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Nov 29, 2023

Scientists reveal liquid metal coating that gives paper a mind of its own

dinn/iStock By subscribing, you agree to our Terms of Use and Policies You may unsubscribe at any time. Have you seen origami videos where the stop-motion effect makes the paper appear to be folding

dinn/iStock

By subscribing, you agree to our Terms of Use and Policies You may unsubscribe at any time.

Have you seen origami videos where the stop-motion effect makes the paper appear to be folding into beautiful shapes by itself? Scientists at Tsinghua University in China have made a breakthrough in the field of smart materials that might make this a reality.

The team developed a new liquid metal coating that can transform ordinary paper into self-adhesive gadgets capable of conducting heat and electricity. Although liquid metal is used in circuits and wearable sensors, the possibility of it being a coating has been unexplored until now.

The researchers successfully formulated a liquid alloy and applied it to paper using a stamping technique, eliminating the need for adhesives. This opens up new possibilities for creating lightweight, flexible smart objects and soft robots that can operate autonomously in various environments. The study, published in the journal Cell Reports Physical Science, examines the potential of liquid metal as a coating and its various applications.

The team, led by Bo Yuan of Tsinghua University in China, experimented with different alloy combinations to test the adhesion of the liquid metal coating. They compared an indium/gallium alloy (eGaIn) with an alloy of bismuth, indium, and tin oxide (BiInSn) and discovered that the latter offered superior adhesion and stability.

Unlike eGaIn, BiInSn does not oxidize when exposed to air, ensuring adhesion that isn’t reliant on an oxide film. Furthermore, BiInSn being solid at room temperature and of a higher melting point offers another advantage in not turning liquid at temperatures under 144° Fahrenheit (62° degrees).

“We needed to ensure the adhesion of liquid metal to be uniform in large scale on different paper, and to maintain the stability of the coating,” Bo Yuan told Ars Technica. “To solve these problems, we changed pressure applied on the stamp as well as the rubbing speed used in the experiments and finally found the most suitable parameters.”

The team folded liquid metal coated paper into origami cubes and observed them to be able to unfold and refold autonomously, thanks to the coating’s self-adhesive nature. Additionally, 3D structures using individual pieces of metal-coated paper maintained their shape without any additional binding agents.

The metal coating, designed to be easily peeled off, allows for recycling and repeated use without affecting the underlying paper.

Bo Yuan and his team look forward to finding a coating that maintains its adhesion once solidified. They believe bio-friendly paint spray based on this technology could revolutionize packaging with boxes that could open and close by themselves; and healthcare, with bandages that come off without leaving a bald patch on your skin.

Soft robots equipped with the liquid metal coating could make navigating challenging terrains easier, with self-adhesion allowing them to fold or unfold as needed and venture into areas unfit for larger rovers.

This study represents a significant advancement in smart material research. “Utilizing our method, one can quickly create smart materials with good thermal and electrical conductivity as well as stiffness-tunable ability,” Yuan added. “I think that this method may provide a new route for designing space explorers.”

Study Abstract

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. Here, 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 mechanism of adhesion between different liquid metals (eGaIn and BiInSn) and non-wetting substrates through force is explained. Furthermore, multifunctional origami structures based on the enhanced paper can switch between several deformation modes and include a shape memory antenna for receiving and transmitting electromagnetic signals. This work offers a generalized approach to developing multifunctional paper for applications in wearable testing platforms, flexible devices, and soft robotics.