Engineering liquid metal-based implantable electrodes toward brain-machine interfaces

Driven by the development of innovative electrode materials, the brain-machine interface (BMI) has witnessed significant advancements in recent years. However, traditional implantable electrodes, such as metal-based and polymer-based electrodes, often face limitations in terms of flexibility, chronic stability, and long-term biocompatibility. Liquid metal-based electrodes have emerged as a promising alternative to traditional electrode materials, offering unique properties and numerous advantages for BMI applications. In this Review, we provided an in-depth overview of liquid metal-based electrodes. Liquid metal-based electrodes demonstrate several desirable features, including high electrical conductivity, flexibility, biocompatibility, and the ability to conform to the irregular and delicate structures of the brain. We also systematically and thematically discussed their material properties, biocompatibility considerations, performance characteristics, and assorted applications in neural recording, stimulation, brain-machine interfaces, closed-loop feedback systems, and neuroprosthetics. Meanwhile, long-term stability, biocompatibility, electrode-tissue interface, miniaturization, signal processing, and ethical considerations are areas that require further research and innovation. Addressing these challenges will contribute to the successful integration and widespread use of liquid metal-based electrodes in BMI systems.