Transforming Everyday Materials into Smart Devices: Chinese Scientists' Breakthrough in Liquid Metal Coating

In a groundbreaking development, Chinese scientists have devised a novel technique for coating common materials like paper and plastic with liquid metal, opening up possibilities for the creation of "smart devices." Published on June 9 in the journal Cell Reports Physical Science, the study demonstrates an innovative method for applying liquid metal coatings to surfaces that traditionally do not bond well with this material. The researchers from Beijing envision potential applications in wearable testing platforms, flexible devices, and soft robotics.

Previously, the high surface tension of liquid metal posed a significant challenge for scientists attempting to combine it with conventional materials such as paper. The existing solution, known as "transfer printing," involved the use of a third material to facilitate binding between the liquid metal and the surface. However, this approach introduced complications and could potentially compromise the electrical, thermal, or mechanical performance of the final product.

To overcome these limitations, Bo Yuan, a scientist at Tsinghua University and the study's first author, and his colleagues pursued an alternative approach that would allow for direct printing of liquid metal onto substrates without compromising its properties. They experimented with two different liquid metals (eGaln and BilnSn) and applied varying levels of force as they rubbed silicone and silicone polymer stamps coated with the liquid metals onto paper surfaces.

"At first, achieving stable adhesion of the liquid metal coating on the substrate was challenging," explained Yuan. "However, after extensive trial and error, we finally identified the right parameters to achieve stable, repeatable adhesion."

Through their experiments, the researchers discovered that applying a small amount of force while rubbing the liquid metal-coated stamp against the paper effectively bonded the metal droplets to the surface. Conversely, applying excessive force prevented the droplets from adhering properly.

The team then demonstrated the technique's versatility by folding the metal-coated paper into a paper crane, highlighting that the surface retained its usual flexibility even after the coating process. The modified paper maintained its typical properties as well.

While the results are promising, Yuan acknowledged that ensuring the stability of the liquid metal coating after application remains a challenge. Currently, a packaging material is required to protect the surface, but the researchers hope to find a solution that eliminates this need.

"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," explained Yuan. "Although the properties of the coating itself will not be significantly affected, objects in contact may become soiled."

Looking ahead, the team plans to expand the method to encompass a wider range of surfaces, including metal and ceramic. Additionally, they aim to utilize the technique for constructing smart devices using materials treated with this innovative approach.

Reference: “Direct fabrication of liquid-metal multifunctional paper based on force-responsive adhesion” by Yuan et al., 9 June 2023, Cell Reports Physical Science.