Advances of liquid metal hydrogel composites in biomedical applications

Liquid metals (LMs) and hydrogels each represent advanced frontiers in emerging biomaterials and biomedicine. Hydrogels as a class of three-dimensional polymer networks formed by hydrophilic materials have broadly tunable physicochemical properties and high similarity with biological tissues. LMs, which possess liquid and metallic properties at normal temperature and pressure, are a new type of conductive material that has gained increasing attention. When integrated into hydrogel polymers, LMs act exceptionally as an 'active' filler and/or responsive element. The presence of LMs in these composites endows the LM hydrogel composites (LMHGs) with intriguing properties such as self-healing, flexibility, responsiveness, and thermal and electrical conductivity, achieving enhanced and even multifunctional synergistic effects. These properties significantly broaden their applications in various fields (figure 1). This review introduces the categories and featured properties of LMs, including biocompatibility, synthesis methods, surface modifications, response behaviors, and reversible deformation. Advanced biomedical applications of LMHGs involving medical electronics, biomedical engineering (BME), and soft electronics actuators are emphatically summarized. According to the integration mode of LMs and hydrogels, the biomedical soft electronics devices based on microfluidic technology and surface patterning technology as well as their properties are introduced in detail, respectively. The applications of LMHGs in BME mainly include tumor embolotherapy, contrast agent, single-cell amber, wound healing, and drug delivery system. Additionally, although in their infancy stage, the excellent performances of LMHGs described in this review contribute to the development of soft actuators and robots in the future. Finally, the present opportunities and challenges associated with the biological applications of LMHGs are also discussed. And, future development directions are proposed. Figure 1.Schematic of liquid metal hydrogel composites (LMHGs) in biomedical applications. The red, green, and orange apples represent the application of LMHGs in medical soft electronics, biomedical engineering, and soft actuators respectively.