In the movie Terminator 2: Judgment Day, the first Terminator (played by Arnold Schwarzenegger) defends Sarah and John Connor from a new generation of liquidators from the future, the T-1000. The first thing that catches the attention of this robot is that it is so stretchy that after a hit or a bullet it will deform and immediately put itself back together. Now Chinese and American researchers have developed something similar to a T-1000 in a reduced version. His creation, made of metal that melts at near room temperature, can transition from a solid to a liquid at the will of its creators. In the experiments they conducted, he was able to escape from a prison by going through the bars, disintegrating completely, and reassembling on the other side. But it has also proven its ability to eject a foreign object in a stomach or solder an LED circuit.
The T-1000 in James Cameron’s film was a prototype which, according to the script, was made from liquid metal by the evil company Skynet using a “mimetic polyalloy” (sic). The robot now presented in the science journal Matter also consists of a matrix of a metal, gallium, which melts at 29.8 degrees in its pure state. I mean it would melt in your hands. They added particles of an alloy of three other elements, neodymium, iron and boron, to this matrix. This enhanced the device’s response to magnetic fields.
The robot is called MPTM, an acronym for Magnetoactive Phase Transition Material. That is, a magnetic field of a certain intensity induces an electric current in the gallium that generates heat and turns it from solid to liquid. Without reaching this threshold, it is also these magnetic fields that allow it to leap twenty times its height, spin around at 1,500 revolutions per minute, or propel itself at a speed of one meter per second. It’s not as big as the T-1000 in the movie – it’s barely an inch off the ground – but it’s a real atomic ant.
“The figure is similar in size to a LEGO character and is melted into a liquid using a magnetic field.”
Carmel Majidi, mechanical engineer from Carnegie Mellon University (USA)
In one of the videos distributed by the researchers (see above), it can be seen escaping from a kind of prison, passing through the bars in a liquid state and then resolidifying outside the prison. Professor of Mechanical Engineering at Carnegie Mellon University (USA) Carmel Majidi explains what they did: “The figure is similar in size to a commercial LEGO figure: about five millimeters wide and one centimeter high. A magnetic field is used to melt it into a liquid and remove it from the case.” Just as gallium melts as it approaches 30 degrees, it freezes below those degrees. And once through the bars, it becomes hard metal again. The fact that it melts in the hand does not prevent it from having the great hardness of other metals.
The scientists devised several experiments to see what their creature could do. In one they turn it into a screw capable of reaching corners, filling the hole in its liquid form and sealing it once solid. In another case, MPTM acts as a soldering iron on an LED circuit, using part of itself as solder. But if it melts at room temperature, what happens if the circuit gets hot during operation?
“The gallium in the material acts both as a solder and as a conductive material. Like other metals, it has a high electrical conductivity and is therefore very suitable for connecting circuits,” explains Majidi, who knows the problem of its state change. “Due to its low melting point, gallium can soften and even melt when the circuit gets hot. It is still conductive in its liquid state, so its performance is not affected. However, to prevent leakage or spillage, it would need to be sealed with rubber or some other soft insulating material,” he says. Majidi is director of the Soft Machines Lab at Carnegie Mellon, so his area of expertise is soft materials, from crystals to liquid metals, so he’s not too concerned about his MPTM melting easily: “Most of my research is focused to liquid metal circuits in which the conductive material remains liquid during circuit operation. As long as the metal is properly sealed and insulated, you generally don’t have to worry about leaks,” he says.
For its creators, MPTM could have relevant medical applications. Using a water-filled model of an artificial stomach, they solved two problems that are widespread in medicine. In one of them, they drove the robot to a foreign object that had to be removed from there. Once next to him, the magnet melted the robot, which hugged the object, a small ball. Once cooled, they quickly retrieved it by playing with the magnets. In the other, they tested the delivery of a drug packaged in MPTM. After taking it to the place where it was needed, it melted and released. In a note, Chengfeng Pan, an engineer at the Chinese University of Hong Kong and a co-author of the article, commented that “giving robots the ability to switch between liquid and solid states gives them more functionality.” Next, Pan says, this system of materials will be promoted to “solve very specific medical and engineering problems.” It’s the other great advantage of magnetism, which is passing through the body or objects to reach places where there isn’t other way is there to do.
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