Liquid metals and their hybrids as stimulus–responsive smart materials

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Liquid metals and their hybrids as stimulus–responsive smart materials Long Ren 1, Xun Xu 1,⇑, Yi Du 1, Kourosh Kalantar-Zadeh 2,⇑, Shi Xue Dou 1,⇑ 1

Institute for Superconducting and Electronic Materials, Australian Institute for Innovative Materials, University of Wollongong, Wollongong, New South Wales 2500, Australia 2 School of Chemical Engineering, University of New South Wales (UNSW), Sydney, New South Wales 2052, Australia

The re-emergence of room temperature liquid metals presents an exciting paradigm for an ideal combination of metallic and fluidic properties. The unique fluid metal features of non-hazardous Gabased liquid metals, including high surface energy, low viscosity, electrical and thermal conductivity, a wide temperature range of the liquid state, and desirable chemical activity for many applications, have led to remarkable possibilities for harnessing their properties and achieving unique functionalities. The realization of their stimulus-responsivity and multi-functionality make Ga-based liquid metals an attractive family of ‘smart materials’ that could act as the basis of countless applications in new frontiers, covering a wide range from materials science and engineering to medicine. Constructing hybrids of Ga-based liquid metals with other functional materials can further extend the fieldresponsive capacity of liquid metals to incredible levels. An increasing number of reports have revealed Ga-based liquid metals and their hybrids as remarkable soft smart-response materials. Nevertheless, the mechanisms underlying their stimulus–response activities, their interactions with other functional entities, and efficient tuning in their intimate integration, still require further exploration. Considering the applications of Ga-based liquid metals and their hybrids, this review focuses on their fieldresponsive physical and chemical properties. The recent field-responsive reports are comprehensively presented. The analysis of their responsive properties and the types of field applied in each case are discussed, so that a critical outlook on this field can be established. Introduction The ongoing technological revolution in robotics and artificial intelligence will have an irrevocable impact on all human lives and the lives of future generations. Robotics in this revolution are ‘smart machines’ with abilities ranging from perceiving and exploring outer space to tracking and eliminating cancerous entities in nanoscale [1]. Herein artificial intelligence directs the progress of this revolution with the algorithms that rule artificial robotic organisms. Making such machines soft and compatible,

⇑ Corresponding authors. E-mail addresses: Xu, X. ([email protected]), Kalantar-Zadeh, K. ([email protected]. au), Dou, S.X. ([email protected]).

and enforcing them to exhibit adaptation, sensitivity, and agility, are essentials for interactions with delicate constituents in living objects as a whole and human body specifically [2]. The key challenge to achieving the full potential of robotics and artificial intelligence is exploring and developing suitable building blocks, namely smart materials which are exceptionally adaptive and smart enough to respond with ultimate freedom to the signal or stimulus from the control center [3,4]. The magic material ‘liquid metal (LM) or mimetic polyalloy’, which can be manipulated to assume various forms, as the main component of the “T-1000” future robots in the Terminator science-fiction movies, is potentially an ideal smart material for scientists and technologists who wish to build artificial robotic

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organisms. Fortunately, the recent breakthroughs in galliumbased LMs (Ga-based LMs) are now making this science-fiction concept a reality. Ga-based LMs, as metals that are liquid at room temperature, offer the most intriguing combination of fluidic and metallic properties. Their unique properties including low viscosity, acceptable safety, high thermal conductivity, and high electrical conductivity, make Ga-based LMs ideal for use in soft electronics for developing components such as ultra-flexible antennas, wearable e-skin, and implanted bio-electrodes [5,6], and in chemical processes for synthesizing functional materials [7–9], as well as for the application of cooling and additive manufacturing [10–13]. It is believed, however, that one of the most attractive and distinct properties of these LMs is their ability to be reversibly shape-reconfigured at room temperature [14,15]. Such smart behavior leads to the possibility of creating devices that can change their functions based on their switchable morphologies, their displacements and perturbations [16–18]. So far, many creative stimulus–responsive properties of Ga-based LMs and their hybrids have been demonstrated, which make these mostly non-hazardous LMs promising tools to meet the requirements of future robotics and artificial intelligence components. Although there have been reviews and progress reports on the topic of LMs, including some comprehensive reports regarding the fundamental concepts underlying LM research and the unique phenomena that emerge in LMs [5,6,19–25], a summary of the stimulus–responsive behavior of Ga-based LMs and analyses of the fundamentals underlying their mechanisms are lacking. In this review, we will focus on establishing the relationships between the basic characteristics of LMs, the stimulus applied, and the responsive activities. The mechanisms and possible physic-chemical effects will be discussed. Applications engineered to take advantage of particular properties of such smart materials will be presented. The outlooks for the perceived stimulus–responsive activities and promising applications based on the integration of smart LM systems will also be discussed. To make the scope succinct, we will focus on Ga and the most recognized Ga-based eutectic alloys, mainly EGaIn and Galinstan, and the hybrids composed from these LMs with other functional materials, which are non-hazardous and liquid at room temperature.

Basic characteristics of Ga-based LMs and their hybrids Smart materials are defined as those with the ability to form smart structural systems that sense their environment and respond to that external stimulus via an active control mechanism [26,27]. This means that one or more properties of the smart materials should be malleable to be significantly and reversibly changed in a controlled fashion in response to external stimuli, such as changes in compressive or tensile mechanical stress, temperature, electric or magnetic fields, light, moisture, or pH value. Ga-based LMs offer the properties that make them ideal materials for creating stimulant responsive components. Ga-based LMs are the most suitable metallic alloys for developing practical apparatus in stimulant responsive systems. Out

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of the five known metallic elements, Fr, Cs, Rb, Hg, and Ga, which are in the liquid state at or near room temperature, only Ga is suitable for safe robotics for room or near human body temperature applications. Featuring the virtually no vapor pressure at room temperature and low toxicity, Ga can be handled safely [28]. The melting point (
30 °C) of Ga is slightly higher than the nominal office temperature, but the addition of other posttransition metals such as In and Sn make the Ga-based eutectic alloy of EGaIn (75% Ga, 25% In) and Galinstan (68.5% Ga, 21.5% In, and 10% Sn) melt at lower temperatures [6]. EGaIn has a melting temperature of 15.7 °C, and the melting temperature of Galinstan is slightly lower [24]. The other advantage of Ga-based LM is its ultimate changeability as a stimulus–responsive material. Ga-based LM can offer great synergy with many organic and inorganic compounds for creating incredible LM hybrids as smart materials. The surface tension of Ga-based LM, for accommodating various functional materials, can be regulated through various strategies [29]. Gabased LMs exhibit a high surface tension (EGaIn: 624 mNm1 and Galinstan: 534 mNm1). Interestingly, when they meet an oxygenated environment (including ambient air and water), Ga-based LMs form a self-passivating Ga-oxide skin that dramatically reduces the surface tension as a responsive reaction [6]. The existence of this distinct property offers remarkable possibilities to manipulate Ga-based LMs by applying stimuli such as chemical reactions, the pH modulations, or electrochemical changes. Until now, some exciting on-demand responsive activities of Ga-based LMs and their hybrids have been reported, which have accelerated the application of LMs in smart systems and offered the snippets for the construction of intelligent robots. A summary has been depicted in Fig. 1. The discussions of the responsive activities of Ga-based LMs and their hybrids will be classified by the type of the stimulus applied and presented in this section. We expand Ga-based LMs in many different areas, although not directly described under the banner of stimuli responsive category by other researchers, across this concept.

Temperature dependence properties and responsivity As a liquid, the first explored stimulus–responsive characteristic of a Ga-based LM was volume expansion with increasing temperature [30]. As a replacement for the toxic Hg, commercial thermometers and thermocouples were the initial products in which Galinstan was used. Ga has the second widest liquid state range of any metal (30–2400 °C), with a peculiar thermal expansion coefficient that increases by several hundred percent after melting. The volumetric-expansion coefficient of liquid Ga in the macroscopic state is 101.5  106 per °C at 30–977 °C [31], larger than those of most solid metals, but it is still relatively lower in comparison to many other liquid substances and soft polymers. The volumetric coefficient of thermal expansion (a) is given by:   1 @V a¼ ð1Þ V @T p where the subscript p indicates that the pressure is held constant during the expansion. Therefore, the expansion of Ga or Ga-based LMs can be conspicuous when storing them in a small container, especially in nanosized containers [30].

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FIGURE 1

Summary of the stimuli and the possible responses reported for Ga-based LMs and related hybrid systems.

Besides thermometers, thermally driven responsive activities have rarely been reported for Ga-based LMs in liquid form, which may be due to the low thermal expansion coefficient of Ga near room temperature, which results in relatively low sensitivity and small response of bulk LMs without small container restriction. By forming hybrids in conjunction with the stimulus–responsive activities of other materials, however, the thermally driven properties of Ga-based LM have been used in practical applications. Stretchable liquid–metal composites of EGaIn mixed with silicone elastomer and other rubbery polymers have been developed, which are capable of transforming and stretching into complex and desirable deformations in response to heating [32]. The addition of EGaIn LM remarkably improves the thermal conductivity and reduces the elastic modulus of the primary elastomer composites, resulting in remarkably large volume changes in response to a thermal stimulus. We believe that Ga-based LM hybrids can play great roles in developing temperature modulation or heat-driven smart behavior of soft robots. For such explorations, the surface characteristics of LMs need to be further explored and verified, and different methods for polymerization of the elastomers on such surfaces are also worth investigation. For maintaining the advantages of the LM thermal conductivities, further studies need to be done to assure that the selflimiting oxides of Ga-based LMs are not a source of negative interference.

In addition to the thermal expansion in liquid form, most properties of Ga, and its LM alloys are temperature-dependent, particularly near the melting point, such as their supercooling/superheating behavior [33], their negative thermal expansion in the transition from liquid to solid [34], and their peculiar superconductivity at very low temperatures [35]. With the thermo-responsive phase transition behavior of LMs in mind, a highly stretchable (680% strain) LM-polymer composite has been designed, exhibiting a reversible insulator–metal transition in response to the temperature change. As shown in Fig. 2(a), the prepared LM-polymer sample (denoted as TIC, from the abbreviation for transitional insulator and conductor) is electrically insulating at room temperature, and when completely cooled, it becomes electrically conductive instantly. Colossal reversible resistivity changes (more than 4  109 times) can be realized through tuning the temperature for warming and freezing this composite, as evidenced by the resistivity measurements shown in Fig. 2(b) and (c). This reversible insulator–metal transition in response to temperature changes is mainly due to the negative thermal expansion of Ga-based LMs in the transition from liquid to solid. Fig. 2(d) and (e) demonstrates that the LM droplets are able to bulge out from a rigid silicone shell after freezing and connect with each other to form a conductive path. After the temperature warms up, the droplets shrivel in form of a liquid and are enclosed by the soft silicone shell again, breaking the 3

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(a) Photographs of temperature-induced reversible transitional insulator and conductor (TIC) prepared by dispersing LM droplets in silicone polymer. This TIC composite is electrically insulating (R > 2  108 X), and becomes conductive (R = 0.05 X) after freezing by liquid nitrogen. The transition between insulator and conductor can be reversed. Scale bar: 2 cm. (b) Plot of the resistivity of TIC as a function of temperature. (c) Resistance change between electrically insulating and conducting for 100 cycles under temperature regulation. (d) Phase change of LM droplets in response to freezing and warming along with volume change. Scale bar: 2 mm. (e) Schematic illustration of the mechanism in the electrical transition between insulating and conducting of TIC in response to a temperature change [36]. (Reproduced with permission from Ref. [36], copyright 2019, John Wiley and Sons.)

conductive path. Such exploration of a temperature stimulated insulator–metal transition offers tremendous possibilities for numerous applications, such as stretchable switches, semiconductors, temperature sensors, and resistive random-access memories. The low working temperature (transition temperature of 61 °C for whole system) may limit its wide application to some extent, however, so developing obvious thermal stimulus– responsive behaviors based on LM and hybrids for which the operating temperature is not far from room temperature is in strong demand.

Mechanically induced response As an intrinsic characteristic of a fluid, LMs can spontaneously flow, and subsequently deform, in response to stress. Taking advantage of its deformability and intrinsic high electric conductivity, and the as-produced electromagnetic field, functional behavior of LM constructed telecommunication and electronic systems can be altered by applying mechanical forces. This has been demonstrated in tuning the resonant frequency of LM based antenna in response to the pressure-induced conductive shape modulations [37,38]. Moreover, LM flexible electronics

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for sensing touch, pressure, and strain can be developed through changes in resistance or capacitance in response to mechanical stimuli. As a simple model, an ultra-stretchable conductive fiber was constructed by injecting EGaIn LM into elastic tubing, which can be stretched up to 800% strain (Fig. 3(a) and (b)) before failure [39]. Meanwhile, the electrical resistance of this fiber is increased in response to the applied strain, since the length of the fiber increases (Fig. 3(c)), while the cross-sectional area decreases. Using mechanically applied stimuli mechanisms, various other soft sensors and antennas have been developed [5,40–42]. Besides the mechanically induced conductive shape changes in LM-based integrated electronic devices and systems, another interesting smart behavior of LM in response to extreme mechanical stimuli is self-healing. Several examples of selfhealing circuits have been realized by filling polymer made microchannels with Ga-based LM [43–45]. As Ga-based LM’s surface is very sensitive to oxygen, and once the microchannel is cut, the filled LM neither leaks out nor retreats into the microchannel as a result of forming a protective oxide layer.

Once the damaged parts are back in contact, the boundaries of LM merge by breaking the thin oxide layers and form a continuous conductive path. As an advanced smart self-healing conductor, for example, autonomously electrically self-healed composites have been achieved by constructing LM-based microcapsules in polymer matrices. In response to cut damage on the circuit (Fig. 3(d)), the embedded microcapsules nearby are disrupted, and consequently, the LM fluid core is released from the microcapsules into the damaged area under the influence of gravity, restoring the electrically conductive path [43]. There are also other examples of such self-healing behavior of Gabased LM composites [46]. These self-healing systems can be potentially used in applications for which constant human surveillance is either too costly or impossible due to the harsh environment.

Electrical and electrochemical modulations Surface tension, another characteristic of a liquid, is the tendency of a fluid surface to shrink into the possible minimum surface area. Ga-based LMs exhibit very high surface tension (>500

FIGURE 3

(a) and (b) An ultrastretchable conductive fiber, made from thermoplastic elastomer fibers, with a hollow core filled with LM, can be stretched to extreme strains while maintaining metallic conductivity. (c) The experimental (filled squares) and theoretical resistance (empty triangles and circles) of the conductive fibers obtained at two different spinning rates of 100 (left) and 1000 (right) m/min [39] (reproduced with permission from Ref. [39], copyright 2012, John Wiley and Sons). (d) Micro computed-tomography (CT) data (corresponding to the shape of the Ga-In LM) with schematic illustration (the Au-coated glass substrate with a crack in the middle) showing a superimposed image of LM released into the crack plane from the microcapsules. The following schematic illustration below shows that the crack damage breaks the conductive pathway, but the electron transport is soon restored by simultaneously rupturing the capsules to enable LM to fill the area of damage [43] (reproduced with permission from Ref. [43], copyright 2011, John Wiley and Sons). 5 Please cite this article in press as: L. Ren et al., Materials Today, (2019), https://doi.org/10.1016/j.mattod.2019.10.007

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mNm1), which generally limits Ga-based LMs to spherical shapes, especially when the surface oxide is removed and the bulk is large with reference to the surface area. The high surface tension of Ga-based LM offers the possibility of controlling its flow behavior via surface or interface tension regulation. When a LM is immersed in an electrolyte or brought into contact with a dielectric, the Lippmann's and Lippmann– Young’s equations govern the electrical change of its surface, and hence, its wettability and dielectric properties, generating modification on surface or interfacial tension [47,48]. The Lippmann's equation is presented as follows:   @c ¼ Q A ð2Þ @EA T;p;li –l where c is the interfacial tension, EA is the potential of a cell in which the reference electrode has an interfacial equilibrium with one of the ionic components of the interface, A, QA is the charge per unit area of the interface, mi is the chemical potential of the combination of species i whose net charge is zero, T is the thermodynamic temperature, and p is the external pressure. The Lippmann–Young’s equation is presented as follows:

cos ðhÞ ¼ cosðh0 Þ þ

ed v2 2rgl t d d

ð3Þ

where h is the apparent contact angle of the liquid under applied field, h0 is the native contact angle (without field), rgl is the surface tension of the gas–liquid interface, ed is the permittivity of the dielectric, td is the thickness of the dielectric layer, and vd is the voltage across the layer.

The stimulus methods, as schematically shown in Fig. 4, include continuous electrowetting, electrowetting on a dielectric, electrocapillarity, and electrochemically controlled capillarity [5,49]. Formation of an electrical double layer on the surface of LM governs its actuation in response to the induced electrochemical charges. The presence or absence of the interfacial oxide layer also plays a very important part. Electrocapillarity is one of the earliest reported methods for tuning the interfacial tension of LMs, which utilizes interfacial charges between the metal and electrolyte to lower boundary tensions. This change in interfacial tension is induced by a

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spontaneously formed electrical potential at the boundary between the LM and the liquid electrolyte, and the potential gradients lead to gradients in the interfacial tension. Although the electrocapillary phenomenon results in modest changes in interfacial tension, constructing unique geometries for LMs (e.g. flowing LMs into desirable channels) is able to increase their sensitivity to electrocapillary actuation [50,51]. In addition, the possible electrochemical reactions that take place at the surfaces of Ga-based LM can significantly lower the surface tension, achieving adjustment of the interfacial forces over a wide range. As a result of applying a voltage between a reference electrode and LM, the breaking of the symmetry on Ga-based LM induces the actuation. A very beginning attempt towards electric-fieldinduced transformation of GaInSn LM was conducted by simply applying an electric field to a LM droplet or bulk film immersed in, or sprayed with, water, incorporated into a two-electrode system. Different morphologies and configurations have been discovered with a constant 12 V voltage supply [14]. Fig. 5(a) shows an example of how an EGaIn LM sphere disperses into an asymmetric shape when a potential from in the range of 1.5 to 0.5 V is applied in a solution of 1 M NaOH, with the surface area is several times that of its original drop. The potential applied at the boundary between the LM and the NaOH electrolyte leads to electrochemical oxidation of the metal, which further lowers the interfacial tension besides the standard electrocapillary effects [15]. Due to the formation of an oxide layer on the surface after increasing the potential, the interfacial tension dramatically drops, and even further approaches near-zero, after further increasing the potential to positive values (Fig. 5 (b)). The high-energy interface between the metal and electrolyte is then replaced by two new interfaces, metal–metal oxide, and metal oxide–electrolyte, resulting in very low interfacial tension and yielding to gravity. The continually growing oxide layer, competing with its dissolution by the basic electrolyte, allows the LM droplet to flow and spread despite being covered with a solid film. In this example, further experiments and mechanism studies have been carried out by characterizing changes in morphology

FIGURE 4

Summary of primary methods for electro-induced LM modulations. (a) Here, continuous electrowetting creates surface tension gradients to actuate the LM within a channel. (b) Electrowetting-on-dielectric uses large voltages to achieve modest changes in wetting behavior on a substrate. (c) Electrocapillarity utilizes charges in the electrical double layer to realize modest changes in surface tension. (d) Electrochemically controlled capillarity utilizes interfacial reactions to achieve enormous changes in surface tension [49] (reproduced with permission from Ref. [49], copyright 2016, AIP Publishing). 6 Please cite this article in press as: L. Ren et al., Materials Today, (2019), https://doi.org/10.1016/j.mattod.2019.10.007

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FIGURE 5

(a) A drop of LM being oxidized electrochemically in 1 M NaOH. (b) Surface tension of a eutectic EGaIn drop in 1 M NaOH. The vertical dotted line represents the electrochemical formation of the oxide layer [15] (reproduced with permission from ref. [15], copyright 2014, National Academy of Sciences.). (c) Schematic diagram of apparatus and experimental images of a 30 lL EGaIn droplet undergoing the branching instability; (d) box-counting plot shown as a function of time for the same droplet, spreading at 1.3 V [52] (reproduced with permission from Ref. [52], copyright 2017, American Physical Society).

and dynamics as functions of the droplet volume and applied electric potential, as shown in Fig. 5(c) and (d). This indicates that surface electrochemical oxidation generates compressive interfacial forces that oppose the tensile forces at a liquid interface and drive instabilities in the Ga-based LM alloy, until the oxide grows too thick and retards further oxidation [52]. Harnessing the competition between this oxidation induced compressive stress and the tension at the interface offers vast potential to develop reconfigurable and stimuli-responsive electronic, electromagnetic, and optical devices based on metallic LMs. The results show that the formation of the oxide was the cause of the spreading, rather than standard electrocapillary effects. Similar to the revealed mechanism, on the other side, it is also possible to use electrochemical reactions to estimate the oxide layer, leading the system back to the state of high interfacial tension [51]. The electrolyte surrounding and the voltages applied to the LM droplets play key roles in the dynamic process of morphological manipulations [53], while the shapes and compositions of the electrodes are also effective influences. With the help of electrode arrays, with a particular design, a simple LM droplet can be dynamically transformed into some complex geometry on demand (even a nonlinear shape such as the alphabet “S” [54]) in a controllable manner. Moreover, the electrode composite may also have a significant influence due to the possible alloying process between Ga and many other metals. Electrochemically

enabled reactive wetting, such as the formation of an intermetallic CuGa2 compound when the Ga-based LM comes into contact with a Cu substrate [55], can accelerate the spreading rate of LM on the substrate and even produce a continuous perfusion of LM within the three-dimensional (3D) porous substrate. Even without direct contact of the LM with an electrode, applying an external electric field to discrete drops of the LM in aqueous solution can create a surface charge difference and subsequently a tension gradient across the LM surface. This gradient is caused by a potential drop across the electrolyte surrounding the metal, and according to the principles of electrocapillarity, can drive fluid motion inside of the channel with no direct contact of the LM with an electrode. The complexity and diversity of such an electrochemically induced actuation can be further enhanced by integrating the LM with semiconducting nanoparticles. LM marbles, which may be coated with different types of nanoparticles (e.g. p-type or n-type), offer an extra dimension for affecting the bipolar electrochemically induced actuation [56–59]. Such hybrid systems offer certain opportunities to build smart actuators that can respond differently to the same stimulus, for applications including precise and selective electronically controllable soft robots. Something that is generally overlooked in electrochemical considerations is the capability of LMs to reduce metal ions and alloy them into the bulk electrochemically. For instance, for each Ga atom, two Li atoms can be inserted into liquid metal

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bulk [60]. Ga-based LM can also be filled up with heavy metal ions such as Cd and Pb at specific applied voltages [29,61]. As such in electrochemical actuations, the competitions between the surface charge changes, surface oxidation, and the insertion of the ions into the bulk of LM, should be carefully taken into consideration.

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To regulate the surface tension of a liquid, surface functionalization using chemicals such as surfactants are usually applied in order to change the interfacial tension. It is important to consider that Ga forms an oxide surface in oxygen ambient or aqueous environment. This oxide layer, like any other twodimensional material, has a strong propensity to physisorbed other entities in its affinity [7,9]. At the same time, the oxide surface can also be reduced, leaving oxygen vacancies that allow the exposure of metallic Ga and chemical functionalization [62]. Thus, this particular surface of Ga-based LM can be functionalised with a variety of surfactants such as salines and thiols, depending on whether the target is Ga or oxygen elements, to achieve the modulation of the interfacial tension [9,63–67]. The native oxide layer of Ga-based LM, as an intrinsic surfactant, can significantly lower the interfacial tension from larger than 500 mNm1 to nearly zero [15]. In addition, this amphoteric oxide layer offers the possibility of regulating the interfacial tension between the LM and its surroundings by implementing chemical reactions, without the help of other stimuli [68]. Due to the benefits of their liquid and soft characteristics, LMs are able to withstand significant bending, stretching, and deformation via chemically created competition with other forces including of electrical, mechanical and electrochemical stimulants. There are many reports on the development of LM systems showing self-actuation, which can convert the energy from spontaneous chemical reactions into mechanical actuations, resulting in autonomous movement of LM droplets [18,69]. For examples, it has been demonstrated that LM objects are able to efficiently self-actuate with a placement of a small Al flake on their surface as fuel (Fig. 6(a)). The actual self-propulsion mechanism of the fuelled LM motor is the force from the generated bubbles as a result of Al reaction in the solution and also the imbalance in the surface tension caused by the bipolar electrochemical reaction that is promoted by the alloyed metal. Metals that are able to alloy with LM at high ratios including Al [18,69–72] and to a lesser extent Cu [73] and Ag [74] have been shown as fuels for actuation. In addition, biomolecules have also displayed the ability to trigger external manipulations for biotechnological applications by localized molecular recognitions and enzymatic reactions in physiological environments [63]. Different styles of actuation can be achieved through the design of fuel distribution, such as the realization of selfpowered oscillators by using metal wires covered by Al granules [70]. This type of metal-fueled LM-based actuator can offer excellent candidates for use in the fabrication of future soft and self-powered biomimetic robots. Besides chemical and biochemical reactions driven actuations, ionic charge symmetry breaking can also influence LM via the modification of the liquid electrolyte surrounding it (pH stimulus) to generate a differential pressure [17]. As seen in

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Fig. 6(b), the highly controlled pH imbalance between the two opposing hemispheres of the LM droplet generates an ionic concentration gradient across the surface of the droplet, which can result in deformation and/or surface Marangoni flow dynamic changes, depending on the pH values across the two separated sides. The kinetic energy from these two types of dynamic propulsions allows the LM droplets either to continuously deform or to be surface rotating without the need for an external electric potential (Fig. 6(c)–(e)). The self-propelling LM droplet offers a responsive mechanism to environmental changes that can be used for switching and controlling flow as well as many other future applications. In addition to the design of self-propelling actuators, the morphology of the LM can be manipulated through adjusting the interfacial tension that is induced by modification of the chemical surroundings [75–81], which could be useful for establishing flexible electronics such as switches, resistors, capacitors, and reconfigurable antennas. Building galvanic cells with LM and conductive chemical surroundings is another effective strategy to activate redox chemical reactions on the surfaces of LMs. Then, the surface tension of LMs can be regulated to trigger the motion or deformation of the LMs. Fig. 7(a) shows a typical galvanic cell design for self-actuation of a LM, in which the LM acts as anode and the copper electrode acts as cathode [82]. The NaOH solution filling the channel between the reservoirs serves as an electrolytic bridge between the two half-cells. When an external electrical connection between the LM and the copper electrode is established, a spontaneous redox reaction is triggered with oxidation of Ga at the anode and oxygen reduction at the cathode. As a result, the oxides on the surface of the LM grow, and the localized interfacial tension becomes lower, inducing LM flow into the channel. When the external electrical connection is broken, the redox reaction halts. The Ga-oxides are then dissolved in NaOH, resulting in the flow back of LM from the channel. With the build-up of a galvanic cell, intense deformation or displacement can be achieved. In alkaline solutions, as an example shown in Fig. 7(b)–(e), hydrogen peroxide and the graphite substrates can act synergistically to simultaneously oxidize the upper and lower surfaces of the LM [76]. The induced surface tension gradient on the surface of the LM causes a Marangoni effect, leading to the quick spreading of LM and fractal behavior with remarkable morphologies [76]. In acid surroundings, as shown in Fig. 7(f) and (g), the shape-shifting and fractal phenomenon could also be achieved by adding ions such as Cu2+ ions to form a Cu-Ga galvanic cell and to trigger the displacement reactions on the surface of the LM, inducing an imbalanced interfacial tension [77]. The influences of different electrolytes, including acidic, alkaline, and neutral, regarding the oxidation formation on the surface of LM droplets should be explored and quantified to better understand and achieve accurate on-demand mobility control of LM droplets [75].

Magnetic field induced characteristics Magnetic fields can be remotely applied to materials, which avoid the inference of potentially invasive fuel-like surroundings, chemical reactions and electrolyte splitting by electrodes. As a noninvasive and more selective stimulus, magnetic fields exhibit very low interactions with nonmagnetic materials, and

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FIGURE 6

(a) Motion of a self-fuelled LM motor in a circular channel, and the schematic operating mechanism of the LM motor. Hydrogen continually bubbles from the Al flake-propelled locomotion of the LM, various forces then affect the velocity of the LM droplet, and the lateral motion of the LM motor heads opposite to the direction of the bubble departure [18] (reproduced with permission from Ref. [18], copyright 2015, John Wiley and Sons). (b) Top-view schematic illustration of a LM droplet, with the arrangement of ions forming an electrical double layer, induced by ionic variations, on the surface. The actual experimental setup with a schematic of the position of the droplet, two channels, and different types of electrolytes is also enclosed on the right side. Scale bar, 5 mm. (c) Schematic of the droplet deformation ratio measurements for D1/D2 assessment. (d) The demonstration of Marangoni flow when there is no deformation observed for the LM droplet (e) Selected enlarged images showing six typical types of motion in response to the various concentrations of NaOH (base) and HCl (acid), which can be assigned to three regions (deformation, Marangoni flow, and both deformation/Marangoni flow). The blue arrows indicate droplet deformation towards NaOH, while red arrows show the Marangoni flow direction towards HCl. Scale bars, 1 mm [17]. (Reproduced with permission from Ref. [17], published by Springer Nature under the Creative Commons CC BY license.)

have the ability of deep penetration into most materials including bio-materials. The magneto-active smart materials show great potential applications for microfluidics, bioengineering, and healthcare [83]. Although the Ga-based LMs are almost unresponsive to permanent magnetic fields at room temperature, as conductive fluids, they are still able to be driven by the Lorentz force [84]. As shown in the example presented in Fig. 8(a), an EGaIn LM droplet without any added magnetic element can be actuated by moving a permanent magnet nearby the droplet, and the selfrotation and circular motion were both observed. The force and torque, as the results of the interaction between the induced eddy current and the rotating magnetic field, drive the LM droplets moving towards the direction of travel of the magnet [85]. By applying an external alternating magnetic field to an EGaIn LM, various physical phenomena, including exothermic behavior, controlled locomotion, electromagnetic levitation, and shape transformation of LM, can be observed [86]. As seen

in Fig. 8(b), a droplet of LM positioning above the center of a magnetic coil, has been shown to exhibit ‘stand-up’ reversible deformation behavior when turning the alternating magnetic field on or off. This indicates that the eddy currents induced by the alternating magnetic field in the LM, according to the Lenz’s law, can enforce a sufficient repulsive force for the LM droplet to overcome gravity [86]. As another common strategy to achieve manipulation by a magneto-stimulus, physical coating of ferromagnetic materials on the surfaces of LMs to form LM marbles has been used for studying magnetic field responsive ability [87–89]. The introduction of magneto-stimulus responsive properties allows them to change the motions and shapes by the application of the magnetic materials as a force mediator under the magnetic field. Dispersing ferromagnetic materials, even in the form of nano or micro particles into the liquid phase is another synthesis route to magneto-responsive LM marbles [90–95]. A typical example is about mixing Fe particles with EGaIn LM to form a biphasic 9

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Research: Review FIGURE 7

(a) Schematic illustration of the experimental set up and mechanism for the self-actuation of LMs via redox reaction by forming a galvanic cell system with LM and copper as electrodes and NaOH solution as electrolyte [82] (reproduced with permission from Ref. [82], copyright 2016, American Chemical Society). (b) Hydrogen peroxide-induced deformation of LM immersed in an alkaline solution on a glass plate and a side view of the contact angle in this case. (c) Deformation of LM immersed in an acidic electrolyte on a graphite substrate with the addition of H2O2 and a side view of the contact angle in this case. (d) Fractals observed for LM immersed in an alkaline solution on a graphite plate and its contact angle. (e) Schematic diagram of electron transfer between the graphite surface and the LM, forming a LM-graphite galvanic cell [76] (reproduced with permission from Ref. [76], copyright 2018, Elsevier). (f) Schematic illustration of LM fractals created by adding acid to CuSO4 solution. (g) Diagram of ionic distributions in a Cu–Ga galvanic cell and the direction of surface convection of the bulk LM, where the surface potential of the LM droplet was altered due to the formation of the galvanic cell, resulting in the appearance of a surface tension gradient [77] (reproduced with permission from Ref. [77], copyright 2018, American Chemical Society).

colloidal suspension, as shown in Fig. 9(a). In the presence of a magnetic field, these inner colloidal magnetic particles align along the magnetic field, providing deformability of the LM marbles, also leading to increased viscosity (Fig. 9(b)) in comparison to the condition without applied magnetic field [90]. Most of the ferromagnetic particles have low solubility in Ga-based LM, but even a relatively low concentration may result in good magneto-induced applications. The developed ferro-fluid containing Gd nanoparticles (Fig. 9(c)) has displayed spontaneous magnetization and a large magnetocaloric effect. This biphasic LM-based ferro-fluid offers promise for applications in future fluidic magnetocaloric devices, which can absorb heat when exposed to a negative magnetic field strength gradient, and release heat in a positive gradient mode [91].

Light-induced responsive properties Light, as a high frequency electromagnetic radiation, can also be applied to materials remotely for their stimulation. In addition, the use of light offers unique advantages in tuning the external stimulus, including regulation of light intensity and frequency, as well as fine-tuned control of the irradiation direction, position, area and duration. Generally speaking, for metals, light is either re-emitted or reflected when the electromagnetic energy interacts with the electronic band structure, depending on the frequency of the incident light. The plasma frequency, namely, the frequency of oscillation of an electron cloud, for the Ga-based LM lies in the energy range that is comparable to ultraviolet (UV) light [6,96,97]. Therefore, the response of a Ga-based LM to light in the visible and infrared

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FIGURE 8

(a) Locomotion of LM droplets under a rotating magnetic field, in comparison to the motion of solid metal spheres; the schematic mechanism of the forces induced on the LM in the system is also demonstrated [85] (reproduced with permission from Ref. [85], copyright 2018, Royal Society of Chemistry). (b) Alternating-magnetic-field (AMF) induced manipulation of LM blobs, and the demonstration (bottom) of AMF-modulated reversible deformation behavior of LM (LM volume: 100 lL; AMF strength: 250 Am1, 245 kHz) [86] (reproduced with permission from Ref. [86], published by Cell Press under the CC BY-NC-ND license).

FIGURE 9

(a) Photographs of EGaIn containing different amounts of Fe with and without an applied magnetic field; the addition of Fe in EGaIn induce the changes on viscosity of samples, resulting in a transformation from liquid to solid when the volume fraction of Fe in EGaIn reaches 42%; and the samples containing Fe exhibit magnetorheological effect in response to the magnetic field. (b) shear stress response to a magnetic field of EGaIn containing
40% Fe [90] (reproduced with permission from Ref. [90], copyright 2017, American Physical Society). (c) Schematic illustration of the magnetocaloric refrigeration process based on the developed magnetic LM [91] (reproduced with permission from Ref. [91], copyright 2017, American Chemical Society).

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Research: Review

region is reflection, so that it behaves like a mirror. When the photons with wavelengths shorter than UV light penetrate through the Ga-based LM, localized surface plasmon resonances may occur as a result of the interaction between the free electrons and the incident light [98]. It is attractive to design plasmonic devices based on these soft LMs, and well-defined plasmon resonance extinction peaks have been demonstrated with a film covered by nanoparticles smaller than 100 nm [99]. Strategies involving particle size adjustment [100,101], coupling nanoparticles [102], and surface oxide thickness control [103] have been utilized to successfully manipulate the resonance frequency, even shift the resonance peak from the UV region towards the visible region. The heat or thermal effect generated by light irradiation is another common stimulus–responsive result, which also can be applied to manipulate Ga-based LM. It has been demonstrated that EGaIn-based nanodroplets show a great morphological transformation in response to simple light irradiation of the aqueous dispersion [104,106]. Actually, as shown in Fig. 10(a), the light applied on the surface of LM in the aqueous solution triggers a localized chemical reaction in which the Ga-based nanospheres convert to high-aspect-ratio Ga-oxide nanorods [104]. Taking advantage of this photochemistry strategy, more complex LM-based functional materials could be obtained. With irradiation by UV light, a photo-polymerization reaction could take place at the surface of an EGaIn droplet, forming a LMbased nanocapsule with high water dispersibility (Fig. 10(b)). In

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response to near-infrared (NIR) laser irradiation, as shown in Fig. 10(c) and (d), the LM nanocapsules can generate thermal energy and reactive oxygen species, resulting in the destruction of the capsule structure. Using these photo-stimulus responsive properties, applications including remotely controlled drug releasing, optical manipulation of a microfluidic blood vessel, and spatiotemporally-targeted marking in organs and in a living mouse have been demonstrated based on LM droplets [105]. Constructing hybrids of Ga-based LMs with other functional materials can further enhance the photo-induced effects. For example, the addition of Mg to the EGaIn LM results in a 61.5% increase in photo-thermal conversion in comparison to the pure EGaIn LM, enabling it to display efficient photothermal therapy of skin tumors (Fig. 11(a) and (b)) [107]. The formation of a new intermetallic phase of Mg2Ga5 in the LM marbles was believed to enhance the photo-thermal effect [107], but the detailed mechanism is not completely understood. Photo-induced locomotion can also be realized by combining the photo-active semiconductors with LMs. As shown in Fig. 11(c), LM marbles coated with WO3 nanoparticles could exhibit photocatalytic properties in response to light irradiation with a wavelength smaller than 460 nm. As a result of a photocatalytic reaction in H2O2 solution, oxygen bubbles that generated in the localized area from the decomposition of H2O2, cause a rolling force to actuate the LM marble towards the opposite side from where the bubble evolution is occurring [108]. Altogether the motion induction of LMs using light irradiation

FIGURE 10

(a) Shape transformation of LM in aqueous solutions. The morphology is transformed from nanospheres to nanorods, due to the localized chemical reaction triggered by the light irradiation [104] (reproduced with permission from Ref. [104], copyright 2017, American Chemical Society). (b) Schematic illustration of polymeric encapsulation of LM particles under 254 nm ultraviolet irradiation; (c) visible and thermographic images of a LM droplet before and after irradiation with a 785 nm laser, as well as the thermal response of the LM solution; (d) thermal expansion of a laser-driven LM droplet in air and in solution [105] (reproduced with permission from Ref. [105], published by Springer Nature under the Creative Commons CC BY License). 12 Please cite this article in press as: L. Ren et al., Materials Today, (2019), https://doi.org/10.1016/j.mattod.2019.10.007

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FIGURE 11

(a) Schematic illustration of Mg-EGaIn in vivo cancer photo-thermal therapy; (b) the thermal response of 2 g EGaIn and 2 g Mg-EGaIn after irradiation with an 808 nm NIR laser [107] (reproduced with permission from Ref. [107], copyright 2018, John Wiley and Sons). (c) SEM image of a WO3-coated Galinstan marble, with the inset showing higher magnification (top) and schematic illustration of the light-induced motion of such a LM marble (bottom) [108] (reproduced with permission from Ref. [108], copyright 2013, AIP Publishing).

offer great practical opportunities for the development of makeshift optical and electronic systems, which still need further explored.

Multi-stimuli responsive behaviors Exploring the synergistic effects of multi-factor stimulation is an interesting topic for the design of LM based units as smart materials. The development of LM systems that are responsive to multiple stimuli is particularly attractive, as in such systems, the extra stimuli can help in enhancing the responsivity and improving the degree of precision. Perhaps, the first issue to be considered in multi-stimuli responsive behavior of LMs is the possibility of their manipulation via regulation of surface properties and interfacial tension. In this context, the competition between surface tension modulation, density, and gravity plays an essential role in stimulus–response behavior. During the stimulus–response process, the gravity applied on LMs generally remains unchanged. The density of LMs and their hybrids, however, as one of the possible parameters affecting the response (Fig. 1), indirectly brings the impact of gravity into the equation. The density can be initially designed by the choice of metals to be mixed together. The addition of different nano- and micro-particles can also change the density, depending on the wetting and alloying properties of the added components [90,109,110]. During the stimulation, the density can be manipulated via applying certain stimuli, such as temperature [111] and voltage that influence the electrical double layer and surface tension [112], as well as other stimuli.

In addition to the considerations regarding gravity and density, only few reports have so far focused on exploring the possibility of multi-stimuli responsive behavior of LMs and their hybrids. One representative example is the realization of precise control and locomotion of LM hybrids under the synergistic effects of the combined magnetic and electrical stimuli [113]. In this work, the LM hybrid was a composite made by modifying LMs with copper–iron magnetic nanoparticles. Therefore, the obtained LM hybrids could both exhibit motion under electrochemical control and also show rapid responses to an applied magnetic field. As a demonstration of advanced behavior, in contrast to sole stimulus–responsive systems, the interworking of both electric and magnetic fields enabled the precise manipulation of the actuation direction within a cross-linked channel, as well as climbing type locomotion. This design presented the potential for developing soft tools for surgery and drug delivery, which could be untethered and remotely controlled. Altogether, the exploration of applying multiple stimuli on LMs represents a great avenue to extend the regulation range and responsivity of LMs that may also offer unexpected responsive properties.

Applications as smart materials and systems Based on the stimulus–responsive properties, many applications, including those in engineering and biomedicine, have been developed. For depicting representative applications of the stimulus–responsive properties of LMs and their hybrids, selective demonstrations close to the practical applications, such as smart electronics, smart mechanical devices, and bio-medicine, are 13

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discussed in this part. The aim has been to present the unique properties and advantages of LMs for these applications, especially LM-based smart machines or devices.

Smart electronics

Research: Review

High electrical conductivity combined with the stimulus– responsive properties of Ga-based LMs make them suitable as key components in manufacturing soft electric components and sensors. Switches and resistors are the main components of electronics. As we have discussed so far, LMs can be deformed and their location can be controlled by mechanical, magnetic, electrical stimuli and light irradiation. The deformation and displacement can be used for either opening or closing the conductive lines between two contact points as a switch. The resistance between the two points can also be controlled by changing this distance or oxidation of the LM surface and consequently create resistive elements that are a function of the applied electrical modulation, imitating the operation of a transistor [114]. Changeable capacitors and antennas are other components that have so far been demonstrated with stimulus-controlled LMs [13,37,115–118]. Sensors made of LMs, depending on the choice of channel geometry and constitution of embedded droplets as well as the encapsulating elastomer, can display highly tunable sensitivity for various applications ranging from tactile sensing to sensing the sudden changes of acceleration in car accidents [119–121]. Fig. 12(a) shows an intricate design for a soft tactile diaphragm pressure sensor based on microfluidic channels filled with galinstan LM encased in an elastomer. Utilizing a Wheatstone bridge circuit design (Fig. 12(b)), this pressure sensor can measure small differential resistance changes arising from the deformation of LM [122]. Moreover, this Wheatstone bridge design also provides temperature self-compensation in the operation range of 20–50 °C, which is critical for practical use, considering that the temperature could have a significant influence on the performance of resistive pressure sensors, such as the body changes in wearable sensing. Building capacitors based on LMs with dielectric materials is another choice to realize a mechanical response, either in self or mutual capacitive modes [115,123,124]. Fig. 12(c) and (d) present a simple but effective stretchable capacitive sensor for torsion, strain, and touch by using double helix LM fibers. The capacitance between the two LM filled fibers can be predictably changed by shifting the geometry in response to twisting or elongating of the fibers. Besides the field of sensors, LM-based smart systems have also shown the potential for harvesting mechanical energy and its conversion to electrical power. For instance, a constructed LM-based triboelectric nanogenerator [125], which allows total contact between the metal and the dielectric, has shown to generate high power in response to a vibration, demonstrating as high as 70.6% instantaneous energy conversion efficiency. As mentioned in the previous part, the self-healing behavior of LM is a valuable property for practical applications. For example, an autonomously electrically self-healing composite has been achieved by building a novel material architecture composed of LM droplets suspended in a soft elastomer, exhibiting a good resilience to repeated mechanical damage [46]. With

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above percolation-limit distribution of LM droplets in the soft elastomer, when a local mechanical damage is applied, the liquid droplet units would rupture and re-route new electrical connections with the neighboring droplets without interruption (Fig. 13(a) and (b)). Such advanced smart self-healing conductor matrices offer great potentials for power and data transmission systems that can instantaneously repair themselves and provide undisrupted operation. Besides these LM-based soft smart electronics, for some particular electrical systems, the mechanical properties of the conductive matters are also expected to be regulated. For example, biological materials display highly diverse combinations of mechanical properties, from rigid bones to very soft tissues, which mean that the mechanical properties of bio-electronics should match such a complex system. As shown in Fig. 13(c) and (d), an advanced LM-based magneto-active electrode with good electrical conductivity and reversible regulation properties for both viscosity and stiffness was designed and built [126]. This electrode could become rigid enough under applied magnetic field for penetration of the membrane during implantation. Without the applied magnetic field, however, it became soft and bendable. This magnetoactive composite was formed by dispersing magnetic iron particles in a Ga-based LM matrix, and the range of the corresponding Young's modulus can be regulated from kPa to GPa under the magnetic field stimulus (Fig. 13(e)). The operation principle of the modulus change for LM-based magneto-active materials is similar to what happens in a traditional magnetorheological fluid. This example depicts how an efficient design and application of mechanically stimulus–responsive bioelectrodes with multi-functionality for different types of biosystems and interfaces can be achieved using LM hybrids built electronics. It is also believed that similar possible smart behaviors of the LMs and their hybrids could offer the opportunity for novel electronic designs with compatibility to both soft robotics and human tissues.

Smart mechanical devices Their possible stimulus-driven actuation and shape-shifting properties enable the Ga-based LMs to play unique roles in constructing intelligent in soft robotic actuators and self-driven machines. As shown in Fig. 14(a), a LM enabled pump was designed based on the regulation of surface tension by application on electrical stimuli. The imbalance of the surface tension, after applying a potential, induces a pressure difference across the surface of the Galinstan droplet, resulting in a flow motion (Fig. 14(c)) which can be explained by electrowetting [16]. Gradually a thin layer of Ga-oxide is electrochemically formed on the downstream hemisphere of the droplet, reducing the surface tension on that side and eventually halting the motion. Instead, by applying an alternating voltage, to avoid the formation of the oxide layer on the Galinstan surface, continuous pumping could be achieved, as demonstrated in Fig. 14(b). Moreover, without the channel for LM droplet actuation, Yuan et al. developed an autonomous oscillation system by creating a contact between a copper wire and the LM [70]. Such an oscillation could be easily regulated by touching a steel needle on the LM surface, which

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FIGURE 12

Soft smart sensors using LM. (a) Optical image of a microfluidic diaphragm sensor based on a Wheatstone bridge design. (b) Schematic layout of the diaphragm sensor and the equivalent circuit (upper); simulation of the normal stress for the radial sensing grids and the tangential sensing grids under applied pressure, as well as a schematic diagram indicating the testing conditions (bottom) [122] (reproduced with permission from Ref. [122], copyright 2017, John Wiley and Sons). (c) LM based torsion sensor which changes the capacitance between the intertwined LM filled fibers when the geometry is changed by twisting. (d) Schematic illustration of this LM based torsion sensing mechanism [115] (reproduced with permission from Ref. [115], copyright 2017, John Wiley and Sons).

provides a unique strategy toward fabricating self-fueled oscillator machines with no rigid bodies. A more complicated wheeled robot with the operability to move the robot outside the liquid environment has been developed by using LM droplets (Fig. 14(d) and (e)). The LM droplet inside the robot is actuated using a voltage to alter the robot's center of gravity, which in turn generates a rolling torque and induces continuous locomotion at a steady speed [127]. Similar modulations have been applied to different smart mechanical systems, such as soft actuators [128,129], electrical switches [114], and delivery [130], while nearly all optimum operation

environments are neutral or alkaline. It was demonstrated that the Galinstan droplets in acidic solutions weakly affected by electrowetting effect, since Marangoni-effect induces a rotation that competes with deformation. Many of the mechanically actuated devices based on Ga-based LMs are driven by electrical and/or chemical stimuli. However, it is a significant challenge to remotely control LM droplets using electrical and chemical stimuli, and acceptable deformation and motion of LM soft robots only appear in aqueous environments, which would somewhat limit their functionality for various needs. Talking remote stimulation limitation,

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Research: Review FIGURE 13

(a) Samples of as-prepared stretchable LM-elastomer that remains electrically conductive after inducing severe mechanical damage; (b) schematic illustration of the self-healing mechanism in response to different damage mitigation strategies [46] (reproduced with permission from Ref. [46], copyright 2018, Springer Nature). (c) Demonstration of the penetration of a bio-electode built from Fe/LM hybrids without and with a magnetic field applied. (d) Electrochemical current response to this electrode inserted into and withdrawn from the electrolyte (LMMS is used as an abbreviation for the as-prepared liquid–metal-based magnetoactive slurries, L50% means that the weight percentage of large iron particles in the composite is 50%). (e) The magnetic field dependence of G0 and the complex viscosity at 0.2% strain amplitude for a sample containing 30% large-size Fe particles (LMMS-L30%) [126] (reproduced with permission from Ref. [126], copyright 2018, John Wiley and Sons). 16 Please cite this article in press as: L. Ren et al., Materials Today, (2019), https://doi.org/10.1016/j.mattod.2019.10.007

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FIGURE 14

(a) Working mechanism of the LM enabled pump. Schematic diagrams of the experimental setup, the Galinstan droplet surface charge distribution when placed in the droplet chamber in a NaOH solution, and the Galinstan droplet surface charge distribution when an electric field is applied between the graphite electrodes. (b) Sequential snapshots of the pumping effect of a Galinstan droplet. (c) Calculations of the formation of vortices along the droplet surface coloured by the velocity magnitude of the flow [16] (reproduced with permission from Ref. [16], copyright 2014, National Academy of Sciences). (d) Working mechanism of the wheeled robot, and schematic illustration of the driving module and the operation mechanism. (f) Sequential snapshots of the actuation of a real wheeled robot moving at a steady speed, with the inset showing the time-displacement plot [127] (reproduced with permission from Ref. [127], copyright 2018, John Wiley and Sons).

photo-induced locomotion from a distance, has been realized by combining the photo-active semiconductors with LMs [108]. A stable and efficient photocatalytic process is highly dependent on hybrid semiconductors, however, which may require complex triggering conditions. A recent attempt to achieve remotely driven actuation has been successfully realized by utilizing ultrasound waves as stimuli. As illustrated in Fig. 15(a), upon exposure to an ultrasound field, the generated acoustic radiation force in the levitation plane propels an as-prepared Ga-based core–shell nanorod to move autonomously [131]. Analysis of the propulsion mechanism (Fig. 15(b) and (c)) shows that the main driving force is the primary acoustic radiation on the asymmetric shape of the liquid Ga nanorods in the levitation plane, instead of the acoustic streaming force. Another recent example of remotely controllable mechanical motion is utilizing magnetic driving forces to realize climbing locomotion during the parallel motion of the LM droplet, which exhibits magnetic responsiveness after modifying the droplet with copper–iron magnetic nanoparticles (Fig. 15(d)) [113].

These designs all made some modification to the Ga-based LM and successfully introduced stimuli-responsive properties, which present viable pathways for the controllable soft-mechanical devices. Nevertheless, the manipulations of LM-based actuator, or robot, still need to be further improved in terms of intelligence and reliability for practical applications.

Bio-medicine Soft structures are always priority materials in bio-applications and as such LMs offer themselves as potential candidates in bio-systems. Unlike the toxic Hg, Ga-based LM presents much better biocompatibility. As a liquid at or near room temperature, which is distinct from the conventional grid metal, Ga-based LM shows a great combination of conductivity and transformability, and therefore, it offers more possibilities to expand its utilization in many bio-applications. A representative example for the Gabased LMs as smart materials for bio-applications is the drug delivery that has been demonstrated by GaIn LM nanoparticles. This LM transformable nanocarriers [132], as named by Lu et al. 17

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(a) Schematic illustration and time-lapse images of acoustically propelled rodlike LM Ga nanomachine (LGNM) for actively seeking and targeting cancer cells. (b) Schematic illustration of the autonomous motion of the asymmetric Ga nanomachine in the levitation plane by acoustic energy that is generated at the base of the cell. (c) Time-lapse image showing the trajectories of two LGNMs (red lines) over 4 s in response to ultrasound. The blue arrow lines represent the directions of motion of the traced liquid Ga particles. Scale bar, 20 lm [131] (reproduced with permission from Ref. [131]. copyright 2018, American Chemical Society). (d) Demonstration of climbing locomotion of the copper–iron magnetic nanoparticles/LM hybrid droplets (FLM: ferromagnetic liquid metal) with the assistance of a magnetic force [113] (reproduced with permission from Ref. [113], copyright 2019, John Wiley and Sons).

(as shown in Fig. 16), is a core–shell nano-structure with liquidphase GaIn alloy as its core and thiolated polymer as the shell. This structure can be easily synthesized by ultra-sonication and tailored through ligand-mediated self-assembly. After accumulating in the tumor site through binding by a tumor-target ligand, after the oxide layer of the LM nanostructure dissolves in the mild acidic environment, the drug-loaded nanocarriers with an initial average diameter of about 100 nm fuse into large nanoaggregates and decompose in 72 h. The regulation of the oxidation layer of LM, in this case, both helps building soft motions for spreading in the organic system and is also used for stimuli triggered switching that can separate or expose the metal core from the outside. The endocytosis and fusion process of the LM nanocarriers in the cells were confirmed and observed by transmission electron microscopy (TEM) [132]. Herein the biochemical environment induces fusible and degradable behavior that provides remarkable strategies for smart drug delivery and tissue engineering. Light, mainly NIR light or laser irradiation, is also used as a stimulus to activate the Ga-based LM nano-carriers, driving the drug delivery process. Lu et al. prepared stimuli-responsive LM hybrids nanodroplets by mixing EGaIn nanoparticles with graphene quantum dots photosensitizer to mechanically disrupt the cellular structures by a remote trigger [104]. Upon the light irradiation, the morphology of the LM hybrid nanodroplets transformed from zero-dimensional (0D) spheres to onedimensional (1D) hollow nanorods, which can physically disrupt the endosomal membrane to promote the endosomal escape of the payload (drugs). Meanwhile, Chechetka et al. used 30 min of NIR laser irradiation to directly break the structure of EGaIn

nanocapsules for drug delivery and released carmofur encapsulations inside the EGaIn carriers [105]. The shape transformable response under varying external stimuli of these LM and hybrids offers a novel means to achieve spatiotemporally controlled intracellular drug delivery, considering the low toxicity of Gabased LMs. In addition, the thermal effect induced by the light irradiation in the LM and its hybrids has been successfully used for cancer therapy [133]. Functionalized EGaIn nanocapsules were developed for photothermal therapy of a tumor, which was completely removed on the third day after laser irradiation [105]. The temperature change of these EGaIn nanocapsules in the bio-system can also be readily controlled by the irradiation power, irradiation distance and the dosage of the formulation, and importantly, without evoking many side effects. The phototherapy effect of the Ga-based LM was much more efficient than that of those reported with nanocarbons or gold nanomaterials, and moreover, it has also been proved that the light induced photo-thermal conversion can be further enhanced by the addition of other materials (e.g. Mg [107] or silica [134]) to Ga-based LMs.

Outlook and conclusion Materials featuring both metallic and fluidic characteristics, which are capable of creating responses to external stimuli, no doubt represent one of the most exciting areas of scientific interest and possess strong potentials for many unexplored applications. We have reviewed and discussed representative manipulations of Ga-based LMs and the applications of these

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FIGURE 16

(a) Schematic of the transformable LM nanodroplets drug delivery system, including the preparation, delivery process, targeted cancer therapy, and acidtriggered fusion and degradation process. Chemical structures of thiolated (2-hydroxypropyl)-b-cyclodextrin (designated MUA-CD) and thiolated hyaluronic acid (designated m-HA). (b) In vivo fluorescence imaging and ex vivo fluorescence imaging of the tumour and normal tissues, with the related analysises also included [132] (reproduced with permission from Ref. [132], published by Springer Nature under the Creative Commons CC BY License).

stimulus–responsive properties. Continuous and vigorous developments in this area are expected and exciting results yielding many innovations are also highly anticipated. Table 1 presents

a summary of selected representative reports on stimulus– responsive mechanisms applied to LMs and their hybrids, presenting some of their key advantages and disadvantages.

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TABLE 1

Summary of past reports on major stimulus–responsive behaviors and mechanisms of LMs and their hybrids. Stimulus

Mechanism

Subjects

Application

Advantages

Disadvantages

Ref

Temperature

Thermal expansion

Liquid Ga

Height of liquid Ga inside a carbon nanotube varies linearly and reproducibly in the temperature range of 50– 500 °C Shape can be changed and resumed to the original state again

Suitable for use in a wide variety of microenvironments

High-level of difficulty for processing

[30,135]

Easy to construct different shapes in large magnitude (11 times) transformation Easy to manufacture stretchable and flexible superconductive devices Quick response (few seconds) and colossal resistivity changes (more than 4  109 times)

More thermal expansion is needed

[32]

Low operating temperature(- 266.4 °C)

[35]

Low operating temperature (61 °C)

[36]

EGaIn-silicone elastomer Research: Review

Phase change

EGaInSn

EGaIn dropletsilicone polymer

Mechanical

Electric field

Chemical

Magnetic field

Fluidity

Surface tension regulation in the form of electrocapillarity Surface tension regulation in the form of electrochemical reactions

Exhibit no-resistivity electrical loss of the as-printed coil under the critical temperature for superconduction Reversible liquid–solid transformation and volume expansion

LMs-soft container (mainly soft polymer) LMs or LM droplets in joint point

Ultra-stretchable and flexible electronics

Unlimited deformability and high conductivity

Possibility of leaking LM out of the container

[39–42]

Self-healing electrical conductivity

Fast recovery of conductive paths and highly stable healing ability

Mechanical force needs to be applied in a certain selected directions

[43–46]

LMs or LM marbles

Motion and shape shifting

Easy to control

Less complexity and diversity

[50,51]

LMs or LM marbles

Motion and shape shifting

Large deformation and compliant movement style

Consuming Ga or producing other unexpected chemicals

[52,53,55– 57]

LMs

Motion and shape shifting

The surface of Gabased LM can be functionalized with a variety of surfactants

Less complexity and diversity

[63,69– 71]

Requiring unique devices with aqueous chemical surroundings Less control of shape and movement

[17]

Surface tension regulation via triggering reaction taking place at the surface pH imbalance and surface charge imbalance Surface tension regulation via forming galvanic cell

LMs

Motion and shape shifting

Highly controllable

LMsconductive substrate or LM marbles

Motion and shape shifting

Large deformation and compliant movement style

Lorentz force

LMs

Locomotion

Require rotating magnetic field or alternating magnetic field

[84–86]

Ferromagnetic force

LM marbles or composites containing ferromagnetic matter LM hybrids containing magnetic matter

Viscosity and stiffness change, motion,

Only use LMs, no need for the assistance of magnetic materials On-demand movement or change in reversible mechanical properties

Most of the ferromagnetic particles have low solubility in Ga-based LMs

[87–90]

Bonus fluidic property in comparison to traditional magnetocaloric

Relatively low efficiency

[91]

Magnetocaloric effect

Temperature change

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[76,77,82]

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TABLE 1 (CONTINUED)

Stimulus

Mechanism

Subjects

Application

Advantages

Disadvantages

Ref

Light

Surface plasmonic effect

Light induced heating effect

Light induced chemical reactions (photo-induced electron-hole generation)

LM nanodroplets or surface modified LM nanodroplets LMs or LM marbles

Light emission

Soft platform with surface for plasmonic applications

Most are UV range plasmonic resonances for Ga-based LM droplets

[98,99]

Temperature and morphology changes

The stimuli can be applied remotely at a localized area

[104–107]

LMs or LM marbles

Motion, morphology change

Precisely switch the reaction on/off

Only applying to the surface of LM, the core, with the major percentage of total weight is not very useful Relatively low efficiency

Although increasing advances regarding the development and applications LMs have been made, research gaps are still aplenty in the field. Many fundamental questions on the reported stimulus–responsive behaviors of LM-based smart materials remain unanswered and require further in-depth studies. Moreover, as an emerging smart material, the current discoveries regarding stimulus–responsive properties of LM systems are still insufficient, the fine manipulation of LM by an applied stimulus is still full of challenges, and the integration of different stimulus– responsive modes and the construction of highly intelligent systems are still lacking. We saw that the precise control of LMs for many applications is still a challenge to be met. High surface tension of LMs limits the creation of a variety of morphologies, and also results in the unintended spread of the applied force across many areas of the bulk of LMs. This means that lack of precisions in stimulation should be tacked in future investigations. We also saw that most of the methods for the manipulations of LMs are still controlled by applying a stimulus (e.g. mechanical, chemical and electrical) at close ranges. However, this limits their functionality for various requirements. For precisely and remotely manipulating LMbased materials and systems, in order to meet the requirements of certain applications, stimulus like magnetic field and light irradiation would be ideal choices. Some demonstrations have been established magnetic field and light as stimuli to manipulate LMs with sensitive and reversible stimuli-responsive ability. Nevertheless, building such smart systems remains elusive. To achieve the next frontiers, high complexity of design and synthesis should be implemented to LMs to allow the creation of fine stimulus–responsive entities that meet the integration requirements, and also more work is needed for understanding of the mechanisms underlying the stimulus–responsive activities and for devising strategies for improving the efficiency of the whole system in different applications. Another important extension of the research on LM-based stimulus–responsive smart materials is exploring and developing multi-stimuli responsive LMs and hybrids. There are some considerations in this context. The developments of multiresponsive LM-based systems that can be actuated and be

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devices

[108]

disrupted on demand by an independent stimulus are desirable and would enable advanced applications for these materials in the future. There are many current examples in which Ga-based LMs are incorporated and respond to different stimuli, including mechanical switches, thermometers, thermocouples, and a number of high precision scientific instruments. There are also great opportunities for commercialising systems that are based on LMs that are controlled by stimulation in the near future. For example, pumps [16] and mixers [128] using LMs can be implemented into point-of-care systems for creating small-scale diagnostic systems, which take samples of body fluids and can be connected to mobile phones. Pumps and mixers based on LMs are indeed low-cost and efficient in comparison to many of the current microfluidic technologies. Another possibility of using stimuli is for the actuation of LMs for writing electronic tracks with dimensions smaller than is possible with current technologies. Actuation of LMs in either electrolytes or air can take advantage of surface tension for creating ultra-fast and fine printing of electronic tracks [136]. Such tracks can also be printed on the top of each other by controlling their positions using magnetic, electrochemical, and mechanical stimuli. The manipulation of their surface oxides at the same time can be used for producing 3D electronic tracks [136]. Stimulation of Ga-based LMs can also be potentially used in many pharmaceutical applications. LMs can be moved smartly and on-demand using electromagnetic, thermal, and mechanical stimuli around and inside the human body and then can be stimulated to release the drugs. Ga melts at 30 °C and is liquid inside the human body while offering compatibility with our body fluids. Solid metals cannot be as efficiently stimulated as we can excite the clouds of ions and electrons in LMs. Examples of such manipulations have been shown for applications that can be promoted to commercial levels in a relatively short time [131]. Certainly, stimulation of LMs can play important roles in the future of energy storage [137] and catalysis [138] by increasing their interaction efficiencies (mechanically and electromagnetically), especially in small dimensions [139]. These are just a few examples of future commercial and real-life possibilities where stimuli can play signifi21

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cant roles and many others can also be imagined as the main components of advanced shape-shifting intelligent components and intelligent soft robots. There is a long way to go for advancing LM based smart materials that can be nearly as good as the one shown in the ‘Terminator’ movie series, and in building such a supremely intelligent shape-shifting robot that can operate with agility and accuracy. The aim of the research in this field is not to make the sciencefiction fact, but once similar achievements have been made, revolutionary advances in materials science, as well as ubiquitous applications relevant to electronics, optics, materials science, chemistry, and medicine can be gained. The current progress on Ga-based LMs as smart materials shows an abundance of possibilities, but the definition of bio-compatible LMs should be further extended to go beyond Ga-based mixes with the investigation of many hybrids. The stimulus and response domains are expected to cover all domains, including mechanical, electrical, chemical, optical, and thermal. It is expected that by further understanding fundamental relationships between these stimuli and LMs, research on stimulus–responsive properties of LM can be strongly promoted in a search of new platforms that will help us in developing even smarter materials, serving this epochal technological revolution.

Acknowledgements The authors thank Dr. Tania Silver for critical reading of the manuscript. This work was financially supported by Australian Research Council (ARC) Discovery Projects (DP160102627 and DP170101467), ARC Linkage Project (LP180100722), an ARC Laureate Fellowship Grant (FL180100053), the International S&T Cooperation Program of China (2015DFA13040), and a BUAA-UOW, Joint Research Centre Small Grant. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]

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