Cornell University researchers have created microscale robots less than a millimetre in size that are printed as a 2D hexagonal ‘metasheet’ but, with a jolt of electricity, morph into preprogrammed 3D shapes and crawl.
The robots’ versatility is due to a novel design based on kirigami, a cousin of origami in which slices in the material enable it to fold, expand and locomote.
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In a sense, the origins of the kirigami robot were inspired by ‘living organisms that can change their shape’, said postdoctoral researcher Qingkun Liu. ‘But when people make a robot, once it’s fabricated, it might be able to move some limbs but its overall shape is usually static. So we’ve made a metasheet robot. The “meta” stands for metamaterial, meaning that they’re composed of a lot of building blocks that work together to give the material its mechanical behaviours.’
The robot is a hexagonal tiling composed of about 100 silicon dioxide panels that are connected through more than 200 actuating hinges, each about ten nanometres thin. When electrochemically activated via external wires, the hinges form mountain and valley folds and act to splay open and rotate the panels, allowing the robot to change its coverage area and locally expand and contract by up to 40 per cent. Depending which hinges are activated, the robot can adopt various shapes and potentially wrap itself around other objects, and then unfold itself back into a flat sheet.
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The project was led by Itai Cohen, professor of physics in the College of Arts and Sciences. His lab has previously produced microrobotic systems that can actuate their limbs, pump water via artificial cilia and walk autonomously.
‘In origami, if you wanted to create three-dimensional shapes, usually you have to hide the excess material inside the 3D object that you’re making,’ Cohen said. ‘But with kirigami, you don’t have to hide anything. Of course, it’s not a contiguous sheet, so there are holes in it, but you don’t have to lose any material. It’s a much more efficient way of generating a three-dimensional shape.’
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Creating this kind of a machine at the microscale was a long, intricate process, from figuring out how to thread electrical wires through the various hinges to determining the ideal balance of floppiness and rigidity for the robot to make and hold its shape. Among the most significant challenges was devising a way for something with so many moving parts to move itself.
‘When you have a kirigami sheet, you have hundreds of potential contact points with the ground. And so for the longest time, we were confused about which parts of the robot were contacting the ground to make the robot move,’ said postdoctoral researcher Jason Kim.
Kim eventually realized that if, instead of using friction, they could make the robot swim through its environment by changing its shape, the forces became much more consistent. Of course, swimming at the microscale is very different than swimming in a pool. At that scale, it’s more akin to swimming through a vat of honey.
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‘By changing the robot’s shape so that different parts were closer to the ground at different points in the swimming gait, we could reliably use fluid drag forces to propel the sheet forward,’ Kim said.
That’s one of the unique things about making microscopic robots, Cohen said. ‘The physics of locomotion at the microscale is often different from the physics of locomoting robots that are macroscopic.’
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Cohen’s team is now considering the next phase of metasheet technology. They anticipate combining their flexible mechanical structures with electronic controllers to create ultra-responsive ‘elastronic’ materials with properties that would never be possible in nature. Applications could range from reconfigurable micromachines to miniaturised biomedical devices and materials that can respond to impact at nearly the speed of light, rather than the speed of sound.
‘Because the electronics on each individual building block can harvest energy from light, you can design a material to respond in programmed ways to various stimuli. When prodded, such materials, instead of deforming, could “run” away, or push back with greater force than they experienced,’ Cohen said. ‘We think that these active metamaterials – these elastronic materials – could form the basis for a new type of intelligent matter governed by physical principles that transcend what is possible in the natural world.’
The research has been published in Nature Materials.