Every time you sit down with your phone in your back pocket, you’re reminded of one basic truth: 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 functionality of electronics and make it 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 with conventional metals. But solid metal can fatigue and break, and the more you add 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 just one of the useful properties of gallium. As it is a metal, it easily conducts heat and electricity. Unlike the better known liquid metallic mercury, it has low toxicity and a low vapor pressure, so it does not evaporate easily.
Gallium flows about as easily as water. But in air it also quickly forms a rigid outer oxide layer, allowing it to be easily shaped into semi-solid forms. The surface tension, which is 10 times that of water, can even be changed by immersing the liquid metal in salt water and applying tension.
“I’m biased, so take it for what it’s worth. But I think it’s one of the most interesting materials on the periodic table because it has so many unique properties,” says Dickey, co-author of a gallium overview in the 2021 Annual review of materials research.
Interest in gallium has lagged in the past, partly because of the unfair association with toxic mercury, and partly because its tendency to form an oxide layer was considered negative. But with an increased interest in flexible and, in particular, wearable electronics, many researchers are paying new attention.
To make bendable circuits with gallium, scientists shape it into thin wires embedded between sheets of rubber or plastic. These wires can connect tiny 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 movements, speed or vital signsfor 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 headphone wires that can stretch up to eight times their original length without breaking. Other circuits can repair themselves when torn – when the edges are positioned 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, one used to monitor heart or brain activity, says Majidi, who created such a circuit in print metal on soft material. The tattoos are more flexible and durable than existing electrodes, making them promising for long-term use.
The shape-shifting quality of liquid metal opens up other potential uses. When metal is pressed, stretched and twisted, its shape changes and the change in geometry also changes its electrical resistance. Thus, passing a small current through a mesh of gallium wires allows researchers to measure how the material is twisted, stretched and squeezed.
This principle could be applied to create motion-sensing gloves for virtual reality: if a mesh of gallium wires were embedded inside a thin, soft film inside the glove, a computer could detect Resistance changes as the wearer moves their hand. .
“You can use it to track the movements of your own body or the forces you’re in contact with, and then transmit that information into the virtual world you’re living in,” Majidi says.
This property even raises the possibility of machines using what Dickey calls “soft logic” to operate. Rather than requiring calculations, machines using soft logic have simple reactions based directly on changes in electrical resistance across the grid. They can be designed so that pushing, pulling or bending different parts of the grid activate different responses. As a demonstration, Dickey created a device that can turn motors or lights on and off based entirely on where the material is pressed.
“There are no semiconductors here. There’s no transistors, there’s no brain, it’s just based on how the material is affected,” says Dickey.
Low-level touch logic like this could be used to build responsiveness in devicessimilar to the construction of 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 energy. ‘electricity.
And that outer oxide layer that forms when gallium is exposed to air is now being exploited. The oxide layer allows the metal to hold its shape and opens up all sorts of possibilities for modeling and manufacturing. Tiny drops of gallium can be stacked on top of each other. A drop of gallium can be trailed along a surface, leaving a fine oxide trail that can be used as a circuit.
Also, in water, the oxide layer can form and disappear by applying a small amount of voltage, causing the beads to instantly form and collapse. By alternating, Dickey can move the beads of a weight up and down. With refinement, this property could form the basis of artificial muscles for robots, he says.
Dickey admits the technology is still in its infancy and the work so far just suggests how it could be commercialized. But gallium has so many interesting properties that it’s bound to be useful in soft electronics and robotics, he says.
He compares the field with the beginnings of computing. Although the first experimental computers made with vacuum tubes and mechanical switches were rudimentary by today’s standards, they established principles that gave rise to modern electronics.
Majidi says he also expects the liquid metal to be used commercially in the near future.
“Over the next few years, you’ll see more and more of this transition of liquid metal technologies in the industry, in the marketplace,” he says. “It’s not really a technical bottleneck at this stage. It’s about finding commercial applications and uses for liquid metal that really make a difference.
Knowable magazine is an independent journalistic company of annual reviews.