Flexible electronics and 3D printing are quickly reshaping the world in many aspects spanning from science, technology to industry and social society. However, there still exist many barriers to impede further progress of the areas. One of the biggest bottlenecks lies in the strong shortage of appropriate functional inks. Among the many printable materials ever tried such as conductive polymers, powdered plastic, metal particles or other adhesive materials, the liquid metal or its alloy is quickly emerging as a powerful electronic ink with diverse capabilities from which direct printing of flexible electronics and room temperature 3D printing for manufacturing metal structures are enabled. All these fabrication capabilities are attributed to the unique properties of such metal's low melting point (generally less than 100 °C), flowable feature and high electrical conductivity etc. To better push forward the research and application of the liquid metal printed electronics and 3D manufacture, this article is dedicated to present an overview on the fundamental research advancements in processing and developing the liquid metal inks. Particularly, the flow, thermal, phase change and electrical properties of a group of typical liquid metals and their alloy inks will be systematically summarized and comparatively evaluated. Some of the practical applications of these materials in a wide variety of flexible electronics fabrication, 3D printing and medical sensors etc. will be briefly illustrated. Further, we also explained the basic categories of the liquid metal material genome towards discovering new functional alloy ink materials as initiated in the authors' lab and interpret the important scientific and technical challenges lying behind. Perspective and future potentials of the liquid metal inks in more areas were also suggested.
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