Folded paper design inspired flexible electronics.

Kirigami, the Japanese art of folding and paper cutting, has inspired a team of engineers at the University of Michigan, Ann Arbor, to create flexible, stretchable electronics. They say that future electronics will be able to be rolled up, folded, or embedded in flexible objects because of a stretchable conductor, which would make up components like wires and electrodes.

Fig. 1 – Terry Shyu demonstrates a stretchable conductor’s ability to conduct. (Credit: Joseph Xu, Michigan Engineering)
Conductors that stretch are difficult to design, and among those that are known, they either don’t expand by much or the conductivity lessens dramatically when they do. By developing a conductor inspired by kirigami, the Japanese art of paper cutting, conductivity is sacrificed up front. The cuts become barriers to electrical conductivity, but when stretched, the conductors perform steadily.

“The kirigami method allows us to design the deformability of the conductive sheets, whereas before it was very Edisonian process with a lot of misses and not a lot of hits,” said Nicholas Kotov, the Joseph B. and Florence V. Cejka Professor of Engineering, referring to Thomas Edison’s trial-and-error approach to invention.

This is because when materials are stretched to the max, it’s difficult to predict when and where rips will occur. However, if the tears are designed in a decisive way, the material’s ability to stretch and recover becomes reliable.

The first prototype of the kirigami stretchable conductor was tracing paper covered in carbon nanotubes. The layout was very simple, with cuts like rows of dashes that opened to resemble a cheese grater. Rigged up in an argonfilled glass tube, the paper electrode turned the gas into a glowing plasma. The voltage across the electrode sent free electrons running into the argon atoms, causing them to emit light. Kotov explained that arrays of such electrodes could control the pixels of a stretchable plasma display.

The researchers wanted to understand exactly how design choices affected the behavior of the stretchable conductor, so a group of chemical engineers, performed computer simulations, which hinted at what kinds of behaviors to expect from different cut patterns. Then, the simulation team explored how details like the length and curvature of the cuts, and the separation between them, related to the stretchiness of the material.

To produce the microscopic kirigami, Terry Shyu, a doctoral student in materials science and engineering, made special “paper” out of graphene oxide, a material composed of carbon and oxygen just one atom thick. She layered it with a flexible plastic, up to 30 layers of each. The difficult part, she explained, was making the cuts just a few tenths of a millimeter long. (See Figure 1)

Shyu first coated the high-tech paper with a material that can be removed with laser light, then burned the dashes out of that material, which turned it into a mask for the etching process.

A plasma of oxygen ions and electrons broke down the “paper” that wasn’t hidden under the mask, creating the neat rows of microscopic dashes. This material behaved as predicted by the simulations, stretching with no additional cost in conductivity.

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