Liquid metal robots may no longer be limited to science fiction.
Researchers reported in Matter that miniature machines can go from solid to liquid and back again to squeeze into tight spaces and perform tasks such as soldering a circuit board.
Gallium's ability to control a magnetic field remotely is credited to researchers. Researchers incorporated magnetic particles to guide the metal's movements with magnets. This new material might help scientists construct soft, flexible robots that can maneuver through narrow passages and be guided externally.
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For years, scientists have been working on magnetically controlled soft robots. Most existing materials for these robots are either stretchy but sturdy materials, which are unable to maneuver in the narrowest of spaces, or magnetic liquids, which are fluid but incapable of carrying large objects (SN: 7/18/19).
After discovering inspiration from nature, researchers merged both approaches (SN: 3/3/21). Sea cucumbers, for instance, "can very rapidly and reversibly change their stiffness," says mechanical engineer Carmel Majidi of Carnegie Mellon University in Pittsburgh. "We as engineers must emulate that in soft materials systems."
The researchers used gallium to melt at around 30° Celsius — roughly below room temperature. The rapidly changing magnetic field causes the metal to heat up and melt. The material resolidifies when left to room temperature.
A permanent magnet can sift out magnetic particles throughout the gallium at a rate of 1.5 meters per second. The upgraded gallium can also support about 10,000 tons of weight.
External magnets are still capable of manipulating the liquid form, such as stretching, splitting, and merging. However, controlling the fluid's movement is more difficult, because the gallium particles can freely rotate and have unaligned magnetic poles as a result of melting.
Majidi and colleagues created small machines that performed different tasks. In a scene from Terminator 2, a toy person escapes a jail cell by melting through the bars and resolidifying in its original form with a mold placed just outside the bars.
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Gallium is a liquid at body temperature, about 37° C, and one machine removed a small ball from a model human stomach, according to the authors. A few more metals, such as bismuth and tin, would be added to the gallium in biomedical applications to increase the material's melting point.
According to biomedical engineer Amir Jafari of the University of North Texas in Denton, this phase-shifting material represents a significant advance in the field. One major challenge, he says, is precisely controlling external magnetic forces inside the human body.
"It's a powerful tool," says Harvard University robotics engineer Nicholas Bira, who was also not involved in the research. However, scientists who study soft robotics are constantly developing new materials, according to him.
"The true innovation that will follow is in combining these different innovative materials."