These shape-shifting entities melt and transform thanks to magnetic fields

Shape-shifting liquid metal robots may no longer be confined to science fiction.

Miniature machines can switch from solid to liquid and back again to squeeze into tight spaces and perform tasks like soldering a printed circuit board, researchers report Jan. 25 in Fabric.

This phase-changing property, which can be controlled remotely with a magnetic field, is thanks to the metal gallium. Researchers embedded the metal with magnetic particles to control the movements of the metal with magnets. This new material could help researchers develop soft, flexible robots that can slide through narrow passages and be controlled remotely.

Researchers have been developing magnetically controlled soft robots for years. Most existing materials for these bots are made of either stretchable but solid materials that cannot pass through the narrowest of spaces, or magnetic fluids that are fluid but unable to carry heavy objects (SN: 18/7/19).

In the new study, researchers mixed both approaches after finding inspiration from nature (SN: 3/3/21). Sea cucumbers, for example, “can very quickly and reversibly change their stiffness,” says mechanical engineer Carmel Majidi of Carnegie Mellon University in Pittsburgh. “The challenge for us as engineers is to mimic that in systems with soft materials.”

So the team turned to gallium, a metal that melts at around 30° Celsius – slightly above room temperature. Instead of attaching a heater to a part of the metal to change its state, the researchers expose it to a rapidly changing magnetic field to liquefy it. The alternating magnetic field generates electricity in the gallium, causing it to heat up and melt. The material solidifies again when left to cool to room temperature.

As magnetic particles are sprinkled through the gallium, a permanent magnet can pull it around. In solid form, a magnet can move the material at a speed of about 1.5 meters per second. The upgraded gallium can also support about 10,000 times its weight.

External magnets can still manipulate the liquid form, causing it to stretch, split and coalesce. But controlling the fluid’s motion is more challenging because the gallium particles can rotate freely and have uneven magnetic poles as a result of melting. Because of their different orientations, the particles move in different directions in response to a magnet.

Majidi and colleagues tested their strategy in tiny machines that performed various tasks. In a demonstration straight out of the movie Terminator 2escape a toy person from a prison cell by melting through the bars and resolidifying into his original form using a mold placed just outside the bars.

On the more practical side, a machine removed a small sphere from a model of a human stomach by melting slightly to wrap around the foreign object before exiting the organ. But gallium itself would become unwieldy inside a real human body, as the metal is a liquid at body temperature, around 37°C. A few more metals, such as bismuth and tin, would be added to gallium in biomedical applications to raise the material’s melting point, the authors say. In another demonstration, the material was liquefied and rehardened to solder a printed circuit board.


Using variable and permanent magnets, researchers turned chunks of gallium into shape-shifting devices. In the first clip, a toy figure escapes his prison cell by liquefying, sliding through the bars, and solidifying again using a mold placed just outside the bars. In the second clip, one device removes a bullet from a model of a human stomach by melting slightly to wrap around the foreign object and exit the organ.

Although this phase-changing material is a major step in the field, questions remain about its biomedical applications, said biomedical engineer Amir Jafari of the University of North Texas at Denton, who was not involved in the work. A major challenge, he says, is precisely controlling magnetic forces inside the human body that are generated from an external device.

“It’s a compelling tool,” says robotics engineer Nicholas Bira of Harvard University, who was also not involved in the study. But he adds that researchers studying soft robotics are constantly creating new materials.

“The true innovation that comes is in combining these different innovative materials.”

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