Stretchable Liquid Metal Keeps Conductivity Even Under Strain

USAF developed a liquid metal conductor that could power wearables and textiles.

The Air Force Research Laboratory developed Polymerized Liquid Metal Networks that rupture to transform into a highly stretchable design, autonomously increasing conductivity with strain. (Image courtesy of Second Bay Studios.)

The USAF recently announced that it has created a class of liquid metal electronics that seemingly defy the laws of electromechanics by becoming better conductors when they are stretched or strained. The new material has been called Polymerized Liquid Metal Networks (PLMN), and autonomously changes its structure in a way that increases conductivity.

“This response to stretching is the exact opposite of what you would expect,” said Dr. Christopher Tabor, AFRL lead research scientist on the project. “Typically, a material will increase in resistance as it is stretched simply because the current has to pass through more material.”

But the PLMNs don’t operate the same way, thanks to their underlying structure. “Experimenting with these liquid metal systems and seeing the opposite response was completely unexpected and frankly unbelievable until we understood what was going on.” 

Wires that can maintain their properties despite changing mechanical conditions have many applications, such as next-generation wearable electronics. As an example, textiles using these wires could be used for a long-sleeved shirt that transfers power across the body and through the material in such a way that the movements of bending an elbow or rotating a shoulder won’t change the power being transferred.

AFRL researchers also evaluated the material’s heating properties in a form factor resembling a heated glove. They measured thermal response with sustained finger movement and retained a nearly constant temperature with a constant applied voltage, unlike current state-of-the-art stretchable heaters that lose substantial thermal power generation when strained due to the resistance changes. 

PLMNs are built starting with tiny particles of liquid metal enclosed in a shell like a microscopic “water balloon.” Using a polymerization process, these particles are chemically attached to one another like a chain, where all the particles are connected to each other. When strained through stretching, the “balloons” burst and spill out a portion of their liquid metal, making it possible to retain conductive properties that are nearly identical to their previously unstrained state. What’s more, when strain is released, PLMNs can fall back into place and operate as if the strain never occurred. 

The research team says this process can be repeated at least 10,000 times without any hint of performance fatigue.

“The discovery of Polymerized Liquid Metal Networks is ideal for stretchable power delivery, sensing and circuitry,” said Capt. Carl Thrasher, research chemist at AFRL. “We think this is really exciting for a multitude of applications. This is something that isn’t available on the market today, so we are really excited to introduce this to the world and spread the word.”

“Human interfacing systems will be able to operate continuously, weigh less and deliver more power with this technology,” Thrasher added.

At present, PLMNs are only in the basic research stage, and the USAF is looking to partner with both private companies and universities to explore scale-up and development into a meaningful technology.