Unlike conventional rigid machines, soft robots generally have unique operation styles that rely heavily on soft matter engineering and smart material systems. Owing to the superior merits of both metals and fluids, liquid metal smart materials are increasingly being innovatively used as soft actuators and sensors to construct soft robots. To promote further development and prosperity in this field, this review organizes and summarizes the typical progress in liquid metal smart materials, with a special focus on their robotic response behaviors. In particular, the article emphasizes the concept of smart composite systems consisting of liquid metals and synergistic substances (e.g., solutions, particles, and polymers). The response behaviors of liquid metal smart materials under the actions of external factors (electricity, magnetism, ultrasound, light, heat, etc.) are examined and the smart properties occurring during these responses, such as motion, transformation, self‐organization, adaptive healing, and autonomous sensing, are identified. Furthermore, soft actuators, control systems, and robots based on liquid metal smart materials are summarized and elaborated upon. Finally, the potential directions worth pursuing and challenges are outlined. It is expected that this review will stimulate further investigations into liquid metal smart materials with the aim of building a new generation of soft robots.

Liquid materials with the ability to transit between conductor and insulator are of great scientific and practical significance. However, achieving the conduction of a liquid metal droplet network is still a challenge. To solve these problems, a generalized method is proposed to fabricate temperature tunable liquid conductor–insulator transition composites, which is achieved firstly via freezing and thawing liquid metal droplets dispersed in dimethicone. Such composites also impart conductivity to the dispersed liquid metal droplet network. To illustrate the typical application of the thus realized materials, a visualized circuit is constructed based on the relationship between the color and the conduction. In addition, reconfigurable and repairable circuits are fabricated depending on the inherent liquid properties of these materials. Furthermore, this universal mechanism has been revealed via the abnormal volume expansion ratio of the liquid metal droplets during the phase change. By calculating the volume change ratio of all metals, we speculate and confirm that gallium-based alloys and bismuth-based alloys can be used to prepare such conductive transition materials. Accordingly, we identify more eligible materials with suitable phase transition points, which significantly extends the transition temperature from insulator to conductor. The liquid material preparation strategy proposed here provides a novel paradigm for achieving the conductor–insulator transition at a wide temperature range and offers promising potential for future applications.

Conventional thermal interface materials (TIMs) as widely used in thermal management area is inherently limited by their relatively low thermal conductivity. From an alternative, the newly emerging liquid metal based thermal interface materials (LM-TIMs) open a rather promising way, which can pronouncedly improve the thermal contact resistance and offers tremendous opportunities for making powerful thermal management materials. The LM-TIMs thus prepared exhibits superior thermal conductivity over many conventional TIMs which guarantees its significant application prospect. And the nanoparticles mediated or tuned liquid metal further enable ever conductive LM-TIMs which suggests the ultimate goal of thermal management. In this review, a systematic interpretation on the basic features of LM-TIMs was presented. Representative exploration and progress on LM-TIMs were summarized. Typical approaches toward nanotechnology enhanced high performance LM-TIMs were illustrated. The perspect of this new generation thermal management material were outlined. Some involved challenges were raised. This work is expected to provide a guide line for future research in this field.

Flexible electronics is one of the most overwhelming and promising technologies available today, receiving widespread attention from both academia and industry. As it continues to evolve, demands on flexible conductive materials are becoming increasingly prominent. Liquid metals (LMs), which combine the compliance of fluids with the electrical conductivity of metals, are excellent candidates among various material options for fabricating flexible electronics. Following more than a decade of development, LM flexible electronics has become a rather promising research field. This review aims to provide a comprehensive summary and interpretation of the typical progress in LM flexible electronics so as to promote further advancement in the area. First, modification strategies of LMs, such as oxidation and composite approaches (doped particles or mixed polymers), are systematically digested to improve their performances and increase the formability involved. Furthermore, we divide the LM flexible electronics into three major architectures (LM flexible electronics with channels, LM printed electronics, and LM/polymer flexible electronics) and introduce the core patterning methods for each type. In addition, we provide an overview of the representative applications of LM flexible electronics in the categories of soft sensors, biomedicine, flexible energy, electronic fabrics, etc. Finally, the current challenges and potential directions of LM flexible electronics are discussed. Overall, the past progress, current situation, and future outlook as outlined in full view in the present article is expected to inspire tremendous new opportunities for further fundamental research or practical explorations of LM flexible electronics in the coming time.