Digital Shear Printing of Mechanically Robust Liquid Metal Circuits with Hierarchical Embedded Structure for Paper Electronics

The seamless integration of recyclable and deformable liquid metal (LM) conductors into paper is attractive to developing flexible, breathable, and green electronics. However, the weak adhesion between paper and LM complicates the patterning. In addition, the low damage endurance of LM, an open problem for on‐surface conductors, restricts the practical applications. Here, a simple yet efficient approach of shear printing is reported to directly pattern hierarchical embedded LM circuits with erasure resistance onto paper. This is achieved by digitally applying the shear force to the solid gallium film to induce its adhesive wear with the paper, allowing the gallium to be embedded into the paper's fiber networks. Meanwhile, the pressure‐induced formation of microgrooves on paper allows the LM circuits to be surface‐embedded onto paper. The hierarchical embedded structure endows the LM circuits with enhanced mechanical damage endurance that they can even withstand eraser rubbing and tape peeling. Applications of the hierarchical embedded, mechanically robust, and breathable LM‐enabled paper electronics in enhanced humidity sensing, electrophysiology monitoring, and digital droplet microfluidics are also shown. This work opens doors to developing sustainable yet robust paper electronics by rationally utilizing the solid properties of low‐melting‐point metals.