High-Resolution Stretchable Soft Liquid Metal Circuits Based on Cu-Ga Alloying and Femtosecond Laser Ablation.

Flexible electronic circuits are critical in biomedical devices, human-machine interfaces, and wearable sensing systems, which further require flexible conductive materials with high conductivity, stretchability, and electrical stability. Liquid metal (LM) has attracted much attention due to its unique metallic conductivity and room-temperature fluidic properties. However, LM's high surface tension properties increase the difficulty of patterning processing. Here, we report a scalable and simple fabrication method based on femtosecond laser ablation for the facile fabrication of patterned LM and Cu composite electrodes (LM@Cu) on flexible substrates. The LM@Cu electrodes, fabricated utilizing the exceptional micro-nanoprocessing precision and three-dimensional fabrication capabilities of femtosecond lasers, exhibit high resolution (approximately 5 μm), superior electrical conductivity (4.08 × 10 S/cm), and enhanced stability. In addition to planar circuits, we successfully fabricated 3D-patterned LM@Cu electrode circuits on PDMS hemispheres. The presence of ultrathin copper foils significantly improves the wettability of LM on the substrate, and the occurrence of alloying reactions between LM and Cu circumvents the challenges posed by the high surface tension of LM in pattern fabrication. We further investigated the electromechanical properties of the patterned LM@Cu electrodes under twisting, bending, and stretching in detail. In addition, the LM@Cu electrodes serve as an interface between rigid electronic devices and flexible substrates. When suffering external damage, LM@Cu electrodes remain working after simple brush coating due to the excellent fluidity of LM. To explore this fabrication approach's potential, we demonstrate various applications in wearable electronics, including stretchable luminous wristbands, flexible wearable strain sensors, and "visible" thermotherapy panels for relieving aching joints.