3D-printable liquid metal-based hydrogel for use as a multifunctional epidermal sensor

Conductive hydrogels have wide application prospects in flexible electronics, biosensors, and soft robotics because of their high flexibility, adjustable mechanical properties, and excellent electrochemical properties. However, it is difficult for a pure conductive polymer or rigid conductive filler hydrogel to meet the application requirements regarding electricity, mechanics, biocompatibility, and stability. To solve this problem, a special combination of polyacrylic acid (PAA) and liquid metal (LM) was adopted. Here, PAA was used as the interlayer phase, through which, on the one hand, the LM nanoparticles were coated through rich surface carboxyl groups and, on the other hand, the hydrogel framework was formed by chelating with calcium ions (Ca²⁺). The Ca-PAA-LM conductive hydrogel prepared in this way combined the multifunctional properties of plasticity, stretchability, printability, self-healing, and multiple sensing capabilities. Therefore, the epidermal sensor based on this hydrogel showed stable monitoring of human health conditions, including all-around body movements and electrophysiological signals, such as electrocardiography and electromyography. The conductive hydrogel developed in this study provides an ideal material choice for personalized health monitoring devices and also provides a path for the development of a new generation of multifunctional flexible sensors.