(Nanowerk News) MIT scientists have developed a small, soft-bodied robot that can be controlled by a weak magnet. The robot, formed from a spiral of rubber magnets, can be programmed to walk, crawl, swim — all in response to a simple, easy-to-implement magnetic field.
“This is the first time it has been done, to be able to control the three-dimensional locomotion of a robot with a one-dimensional magnetic field,” said Professor Polina Anikeeva, whose team published an open access paper on robot magnets in the journal Advanced Materials (“Magnetically Driven Fiber-Based Soft Robot”). “And since they are mostly made up of polymers and soft polymers, you don’t need a huge magnetic field to activate them. It’s actually a very small magnetic field that drives these robots,” added Anikeeva, who is a professor of materials and engineering sciences and brain and cognitive sciences at MIT, associate investigator of the McGovern Institute for Brain Research, and associate director of MIT’s Research Laboratory of Electronics and director of MIT’s K. Lisa Yang Brain-Body Center.
The new robot is well-suited for transporting cargo through confined spaces and its rubber body is soft in fragile environments, opening up the possibility that the technology could be developed for biomedical applications. Anikeeva and her team have made their robots millimeters long, but she thinks the same approach could be used to produce much smaller robots.
Magnetic robot engineering
Anikeeva said that until recently, magnetic robots moved in response to a moving magnetic field. He explains that for this model, “if you want your robot to walk, your magnet will walk with it. If you want to rotate, you rotate your magnet.” That limits the settings in which such robots can be used. “If you are trying to operate in a really confined environment, moving magnets may not be the safest solution. You want to be able to have a stationary instrument that applies a magnetic field to the entire sample,” he explains.
Youngbin Lee PhD ’22, a former graduate student in Anikeeva’s lab, engineered a solution to this problem. The robots he developed in Anikeeva’s lab are not uniformly magnetized. Instead, they are strategically magnetized in different zones and directions so that a single magnetic field can enable a motion-driven profile of magnetic forces.
However, before being magnetized, a flexible and lightweight robot body must be created. Lee started this process with two types of rubber, each with a different stiffness. These are sandwiched together, then heated and stretched into long, thin fibers. Due to the different properties of the two materials, one rubber retains its elasticity through this stretching process, but the other rubber deforms and cannot return to its original size. So when the tension is released, one layer of fiber contracts, pulling on the other side and pulling the whole thing into a tight coil. Anikeeva said the helical fibers were modeled after the twisty tendrils of cucumber plants, which twist when one layer of cells loses water and contract faster than the second layer.
A third material – whose particles have the potential to become magnets – is incorporated in channels that run through the rubber fibres. So once the spiral is made, magnetization patterns that allow certain types of movement can be introduced.
“Youngbin thought very carefully about how to get our robot to move the way he programmed it to move,” says Anikeeva. “He made calculations to determine how to build such a force profile on it when we applied a magnetic field that it would actually start walking or crawling.”
To form a crawling robot like a caterpillar, for example, the helical fibers are shaped into gentle undulations, then the body, head, and tail are magnetized so that a magnetic field applied perpendicular to the robot’s plane of motion causes the body to move. compress. When the field is reduced to zero, the compression is released, and the crawler robot stretches. Together, these movements propel the robot forward. Another robot in which two leg-like helical fibers are connected by joints magnetized in a pattern that allows for more walking-like movements.
This precise magnetization process generates a program for each robot and ensures that once the robots are built they are easy to control. Weak magnetic fields activate each robot program and set in motion certain types of motion. A single magnetic field can even send multiple robots traveling in opposite directions, if they’ve been programmed to do so. The team found that one small manipulation of the magnetic field had a useful effect: By flipping a switch to reverse the field, a cargo-carrying robot could be made to gently wiggle and release its payload.
Anikeeva said she could imagine these soft-bodied robots — whose live production would be easy to scale up — delivering materials through narrow pipes, or even inside the human body. For example, they may carry medication through narrow blood vessels, releasing it exactly where it is needed. He says magnetically driven devices have biomedical potential beyond robotics as well, and may one day be incorporated into muscles or artificial materials that support tissue regeneration.