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MIT scientists have created tiny, soft-bodied robots that can be controlled with a weak magnet. The robots are formed from rubbery magnetic spirals and can be programmed to walk, crawl, and swim in response to an easy-to-apply magnetic field. 

The MIT team published their findings in an open-access paper in June in the journal Advanced Materials. Polina Anikeeva, a professor of materials science and engineering and brain and cognitive sciences at MIT and the associate director of MIT’s Research Laboratory of Electronics, led the research. 

According to Anikeeva, this is the first time someone has been able to control three-dimensional locomotion with a one-dimensional magnetic field. And because the robots are composed of a soft polymer, the team didn’t have to use a large magnetic field to control them. 

Magnetic robots typically move in response to moving magnetic fields, according to Anikeeva. This means that if you want a robot to walk, the magnet needs to walk with it. This limits the settings where the robots can be deployed, as it may not be safe to move a magnet in constrained environments. The team sought to make a robot that moves when a stationary instrument applies a magnetic field to the whole sample.

Developing the robots 

The robots used by the team were developed by Youngbin Lee, a former graduate student in Anikeeva’s lab. They work by not being uniformly magnetized. Instead, the robots are strategically magnetized in different zones and directions. This allows a single magnetic field to enable movement. 

Lee’s development of the robots started with two kinds of rubber of different stiffness. Lee sandwiched these together, heated them, and then stretched them into a long, thin fiber. Because of the different properties of the fibers, one of the rubber pieces retains its elasticity through the process, while the other deforms and cannot return to its original size. 

When the strain is released, one layer of the fiber contracts, pulling the other side, and the entire structure, into a gith coil, similar to the tendrils of a cucumber plant that spiral when one layer of cells loses water and contracts faster than another layer. 

The team then incorporated a material whose particles have the potential to become magnetic into a channel that runs through the rubbery fiber. After this, they can apply a magnetization pattern that enables a particular type of movement. 

“Youngbin thought very carefully about how to magnetize our robots to make them able to move just as he programmed them to move,” Anikeeva said. “He made calculations to determine how to establish such a profile of forces on it when we apply a magnetic field that it will actually start walking or crawling.”

For example, to create a caterpillar-like crawling robot, the helical fiber had to be shaped into gentle undulations. The body, head, and tail are then magnetized so that a magnetic field applied perpendicular to the robot’s plane for motion will cause the body to compress.

When this magnetic field is reduced to zero, the compression releases and the robot stretches. Putting these movements together results in the robot propelling forward. 

The team found that this kind of movement worked well for releasing payloads, and because the robots are made from a soft polymer, they could be used in biomedical applications in the future. While the teams’ robots are millimeters long, the same approach could be used to make much smaller robots better suited for medical scenarios.