Giant electrical conductivity difference enabled liquid metal-hydrogel hybrid printed circuits for soft bioelectronics

Soft printed circuits is a vital component in electronic devices, which is designed to connect electrical components to exert certain functions. However, the design and fabrication of soft printed circuits still suffer from significant limitations including interconnection difficulties among varying materials with different stiffness, intrinsically low conductivity of typical soft materials like hydrogels, and the multi-material assembly and integration issues involved in the wiring and encapsulation process. Here we report a novel design paradigm of a soft printed circuit termed as liquid metal (LM)-hydrogel hybrid printed circuit (LMH-HPC), in which highly conductive LM functions as electrical wires while hydrogel can simultaneously work as the electrical junction, encapsulation, and bio-interfacing. The whole LMH-HPC maintains completely mechanically adapted (∼1–30 kPa) to soft tissues. Based on this LMH-HPC, we design soft hybrid circuits with movable but stable electrical connections with conventional solid electrical components. Owing to the giant electrical conductivity difference between LM and hydrogel, the current through the hydrogel can be tuned with density distribution at the hydrogel junction, which can be further utilized as the soft and safe bioelectronic interfaces for applications such as soft electrical stimulators for excitable cells and tissues, and iontophoresis-assisted wound dressing for antibacterial therapy. Overall, this LMH-HPC may set up a multifunctional platform for the design of hydrogel electronic devices.