Fold me a machine

By Nick Carne

For some time, engineers and mathematicians have been intrigued by the ancient Japanese art of kirigami (similar to the better-known origami, but with scissors allowed).

Four years ago, for example, Cosmos reported that it had inspired a simple rooftop solar cell design and been used to create nanoscale machines out of graphene.

In just the past few months, other research teams have explored its underlying mathematical principles and designed a kirigami-inspired sensor patch.

Now researchers from North Carolina State University in the US are reporting another step forward – thin sheets of material that automatically reconfigure into new 2D and 3D structures in response to environmental stimuli.

“This is the first case that we know of in which 2D kirigami patterns autonomously reshape themselves into distinct 3D structures without mechanical input,” says Jie Yin, first author of a paper in the journal Proceedings of the National Academy of Sciences.

“Instead, we apply energy in the form of heat, and the material rearranges itself.” {%recommended 9156%}

The material has three layers: two outer layers that are not responsive to heat and a polymer middle layer that contracts in response to heat. 

Through-cuts, which penetrate all three layers, control the material’s range of motion. Etchings, which penetrate the outer layers and expose the heat-responsive polymer, control the angle and direction at which the material folds, as well as how far it folds. 

As the material folds, it opens the through-cuts, shifting the shape of the sheets into 2D or 3D designs.

“We can make a 2D template with the same pattern of through-cuts and use it to create many different 3D structures by making slight changes in the etching,” Yin says. “This effectively makes the active kirigami sheets programmable.”

As part of their proof of concept, the researchers created a suite of thermoresponsive kirigami machines, including simple gripping devices and self-folding boxes, and a soft robot with a kirigami body and pneumatic legs. 

By switching the orientation of the body, they could rapidly reposition the legs, changing the robot’s direction of movement. 

“We used a temperature-responsive polymer for this work, but there’s no reason to think that other stimuli-responsive polymer materials, like photoactive liquid crystals, wouldn’t work as well,” Yin says. 

CREDIT: Jie Yin, NC State University