High Solid Content Liquid Metal Ink for Flexible Printed Circuits: Formulation, Stability, and Multi‐Layer Integration

Abstract This study introduces a simple formulation process for high solid content liquid metal (LM) ink for flexible printed circuits. The formulation process begins by sonicating gallium liquid metal in ethylene glycol (EG) to create sub‐micron particles, forming a stable colloidal suspension. Cellulose nanofibers (CNF) are then added to encapsulate the particles, resulting in a uniform particle size of ≈300 nm and a high zeta potential of −60 mV, ensuring the ink stability for long term storage. The optimal ratio of LM to CNF is determined to be 10:0.75, balancing suspension stability and conductivity. The optimized LM‐CNF/EG ink demonstrates excellent printability, substrate adhesion, and self‐sintering capabilities, achieving a high electrical conductivity (10 5 S m −1 ) without conventional mechanical sintering. The printed circuits exhibit remarkable mechanical resilience, maintaining the same conductivity after over 1000 cycles of bending and stretching. Using a direct writing method, multi‐layered flexible printed circuit boards are successfully integrated using lamination and via‐filling techniques for the continuity of layer structure. This integration highlights the potential for portable, lightweight wearable technologies. This LM‐CNF/EG ink and novel fabrication technique offer a robust solution for advanced multi‐layered flexible electronics, marking a significant step toward the development of integrated electronic devices.