Liquid metal for flexible smart materials

Flexible technologies are emerging, from foldable smartphones to freeform public information displays, and Chinese researchers have made a discovery that could elevate this technology to the next level.

The high electrical conductivity and flexibility of liquid metals are the keys to this technology, with potential applications in wearable sensors, actuators, smart switches, printable circuitry for robotics, and technologies that not only move but stretch, such as prosthetics.

Previous efforts to combine liquid metals with flexible materials have been impeded by extremely high surface tension, which naturally resists adhering to most materials, even porous paper.

Earlier attempts mainly focused on transfer printing, which involves using a third material to bind the liquid metals to the surface. But mixing or coating the metal with binders often weakens electrical, thermal, and mechanical performance.

A new study published in the journal Cell Reports Physical Science, showcases an inexpensive, efficient technique for coating surfaces that do not easily bond with liquid metal.

"Before, we thought that it was impossible for liquid metal to adhere to non-wetting surfaces so easily, but here it can adhere to various surfaces only by adjusting the pressure, which is very interesting," said Bo Yuan, a scientist at Tsinghua University in Beijing, China, and the first author of the study.

Liquid metal alloys

To directly apply liquid metal to substrates without using adhesives or sacrificing the metal's properties, Yuan and his team tested two types – an alloy of bismuth, indium, and tin oxide with a relatively higher melting point and an indium-gallium alloy using various silicone and silicone polymer applicators and different levels of application force.

The new liquid metal coatings developed by Yuan and fellow researchers are designed to work at a large scale where everyday materials like sheets of paper or plastic could be transformed into wearable sensors, smart materials, and soft robotics.

By selecting the proper application temperature, previously delicate substrates can handle more wear and weight, achieve self-healing properties and shape memory capabilities, all by heating and cooling.

"At first, it was hard to realize stable adhesion of the liquid metal coating on the substrate," said Yuan. "However, after a lot of trial and error, we finally had the right parameters to achieve stable, repeatable adhesion."

Enhanced origami

Once an effective coating technique was developed, the team folded an "enhanced" sheet of paper into an origami crane, demonstrating that the material can maintain its original properties and retain conductivity, even in the folds.

In the ancient art of origami, one thin sheet of paper can be folded into elaborate 3D structures with unique functions without cutting or gluing. The principles of origami are becoming more widely adopted in bioengineering and mechanical metamaterials, realizing complex functions such as cutting-edge architectural modeling and controlled drug delivery in nanomedicine.

Yuan acknowledged that while results of the team's advanced material origami are promising, the liquid metal coating still requires protection after application. The current solution involves adding a packaging material to the surface, but the team is determined to eliminate this additional requirement.

"Just like wet ink on paper can be wiped off by hand, the liquid metal coating without packaging here also can be wiped off by the object it touches as it is applied," said Yuan. "The properties of the coating itself will not be greatly affected, but objects in contact may be soiled."

Unfolding applications

Using liquid metal-coated materials, the Chinese research team developed a reconfigurable antenna that could receive and transmit electromagnetic signals at different frequencies. With heat, the antenna can be folded and unfolded, maintaining either position. They said this type of shape memory technology is more advantageous than telescoping or mechanically programmed antennas due to the ease of manipulation and lack of moving parts.

Additionally, the surface-to-surface adhesion capability of liquid metal enhancement made it possible for separate components to be combined in various reversible configurations using temperature changes, without extra adhesive, permitting complicated structures that otherwise couldn't be realized by untreated paper materials.

The liquid metal research team plans to expand the method to test a greater variety of surfaces commonly used in new technologies, robotics, and electronics, including metal and ceramic.

"We also plan to construct smart devices using materials treated by this method," said Yuan.