Shape-shifting liquid metal robots may no longer be the stuff of science fiction.
Miniature machines can go from solid to liquid and back again to squeeze into tight spaces and perform tasks like soldering a circuit board, researchers report Jan. 25 in material.
This phase shift property, which can be controlled remotely with a magnetic field, is due to the metal gallium. The researchers embedded the metal with magnetic particles to direct the movements of the metal with the help of magnets. This new material could help scientists develop soft, flexible robots that can pass through narrow passages and be externally guided.
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Scientists have been developing magnetically controlled soft robots for years. Most existing materials for these robots are made from either elastic but solid materials that cannot fit through the tightest of spaces, or magnetic fluids that are fluid but unable to carry heavy objects (SN: 7/18/19).
In the new study, the researchers combined both approaches after taking 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 soft material systems.”
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 piece of 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 resolidifies when allowed to cool to room temperature.
Because the magnetic particles are sprinkled in the gallium, a permanent magnet can pull it around. In solid form, a magnet can move material at a speed of about 1.5 meters per second. The improved gallium can also carry about 10,000 times its weight.
External magnets can still manipulate the liquid form, causing it to stretch, split, and coalesce. But controlling the motion of the fluid is more difficult because gallium particles can spin freely and have misaligned 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 performing different tasks. In a straight-from-the-movie demo Terminator 2a toy person escaped from a prison cell by melting through the bars and resolidifying into his original form using a mold placed just outside the bars.
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On the more practical side, a machine removed a small ball from a model of a human stomach, melting gently to wrap around the foreign object before exiting the organ. But gallium by itself would turn into carbohydrates inside a real human body, since the metal is a liquid at body temperature, about 37 °C. More metals, such as bismuth and tin, would be added to gallium in applications biomedical. to raise the melting point of the material, say the authors. In another demonstration, the material liquefied and resolidified to glue a circuit board.
Although this phase-change material is a big step in the field, questions remain about its biomedical applications, says biomedical engineer Amir Jafari of the University of North Texas at Denton, who was not involved in the work. A big challenge, he says, is precisely controlling the magnetic forces inside the human body that are generated by an external device.
“It’s a compelling tool,” says robotics engineer Nicholas Bira of Harvard University, who was not involved in the study. But, he adds, scientists studying soft robotics are constantly creating new materials.
“The real innovation to come lies in combining these different innovative materials.”