500533, say they 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.”