Mini-robot shifts from solid to liquid to escape its cage—just like the T-1000

one of the many iconic moments of Terminator 2: Judgment Day I watched as the T-1000 briefly morphed into liquid and passed through the metal bar, away from its target, a teenage John Connor. A team of engineers mimicked that famous scene with his soft robot in the form of a Lego minifigure. The robot “melts” into a liquid in response to a magnetic field, oozes out between the bars of the cage, and re-solidifies on the other side. The team described their research in a recent paper published in the journal Matter.

As we reported previously, we traditionally think of robots as being made of hard, rigid materials, but the subfield of soft robotics takes a different approach. We are trying to build robotic devices out of more flexible materials that mimic properties found in living animals. Making a robot entirely out of soft materials has significant advantages, such as being flexible enough to squeeze into tight spaces to search for survivors after a disaster. Soft robots also have great potential as prosthetics and biomedical devices. Even rigid robots rely on some soft components, such as foot pads that act as shock absorbers and flexible springs to store and release energy.

For example, researchers at Harvard University built an octopus-inspired soft robot in 2016, which was composed entirely of flexible materials. Soft robots are so flexible that they are more difficult to control precisely. Therefore, in “octobot”, we replaced rigid electronic circuits with microfluidic circuits. Such circuits regulate the flow of water (hydraulic) or air (pneumatic), rather than electricity, through microchannels in the circuit, allowing the robot to bend and move. In 2021, engineers at the University of Maryland built his hand, a three-fingered soft robot that is agile enough to interact with Nintendo’s controller’s buttons and directional pad. Super mario bros. As a proof of concept.

This latest robot belongs to a class known as magnetically actuated miniature machines, typically made of soft polymers (such as elastomers or hydrogels) embedded with ferromagnetic particles with programmed magnetization profiles. This kind of robot can swim, climb, roll, walk, jump, as well as change shape simply by changing the corresponding magnetic field. As such, it is ideal for several biomedical applications such as targeted drug delivery and ulcer treatment. However, according to the authors of Matter’s new paper, such elastomer-based composites, which are inherently solid and have limited deformability, are limited to very narrow openings smaller than the dimensions of the material. It is difficult to get through the closed space.

Eager to find a solution, they looked to the humble sea cucumber for inspiration. Sea cucumbers are fascinating creatures with soft cylindrical bodies and mouths surrounded by retractable tentacles. Some species can even spit out toxins as a means of self-defense. But what piqued the interest of these engineers was the sea cucumber’s remarkable ability to loosen and tighten the collagen that forms its body’s walls at will. This causes the sea cucumber to essentially “liquefy” its body and push it through small cracks and crevices before joining all the collagen fibers together to form a solid body again.

The new minirobot is made of magnetically active phase-change material (MPTM) and can switch between solid and liquid states. When MPTM is heated in an alternating magnetic field, it melts into a liquid, and when the magnetic field is removed, it resolidifies due to ambient cooling. MPTM consists of ferromagnetic neodymium-iron-boron microparticles embedded in pure gallium. The resulting material has a melting point of 30.6° C (about 87° F), so it remains solid at room temperature. The MPTM has a rugged shape, good mechanical strength, can withstand high loads and has a wide range of maneuverability. In the liquid phase, the microparticles can rotate and reorient their magnetic polarities to extend, split and fuse as needed.

“The magnetic particles here have a dual role,” says co-author Carmel Majidi, a mechanical engineer at Carnegie Mellon University. “One thing is that by making the matter responsive to alternating magnetic fields, we can heat the matter by induction and cause a phase change. It also gives you the ability to move with your hands.”