Directly-Printed Liquid Metal Can Turn Paper Into a Tunable Smart Material for Robotics and More

A research team from Tsinghua University, Beihang University, and the Chinese Academy of Sciences has come up with a generalized approach to turning basic paper into a functional material good for wearables, soft robotics, and more — by combining the direct application of liquid metal with origami.

"The introduction of paper and origami into industry and daily life has been a hallmark of human civilization. However, origami’s performance is limited by the properties of the constituent materials, including long-term bending, electrical, and thermal conductivity," the team explains. "We report a new method for directly adhering liquid metal onto non-wetting substrates on a large scale, allowing for the regulation of the mechanical and electrical properties of the enhanced paper by controlling the applied force during fabrication."

The idea: adjusting the electrical and thermal conductivity of paper by applying a liquid metal, but in a way that would allow for the mass-production of the resulting functionalized paper. That's not as easy as it sounds: liquid metal has a very high surface tension, much higher than most liquids, meaning that it has a tendency to simply roll off surfaces like paper rather than soak in.

The team's solution is a fast fabrication method in which liquid metal is printed directly onto the paper using a force-responsive adhesive. As the force is increased, more of the metal remains joined to the paper — providing an easy way to change the conductivity and physical stiffness of the underlying material along a continuous spectrum. Once printed, the metal and the paper remain bonded — even if the material is cut or folded, allowing for complex shapes to be produced from flat sheets using origami or kirigami (the Japanese arts of paper folding and paper cutting, respectively) methods.

To prove the concept the team set about building a range of prototypes from the enhanced paper, including an origami crane and a stiff box — and, more impressively, a reconfigurable antenna which can be folded then automatically unfolded through exposure to heat. "The antenna owns three working modes including complete unfold, complete fold, and fold-to-unfold status after heating," the researchers explain. "The LM [Liquid Metal] origami monopole antenna switches the working state of the antenna through temperature adjustment and realizes the reconfigurability of the antenna frequency."

The team's work has been published in the journal Cell Reports Physical Science under open-access terms.