Gallium finds a new use in soft electronics

Gallium finds a new use in soft electronics

Fold it. Stretch it out. Use it to conduct electricity. Researchers are exploring a range of applications that take advantage of the element’s unusual properties.

Whenever you sit down with your phone in your back pocket, a fundamental truth comes to mind: Human bodies are soft and flexible. Electronics no.

But soon there may be devices that can stretch, bend, and even repair themselves when damaged. By harnessing the unusual properties of a liquid metal called gallium, materials scientists aim to create a new generation of flexible devices for virtual reality interfaces, medical monitors, motion sensing devices and more.

The goal is to take the capabilities of electronics and make them softer, says Michael Dickey, a chemical engineer at North Carolina State University. “I mean, the body and other natural systems have figured out how to do it. So surely we can do it. “

Foldable electronics can also be made from conventional metals. But solid metal can fatigue and break, and the more it is added to a soft material, the stiffer the material becomes. Liquid metals don’t have this problem, says Dickey – they can be bent, stretched, and twisted with little or no damage.

Flexibility turns out to be only one of the useful properties of gallium. Since it is a metal, it easily conducts heat and electricity. Unlike the more well-known liquid metallic mercury, it has low toxicity and low vapor pressure, so it does not evaporate easily.

Gallium flows easily like water. But in the air it also quickly forms a rigid outer oxide layer, allowing it to be easily formed into semi-solid forms. The surface tension, which is 10 times that of water, can also be varied by immersing the liquid metal in salt water and applying a tension.

“I’m biased, so take this for what it’s worth. But I think this is one of the most interesting materials on the periodic table because it has so many unique properties, “says Dickey, co-author of an overview of gallium in 2021. Annual review of materials research.

Interest in gallium has lagged behind in the past, partly due to its unfair association with toxic mercury and partly because its tendency to form an oxide layer was considered negative. But with the growing interest in flexible and especially wearable electronics, many researchers are paying new attention.

To make foldable circuits with gallium, scientists form it into thin threads embedded between sheets of rubber or plastic. These wires can connect small electronic devices such as computer chips, capacitors, and antennas. The process creates a device that could wrap around an arm and be used to track an athlete’s movement, speed or vital signs, for example, says Carmel Majidi, a mechanical engineer at Carnegie Mellon University.

These liquid metal wires and circuits can withstand significant bending or twisting. As a demonstration, Dickey has made earphone cables that can stretch up to eight times their original length without breaking. Other circuits can heal if torn: when the edges are placed against each other, the liquid metal flows back together.

Gallium circuits can also be printed and applied directly to the skin, like a temporary tattoo. The “ink” works like a conventional electrode, the kind used to monitor heart or brain activity, says Majidi, who made such a circuit by printing the metal onto a flexible material. Tattoos are more flexible and durable than existing electrodes, which makes them promising for long-term use.

The shape-shifting quality of the liquid metal opens up other potential uses. When the metal is crushed, stretched and twisted, its shape changes and the change in geometry also changes its electrical resistance. Then running a small current through a gallium wire mesh allows researchers to measure how the material is twisted, stretched and pressed.

This principle could be applied to create motion sensing gloves for virtual reality: if a gallium wire mesh were embedded within a thin, soft film inside the glove, a computer could detect changes in resistance while the wearer moves his hand.

“You can use it to track the movement of your body, or the forces you are in contact with, and then transmit this information to whatever virtual world you are experiencing,” says Majidi.

This property also increases the possibility of machines using what Dickey calls “soft logic” to function. Rather than requiring computation, machines using soft logic have simple reactions based directly on changes in electrical resistance across the grid. They can be designed in such a way that pushing, pulling or bending different parts of the grid activates different responses. For demonstration purposes, Dickey has created a device that can turn motors or lights on and off depending on where the material is pressed.

“There are no semiconductors here. There are no transistors, there is no brain, it’s just based on how the material is touched, ”says Dickey.

Low-level tactile logic like this could be used to create reactivity in devices, similar to creating reflexes in soft robots: such reactions do not require a complex “brain” to process information, but can react directly in response to environmental stimuli. changing color or thermal properties or redirecting electricity.

And that outer oxide layer that forms when gallium is exposed to air is now being harnessed. The oxide layer allows the metal to hold its shape and opens up all sorts of possibilities for modeling and fabrication. Small drops of gallium can be stacked on top of each other. A drop of gallium can be dragged along a surface, leaving a thin oxide trail that can be used as a circuit.

Additionally, the oxide layer in water can be caused to form and disappear by applying a small amount of tension, causing the beads to form and collapse instantly. By switching back and forth, Dickey can make the beads move a weight up and down. With refinement, this property could form the basis of artificial muscles for robots, he says.

Dickey admits that the technology is still in its early stages and that the work so far simply suggests how it might be commercialized. But gallium has so many interesting properties that it’s bound to be useful in soft electronics and robotics, he says.

Compare the field with the early days of computing. Although the first experimental computers made with vacuum tubes and mechanical switches are crude by today’s standards, they established principles that gave rise to modern electronics.

Majidi says she also expects to see the liquid metal used commercially in the near future.

“In the next few years, you will see more and more of this transition of liquid metal technologies in industry, in the market,” he says. “It’s not really a technical bottleneck at this point. It’s about finding commercial applications and uses for liquid metal that really make a difference. “

This article originally appeared in
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