Liquid Metal Smart Materials toward Soft Robotics

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.

DOI: 10.1002/aisy.202200375 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. smart polymers, [24,25] shape memory alloys, [26,27] shape memory polymer (SMP), [28,29] magnetorheological (MR) fluids, [30] and hydrogels. [31,32] The different characteristics of materials enable diverse actuation strategies and functionalities, endowing soft robots with a greater range of possibilities. Applications of smart materials have resulted in prototypes and have successfully broadened the recognition of soft robots.
However, typical soft robots (such as those listed in Figure 1b-g) are evidently made of soft matter to ensure their softness. The Young's modulus of such soft materials is usually below 1 GPa, whereas the Young's moduli of the materials making up most current robots are greater than this value (Figure 1h). A rigid material with a large modulus helps maintain the mechanical properties of a robot and is beneficial for achieving precise control of movements. Therefore, identifying a material able to transition from soft to hard is important to providing flexibility and precise control to the robot. Fortunately, as illustrated in Table 1, liquid metal provides multiple advantages, including a low melting point, small viscosity, high electrical conductivity, and thermal conductivity. The low melting point means that the liquid metal readily changes between solid and liquid. This suggests that the Young's modulus of a liquid metal-based functional entity covers a large range. Moreover, liquid metals with the desirable ability to switch between various morphologies can exhibit a wealth of smart response behaviors to external stimuli; thus, the concept of liquid metal smart materials has Figure 1. Origins, prototypes, and materials of soft robots. a) Creatures in nature distributed in the sea, land, and air have inspired and continued to promote the development of soft robots. b) Pneumatic soft robotics based on the elastomer material and network structure. Reproduced with permission. [17] Copyright 2014, Wiley-VCH Verlag. c) Entirely soft, autonomous robot actuated by self-powered gases. Reproduced with permission. [18] Copyright 2016, Springer Nature. d) Soft thermoactuator assisted by liquid crystal elastomers. Reproduced with permission. [21] Copyright 2017, Royal Society of Chemistry. e) Light-driven inching soft robot. Reproduced with permission. [22] Copyright 2018, Wiley-VCH Verlag. f ) Shape memory alloy-based soft gripper. Reproduced with permission. [26] Copyright 2017, Elsevier. g) Untethered fast-transforming soft robot made of smart materials with ferromagnetic substances mixed. Reproduced with permission. [180] Copyright 2018, Springer Nature. h) Approximate Young's moduli of typical engineering and biological materials.
gradually emerged and favorably contributed to the development of liquid metal-based soft robots. Consequently, liquid metal smart materials are expected to solve the trade-off between softness and accuracy, opening up new possibilities for soft robots. [33][34][35] Thus far, liquid metal soft robots have received continuous attention, leading to higher demands for intelligent properties from liquid metal smart materials. Considering this historical node, it is important to review the progress of liquid metal smart materials toward the development of soft robots in recent years. Therefore, this article systematically summarizes and analyzes the responsive behaviors of liquid metal smart materials under the stimulation of various external factors. Before presenting the specifics, we illustrate the lineage of the development by plotting the typical works highlighted in this field on a timeline. Subsequently, the responsive behaviors and intelligent properties of the liquid metal smart materials are elaborated upon and analyzed. Furthermore, the existing soft actuators, control systems, and robots are summarized and interpreted. Finally, we present the challenges and outlook for liquid metal smart materials aimed toward robots to guide future developments.

Intelligent Properties and Advancement of Liquid Metal Smart Materials
After natural materials, synthetic polymer materials, and artificially designed materials, smart materials have become a new class of materials and attracted extensive attention in recent years. [36][37][38][39][40][41] Smart materials generally feature a variety of functions such as response characteristics, sensing, feedback, self-diagnosis, self-healing, and self-adaptation. Owing to their high electrical conductivity, [42][43][44] high thermal conductivity, [45,46] low phase transition points, [47] high surface tension, [48,49] and active chemical interfaces, [50,51] liquid metals not only respond to multiple external fields (such as electric and magnetic fields, ultrasound, light, and heat), but also demonstrate smart characteristics. Owing to these characteristics, liquid metals have recently emerged as a new class of smart materials.
Notably, it is difficult for a single liquid metal to exhibit smart properties. Thus, the concept of systems should be emphasized. In other words, the liquid metal smart materials discussed in this review refer to a class of composite systems consisting of liquid metal and synergistic substances (e.g., solutions, particles, and polymers). Such systems exhibit diverse smart behaviors, such as multiexternal field responses, motion, deformation, selforganization, adaptive healing, and autonomous sensing. The characteristics of the liquid metals play significant roles in these response processes. For example, the high surface tension of a liquid metal is largely responsible for its responsive behavior in liquid metal-solution composite systems. [52] High electrical and thermal conductivities also have noticeable impacts on the response of liquid metal-polymer composite systems. [53,54] Therefore, it is appropriate to use liquid metal smart materials to generalize these types of material systems. Furthermore, based on these smart features, the development of liquid metal smart materials has contributed to progress in several fields (particularly soft robotics).
Important advances have been made over the last two decades in the development of liquid metal smart materials for soft robots. To better understand the developmental trend, we first consider the timeline of typical contributions in this area ( Figure 2). Previously, researchers demonstrated an electric field-driven movement of Hg via a continuous electrowetting effect. [55,56] Later, Tang et al. [57] conducted an experiment by replacing toxic Hg with Ga. In 2014, Sheng et al. [58] proposed and demonstrated the concept of liquid metal machines based on their fundamental discovery of the diverse transformation phenomena of liquid metals with different morphologies under electric fields. Such liquid metal transformers were regarded as signaling a new era of soft robots. Furthermore, an electrically driven liquid metal pump was presented. [59] Perhaps one of the most intriguing phenomena of liquid metal matter concerns self-fueled liquid metal mollusks, [60] which have aroused worldwide attention and generated the impetus for the research endeavors toward liquid metal-based soft machines. It was revealed that liquid metal droplets that "swallowed" aluminum could spontaneously move for long periods, similar to the actions of a simple life. Through continuous investigations, the concept of liquid metal robots has become clearer. [61][62][63] The realization of fully soft or liquid robots based on liquid metal is one target of this research direction. However, solidliquid coupling is more practical for now and allows for wider imagination in the field of liquid metal-based robots. One start-up work in this regard concerns the periodic oscillation of Cu wires inside a liquid metal machine. [64] Such liquid metal-enabled smart machines are tunable and demonstrate multiple application prospects. For instance, they can be adopted as switches for electricity, optics, and fluids. Researchers have also demonstrated an ionic imbalance-induced motion of liquid metal droplets. [65] In the future, additional movements and transformation behaviors of liquid metal or its extended machines are expected.
Xiong et al. [66] successfully doped Ni nanoparticles into Ga (or its alloy) and tested their interaction properties with magnetic fields. Subsequently, researchers successfully doped Fe particles into liquid metal droplets under acidic conditions. [67] Such a preparation method avoids the oxidation of the liquid metal and results in a magnetic fluid with desirable fluidity. A liquid metal electromagnetic actuator based on the conductive and flexible properties of liquid metal has been proposed. [68] In addition, Ga [183] 29.8 1.37 6.73 29.3 In [184] 156.6 -1.25 Â 10 7 81.6 (at 27°C) Sn [184] 231.9 -8.7 Â 10 6 66.6 (at 27°C) Bi [184] 271.4 -9 Â 10 5 (at 25°C) 7.87 (at 27°C) GaIn 21.4 (EGaIn) [185]  an acoustic field has been used to drive liquid metal droplets with controllable motion directions. [69] In general, liquid metals are driven by multiple fields. This provides a technical basis for the development of liquid metal soft robots. To help create the field, the first book related to liquid metal soft machines was published in 2018. [65] Based on the strength of the physicochemical effects of liquid metal previously realized, advances in liquid metal-based robots have been continuously attained. By introducing an electrically driven liquid metal wheel and 3D printed structure, Yao et al. [70] demonstrated a group of wheeled vehicles based on liquid metal. Later, the researchers designed wheeled robots relying on the responsive properties of the liquid metal. [71,72] When the liquid metal inside a ring moved, the center of gravity of the device shifted, thereby achieving the overall motion. Though the functions of robots based on liquid metal remain relatively simple, they demonstrate innovations over traditional robots. Recently, hard magnetic particles were successfully doped into liquid metal. The resulting composites remained magnetic after the magnetic field was removed, [73] providing not only magnetic response characteristics but also properties like magnetic pole reconfiguration and self-assembly. In addition, progress has also been made in liquid metal composites. For instance, a dielectric elastomer actuator comprising liquid metal and polydimethylsiloxane has been demonstrated. [74] Liquid crystal elastomers have been used to form composites with liquid metal to achieve enhanced thermal response properties. [75] Liquid metal artificial muscles for mimicking the contractions and diastolic functions of muscles have been developed based on electrochemical oxidation. [76] In the future, liquid metal smart composites are expected to usher in a new stage of development in this context.
From the above description of the typical progress in liquid metal smart materials, it is clear that liquid metal exhibits a wealth of responsive properties. Accordingly, the expansion of smart properties based thereon will open up new applications for liquid metal in the field of robotics. According to incomplete statistics, the factors able to stimulate responses to liquid metals include electrical, magnetic, acoustic, optical, thermal, mechanical, and chemical stimuli, which display different mechanisms, functions, advantages, and disadvantages ( Table 2).
As discussed earlier, a synergistic substance is typically required for the responses of liquid metals, including solutions, polymers, and particles (Table 1). In addition, it can be observed that there are different mechanisms behind the responses of liquid metal smart materials to different external fields. For example, liquid metal has by far the highest surface tension of any room-temperature fluid; thus, large surface tension differences can be achieved on its surfaces, thereby resulting in locomotion and deformation. To achieve this effect, liquid metal usually needs to be placed in a solution and have electricity applied to it. Another major category of substances obtains additional response properties by compounding them with functional materials such as magnetic particles or polymers. This feature is mainly based on the fact that a liquid metal is suitable for acting as both a matrix and filler in a composite, effectively expanding the response range and application space. The table lists a variety of other external factors for stimulating smart behavior in liquid metal. The corresponding response characteristics exhibit unique advantages and disadvantages owing to the differences in the excitation sources. For instance, the widely used electric field depends on the electrolyte. The acoustic field enabling the manipulation of liquid metal droplets to achieve self-organizing patterns requires complex equipment. These aspects are discussed in more detail below to promote the application of liquid metal smart materials in the field of soft robotics.

Electric Field-Excited Response Behaviors
The electric field was one of the earliest external fields used to drive liquid metal. In the early days, researchers found that liquid metal droplets could be driven by the electric fields in electrolyte solutions in a process defined as continuous electrowetting. [56] The liquid metal droplets were toxic Hg droplets with high vapor pressure, [77] making them unsuitable for potential use owing to their toxicity. As an alternative, Tang et al. [57] adopted Ga-based droplets for providing movement based on the electrochemically induced actuation of liquid metal marbles (Figure 3a). Sheng et al. [58] found that the liquid metal underwent diverse large-scale transformations among different morphologies under electric fields. This profiled a clear picture of liquid metal transformer machines. Since then, many studies on the movement behaviors of liquid metals under electric fields have gradually emerged. [78][79][80][81] Droplet driving via direct current in electrolyte solutions inevitably generates appreciable amounts of gas. To address this problem, Yang et al. [82] used an AC power source to move droplets. As shown in Figure 3b, the amount of gas produced by electrolysis decreased significantly with increasing frequency. In addition, providing liquid metal with antigravity movements is another important task. In response to this, researchers demonstrated that a graphite plate could electrochemically oxidize a liquid metal, providing antigravity movement at a small angle with the participation of the electric field ( Figure 3c). [83] Further, Chen et al. [84] achieved 3D actuation of liquid metal droplets with a foam-core structure. By adhering liquid metal to a hollow sphere, the density of the resulting liquid metal droplets was reduced, allowing them to move against gravity in solution under an electric field.
Numerous studies have shown that the motion of a liquid metal under the action of an electric field always requires electrolyte solutions. Previous studies have mostly used alkaline electrolytes represented by sodium hydroxide. In an experiment performed by Handschuh-Wang, [85] it was shown that liquid metal could also be driven in acidic solutions with the addition of surface-active anions. As illustrated in Figure 3d, owing to the high surface charge density, the interfacial tension of the liquid metal would be altered by the formation of a new electrical double layer, thereby driving the motion of the liquid metal droplets. Notably, the direction of motion of the liquid metal droplets achieved in this acidic electrolyte was opposite to that achieved in alkaline solutions. Further, based on the electric field driving mechanism, Ren et al. [86] achieved discontinuous droplet patterns based on the light-controlled manipulation of liquid metal droplets ( Figure 3e). The basic mechanism involved the manipulation of liquid metal droplets by an electric field; however, the innovation was that the switching of the electric field could be controlled by a laser that selectively activated the phototransistor.
In addition to causing the movement of liquid metal, electric fields can excite its deformation. The basic principle involved lies in the oxidation of a liquid metal by a positive electrode in a process known as electrochemical oxidation. [87] As shown in Figure 3f, the negative terminal of the power supply is placed in the solution, whereas the positive terminal is in direct contact with the liquid metal. Consequently, when a certain voltage is applied to the liquid metal, it is oxidized, causing it to spread out into a dendritic structure. When the positive electrode is removed, the liquid metal returns to a spherical shape, indicating that the liquid metal has achieved reversible deformation via this mechanism. Furthermore, the negative electrode can be used to guide the movement of the liquid metal while the positive electrode oxidizes the liquid metal. Based on this principle, a typical early exploration showed that a continuous pattern could be  [124,190,191] Light Polymer Solution Photothermal conversion, photocatalysis Locomotion, transformation Remote activation Fabrication complexity [113,192] Acoustic Solution Acoustic radiation force Locomotion, self-organization Remote activation Complex equipment [69] Chemical stimuli Solution Redox, Marangoni effect Transformation, locomotion Self-fueled, continuous movement Need electrolytes [60,102,193,194] www.advancedsciencenews.com www.advintellsyst.com formed by changing the polarity of the electrodes. [88] Multiple electrodes were used because elongated liquid metal exhibits fluid instability. Aiming to overcome this challenge, Li et al. [89] proposed that a liquid metal doped with Fe could suppress instabilities owing to the decrease in surface tension from electrochemical oxidation, thereby allowing the liquid metal to be stretched to tens of times its original length. This allowed it to be freely manipulated in a 2D plane to form complex patterns ( Figure 3g). As a result, programmable and controllable liquid metal patterns could be realized, facilitating their applications in robotics.
With the help of the two basic types of principles described above, a series of movements and deformation phenomena have been generated. [90,91] Electric fields are a class of easily accessible energy sources exhibiting controllable properties; thus, the responses of liquid metal to electric fields have been extensively and intensively studied. However, to provide a steady supply of electrical energy, power supply equipment is often required, adding to the complexity of the overall drive unit. Moreover, it is difficult to avoid the gases generated by the electrolysis of the solution during the response. Related integrated machines based on electric field driving have been realized by encapsulating the solution and liquid metal, [71,92] but the resulting integral machine may lose the inherent deformability of the liquid metal.

Chemical Stimuli-Induced Smart Responses
To achieve the aspirational goals of liquid metal robots, it is meaningful to study approaches to causing the movement of their droplets. [93] This type of basic contribution will lead to significant scientific progress. In addition to driving liquid metal droplets with the help of an electric field, chemical stimuli are also suitable for driving their motions and transformations. Both of these different strategies have a similar driving mechanism, that is, the surface tension of the liquid metal as affected by the electric double layer (EDL). Notably, the solution plays an indispensable role in this process as it ensures the formation of the EDL on the surface of the liquid metal. The earliest research in this area can be traced back to Lippman who found that the surface tension of liquid droplets and nature of the EDL were closely related [94] and revealed the relationship between the surface potential (ψ) and interfacial tension (σ) as follows In the above, C stands for the EDL capacitance per unit area, ψ 0 stands for the potential of zero charge, and σ 0 is the maximum interfacial tension at the potential of zero charge. As shown in Figure 4a, the EDL includes a compact layer and diffuse layer. The compact layer consists of inner and outer Helmholtz planes. The surface potential provides a quantitative description of the structural properties of an EDL. As it is specific to liquid metal, the capillary curves of Ga were studied to reveal the variation of its surface tension (σ) with the surface potential (ψ). [95] The electrocapillary curves for Ga in solutions of 1 N HCl and 1 N . Reproduced with permission. [95] Copyright 1965, Elsevier. c) Liquid metal droplets driven by the acid-base concentration difference. Scale bars: 1 cm. Reproduced with permission. [65] Copyright 2016, Springer Nature. d) Surface convection of liquid metal excited by Cu particles. Reproduced with pemission. [96] Copyright 2017, Wiley-VCH. e) Self-growing and snake-like locomotion of liquid metal induced by Cu ions. Reproduced with permission. [97] Copyright 2018, American Chemical Society. f ) Spontaneous dispersion and deformation of liquid metal excited by Fe ions. Reproduced with permission. [98] Copyright 2019, American Chemical Society. g) (i) Spontaneous oscillation of liquid metal droplets semisubmerged in alkaline solutions; (ii) different states of liquid metal in air and alkaline solutions. Reproduced from permission. [100] Copyright 2016, Royal Society of Chemistry. h) Graphite-induced periodical self-actuation of liquid metal. Scale bars: 3 mm. Reproduced with permission. [101] Copyright 2016, Royal Society of Chemistry. i) Liquid metal fractals induced by synergistic oxidation. Reproduced with permission. [102] Copyright 2018, Science China Press.
www.advancedsciencenews.com www.advintellsyst.com KC1 þ HCl are illustrated in Figure 4b. They indicate that the interfacial tension of Ga has the largest value at a particular potential. Therefore, the interfacial tension can be altered by changing the surface potential; this was the underlying mechanism in a variety of subsequent studies. As a typical demonstration of chemically responsive liquid metal smart materials, researchers induced motion in liquid metal droplets based on the difference in an acid-base concentration ( Figure 4c). [65] The mechanism underlying this phenomenon involved a change in the EDL of the liquid metal droplets. In addition, the interactions between other metals and the liquid metal represent another highly effective strategy for altering the EDL to move and deform it. For instance, Tang et al. [96] realized large-scale convection on liquid metal surfaces excited by Cu particles ( Figure 4d). In addition to Cu particles, a Cu ion solution is also feasible for changing the structure of an EDL. Chen et al. [97] achieved self-growing serpentine locomotion in a liquid metal. In this study, the liquid metal was placed in a Cu salt solution that could react with Ga and its entire surface was subjected to surface pressure differences owing to the unique reaction environment ( Figure 4e). The generated surface pressure difference (ΔP) was described as follows Here, R 1 and R 2 represent the principal radii of curvature at the interface. Therefore, it is clear that the large surface pressure difference is owing to the change in the interfacial tension caused by the Cu-Ga primary cell reaction. The specific chemical reactions are as follows Hydrochloric acid was added to adjust the pH of the solution and obtain the desired results. To eliminate the influence of the added acid, a ferric chloride solution (acidic owing to strong hydrolysis) was chosen. The experimental results are shown in Figure 4f. An intriguing spontaneous dispersion phenomenon was discovered. [98] Oxidation is another mechanism by which chemical stimuli can change the shape of a liquid metal. It is well known that liquid metal is immediately oxidized in air [99] and that the surface tension of the oxidized liquid metal is reduced. Therefore, the motion and deformation behavior of the liquid metal can be regulated by controlling the oxidation and deoxidation. For example, as illustrated in Figure 4g, Yi et al. [100] found that a liquid metal droplet semisubmerged in an alkaline solution oscillated regularly, similar to the breathing of living organisms. The semisubmerged design allowed the liquid metal droplet to be exposed to air, which oxidized it. Alkaline solutions dissolved its oxides, thereby periodically changing the surface tension of the liquid metal. In addition to air, a graphite plate was used to more efficiently oxidize the liquid metal. Based on the electrochemical oxidation provided by graphite plates, Wang et al. [101] found that liquid metal droplets in contact with graphite plates undergo periodic contraction and expansion (Figure 4h). Furthermore, Chen et al. [102] achieved a high-dimensional liquid metal fractal by introducing a green oxidant (hydrogen peroxide) onto graphite plates (Figure 4i). The mechanism behind this lies mainly in the competition between the oxidative stress provided by the synergistic oxidation of the graphite plate and hydrogen peroxide, as well as the surface tension released by the alkaline solution dissolving the liquid metal.
Overall, in addition to droplet motion, chemical stimuli can be used to achieve large-scale deformation. Notably, the response of the liquid metal to chemical stimuli implies that the liquid metal can obtain energy from the surrounding environment to achieve a regular morphology or self-driven behavior in the composite system; this effectively demonstrates the concept of liquid metal smart materials. Although progress has been achieved, more quantitative research is required to achieve controlled deformation.
Furthermore, the highest level of freedom for soft robots is perhaps self-driving robots, similar to human beings or animals that obtain their energy by eating. One goal of liquid metal machines is to realize autonomous movement. Fortunately, liquid metal smart materials have been found to exhibit self-driven behaviors. [60] As illustrated in Figure 5a, liquid metal droplets immersed in sodium hydroxide solution spontaneously "ate" an Al foil and then began a self-driven motion lasting for a significantly long time. Furthermore, a mutual pursuit motion of multiple liquid metal droplets swallowing Al was also observed [103] (Figure 5b), along with mutual collisions and bounces of multiple liquid metal droplets containing Al. [104] With an increase in the number and decrease in the size of the droplets, liquid metal micromotors based on this strategy were successfully fabricated as illustrated in Figure 5c and displayed a trajectory similar to Brownian motion. [105] The speed of these liquid metal droplets could be increased up to 40 times by applying an electric field (Figure 5d) and the response time was rather fast. [106] Tan et al. [107] introduced a magnetic field to regulate the motion of liquid metal micromotors and revealed a liquid metal magnetic trap effect (Figure 5e). The mechanism behind these movements was attributed to the "activation" of the liquid metal droplets by Al; that is, the addition of Al changed the interfacial tension of the liquid metal, thereby causing interfacial tension differences to form on its surface, which, in turn, drove the movement of the droplets.
Solid-liquid coupling machines have also been fabricated by adding a Cu wire to liquid metal droplets [64] (Figure 5f ). In one example, a Cu wire oscillated periodically inside a droplet and a steel needle regulated its oscillation period. In this process, the Al in the liquid metal also played a crucial role, causing bubbles to be generated on the surface of the Cu wire. The resulting unbalanced force dove the periodic oscillation motion. Interestingly, the behavior of liquid metal droplets differed when the base was changed to an Fe substrate in such a composite system. [108] In this situation, the liquid metal droplets underwent highfrequency oscillations rather than self-driven directional motion owing to the binding effect of the substrate to the liquid metal droplets (Figure 5g). Furthermore, when the substrate was replaced by graphite with a unique oxidation capacity, the amoeba behavior of the liquid metal placed on the graphite plates was revealed and multiple pseudofoots were spontaneously generated ( Figure 5h). These studies suggest that the substrate has a crucial influence on the movement and deformation of an Al-eating liquid metal.
www.advancedsciencenews.com www.advintellsyst.com In conclusion, a liquid metal doped with Al facilitates the elimination of its surface oxides [109] and allows it to be in an activated state, thereby stimulating a wide variety of self-fueled motions and deformation behaviors. Further analysis revealed that the deeper mechanism lied in the electron transfer behavior between the Ga and Al. Similarly, in addition to Al, the driving motions of the liquid metal droplets were implemented by means of the good wettability between the liquid metal and other metals. In this way, the liquid metal droplets were induced to move at a high speed with the help of a Ag foil; [110] a Cu plate [111] could induce the surface convection of a liquid metal; and solid Ni particles were used to trigger jumping behaviors in liquid metal droplets in an electrolyte. [112] In the future, the application of such rich liquid metal self-driven motions and deformations and their integration with robotics will be the focus of this research.

Response Behaviors Induced by Other Fields
In addition to the fields considered in the aforementioned studies, liquid metals can be controlled by other factors. As illustrated in Figure 6a, one study demonstrated that light can induce the locomotion of a liquid metal by promoting bubble generation. [113] However, to achieve light-controlled motion, the solution was confined to hydrogen peroxide (H 2 O 2 ) and the liquid metal surface was coated with tungsten trioxide (WO 3 ), making it not conducive to practical application. Researchers have also demonstrated laser-excited liquid metal uplift (Figure 6b). [114] This is because liquid metal doped with Fe particles generates a large amount of gas under laser irradiation, leading to the creation of a porous liquid metal and driving it upward. Therefore, it can be concluded that the light control mechanism includes both photocatalysis and photothermal conversion. For liquid metals at Figure 5. Self-fueled motion and deformation of liquid metal droplets that have swallowed Al. a) Self-fueled movement of an Al-liquid metal droplet in the channel. Reproduced with permission. [60] Copyright 2015, American Chemical Society. b) Mutual chasing motion of multiple liquid metal droplets that have swallowed Al in the channel. Reproduced with permission. [103] Copyright 2015, Science China Press. c) Brownian-like motion of Al-liquid metal micromotors. Reproduced with permission. [105] Copyright 2015, Science China Press. d) Applying an electric field to accelerate the speed of Al-liquid metal micromotors. Reproduced with permission. [106] Copyright 2015, The Royal Society. e) Magnetic fields restrict the movement of Al-liquid metal droplets. Reproduced with permission. [107] Copyright 2015, Science China Press. f ) Self-oscillating motion of a Cu wire inside Al-liquid metal droplets. Scale bar: 5 mm. Reproduced with permission. [64] Copyright 2016, American Chemical Society. g) Al-assisted high-frequency oscillation of liquid metal droplets. Reproduced with permission. [108] Copyright 2019, Wiley-VCH Verlag. h) Al-liquid metal droplets on graphite plate spontaneously extending pseudopod. Scale bar: 1 cm. Reproduced with permission. [182] Copyright 2017, Springer Nature.
www.advancedsciencenews.com www.advintellsyst.com the micro-and nanoscale, ultrasound is a feasible strategy for driving their motions. [115] As shown in Figure 6c, in one study, directional movement was induced using an ultrasound field. [69] However, considering that the underlying mechanism is primarily an acoustic radiation force, a large-mass liquid metal cannot be driven. In addition, the ultrasound field induces cluster movements (Figure 6d). [116] These results indicate that it can be used in an active assembly unit to build reconfigurable nanomachine clusters mimicking the emergent self-organizing behavioral characteristics of living organisms, providing a theoretical and technological basis for the next generation of liquid metal-based active soft matter and intelligent robot manufacturing.
In general, liquid materials undergo a phase change behavior at a particular temperature as an inherent response to temperature. However, the temperature response of a liquid metal is accompanied by an abnormal change in the volume. As shown in Figure 6e, the liquid metal droplets paradoxically expand in volume during the cooling process; [117] this is different from the phase changes of most metals. Based on this response to temperature, a circuit with conductive-insulation transition characteristics has been implemented ( Figure 6f ). Moreover, the conductivity of such a circuit can be distinguished by the color of the circuit, as shown in Figure 6g; thus, it was used as a visual circuit. Researchers have also conducted tumor treatments based on volume changes during the process of phase transition, [118] showcasing the promise of liquid metal robots in biomedical applications. However, liquid metals excited by thermal fields respond less frequently. As a class of substances with high thermal conductivity, there is significant scope for exploring such response behaviors and the focus should be on the roles of the liquid metal and its comaterials.
The stimuli for liquid metals include a wide range of factors and the external fields presented in this section are representative of them. The responses of liquid metals to many other factors remain unknown. In addition to the studies combining electric and magnetic fields as well as heat and light, the smart response behaviors of liquid metals in the combined action of multiple external fields require further exploration.

Liquid Metal-Particle Smart Systems
In addition to solutions, the addition of particles can effectively provide a liquid with metal responsive properties and thus transform it into a smart material. For example, Yuan et al. [119] achieved a fast-response water-triggered liquid metal smart material by adding Al power into a liquid metal. For such systems, the most studied method is the doping of magnetic particles into a liquid metal to prepare magnetic fluids. This is because the magnetic force is a noncontact force with evident advantages. The addition of magnetic particles to liquid metals is a feasible strategy. According to incomplete statistics, the doped magnetic particles include Ni, [66,[120][121][122][123] Fe, [67,[124][125][126][127] NdFeB, [73,128] and FePd. [129] These particles can be divided into two main categories: soft and hard magnetic particles. The main difference between them is whether they remain magnetic even after the magnetic field is withdrawn.
After several years of research, various methods for constructing magnetic liquid metals have been developed. These can be grouped into two main categories: coating and stirring. The specific preparation methods are as follows: surface coating via rolling, [130] electroplating, [122] direct stirring, [66,121] and stirring in an acid. [67,127,131,132] The former involves covering the liquid metal with magnetic particles as shown in Figure 7a; the resulting magnetic liquid metal does not need to touch the substrate and avoids the residues of oxides on the substrate. [130] The other method is stirring, which is widely used and can be subdivided into direct stirring in air and stirring in an acid. The main difference between the two methods is the presence or absence of oxides. A schematic of the direct-stirring method is shown in Figure 7b. This method is simple and effective; however, it increases the viscosity of the liquid metal owing to its high degree of oxidation, [67] which is not conducive for its use as a magnetorheological fluid. To address this issue, one study continuously stirred liquid metal and Fe particles under acidic conditions until all Fe particles were swallowed by the liquid metal. [67] Furthermore, the stress variation induced by the magnetic field was tested (Figure 7c) and indicated that the magnetic liquid metal had the characteristics of a magnetorheological fluid.
In addition to this hardness response to magnetic fields, a liquid metal with added Fe particles can be manipulated by magnetic fields without contact, as illustrated in Figure 7d. In particular, magnetic liquid droplets can be manipulated by electromagnets to form specific letters. [133] Li et al. [126] achieved precise control and climbing locomotion in magnetic liquid metal droplets based on the simultaneous interworking of both electric and magnetic fields. In addition, a gripper based on a magnetic liquid metal has also been developed ( Figure 7e); [131] it primarily exploits the difference in adhesion to the contact surface before and after the liquid-solid phase change of the liquid metal, thereby enabling the manipulation of nonmagnetic objects using magnetic fields. Furthermore, a magnetic liquid metal droplet can be used as a machine to manipulate other liquids, [127] as illustrated in Figure 7f. The transport and release of droplets can be achieved by exploiting the differences in the speed of movement of the liquid metal droplets. Recently, by mixing an Ni-doped liquid metal and elastomers, Hoang et al. [123] introduced a liquid metal composite with high electrical conductivity and a low modulus. The resulting magnetic liquid metal composites could be used in soft surgical robotic arms and exhibited promising applications.
Modifying a liquid metal with hard magnetic particles is a viable and common method for obtaining a liquid metal with permanent magnetic properties. Figure 7g compares the appearance of liquid metal droplets doped with soft magnetic particles and hard magnetic particles. [128] In the corresponding study, it was found that different from the other droplets with a smooth surface well, the liquid metal droplets with the addition of large-sized NdFeB particles had an ellipsoidal shape and a rough surface. A further study revealed that adding hard magnetic particles to liquid metal provided favorable mobility, good elasticity, and mechanical robustness. Another study contributing to the construction of soft magnets was conducted by He et al. [73] As illustrated in Figure 7h, a liquid metal composite became magnetic after magnetization; the surface magnetic field distribution is shown in Figure 7i. Notably, the reconfigurable magnetic polarity of micromagnetic particles immersed in a liquid metal matrix was implemented first. The principle behind this lied in the torque resulting from the interaction between the magnetic dipole of the NdFeB microparticles and magnetic field applied by the permanent magnet. The torque (M) can be expressed as follows In the above, M is the torque, ρ and V are the density and volume of the magnetic microparticles, respectively, m stands for remnant magnetization, B denotes the magnetic induction from external permanent magnets, and θ represents the angle between the direction of the remnant field and direction of the external magnetic field. Furthermore, paper-based robots were constructed based on this magnetic liquid metal with a large remnant magnetization (Figure 7j), demonstrating potential applications.
Thus far, it has been confirmed that doping ferromagnetic particles into a liquid metal can impart it with magnetic properties, leading to a magnetic liquid metal well driven by a magnetic field. More importantly, the force exerted by the magnetic field on the liquid metal is a bulk force rather than a surface force; thus, the magnetic field provides a stronger force, thereby providing functions such as 3D driving or antigravity motions. [134] However, imparting magnetic properties to liquid metal while also causing changes in the properties of the liquid metal itself, such as its fluidity, remains an ongoing challenge.

Liquid Metal-Polymer Smart Systems
In addition to the responses of the liquid metal-solutions and complex particle systems to multiple fields, liquid metal-polymer www.advancedsciencenews.com www.advintellsyst.com smart systems have received considerable attention and have been extensively studied. [135][136][137] Following similar logic, we discuss the smart responses of liquid metal-polymer composites to external stimuli. The light response is noncontact, nonintrusive, and precisely controllable, thereby demonstrating large application value. As shown in Figure 8a, liquid metal nanofiber composites respond to light. [138] Using structural design, researchers have achieved light-controlled closures of flower-like structures. [138] Liquid crystal elastomers are a typical class of polymeric materials that respond to heat. [139] In this respect, researchers have exploited the electrical conductivity, high thermal conductivity, and good fluidity of liquid metals to combine them with liquid crystal elastomers to achieve a variety of smart response properties. As shown in Figure 8b, liquid metalelastomer composites have better thermal conductivity and thus a faster response to heat. [75] Liu et al. [140] achieved ultracompliant liquid metal electrodes with self-healing capabilities for dielectric elastomer actuators. The liquid metal was placed on the surface of this material. Moreover, liquid metal-elastomer composites obtained by mixing display a greater blocking force of dielectric elastomer actuators than simple pristine elastomers, owing to the enhancement of the Maxwell stress enabled by the larger dielectric constant derived from the liquid metal (Figure 8c). [74] Similarly, liquid Reproduced with permission. [130] Copyright 2019, Wiley-VCH Verlag. b) Preparation of magnetic liquid metal via direct stirring. Reproduced with permission. [121] Copyright 2018, Wiley-VCH Verlag. c) Stress variation with magnetic field in liquid metal prepared by stirring in acid. Reproduced with permission. [67] Copyright 2017, American Physical Society. d) Control of the movement of magnetic liquid metal more accurately to form letters. Scale bar: 5 mm. Reproduced with permission. [133] Copyright 2020, American Chemical Society. e) Gripper based on phase transitional liquid metal ferrofluid. Reproduced with permission. [131] Copyright 2021, Wiley-VCH Verlag. f ) Magnetic liquid metal droplets for droplet manipulation. Reproduced with permission. [127] Copyright 2022, American Chemical Society. Here, MLMR stands for magnetic liquid metal robots. Scale bar: 2 mm. g) Optical picture comparison of liquid metal droplets doped with soft magnetic particles and hard magnetic particles. Reproduced with permission. [128] Copyright 2021, Elsevier. Here, LMD stands for liquid metal droplets, S-LMD stands for liquid metal droplets with the addition of soft magnetic particles, and H-LMD and H-LMD1 stand for liquid metal droplets with the addition of small-sized NdFeB particles and large-sized NdFeB particles, respectively. h) Doping NdFeB into the liquid metal to form a permanent magnet via magnetization. i) Surface magnetic field distributions of the triangle-shaped. j) Application: crawling movement triggered by a rotating magnetic field. Reproduced with permission. [73] Copyright 2020, Wiley-VCH Verlag.
www.advancedsciencenews.com www.advintellsyst.com metal has been used as a medium for electrical and thermal conduction in soft thermochromic elastomers and researchers have implemented material tactile logic usable as an embedded sensor in a feedback loop. [54] By exploiting the vaporization characteristics of low-melting-point liquids, Wang et al. [141] achieved thermally excited liquid metal-polymer composites doped with liquid metal that were deformable on a large scale. An enhanced responsive behavior is favorable for building liquid metal-based soft robots. Special structures are often required for liquid metal composites with response characteristics. This means that traditional preparation methods cannot meet the performance requirements. Therefore, it is important to explore novel preparation methods. A typical study is demonstrated in Figure 8d-f, [142] where an electrically conductive composition and a nonconductive composition able to achieve a higher actuation strain are combined through multimaterial printing (Figure 8d,e). As shown in Figure 8f, such composites display a voltage-regulated shape change and the differences in the driving performances of different material structures highlight the advantages of multimaterial printing. As a typical application, a gripper has practical application value. [143] In one study, a thermally responsive gripper based on similar metal composites was developed [144] (Figure 8g), further demonstrating the promising prospects of responsive metal composites in the field of soft actuators. In such responsive materials, the source of the responsive properties is the polymer matrix. The liquid metal endows such composites with optical and electrical control properties owing to their high electrical and thermal conductivity.
Self-repair is another representative property of smart materials. Fortunately, owing to the inherent fluidity of liquid Figure 8. Response characteristics and typical preparation methods of liquid metal-polymer smart composites. a) Light-responsive liquid metalnanofibrils composites. Reproduced with permission. [138] Copyright 2019, Springer Nature. b) Thermal-responsive liquid metal-elastomer composites. Scale bar: 3 cm. Reproduced with permission. [75] Copyright 2019, National Academy of Sciences. c) Electrically responsive liquid metal-elastomer composites. Reproduced with permission. [74] Copyright 2019, Wiley-VCH Verlag. d) Printing schematic of the multimaterial printing process of liquid metal composites. Reproduced with permission. [142] Copyright 2019, American Chemical Society. e) Top views of uniaxial print patterns of different contents of liquid metal/liquid crystal elastomers. Reproduced with permission. [142] Copyright 2019, American Chemical Society. f ) Sequential images of a liquid metal/liquid crystal elastomer actuator. Scale bar: 3 mm. Reproduced with permission. [142] Copyright 2019, American Chemical Society. g) Thermalresponsive gripper to grasp a ball. Scale bar: 20 mm. Reproduced with permission. [144] Copyright 2020, American Chemical Society.
www.advancedsciencenews.com www.advintellsyst.com metal, composite systems based on it possess self-healing properties. [145] A schematic diagram of the principle of self-repair is shown in Figure 9a. In this manner, the self-repair function has been achieved in a variety of liquid metal composite systems. [146][147][148][149] As an example application based on such features, when robots encounter puncture damage from, e.g., a hole punch, an electrical pathway could be autonomously reconfigured around the damaged region without loss of electrical conductivity. [150] As another functional property, a conductiveinsulation transformation is indispensable to many applications. Liquid metal composite systems can achieve such smart properties. [151] As illustrated in Figure 9b, at both sides of a critical temperature, the conductivity of the liquid metal composite system undergoes an abrupt change of several orders of magnitude, thereby achieving a conductive-insulation transition. The mechanism lying behind this is shown in Figure 9c. Specifically, the temperature-induced phase transition of liquid metal and accompanying anomalous volume expansion causes the originally separated liquid metal droplets to come into contact with each other. Based on this electrical transition characteristic, researchers have realized a binary signal converter for converting temperature changes into 0 and 1 in binary ( Figure 9d); notably, Figure 9. Smart sensing characteristics of liquid metal-polymer composites. a) Schematic diagram of self-repair of liquid metal composites. Reproduced with permission. [145] Copyright 2021, Springer Nature. b) Conductive-insulation transition of liquid metal composites appears with the change in temperature. Reproduced with permission. [151] Copyright 2019, Wiley-VCH Verlag. c) Schematic diagram of the mechanism behind conductive-insulation transition. Reproduced with permission. [151] Copyright 2019, Wiley-VCH Verlag. d) Schematic diagram of a temperature-controlled digital display. Reproduced with permission. [151] Copyright 2019, Wiley-VCH Verlag. e) Different bending directions of the liquid metal-based artificial muscle obtained by adjusting the laser irradiating direction. Reproduced with permission. [153] Copyright 2022, American Association for the Advancement of Science. f ) Schematic diagram of the principle of deformation. Reproduced with permission. [153] Copyright 2022, American Association for the Advancement of Science. Here, LCE means liquid crystal elastomers and LMPA means low-melting-point alloys. g) Variation in resistance of one liquid metal-based artificial muscle sequentially irradiated at three different positions. Scale bars: 10 mm. Reproduced with permission. [153] Copyright 2022, American Association for the Advancement of Science.
www.advancedsciencenews.com www.advintellsyst.com binary constitutes the underlying foundation of the current vast electronic world. Based on a dual-phase liquid metal, researchers have also achieved a conductive-insulation transition in a liquid metal-polymer composite system. [152] In the future, such material systems are expected to be applied in the storage and calculation of information. Self-sensing denotes that a state is sensed by itself and is a good example of the smart properties of a liquid metal-polymer composite system. Based on this property, researchers proposed a liquid metal-enabled artificial muscle. [153] As illustrated in Figure 9e, the artificial muscle deformed when exposed to laser irradiation. The experimental results showed that the deformation was mainly driven by the contraction of the outer liquid crystal elastomers induced by the laser (Figure 9f ). The solid-liquid transition of the liquid metal was used to control whether the artificial muscle was deformable or not. Specifically, when the liquid metal was in the liquid state, the artificial muscle was deformable. However, when the liquid metal was in the solid state, the artificial muscle was in a fixed orientation. In addition, the study showed that the resistance of the liquid metal in the composite system changed when the artificial muscle bent; this bending degree was further obtained based on the measured resistance values (Figure 10g). This process of sensing the bending angle does not require the Figure 10. Liquid metal-based actuators. a) Schematic diagram of the principle of electrochemical oxidation. Reproduced with permission. [76] Copyright 2021, Wiley-VCH Verlag. b) Driven unit of the liquid metal artificial muscle. Reproduced with permission. [76] Copyright 2021, Wiley-VCH Verlag. c) Liquid metal artificial muscle with a multilayer structure. Reproduced with permission. [76] Copyright 2021, Wiley-VCH Verlag. d) Caudal fin of robotic fish driven by the liquid metal artificial muscle. Scale bar: 2 mm. Reproduced with permission. [76] Copyright 2021, Wiley-VCH Verlag. e) Linear actuators by electrochemical oxidation of liquid metal. Left scale bar: 2 mm and right scale bar: 5 mm. Reproduced with permission. [157] Copyright 2022, Wiley-VCH Verlag. f ) Basic drive units of electromagnetic actuators based on liquid metal. Reproduced with permission. [68] Copyright 2017, Science China Press. g) Fishtail fabricated based on liquid metal electromagnetic actuators. Reproduced with permission. [68] Copyright 2017, Science China Press. h) Flower-shaped liquid metal electromagnetic actuators. Reproduced with permission. [161] Copyright 2020, American Association for the Advancement of Science. i) Liquid metal electromagnetic actuator-enabled ultrafast soft robots. Reproduced with permission. [162] Copyright 2022, Springer Nature.
www.advancedsciencenews.com www.advintellsyst.com intervention of external sensors; thus, the material self-senses its own structural deformation. This self-sensing feature also means that driving and sensing are integrated, facilitating the optimization of soft robot structures.

Liquid Metal-Based Soft Actuators
Actuators are the power sources of robots and important structures. Therefore, building an actuator is an indispensable step in building a robot. Based on their diverse response behaviors, liquid metals are considered as promising materials for soft actuators. Recently, significant progress has been made. [154][155][156] As illustrated in Figure 10, the actuation mechanisms of liquid metal-based actuators fall into two main categories: electrochemical oxidation-enabled actuators and liquid metal electromagnetic actuators. For the former case, the basic principle is that liquid metal placed in solution is oxidized by the positive electrode, [87] thereby causing the interfacial tension to decrease (between approximately 550 mN m À1 to near zero) and leading to spreading ( Figure 10a). Early attempts in this direction were conducted by Majidi et al. [155,156] They designed actuators based on the mechanisms of electrochemical oxidation and reduction and achieved an output of mechanical work. Such an actuator could be endowed with a high work density on a small scale (%10 2 J m À3 at the mm-scale and %10 5 J m À3 at the μm-scale) and no longer required a high voltage (like a dielectric elastomer actuator). Furthermore, a liquid metal artificial muscle was fabricated based on similar principles. [76] The drive unit is shown in Figure 10b. The driving force (F σ ) induced by liquid metal interfacial tension (σ) can be expressed as follows Here, θ stands for the contact angle between the droplet and the Cu pads and r Cu represents the radius of the Cu pad. Thus, voltage-induced changes in the interfacial tension of the liquid metal can alter this driving force and electric fields are used to drive such artificial muscles. In addition, multilayer-structured liquid metal artificial muscles with better performance can be obtained by stacking (Figure 10c). It was further revealed that the caudal fin of the robotic fish driven by the liquid metal artificial muscle could reach a swing angle of 20° (Figure 10d), which makes it possible to prepare an underwater robot fish. Nevertheless, it was demonstrated that stacking at the millimeter scale is only functional for no more than three layers owing to the effect of gravity. Therefore, Shu et al. [157] proposed a new architecture to configure liquid metal droplets to achieve force and structural scalability (Figure 10e). A significant problem with this type of actuator is the generation of H 2 and O 2 bubbles during the charging process owing to oxidation and reduction reactions, which should be handled carefully in subsequent practical applications. However, the forces generated by the electrochemical oxidation and reduction are insufficient. The use of a large number of small liquid metal droplets to implement an integrated design is a possible solution. Because of the Cu plate, the entire actuator is not completely soft, which is a challenge worth addressing.
Another typical class of liquid metal-based actuators is electromagnetic actuators, the principle of which is described below. Liquid metal is inherently electrically conductive and inevitably subjected to Lorentz or Ampere forces when placed in an electromagnetic field. [158][159][160] The underlying mechanism is that when the liquid metal is placed in a variable magnetic field, an electric current is excited within it and a magnetic field is generated in the direction opposite to that of the original magnetic field, thereby providing the driving force. If the current in a liquid metal is supplied by an AC power source, it produces a magnetic field with a variable direction that is periodically attracted and repelled by another magnetic field. The direction of the resulting driving force is perpendicular to the plane where the magnetic field and current are located and can be calculated based on the Lorentz force law as follows where I is the current passing through the coil; dl is an extremely small section of the coil; and B is the magnetic flux density. Based on this principle, Guo et al. [68] designed an electromagnetic actuator whose basic driving unit is shown in Figure 10f. An alternating current was applied to the liquid metal coil, which caused the direction of the excited magnetic field to change, thereby achieving periodic attraction and repulsion with the permanent magnet. Because of this periodic movement, the authors used it as a fishtail to create a bionic fish (Figure 10g). With further optimization, Mao et al. [161] achieved a fully flexible liquid metal electromagnetic actuator by removing a rigid magnetic holder. A flower-shaped electromagnetic actuator with individually controlled petals was achieved by introducing a liquid metal multicoil (Figure 10h), showing potential applications in soft grippers and minimally invasive medicine. Furthermore, researchers have developed a series of small-scale soft electromagnetic robots based on liquid metal (Figure 10i) that display diverse abilities, including swimming, jumping, walking, running, and steering. [162] The key innovation of this study is the high speed of movement resulting from the miniaturization of soft robots based on liquid metal 3D printing. These two types of actuators primarily exploit the electrochemical oxidation of liquid metals and their response to electromagnetic fields, which convert electrical energy into mechanical energy. The next step is to develop more actuators based on the response behavior of liquid metal. For example, an actuator without an external energy supply and drag wires can be realized by taking full advantage of the self-driven motion of a liquid metal swallowing Al or its response to chemical stimuli, which could be an inspiration for achieving integrated liquid metal soft robots.

Liquid Metal-Based Soft Control Systems
Along with material innovations and structural improvements, soft robots have rapidly developed over the last few decades and are increasingly attracting academic attention as a major option for achieving breakthroughs in robotics. Although there have been many representative achievements in the development of smart materials for soft robots, the lack of feasible basic www.advancedsciencenews.com www.advintellsyst.com strategies and components for the control of soft robots hinders their refinement and intelligence. First, the responsive properties of smart materials for building soft robots promise to address this challenge and should therefore focus on improving the responsiveness and adjustability of the materials. Although this is a good strategy, it places considerable demand on the materials themselves.
Another approach focuses on conventional control systems including logic devices and their corresponding control algorithms. If the hardware units can be softened and a series of "soft logic devices" constructed, then the established control theories can be easily adapted for direct application and thus meet the requirements. Liquid metal smart materials also play a vital role in this strategy. Li et al. [163] implemented a soft logic system using the response of a liquid metal to an electric field. The team systematically demonstrated the concept of a soft logic unit based on liquid metal and designed two types of basic logic units (Figure 11a(i-ii)). The principle is to control the magnitude of the switching voltage and to manipulate the movement of the liquid metal droplets in the flow channel to turn the output circuit on or off, thereby providing the device with an "on-off" binary logic feature. Based on this logic unit, a series of liquid logic gates (AND, OR, NOT, NAND, etc.) have been developed (Figure 11a(iii)), enabling a wider range of logic operations and electrical signal outputs. The resulting soft logic system is compatible with soft robots and has been successfully employed to control them (Figure 11a(iv)).
In addition, based on the principle of oxidation and reduction of liquid metal in an electric field, droplets have been used to implement field-controlled electrical switches. [164] Experimental results show that the measured conductance varies by >3 orders of magnitude depending on whether or not drops coalesce (left) or Figure 11. Liquid metal-based soft control system. a) Liquid metal-based switch: (i) D-type; (ii) L-type; (iii) soft logic gates; and (iv) liquid metal-based soft logic system for controlling robot grippers. Reproduced with permission. [163] Copyright 2021, Wiley-VCH Verlag. b) Field-controlled electrical switch with liquid metal: effective output conductance in different states. Reproduced with permission. [164] Copyright 2017, Wiley-VCH Verlag. c) Liquid metal electronic oscillators: (i) the basic structure of the resonance unit; (ii) the "OFF" state or "ON" state of the liquid metal electronic oscillator. Reproduced with permission. [165] Copyright 2022, Royal Society of Chemistry.
www.advancedsciencenews.com www.advintellsyst.com separate (right) (Figure 11b), thus the "on" and "off" of this switch could be determined by whether the droplets are in contact or not. Liquid metal droplets come into contact with each other because electrochemical oxidation causes them to spread out, whereas separation draws on the instability of the fluid. In addition, the oscillation of the liquid metal under the action of an electric field was used to convert DC to AC. [165] A schematic of the converter is shown in Figure 11c(i), where the liquid metal droplet is anchored only to the Cu negative electrode in its initial state. Subsequently, under the action of an electric field, the droplet moved toward the positive electrode and the entire circuit became short-circuited when it touched the positive electrode. Subsequently, the droplet returns to its original state, at which point the circuit breaks and a new cycle begins. Whether the droplet touches the positive pole determines the opening or closing of the circuit, as shown in Figure 11c(ii). In this study, the movement of the droplet is controlled by a DC and the resulting on and off currents can be considered as a periodic AC. Consequently, a DC-AC converter was successfully prepared. The control unit based on the response behavior of the liquid metal presented above is an interesting initial exploration for flexible control systems. The initial exploration of soft control systems based on liquid metal was presented above. These control systems remain in the early stages of development. In the future, there is a need to explore many possibilities in this direction. Harnessing the properties of liquid metals is a promising path toward new advances.

Liquid Metal-Enabled Robots
Based on the intelligent behaviors of liquid metals and development of soft actuators and control systems, integrated liquid metal-enabled robots have been implemented. Some early studies in this direction are shown in Figure 12a, where liquid metal droplets moving in an electric field were used as wheels to support a car plate used to transport goods (Figure 12b). [70] This study primarily utilized the response behaviors of liquid metals to electric fields. Based on a similar drive principle, researchers rationally designed a liquid metal-wheeled robot. [71] As shown in Figure 12c, the movement of an electrically controlled liquid metal droplet inside the robot changed the robot's center of gravity, thereby driving the robot forward. For such a wheeled robot, various strategies could be used to change its center of gravity. In another study, Ye et al. [72] used chemical reactions and electrical heating to generate gas inside a robot as a method to push liquid metal droplets to move, thereby causing the center of gravity of the overall structure to change and movement to occur (Figure 12d). Accordingly, they developed a tripodal-wheeled robot driven by liquid metal droplets. [166] Similarly, Xue et al. [167] proposed a small universal mechanical module driven by a liquid metal droplet placed in an electric field for circulating pumping (Figure 12e). More practically, such electric-field-driven motors were further developed to drive vehicles [92] (Figure 12f ). In general, liquid metal is commonly used as a power source for integrated robots. To achieve this driving effect, the liquid metal must be placed in a solution. Through encapsulation, it is converted into a drive unit for the robot. Therefore, the entire robot can operate in a solution-free environment, effectively expanding the scope of applications of liquid metal-enabled robots.
Notably, several of the previously described studies largely overshadowed the advantages of liquid metals with natural flexibility owing to the involvement of rigid components. Recently, researchers developed liquid metal-enabled biomimetic robotic jellyfish. [168] Liquid metal was injected into a flexible substrate; the other components of the robot were also made of flexible materials to mimic jellyfish as much as possible. As demonstrated in Figure 12g, the liquid metal jelly robot had contracted and diastolic states and the rapid switching between the two ensured that the liquid metal jellyfish robot moved underwater. The underlying mechanism lied in the response of an energized liquid metal coil to a magnetic field, as mentioned earlier; the application of an alternating current allowed for a periodic response. Further experimental results confirmed that the almost fully flexible liquid metal jellyfish robot demonstrated a gentle low-noise motion with little effect on nearby fish in the water (Figure 12h). Nevertheless, the presence of permanent magnets and external power cables implies that further improvements in such robots remain possible. A possible solution for realizing a fully soft, drag-free liquid metal robot is to use its self-driving properties as a drive unit, a direction worth exploring in the future.
Importantly, every significant step forward in the movement and transformation of liquid metal will cause corresponding concern and discussion, as derived from people's expectations of liquid metal humanoid robots. The liquid metal robot in the film Terminator is an example of the excessive shock in response to such expectations. In this context, one cannot help but consider how to appropriate implement such humanoid robots. As a trial, we imagine a strategy for achieving liquid metal humanoid robots. It is believed that a pool of liquid metal needs to be given four functions to construct liquid metal humanoid robots: standing, shaping, moving, and sensing. It is a massive challenge to stand and shape liquid metal in three dimensions owing to the inherent liquid state of the material. Fortunately, a series of advances in liquid metal have shown promise. Previous studies have shown that liquid metal has desirable wettability with other materials (e.g., Cu mesh), [169][170][171][172] thereby ensuring that the liquid metal can be closely attached to these soft solid materials while retaining the ability to be shaped. [173] Such a solid-liquid coupling strategy can help achieve the standing and shaping of liquid metal. In addition, the fast and controllable liquid metal solid-liquid phase changes are considered as beneficial for its standing and shaping. Approaches to achieving fast phase changes include lowering the supercooling degree [174] and applying an external magnetic or electric field. [175] Oxidation is another potential strategy, as the surface tension of an oxidized liquid metal changes dramatically. [176] In summary, the adjustable stiffness of liquid metal and its composite systems shows promise for enabling the standing and shaping of liquid metal.
When the liquid metal humanoid robots can maintain a designed shape in a 3D space, the moving of such robots will become another task. Unlike traditional mechanical driving methods, such robots will require more flexible drive strategies, for example, electromagnetic field or chemical stimuli drives. Here, the self-driving motion of liquid metal without external www.advancedsciencenews.com www.advintellsyst.com fields or devices is worth studying by researchers. Humans rely on a large number of biochemical reactions in their bodies to achieve movement, i.e., what liquid metal humanoid robots are intended to achieve. In this process, providing accurate modeling of the motion behaviors of liquid metal humanoid robots remains a challenge. Moreover, a sensing function is essential for the robots as well. Fortunately, liquid metal integrates the ability to sense itself, [177][178][179] allowing it to act as a smart sensor. Liquid metal humanoid robots fully employing these concepts remain unrealized and the exploration of smart properties will help to achieve this goal.

Outlook and Conclusion
A series of intriguing physicochemical effects of liquid metal have recently been discovered, contributing to the emergence of the concept of liquid metal soft machines or robots. As the smart behaviors of liquid metal continue to be studied, the concepts of liquid metal robots are becoming clear. However, liquid metal soft robots remain in their infancy. The existing liquid metal machines remain far from the concept traditional robots, as they do not have the abilities to sense and give feedback, control and make decisions, etc. At this stage, researchers are Reproduced with permission. [70] Copyright 2016, Royal Society of Chemistry. b) Liquid metal-wheeled small vehicles for cargo delivery. Reproduced with permission. [70] Copyright 2016, Royal Society of Chemistry. c) Liquid metal-wheeled robots. Reproduced with permission. [71] Copyright 2019, Wiley-VCH Verlag. d) Liquid metal two-fluid robots. Reproduced with permission. [72] Copyright 2019, Chinese Academy of Sciences. e) Circulating pumping driven by a liquid metal droplet. Reproduced with permission. [167] Copyright 2021, Royal Society of Chemistry. f ) Vehicles based on a liquid metal motor. Reproduced with permission. [92] Copyright 2021, Elsevier. g) Expansion and contraction of liquid metal jelly robot under the action of alternating current. Reproduced with permission. [168] Copyright 2022, Mary Ann Liebert Inc. h) Liquid metal jelly robot with small disturbances used for filming and surveillance. Reproduced with permission. [168] Copyright 2022, Mary Ann Liebert Inc.
www.advancedsciencenews.com www.advintellsyst.com expected to explore additional smart properties of liquid metal to further design soft actuators and flexible control systems. These efforts will help accelerate the creation of liquid metal soft robots.
To inspire follow-up research, potential development directions or challenges for using liquid metal smart materials in robots are presented below.

Enriching Intelligent Attributes
Liquid metal smart materials show rich responsive behaviors under the actions of various external fields and can be driven thereby. It remains meaningful to explore more complex intelligent response behaviors of liquid metal smart materials to enrich the intelligent attributes of liquid metals and facilitate the implementation of liquid metal-based soft robots. The response behaviors of liquid metals and their composite systems in the acoustic and optical fields require further study. For example, it may be possible to provide liquid metal rotation and levitation based on acoustic or optical fields.

Constructing Composite Systems
By fully utilizing the advantages of liquid metals and other synergistic materials, a series of smart properties of liquid metal composites have been discovered. As the complexity of composite systems increases, unpredictable smart properties are expected to emerge; correspondingly, the effective construction of a liquid metal composite system may become a greater challenge. Thus, studies must be conducted on the construction of liquid metal composite systems able to work in concert with other materials. In addition to solutions, particles, and polymers, exploring the interactions of other synergistic substances with liquid metals for building composite systems is a promising strategy.

Micro-and Nanomachines
For achieving the purposes of liquid metal robots, it is necessary to study the smart behaviors of liquid metal droplets to provide rich methods and strategies for manipulating liquid metal. Small-scale liquid metal droplets, as an imaginary micro-nano robot, are considered as a potential candidate. The use of liquid metal droplets as vascular robots has also recently been proposed.

Dynamic Control
As liquid metal soft robots with flexibility and environmental adaptability appear, improving their motion accuracy will become the focus of further studies. Generally, the stiffness of a robot ensures higher control accuracy to a certain extent. As such, providing dynamic control of soft robots remains a challenge. Thus, the development of programmable liquid metal soft robots has become an important task. During this development process, the control of group behavior represents another difficulty in this research.

Energy Supply
The energy of traditional robots is usually provided from an external input, whereas liquid metal soft robots are anticipated to make full use of their unique smart properties to achieve selfdriving motion. For instance, they use their hydrogen production as an energy supply and use interface reactions to obtain energy from the environment.

Performance Enhancement
The performance of robots needs to be further enhanced for practical applications, e.g., not only to the speed and strength of liquid metal soft robots but also their deformability.

Liquid Robot
It is believed that an imaginative liquid metal robot should be all soft or even entirely liquid, with deformation capabilities and multiple functions. However, liquid robots are different from traditional robots and implementing liquid robots will be challenging. For instance, the 3D shaping of liquid robots is still fraught with difficulties owing to lack of support structures, as robots often need to form specific shapes when performing tasks. As a result, further efforts need to be made.
In conclusion, liquid metal soft robots are evolving from fantasy to reality. The development of liquid metal smart materials is a key step toward achieving this goal (and correspondingly has received extensive attention in recent years). In this review, a timeline of the typical progress of liquid metal smart materials toward soft robots was organized and introduced. Then, the stimuli-responsive behaviors of liquid metal smart materials were analyzed and expounded upon in detail based on applied factors such as electric field, magnetic field, chemical stimuli, light, and ultrasound. More specifically, this review provided a logical presentation and description of relevant advances in each type of responsive behavior with the aim of reflecting on past developments more completely and effectively guiding subsequent research. In addition to the liquid metal-solution and particle systems facing the rapid development trend of liquid metal-polymer smart systems, this article also focused on their smart properties, such as stimulus responses, self-healing, conductive-insulation transformations, and self-sensing. Liquid metal-enabled soft actuators, flexible control systems, and integrated liquid metal robots with specific functions were listed and elaborated upon. Finally, potential future directions and challenges are discussed. It is expected that this review will promote further development in this field, endowing liquid metal with more smart properties and eventually building liquid metal soft robots.