A New Era of Microrobotics: Kirigami-Inspired Metasheet Robots

In a study published in Nature Materials, Cornell researchers created a robot smaller than 1 millimeter in size that is printed as a 2D hexagonal "metasheet" but, when shocked with electricity, transforms into preprogrammed 3D shapes and crawls.

The legacy of Cornell's microscale robotics program is still being shaped, reshaped, and reopened.

The newest addition is a robot that is smaller than a millimeter and is printed as a 2D hexagonal "metasheet"; yet, it can transform into preprogrammed 3D forms and crawl when it receives an electrical shock.

The robot's adaptability stems from a unique design based on kirigami, a relative of origami in which slices in the material (the Japanese term "kiru" means "to cut") allow it to fold, expand, and move.

Postdoctoral researchers Qingkun Liu and Wei Wang, Ph.D. ’24 are the study's co-lead authors.

Itai Cohen, a physics professor at the College of Arts and Sciences (A&S), spearheaded the study. His group has previously created microrobotic systems that can operate their limbs, pump water using artificial cilia, and walk independently.

The next development in that evolution is the kirigami robot, which is the result of longtime collaborations with Paul McEuen, the John A. Newman Professor of Physical Science (A&S); Hadas Kress-Gazit, the Geoffrey S.M. Hedrick Sr. Professor in Cornell Engineering; Nicholas Abbott, a Tisch University Professor in the Robert F. Smith School of Chemical and Biomolecular Engineering in Cornell Engineering; Alyssa Apsel, the IBM Professor of Engineering in Cornell Engineering; and David Muller, the Samuel B. Eckert Professor of Engineering in Cornell Engineering – all co-authors of the study.

According to Liu, the genesis of the kirigami robot was somewhat influenced by “living organisms that can change their shape.”

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 behaviors.

Qingkun Liu, Laboratory of Atomic and Solid-State Physics, Cornell University

According to Wang, it is frequently possible to create such metamaterials to have qualities that are challenging to do with natural materials.

Approximately 100 silicon dioxide panels, each roughly 10 nanometers thin, are joined by more than 200 actuation hinges to form the robot's hexagonal tiling. The hinges allow the robot to alter its covering area and locally expand and contract by up to 40% when electrochemically actuated by external wires. They also generate mountain and valley folds and operate to splay open and rotate the panels.

The robot can take on many forms and even wrap itself around things before unfolding back into a flat sheet, depending on which hinges are engaged. This is how clever kirigami can be.

In origami, if you wanted to create three-dimensional shapes, usually you have to hide the excess material inside the 3D object that you are making, but with kirigami, you do not have to hide anything. Of course, it is not a contiguous sheet, so there are holes in it, but you do not have to lose any material. It is a much more efficient way of generating a three-dimensional shape.

Itai Cohen, Professor, College of Arts and Sciences, Cornell University

It took a lot of time and effort to create this sort of machine at the microscale, from finding out how to thread electrical wires through all of the different hinges to working out how to balance stiffness and floppiness just right so the robot could take on and maintain its shape. Creating a mechanism for something with so many moving components to move itself was one of the biggest obstacles.

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.

Jason Kim, Study Co-Author and Postdoctoral Researcher, Cornell University

Kim finally recognized that the forces became much more constant if they could make the robot swim through its surroundings by altering its form rather than using friction. Swimming in a pool is obviously not the same as swimming at the microscale. At that magnitude, it is more like swimming in a pot of honey.

“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 added.

Cohen described this as one of the unique aspects of creating microscopic robots.

Kim further added, “The physics of locomotion at the microscale is often different from the physics of locomoting robots that are macroscopic.”

Cohen's team is already considering the next generation of metasheet technology. They intend to combine their flexible mechanical structures with electronic controllers to develop ultra-responsive "elastronic" materials with qualities that would never be conceivable in nature. Applications might include reconfigurable micromachines, tiny biomedical devices, and materials capable of responding to impact at virtually the speed of light rather than the speed of sound.

Cohen added, “Because the electronics on each 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. 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.”

Additional authors include Postdoctoral Researcher Itay Griniasty; Michael Reynolds, M.S. ’17, Ph.D. ’21; Michael Cao ’14; and Doctoral Students Himani Sinhmar, Jacob Pelster, and Paragkumar Chaudhari.

National Science Foundation’s Emerging Frontiers in Research and Innovation program (EFRI); the Army Research Office; Cornell Center for Materials Research, which is supported by the NSF’s MRSEC program; the Air Force Office of Scientific Research; and the Kavli Institute at Cornell for Nanoscale Science supported the study.

The researchers used the Cornell NanoScale Facility, a member of the National Nanotechnology Coordinated Infrastructure funded by the National Science Foundation, as well as the Cornell Institute of Biotechnology.

Journal Reference:

Liu, Q., et al. (2024) Electronically configurable microscopic metasheet robots. Nature Materials. doi.org/10.1038/s41563-024-02007-7.