Scientists create liquid metal robot that can pass through bars, Terminator

Dec 13, 2023

In the movie Terminator 2: Judgment Day, the mission of the first terminator – played by Arnold Schwarzenegger – is to protect Sarah and John Connor from the T-1000, a new model of killer robot from the future who is so ductile that it can recover its shape with ease, even after being hit or shot at. Now, Chinese and US scientists have created something akin to a tiny T-1000. The invention, made of a metal that melts at near room temperature, can go from solid to liquid at its creators’ will. In the experiments, the robot was able to escape from a cage by going through the bars: it melted down and then solidified again on the other side. It has able been able to remove a foreign object from a stomach and solder an LED circuit.

The T-1000 in the James Cameron film was a prototype manufactured by the evil company Skynet with a "mimetic polyalloy" of liquid metal. The new robot, which was presented this week in the scientific journal Matter, is also made with a metal matrix: gallium, which, when pure, melts at 85.6ºF (29.8ºC). In other words, it melts in your hands. In addition to gallium, the robot contains an alloy of three other elements (neodymium, iron and boron) to amplify its response to magnetic fields.

The robot is made up of material known as magnetoactive phase transitional matter – or MPTM, for short. A magnetic field, at a certain intensity, induces an electric current inside the gallium, generating heat and turning it from solid to liquid. Without reaching that threshold, these magnetic fields can also make the robot jump 20 times its height, rotate at 1,500 revolutions per minute and move at a speed of one meter per second. It might not be as big as the T-1000 in the movie – it is barely a centimeter tall – but it is quite the bundle of energy.

In one of the videos shared by the researchers (see above), one can see the MPTM robot in the shape of a LEGO minifigure – approximately five millimeters wide and one centimeter high – escape from a small cage by going through the bars in a liquid state and solidifying again once it is free. "A magnetic field is used to melt it to a liquid and remove it from the enclosure," explains Carmel Majidi, professor of mechanical engineering at Carnegie Mellon University. In the same way that gallium melts when it approaches 86ºF (30ºC), it solidifies below that mark, and once it has gone through the bars, it returns to being a hard metal. The fact that it melts in your hand does not mean that it cannot be as hard as other metals.

The scientists devised several experiments to put their creation to the test. In one, they turn it into a screw that can reach corners, entering a hole in its liquid form and then becoming solid, sealing it. In another, the MPTM robot acts as a welder on an LED circuit, using part of itself as solder; gallium works both as a solder and as a conductive material, and like other metals, it has a high electrical conductivity, so it is very effective for connecting circuits. But, if it melts at room temperature, what will happen when the circuit gets hot while it works?

Majidi acknowledges the problem of its change of state. "Due to its low melting point, it is possible for gallium to soften and even melt when the circuit gets hot. It will still be conductive in a liquid state, so it would not affect its performance. However, to prevent it from leaking or spilling, it would have to be sealed with rubber or other soft insulating material," he says. As the director of the Soft Machines Lab at Carnegie Mellon, Majidi's field of expertise is soft materials, from crystals to liquid metals – and he is not overly concerned with the prospect of his MPTM melting: "Most of my research focuses on liquid metal circuits in which the conductive material remains liquid during circuit operation. As long as the metal is properly sealed and insulated, leaks aren't usually a cause for concern," he explains.

The creators of the MPTM think that it could have important medical applications. Using a human stomach model filled with water, they solved two very common problems in medicine: in one, they led the robot to a foreign body that needed to be removed. Once next to it, a magnet melted the robot, which then proceeded to envelop the object. Then, after it cooled down, they quickly extracted it with magnets. In the other, they tested the administration of a drug wrapped in MPTM. Once it got to where it was needed, it melted and released the drug.

Chengfeng Pan, an engineer from the Chinese University of Hong Kong and co-author of the paper, explained that "giving robots the ability to switch between liquid and solid states endows them with more functionality." What comes next, he says, is promoting this system of materials to "solve some very specific medical and engineering problems."

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