Researchers have developed a 3D printer which can make objects with liquid metal.

The breakthrough raises the possibility of far more durable - and potentially deadly - objects being made remotely by individuals with 3D printing machines.

The structures made so far are stable at room temperature, and are free-standing. They are also in principle able to 'self heal', because the outer hard coating is made when the liquid metal is exposed to air.

Developing the technique involved making a metal alloy (75 percent eutectic gallium alloy and 25 percent indium) which is in a liquid state at room temperature, but forms an outer layer of gallium oxide in air allowing it to hold its structure.

The team from North Carolina State University were able to pioneer a number of techniques - principally intended for use in electronics. They include one method in which the machine stacks drops of liquid metal on top of each other which merge, but retain their original shape.

They also injected the metal into a polymer 3D-printed cast, where it is moulded into a pre-defined shape. A third technique is able to make liquid metal wires.

"It's difficult to create structures out of liquids, because liquids want to bead up. But we've found that a liquid metal alloy of gallium and indium reacts to the oxygen in the air at room temperature to form a 'skin' that allows the liquid metal structures to retain their shapes," said Dr. Michael Dickey, an assistant professor of chemical and biomolecular engineering at NC State.

The metal is non-toxic but is expense - about 100 times the cost of the plastic used in 3D printers.

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