Smaller than a chip, a single robot crab stands on the edge of a coin.

Northwestern University

Northwestern University engineers have developed the smallest remote-controlled walking robot ever — and it comes in the form of an adorable little peekytoe crab.

At just half a millimeter wide, the tiny crabs can bend, twist, crawl, walk, spin and even jump. The researchers also developed millimeter-sized robots resembling caterpillars, crickets and beetles. Although the research is exploratory at this stage, the researchers believe their technology could bring the field closer to achieving micro-robots that can perform practical tasks in tightly confined spaces.

The research will be published Wednesday, May 25 in the journal Science Robotics. Last September, the same team presented a winged microchip that was the smallest flying structure ever made by man (published on the cover of Nature).

“Robotics is an exciting field of research, and the development of microscale robots is a fun topic for academic exploration,” said John A. Rogers, who led the experimental work. “You could imagine micro-robots as agents for repairing or assembling small structures or machinery in industry or as surgical assistants for clearing clogged arteries, stopping internal bleeding, or removing cancerous tumors – all in minimally invasive procedures.

“Our technology allows for a variety of controlled movement modalities and can walk at an average speed of half its body length per second,” added Yonggang Huang, who led the theoretical work. “It’s very difficult to achieve on such a small scale for ground robots.”

A pioneer in bioelectronics, Rogers holds the Louis Simpson and Kimberly Querrey Chair in Materials Science and Engineering, Biomedical Engineering, and Neurological Surgery at Northwestern’s McCormick School of Engineering and Feinberg School of Medicine and Director of the Querrey Simpson Institute for Bioelectronics (QSIB). Huang is the Jan and Marcia Achenbach Professor of Mechanical Engineering and Civil and Environmental Engineering at McCormick and a key member of QSIB.

Smaller than a chip, the crab is not powered by complex hardware, hydraulics or electricity. Instead, his power resides in the elastic resilience of his body. To build the robot, the researchers used a shape-memory alloy that changes to its “memorized” shape when heated. In this case, the researchers used a scanned laser beam to quickly heat the robot at various targeted locations on its body. A thin layer of glass elastically returns this corresponding part of the structure to its deformed shape upon cooling.

As the robot moves from one phase to another – deformed to the memorized shape and back again – it creates locomotion. Not only does the laser remotely control the robot to activate it, but the scanning direction of the laser also determines the walking direction of the robot. A swipe from left to right, for example, makes the robot move from right to left.

“Because these structures are so small, the cooling rate is very fast,” Rogers explained. “In fact, reducing the size of these robots allows them to operate faster.”

To make such a small creature, Rogers and Huang turned to a technique they introduced eight years ago – a pop-up assembly method inspired by a children’s pop-up book.

First, the team fabricated precursors to the walking crab structures in flat and planar geometries. Then they glued these precursors onto a slightly stretched rubber substrate. When the stretched substrate is released, a controlled buckling process occurs which causes the crab to “pop” into precisely defined three-dimensional shapes.

With this manufacturing method, the Northwestern team was able to develop robots of various shapes and sizes. So why a peekytoe crab? We can thank the students of Rogers and Huang for that.

“With these assembly techniques and material concepts, we can build walking robots with almost any size or 3D shape,” Rogers said. “But the students felt inspired and amused by the sideways crawling movements of the little crabs. It was a creative quirk.

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