3D printing has started to involve everything from custom plastic casts to duck's feet. Now, researchers at North Carolina State University have developed a 3D printing technique that can create a variety of stable free-standing structures from liquid metal at room temperature.

While 3D printing of plastic objects has become almost commonplace, creating 3D structures out of metal is still a rarity. NASA has demonstrated a technique using “selective laser melting“ to produce intricate metal parts such as rocket engine components, but this requires the use of a high energy laser.

As Dr. Michael Dickey, an assistant professor of chemical and biomolecular engineering at NC State, explains, it is difficult to create structures from liquids, as they have a tendency to bead. "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."

Unlike processes that pattern metal "in plane," – or on one level – the team has developed multiple techniques that allow the creation of liquid metal structures that reach up or down, which is valuable in the connection of electrical components in three dimensions.

A thin (about 1 nm thick), passivating oxide skin forms rapidly on the surface of the liquid metal and stabilizes the microstructures despite the low viscosity and large surface energy of the liquid. Because the structures have a liquid center retained by only a thin film, the shape tends to be fragile while free-standing.

"They won't fall apart on their own weight (which is what makes this cool), but if you touched them or shook them with enough vigor, they would break," says Dickey.

One of the techniques to print these metal structures involves stacking droplets of liquid metal on top of each other, like "stacking oranges at the supermarket," according to the researchers. In this way, the droplets retain their shape as they adhere to each other.

Another method injects the liquid metal into a polymer template and the metal forms take that specific shape. Once injected, the template is dissolved, leaving the liquid metal in the molded shape.

The team is developing similar techniques for creating liquid metal wires, which retain their shape even when held perpendicular to the substrate. There is even potential to use them in various electronics applications and in conjunction with established 3D printing technologies.

The researchers are exploring how the new techniques could be used in conjunction with established 3D printing technologies and attempting to utilize these new structures for embedding in electronics. "For any practical device, they would need to be embedded in some other material," Dickey explains. "In our paper, we show that you can print a wire (in this case, that connects two LEDs) and embed it in elastomer (polymer) to make stretchable electrical connections."

The team's paper appears online in Advanced Materials.

The video below demonstrates the stacking technique.