Some of you must have heard the word “Kirigami”. It’s a fascinating art involving paper and creative folds and cuts, which is commonly associated with Japanese culture. In Japanese, the word “Kirigami” means cutting of paper ( “kiru” = to cut, “kami” = paper). Some of the Kirigami structures are truly amazing, from beautiful paper palaces to stunning dragons, it has captivated minds of millions of people all over the world.
From a materials engineering perspective, Kirigami, presents yet another interesting way of building things. For scientists and engineers, Kirigami is an inspiring new way of building 3D objects with 2D materials. The cutting and folding techniques explained in Kirigami art can be used in building advanced 3D structures with interesting mechanical, optical or electrical properties. Such technique is especially useful for nanotechnology researchers because creating an elaborate structures at nanoscale (about 80,000 times smaller than a human hair) is quite challenging and can benefit greatly from an easy approach like kirigami.
A group of scientists led by Paul McEuen at Cornell University, recently discovered that Graphene behaves much similar to a paper at the nanoscale. The main observation that led to this conclusion is that when graphene sheets are physically crumpled by a tool, they simply relax back to its original shape. Scientists then combined the principles from Kirigami to make micro and nanoscale structures that can be converted in to 3D structures with simple movements.
To make kirigami desings with graphene, scientists first made graphene on a copper surface using chemical vapor depsosition technique. Then they followed a series of transfer and coat procedures to obtain a graphene sheet with a thin aluminum layer on it. To cut graphene sheets in to desired shapes, scientists evaporated protective gold mask on the aluminum layer. Then the graphene sheet was subjected to oxygen plasma, which etched away all the unprotected aluminum and the graphene layer behind it. Finally, scientists subjected the material to a weak acid wash which dissolved the thin aluminum layer revealing the graphene sheet cut in to desired kirigami pattern.
One of the first things that scientists tried was a kirigami in-plane spring. This is essentially a flat sheet of graphene or paper cut at different off set positions that allows the material to be stretched in to a three dimensional spring like structure. This is in fact one of the simplest kirigami models out there. Scientists observe that kirigami graphene sheet showed incredible resiliency, showing no physical damage even after being pulled and pushed close to 10,000 cycles. This is a truly amazing feat by graphene considering its only one atom thickness. Then they looked at more advanced structures such as pyramids and spirals and observed similarly extraordinary behavior from graphene. Group also investigated different ways of folding graphene remotely, instead of using a needle and a mechanical setup to operate graphene kirigami designs. They used minute magnets and heating elements to force simple mechanical movements such as folding, rotating and bending. Once the force is removed, graphene came to its original shape showing its extreme mechanical resiliency. This is yet another achievement by the group, as mechanical operations at nanoscale is still quite challenging.
Team is convinced that graphene kirigami template based structures would spark a novel stream for fabrication of advanced structures at the nanoscale. This would allow us to exploit amazing mechanical, electrical, optical ad thermal properties extending graphene uses in future materials. Such a stream would also give rise to novel graphene applications and revolutionize current graphene uses. The research group who discovered graphene kirigami however, focuses more on super soft electronic devices that are made with kirigami techniques. They point to implantable electronic devices that can perform actions from monitoring of cell activity to diagnosis of cancer and other diseases. They have started working on a kirigami graphene structure that can be implanted near to a neuron to tap in to sensory signals that pass through it.