Applications of Liquid Metals

Low-melting-point metals, especially those that are liquid at room temperature, are being explored for an increasing number of applications. Like solid-phase metals, liquid metals have high electrical and thermal conductivity. However, liquid metals are also conformal, flexible, and stretchable, even when using thick films and large volumes. The shapes and structures that can be achieved with liquid metals span a variety of geometries and scales, ranging from thin films to 3D structures, and from nanoscale to macroscale feature sizes. Furthermore, liquid metals based on alloys of gallium have been shown to have low toxicity, making them suitable for biomedical devices and wearable electronics. These unique properties of low-melting-point metals make them useful materials in a variety of applications such as soft electronics, catalysis, and microfluidics. This Special Section is a collection of ten research articles and one review article, contributed by experts in applications that utilize low-melting-point metals. The articles can be broadly sorted into three themes: 1) liquid metals for stretchable electronics, 2) liquid metals for energy storage devices and recyclable devices, and 3) fabrication processes enabled by or tailored to the use of low-melting-point metals. The topics covered by this special section illustrate the wide applicability of low-melting-point metals, and the utility of this class of materials in important and trending research areas. Liquid metals are inherently suitable for stretchable electronics, as they can be used to realize deformable electrically conductive materials. In addition, repeated cycles of stretching and bending can result in fatigue of solid materials, but liquid–metal components are unaffected. Furthermore, the low toxicity of gallium-based liquid metals makes them suitable for wearable electronics. In this special section, Liu and co-workers demonstrate a wearable sensor that uses a spiral structure of liquid metal to measure a variety of human motion (article number 2300896). Du and co-workers have reviewed the broader field of stretchable and flexible sensors that employ liquid metals (article number 2300431). Lim and co-workers describe a method of fabricating electrodes that use liquid metal in a sponge-like structure, and demonstrate stretchable sensors and flexible electronic breadboards using these "sponge electrodes" (article number 2301589). Malakooti and co-workers have also created stretchable conductors but with a different approach: printing elastomers impregnated with liquid–metal microdroplets (article number 2301324). Under strain, the microdroplets become electrically connected, and remain conductive after the strain is released. Bae and co-workers also used the direct printing of conductive material, but in their work liquid metal was used as an ink to create a stretchable thermoelectric device (article number 2301171). As seen in these papers, not all of the components of stretchable electronics need to be deformable. Jang and co-workers created a stretchable display that consists of an array of light-emitting-diode pixels with liquid–metal interconnects (article number 2301413). It is beneficial to have flexible and stretchable energy sources for stretchable electronics. Low-melting-point metals are also useful in this area of research. As in other stretchable devices, liquid metals can be used for electrodes in energy storage devices. Toward this end, Tavakoli and co-workers show that graphene oxide coatings on eutectic gallium–indium liquid metal films make them more stable in acidic or alkaline solutions (article number 2301428). The coating thus makes electrodes made from these liquid metals more robust, with a higher capacitance per unit area. This is useful for energy storage in devices such as supercapacitors. In a separate article, Tavakoli and co-workers demonstrate a different type of energy storage for stretchable electronics: a strain-tolerant rechargeable battery (article number 2301189). This battery uses a liquid–metal current collector and a gallium-carbon anode, and is capable of self-healing damage to the gallium-carbon electrode. The battery can still be repaired after more extensive damage, and the metals can be recovered and recycled at the end of the battery's lifetime. Handschuh–Wang and co-workers have also developed devices that can be recycled (article number 2301483). In this case, these are transient stretchable circuits made from gelatin biogel substrates with liquid metal conductive elements. The circuits can be quickly and easily degraded, as the biogel substrate dissolves in hot water in less than a minute. The liquid metal and biogel materials can then be recovered and recycled. The articles mentioned above employ a variety of methods to fabricate the devices and circuits that use low-melting-point metals. These fabrication processes have resulted in many types of novel devices and circuits. However, this special section contains two articles that focus on fabrication methods with broader applicability. Gui and co-workers show that molds made of elastomer and polycarbonate membranes can be used to create 3D metal structures with a minimum size of 10 µm (article number 2301625). In this work, a bismuth-indium alloy with a melting point of 72 °C fills the mold in its liquid state, then is cooled to create the final metal structure, forming a variety of 2D or 3D shapes. Lazarus and co-workers describe a fabrication method that integrates the direct laser writing of microfluidic channels with larger features and substrates made by stereolithography (article number 2301980). These multi-scale structures help with the introduction of liquid metal into microchannels, and enable the fabrication of nH-range coil-type inductors. We thank all the authors in this special section for their valuable contributions to this area of applied research. We also appreciate the other experts who have volunteered their expertise and time during the peer review process. We are especially grateful to Dr. Joseph Krumpfer and Dr. Esther Levy for their efforts in making this special section possible. The papers in this special section span a variety of important and interesting topics, all made possible by the use of low-melting-point metals. We hope that this collection of articles informs and stimulates further research using this unique class of liquid–metal materials. Furthermore, because of the variety of applications, and the increasing popularity of liquid metals, this Special Section has been organized jointly with another Special Issue on liquid metals in Advanced Functional Materials. Interested readers are encouraged to also explore this accompanying special issue (see Guest Editorial for details). The authors declare no conflict of interest. Aaron Ohta is a professor in the Department of Electrical and Computer Engineering at the University of Hawaii at Manoa. He received his B.S. degree from the University of Hawaii at Manoa, his M.S. degree from the University of California, Los Angeles, and his Ph.D. degree from the University of California, Berkeley, all in electrical engineering. Aaron's research interests include reconfigurable circuits and systems using liquid metals and other materials, microfluidics, and microelectromechanical systems (MEMS). Michael D. Bartlett is an associate professor and John R. Jones III Faculty Fellow of Mechanical Engineering at Virginia Tech. He received his B.S.E. from the University of Michigan, his Ph.D. from the University of Massachusetts Amherst, and was a postdoctoral fellow at Carnegie Mellon University. Michael leads the Soft Materials and Structures Lab, which investigates multifunctional soft materials and composites with highly controllable mechanical and functional properties for the creation of soft electronics and robotics based on liquid metal, switchable and intelligent adhesives, and adaptive materials. Michael Dickey is the Camille and Henry Dreyfus professor in the Department of Chemical & Biomolecular Engineering at NC State University. He received a BS in Chemical Engineering from Georgia Institute of Technology (1999) and a PhD from the University of Texas (2006) under the guidance of Professor Grant Willson. From 2006–2008 he was a post-doctoral fellow in the lab of Professor George Whitesides at Harvard University. He completed a sabbatical at Microsoft in 2016 and EPFL in 2023. Michael's research interests include soft matter (liquid metals, gels, polymers) for soft and stretchable devices (electronics, energy harvesters, and soft robotics). Kourosh Kalantar-Zadeh is a professor and head of the School of Chemical and Biomolecular Engineering at the University of Sydney. He is involved in research in the fields of analytical chemistry, materials sciences, gastroenterology, electronics, and sensors. Professor Kalantar–Zadeh is best known for his works on ingestible sensors, liquid metals, and 2D semiconductors. He led his group to the invention of an ingestible chemical sensor: a human gas sensing capsule, one of the breakthroughs in the field of medical devices. He has received several international awards for his scientific contributions including the 2017 IEEE Sensor Council Achievement, and 2020 Robert Boyle Prize of RSC.