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Article Cite This: ACS Omega 2019, 4, 2311−2319
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Large-Magnitude Transformable Liquid-Metal Composites Hongzhang Wang,†,⊥ Youyou Yao,†,⊥ Xiangjiang Wang,† Lei Sheng,§ Xiao-Hu Yang,§,∥ Yuntao Cui,§,∥ Pengju Zhang,§,∥ Wei Rao,§ Rui Guo,† Shuting Liang,† Weiwei Wu,† Jing Liu,*,†,§,∥ and Zhi-Zhu He*,‡ †
Department of Biomedical Engineering, School of Medicine, Tsinghua University, Beijing 100084, China Department of Vehicle Engineering, College of Engineering, China Agricultural University, Beijing 100083, China § Beijing Key Lab of Cryobiomedical Engineering and Key Lab of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China ∥ School of Future Technology, University of Chinese Academy of Sciences, Beijing 100049, China
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‡
S Supporting Information *
ABSTRACT: Most of the existing robots would find it difficult to stretch and transform all parts of their body together due to rigid components and complex actuation mechanisms inside. Here, we presented a highly transformable liquid-metal composite (LMC) that is easy to change shape in large magnitude and resume its original state again according to need. When subject to heating, part of the ethanol droplets embedded in the composite would change phase and then actuate. We demonstrate the flexible transformation of LMCmade octopus from a two-dimensional shape into several predictable three-dimensional shapes freely on a large scale (even up to 11 times its initial height) through remote wireless heating, which needs no sophisticated operating system at all. Further, several designed behaviors, such as movement of octopus and entangling objects of soft robots, are also realized. Theoretical analysis of the heating-induced liquid−vapor transition of the embedded ethanol droplet interprets the mechanisms involved. The present findings open a new way to fabricate functional transformable composites that would find significant applications in developing future generation soft robots. mers,19,20 liquid crystal elastomers,21,22 fluidic elastomer actuators,23 and pressurized fluids actuators.10,24 Among the many different soft actuation methods, liquid−vapor phase change materials especially present an attractive performance, such as high expansion strain and simple controls.24−26 Phase change of low-boiling point materials such as ethanol has been proven to be a better actuation method for making soft robots.27,28 However, it seems to be difficult to transform freely into more sophisticated shapes as desirable than inflating on the basis of only the joule heating. Besides, the flexibility, stretchability, and thermal conductivity of phase change material composites should improve significantly to fulfill the need. The room-temperature liquid metals refer to those materials with high conductivity of metals and good mobility of fluids that might be a building block for fabricating soft robots.29 In addition, liquid-metal silica rubber composites30−32 have attracted increasing attention because of their various potential
1. INTRODUCTION Robots are usually designed to be stiff, composed of intrinsically rigid materials, hard links, and joints, which limit their flexibility and deformability.1 However, living beings, from fully soft-bodied octopus to mammals with stiff endoexoskeleton or exoskeletons, all own soft bodies. In recent years, soft robots are emerging as a quickly growing field that focuses on mimicking elastic skin, muscle, and mollusk.2−4 Different from hard-bodied robots, soft robots with deformable structure are capable of bending, twisting, and shape-changing so as to adapt to various unstructured environments.5−7 Earlier, a series of untethered soft robots and mollusk robots have been developed,4,8 including wormlike robot,9 caterpillarlike robots,10,11 octopus-like robots,12−16 etc. As is well known, actuation is one of the main challenges in developing untethered soft robots. Especially, to achieve soft robots like octopus and jellyfish that can freely transform their morphologies and locomotion without relying on a rigid skeleton, soft actuation components deserve careful consideration and implementation. So far, there has already been a variety of ingenious methods to actuate the soft robots, including pneumatic actuators,17,18 shape memory poly© 2019 American Chemical Society
Received: December 11, 2018 Accepted: January 11, 2019 Published: January 30, 2019 2311
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Figure 1. Fabrication process and transformation behaviors. (a) Fabrication process of the liquid-metal composites (LMC). (b) High stretchability of LMC (700%). (c) When heated by a heat gun for 30 s, LMC forms a large bubble shape, whereas the control group (without LM) shows no significant shape change; the scale bar is 2 cm. (d) Elasticity modulus of LMC with different LM weight ratios. (e) Local deformation of LMC in response to the change of heat source location. (f) Through selective heating, the local area of LMC swells sequentially and forms the 3D shape.
operating system. We show the characteristics of the composites and present theoretical interpretation on the transformation mechanism. With this material, we designed and evaluated a group of entangling behaviors and temperature-controlled movements of wormlike robots.
applications and promising properties, including outstanding softness and super elasticity,33−35 enhancing fracture energy,36 high thermal conductivity,30 capability of wireless heating, and three-dimensional (3D) printing feature in multiple styles.37 With outstanding versatile capabilities, liquid-metal composites (LMC) might be well suitable for fabricating soft robots. However, one of the toughest bottlenecks for fabricating soft robots with these is the lack of inherent actuation components. Up to now, there have been no reports about introducing the phase changing materials in the liquid-metal composites. Here, we developed liquid-metal composites that are capable of freely transforming and stretching any parts for performing complex and desirable deformation in response to wireless heating. Such stretchable liquid-metal composites (LMC) are composed of liquid metal (gallium−indium, eutectic; EGaIn) mixed with silicone elastomer and then blend with liquid ethanol to produce extremely large volume changes in response to thermal stimuli. The role of the liquid metal as introduced here remarkably improves the thermal conductivity up to 4 times higher and reduces the elastic modulus of LMC dramatically (43.1%) of the primary elastomer−ethanol composites, both of which contribute most to the free transformation capability. Combining the desired merits of all above materials together, we made a conceptual soft robot that demonstrates significant flexibility, quick response capability, and electromagnetic induction actuation and is freely transformable. We present transformation of LMC-made octopus from a two-dimensional (2D) shape into several predictable 3D shapes (even up to 11 times its original height) freely only with wireless heating and without any sophisticated
2. RESULTS 2.1. Fabrication of Liquid-Metal Composites. The liquid metal is mainly composed of 75.5% gallium and 24.5% indium by weight, which is dispersed in an uncured silicone elastomer. To achieve the soft actuation, the key component for soft robots, phase change agent ethanol, is chosen to realize large volume change via liquid−vapor conversion mechanism. Liquid ethanol has a low boiling point (78.4 °C) and is nontoxic, making it an excellent candidate for actuation. Liquid-metal composites (LMC) are fabricated through shearmixing the uncured liquid-metal elastomer with liquid ethanol (weight ratio 9:1), which is ready to cast and print into different shapes before solidifying. The LMC can be easily fabricated (within 2 min), which is shown in Figure 1a (more details are available in Section 5). The liquid-metal composites thus made display high stretchability (700%, Figure 1b). The microstructure when stretching is shown in Figure S1. Under heating condition, LMC swells and changes shape freely in response to the increasing pressure due to ethanol liquid evaporation. The liquid-metal loading plays an important role in the free transformation of composites. Ultrahigh deformability of LMC is depicted in Figure 1c and Movie S1, where LMC is more inclined to deform its body to a 3D shape compared with the control group. 2312
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Figure 2. Expansion of LMC in response to different heating methods. (a) When heated by microwave, the temperature of LMC with larger LM loading rises more quickly, as shown in the infrared image. The scale bar is 2 cm. (b) When heated on a hot plate, LMC presents an obvious hunching behavior, whereas the control group (without LM) shows no significant shape change. The scale bar is 2 cm. (c) The temperature of LMC with a 60% LM weight ratio rises much faster, compared to 30 and 0% LM weight ratios. (d) Thermal conductivity and heating rates of LMC with different LM ratios. (e) LMC swells in response to wireless heating with a magnetic coil.
to fracture, which can enhance the performance of soft actuators. In a word, liquid-metal inclusions are able to soften the material and open promising opportunities for the development of soft components. Programmable and desirable swellings are challenging capabilities to realize in soft actuation. Herein, controlled swelling of LMC can also be conveniently achieved through selective heating, which broadens the range of its application. Figure 1e demonstrates the local deformation of LMC in response to the change of heat source location. Therefore, it is possible to expand any parts of the composites and achieve programmable 3D structures as desired. Figure 1f shows that the planar LMC swells and uplifts sequentially in a specific area when subject to selective heating (Movies S2 and S3), and finally forms a 3D structure, which can be recovered to its original planar shape when cooled. 2.2. Demonstration of Thermally Responsive Capability. As mentioned above, LMC are able to deform reversibly in response to the heat stimulation. Various heating methods can be applied to actuate the reversible deformation of LMC. In this section, we applied four methods to heat the LMC, including a microwave oven, hot plate, heat gun, and a magnetic coil, all of which can lead to swelling and deformation. First, we come up with the experiments to utilize a microwave oven (Midea L1) to heat up LMC. It is well known that the bulk metal is hard to heat by electromagnetic radiation at microwave frequencies due to the fact that the electromagnetic penetration depth is on the order of microns and most of the electromagnetic energy is reflected. However, our experimental results indicate that the liquid-metal droplets effectively interact with microwaves for heat generation, which is similar to the observation in sintering of metal powders in a microwave field.38 For the microwave frequency f = 2.45 GHz,
As can be seen in the metalloscope micrography image (Figure S2a), liquid-metal droplets are encapsulated in the composites of silicone elastomer. Applying micro CT (computed tomographic, Carl Zeiss Xradia 410 versa), 3D structure of LMC can be scanned, as shown in Figure S2b. White points refer to LM droplets, which are distributed throughout the silicone polymer, along with the ethanol bubbles. The deformation mechanism of this composite can be attributed to the strong extrusion of ethanol evaporation, compelling the sealed elastomer to expand and transform (Figure S2c). Therefore, the high thermal conductivity and flexibility are both critical to maximizing the effectiveness of evaporation and deformation. At the same pressure from the phase transition, materials with a lower elastic modulus are able to generate the larger deformation. Elastic modulus within the limit of proportionality can be described as follows E = (F /S)/(ΔL /L) = σ /ε
(1)
F/S
where, ε = E , F is the force, S is the force acting area, L is the length, σ is the stress, and ε is the strain Therefore, under the same evaporation force, materials with a lower elastic modulus can produce larger strain and deformation. To develop robots with excellent deformation ability, materials with lower elastic modulus should be selected. Liquid-metal inclusions in LMC can decrease the elastic modulus significantly (Figure 1d), making it a better candidate for fabricating soft robots. LMC with 60% LM exhibited the best performance, which reduced the elastic modulus by 43.1%. Besides, high fracture energies of liquid-metal elastomer35 present good tear resistance in the expansion process. Even with extremely large swelling, LMCs are difficult 2313
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Figure 3. Demonstration of different kinds of soft robots: (a) a cylindrical shaped LMC is selectively heated and cooled along with swelling and contraction reversibly. (b) LMC-made octopus transforms from a 2D shape to 3D shape; the scale bar is 2 cm. (c) Temperature change-induced locomotion of the soft robot. When heated, the robot expands and elongates its body and the front leg moves to the right in response to the elongation. When cooled, the robot shortens its body and the posterior leg moves to the right as well as the whole body motion. The scale bar is 2 cm. (d) The process of the LMC robot grabbing objects with contactless heating.
liquid-metal conductivity σ = 3 × 106 S/m and the permeability of free space μ0 = 4π × 10−7 H/m. The released power of the interaction between liquid-metal droplets with microwave is strongly dependent on its radius. The maximum of the volume heat generation is obtained through optimizing the liquid-metal droplet radius39 with Rm = 2.4δ = 14 μm. The stirring time plays an important role in regulating the droplet size. In comparison with Figure S3a (stirring for 50 s), Figure S3b shows the LMCs that have been stirred for a longer time (500 s), whose droplet sizes are smaller. Due to the irregular shape of droplets, the areas rather than radius are calculated. The longer stirring time can reduce the liquid-metal droplets areas efficiently. Therefore, to achieve the best absorption efficiency of microwave energy, we can regulate the stirring time to acquire the appropriate droplet size. Compared with
the silicone mixed only with liquid ethanol, the released power of microwave to the liquid-metal inclusion is much higher and significantly improves the absorption efficiency, which leads to the temperature increasing much faster, facilitating the deformation of the elastomer matrix. To visually demonstrate the expansion effect of soft liquidmetal materials with different LM contents, a middle-level microwave power (600 W) is applied to heat six cubic LMC, of which the liquid-metal proportion ranges from 0 to 81%, as shown in Figure 2a. After heating for 90 s, the thermal images and temperature variations are recorded by an infrared camera, from which one can notice that, with a higher liquid-metal proportion, the ultimate temperature would be higher too. During the microwave heating process, if the temperature of the soft material exceeds 78.4 °C (boiling temperature of 2314
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specific region to expand and hunch again, which can be repeated many times if needed (Movie S4). Further, the LMC ball deforms reversibly after repeated heating and cooling via a heat gun. More interestingly, this soft ball is capable of changing the skin colors of their skin through the temperature modulation. Once temperature increases, the soft robots expand as ethanol evaporates and exposes the liquid-metal droplets inside, changing their color of appearance from gray to silver (Figure S5). When the temperature decreases, the robots contract along with color recovery. Therefore, the soft robot can control its color with temperature like the chameleon in nature. As we know, repeatability and the degree of extreme deformation are critical indicators of soft transformable robots. We record the repeatability of expansion and shrinking (Figure S6). In Figure S6, the blue points represent the initial height (1.4 mm) whereas the red points are the final height of uplifting after heating. The peak value of the uplifting height is 16 mm (more than 11 times the original height). The experiments demonstrate that the cycle of a higher expansion ratio (about 6 times of the original height) can repeat more than 24 times. The ethanol gas releasing from LMC limits the repeatability of expansion, which might be solved by sealing the LMC with airtight materials. These basic actions can be applied to realize controlled actuation as desired. A 2D LMC-made octopus inflates itself in response to the selective heating, and deformation between 2D and 3D structures can be repeated many times only through the temperature regulation (Figure 3b and Movie S5). In addition, we designed a soft robot consisting of LMC, which can move toward a certain direction driven by heating and cooling (Figure 3c and Movie S6). A schematic diagram of how this soft robot proceeds with locomotion is shown in Figure 3b. Both ends of a bar of LMC are inserted in a special socket, whose surface is carved with gear rack pattern. Specifically, the gear rack pattern slants to one side, causing smaller friction when the direction of relative motion is the same as that in which the gear rack pattern slants. The surface of the rail is carved with a similar pattern but in a complementary way. As a result, the friction force of moving toward the left is significantly smaller than moving toward the right. When the robot is placed on the rail and heated with a heat gun, its length would increase. Since moving toward the left is easier than moving toward the right, the force of expansion pushes the left end of the robot to the left, whereas the right parts just remain fixed due to the large friction of moving toward right. Therefore, the centroid of the robot moves toward the left. When the robot is cooled by spraying water, the force of shrinking would pull the right end toward the left rather than the left end to the right due to a friction force difference. The centroid of the robot still moves toward the left. With each circle of heating and cooling, the robot would gradually move toward the left, creating continuous locomotion. From the Movie S6, the robot moves via heating and cooling at a speed of 6 mm/min. The speed should be increased by improving the speed of operation (heating and cooling). This method offers a new way to actuate the movement of soft robots. The soft gripper is one of the important applications in soft robotics.40,41 On heating the target region with a heat gun, the local area would swell and deform controllably to manage different tasks. Figure 3d shows the process of the robot grabbing objects. This flexible disk can gradually fold and grab the object when heated because the two layers of the robots expand at different speeds due to the uneven heating (heat on
ethanol), the ethanol liquid would evaporate and inflate the LMC (Figure S4a,b). We also used a hot plate (150 °C) to heat LMC directly. From Figure 2b, the LMC expands and transforms, whereas materials without liquid metal do not although subject to the same heating time (180 s). The basic reason lies in that LMC with liquid metal warms faster compared to that lacking liquid metal when heated upon a hot plate, as indicated by infrared thermometry images (Figure 2b). Complex deformation behaviors of LMC are achieved by coupling the heterogeneous structure design with targeted heating. As shown in Figure S4c,d, when heated on a hot plate, the LMC sheet curves into a wave shape due to the designed uneven heat distribution. To further validate the assumption that liquid metal accelerates the warming effect, surface temperature variations of LMC with different weight ratios (0, 30, and 60%) are recorded with three thermocouples. We heated LMC for 180 s on a hot plate; the heating curves of three LMCs are shown in Figure 2c. Obviously, the surface temperature of LMC with 60% liquid metal rises most quickly and reaches the highest value (84 °C) compared with the other two. As expected, the temperature of LMC for lack of liquid metal rises slowly. Therefore, experimental results are consistent with the hypothesis that liquid metal enhances heating efficiency. With higher temperature, ethanol evaporates more easily, leading to quicker deformation. Figure 2d (right) shows the temperature rising rate of three materials containing different LM ratios in 90 s. The temperature rising speed of LMC with 60% LM (0.64 °C/s) is almost 4.5 times quicker than that of the materials without liquid metal (0.14 °C/s). Thermal conductivity of the three materials (0% LM, 30% LM, and 60% LM) is 0.2, 0.44, and 0.87 W/(m °C), respectively (tested by a hot disk), which is approximately in agreement with experimental results. Hence, heat-responsive robots made from LMC with more liquid metal will be more responsive and expand faster (at least 4.5 times), which is an important characteristic of soft actuators. In consideration of the good electrical conductivity of liquid metal, an electromagnetic induction heater can also be utilized to heat and deform the liquid-metal soft robots. To achieve a directed deformation pattern, a three-layer configuration is designed to realize electromagnetic induction heating, which contains a noninflatable transparent poly(dimethylsiloxane) layer, a liquid-metal layer with a rectangular net pattern, and an LMC layer. The fabrication process is shown in Figure S4e. Due to the theory of electromagnetic induction heating, the sizes of heating conductors are required to be macrocomparable with the heating copper coil; thus, the eddy current could be generated and transformed into thermal energy. When the three-layer liquid metal is placed upon the coil and the power source is switched on, the heater would induce an eddy current in the liquid-metal layer with a grid shape. Further, the current would be transformed into thermal energy and increase the temperature of LMC. As the temperature exceeds the boiling point of ethanol, the polymer layer would be inflated and swell into a curved shape due to the nonequilibrium of stretchability (Figure 2e). 2.3. Demonstration of Thermally Responsive Soft Robots. LMCs are capable of reversible deformation between planar shape and designed 3D structure. As shown in Figure 3a, a cylindrical material hunches in the target regions when heated selectively. While cooling, the hunching area shrinks to its original shape immediately. As expected, heating causes the 2315
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Figure 4. Analytic relation of the expansion induced by the heating temperature (a) illustration of a single ethanol droplet transition to ethanol bubble. (b) Thermal properties of ethanol saturation vapor pressure. (c) Its saturation liquid and vapor density. (d) The volume expansion ratio and the boiling ethanol vapor mass ratio for the fixed shear modulus of the LMC elastomer μ = 106 Pa and (e) for the fixed limiting stretch λlim = 3.
ÅÄÅ ÑÉÑ ÅÅij R yz3 ÑÑ Å α = 1 + ηÅÅÅjjj zzz − 1ÑÑÑÑ j z ÅÅk R 0 { ÑÑ ÅÇ ÑÖ
the top layer side). As the top layer contains ethanol that creates bubbles when heated, it expands much faster and pushes the lower layer to fold and grab the object (Movie S7). This flexible hand is pretty much like tentacles of an octopus. Once cooled, the soft hand loosens and releases the object and recovers to the plane due to the liquidation of ethanol. Better yet, this gripper can be constructed entirely with soft material and controlled with wireless heating. Many other behaviors are also demonstrated in Figure S7; a three-legged robot starts warping when heated unevenly and begins rolling on the hot plate without any other external manipulation (Figure S7a and Movie S8). The part near the heat source expands faster, causing the material to bend toward the other part. With a simple design, LMC can form different shaped structures with selective heating. After continuous heating, LMC inflates its tummy like a puffer (Figure S7b). With elaborate design, LMCs are capable of achieving more complicated and desirable deformation and locomotion.
(2)
For a small value of volume fraction η (lesser than 20%), we suppose that the interaction of droplets is negligible. Thus, considering a single ethanol droplet with radius R0 embedded in the infinite LMC matrix illustrated in Figure 4a, it would boil and become vapor upon reaching the liquid−vapor transition temperature. This leads to a significant expansion of the whole LMC matrix. The thermal properties of ethanol saturation vapor pressure and its saturation liquid and vapor densities are given in Figure 4b,c. Figure 4d,e shows that the volume expansion ratio and the boiling ethanol vapor mass ratio are affected by the shear modulus of the LMC elastomer and the limiting stretch for the volume fraction η = 20%. The results indicate that the shear modulus of the LMC elastomer has a significant impact on the volume expansion ratio, which could be modulated by changing the liquid-metal inclusion volume. In addition, increasing the limiting stretch through adding liquid metal can also improve the capability of volume expansion. We choose the liquid metal as a filler mainly due to increase of thermal conductivity and decrease of the elastic modulus as well as wireless heating. Besides, former research found that the liquid metal can increase fracture energy significantly36 and alleviate the damage due to the stretching in the expansion process. The experiment found that it is difficult to achieve free transformation into different shapes without the addition of liquid metal. The crucial limit of this material needs to be improved. The release of ethanol vapor leads to a low repeatability, which might be solved by sealing the LMC with
3. DISCUSSION To quantitatively predict the expansion and deformations of LMC induced by heating, we derived the analytical relation of the expansion induced by the heating temperature (the detailed discussion is given in the Supporting Information). The main results are presented here. We assume that the ethanol droplet uniformly embedded in the LMC matrix with the total volume fraction η. The average radius is R0 in the undeformed LMC, and R in the deformed elastomer due to the transition from liquid ethanol to vapor state. Then, the volumetric expansion is estimated by 2316
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novel implementations in biomedical engineering, aerospace engineering, etc.
airtight materials. The experiments demonstrate that the cycle of a higher expansion ratio (about 6 times of the original height) can repeat for more than 24 times. Indeed, vapor pressure increases continuously in the high temperature, leading to the pockets’ rupture eventually. To improve repeatability of LMCs, the heating time should be well controlled. Because there are many pockets inside (from the microstructure image), they can expand many times at a high ratio. From the Movie S2, the LMC can swell many times in the same place. If most of the pockets rupture in the first heating, it is hard to swell again and form a similar shape. Under the same evaporation force, materials with a lower elastic modulus can produce larger strain and deformation. In fact, the heating process of ethanol−polymer composites are always accompanied by the voice of cracking due to the rupture of small pockets. Therefore, ethanol−polymer composites are more inclined to rupture in comparison with LMC. The liquid metal in the LMC might increase the repeatability of actuation compared with the ethanol and polymer composites. As we know, there are many other thermally responsive materials that have better repeatability than LMC, including shape memory alloys and thermally responsive polymers. However, it is difficult for them to achieve very large volume change when compared with liquid−vapor transition actuation. Besides, compared with other solid fillers, liquids are easy to comply with the soft components. On the basis of the properties of deforming in response to the thermal stimulus, LMC implementation would be further improved via programmable control, thus achieving a more precise, quick response and adaptable bulging deformation. To achieve accurate and controllable bending, twisting, or swelling of the composite, we can adhere array soft heating film units on the surface of the composites and control the on/off state of every single unit independently. Thus, the desired heating fields could be realized and further control the deformation of LMC. Besides, the composite can also be threaded by resistance heating wire as the composite itself is insulative. Heating wires can be regularly aligned before the molding of LMC. Both the heating film and heating wire can be adjusted by a programmed circuit board; hence, the deformation of LMC is controllable through this integration. Applying the embedded control system and sensors, as well as localized heating units, different robots with multiple locomotion configurations could be further developed, such as rolling robots or wriggling worm robots. In addition, various energy forms like solar energy or electric energy can be converted to heat to actuate more completed LMC robots. Besides, LMC has potential for being applied in other areas, such as a temperature sensor, pressure sensor, or ultrastretchable soft screen on account of its color changing capability after stretching. As known to us all, the liquid metal integrates properties of both liquids and metals, thus being a good candidate for soft electronics in soft robots. Unique properties of liquid metal endow LMCs with a huge application value. For instance, the LM can be designed as functional circuits on LMC for actuating, lighting, and displaying. In addition, the liquid-metal layer on the LMC has an anti-interference function of electromagnetic radiation. Liquid-metal antenna can also be applied in the LMC-made soft robotics, such as transmitting information when performing tasks. It is expected that this shape transformable in a large-scale composite material could also help researchers in other areas to find
4. CONCLUSIONS To summarize, we proposed large-magnitude transformable liquid-metal composites via combining liquid metal, elastomer, as well as liquid ethanol together, which can reversibly transform their shape at will on a pretty wide scale (even up to 11 times their original height) in response to wireless heating. We demonstrated the transformation of LMC-made octopus from a 2D shape into several predictable 3D shapes only by wireless heating without requiring any sophisticated operating system. A particular finding lies in that in such a composite, it is hard to achieve a freely transformable behavior without the addition of liquid metal. Here, liquid-metal inclusions do work in reducing the elastic modulus by 43.1%, increase the thermal conductivity (up to 450%), and play an indispensable role during wireless heating. Theoretical analysis about heating-induced liquid−vapor transition of the embedded ethanol droplet indicates that the shear modulus and limiting stretch (700% strain) as well as the thermal conductivity of LMC contribute most to the volume expansion ratio, all of which can be improved by the loading of liquidmetal inclusions. Designed behaviors of entangling objects like jellyfish and inching like worm only via heating and cooling are also achieved. The present composite opens doors to realize freely transformable robots, which are completely constructed with fully soft materials and could be entirely controlled by wireless heating. 5. EXPERIMENTAL SECTION 5.1. Preparation Methods of LMC. The raw materials for LMC are composed of 75.5% gallium (Nanjing Jinmei Gallium LTD., 99.999%) and 24.5% indium (Shaoguan Jinyuan Industrial Co., Ltd., 99.999%) by weight and silicone rubber (methyl vinyl polysiloxane and Pt-catalyzed polymethylhydrosiloxane). They are fully mixed to reduce the formation of liquid-metal droplets and suspend in uncured silicone rubber adequately. Thereafter, ethanol is vigorously mixed in the above compound, resulting in a notable expansion capability of the material under heating condition. The proportions of the three elements in LMC can be varied according to the experimental requirement, which are noted in the relative text. Over these steps, different stirring times were chosen to achieve different droplet sizes. LMC 10% wt ethanol and 60% weight ratio liquid metal was chosen when considering the high thermal conductivity and lower elastic modulus (Figures 1d and 2d). 5.2. Electromagnetic Induction Heater. Although the liquid-metal polymer consists of EGaIn metal droplets, the metal could not form strong electrical current due to the distributed and microscale characteristics. The electromagnetic induction heater is a self-designed alternating current generator connected to a copper plate coil, which is 6 mm in thickness and 8 cm in diameter and connected to a water-cooling system. The heater is powered by a 50 V, 50 A constant voltage source (Huawei R4850N6). 5.3. Temperature Recording. Temperature variations of LMC are recorded by an infrared camera (Figure 2a,b). When applying the microwave heating methods, it is hard to test the LMC temperature in real time. Therefore, we took them out from the microwave oven immediately and then measured the 2317
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Notes
temperature by an infrared camera. In Figure 2c,d, T-type thermocouples placed on the upper surface of the LMC were used to measure the temperatures of the LMC, which was placed on a hot plate (150 °C). For the purpose of collecting the real-time temperature data of the LMC, a data acquisition instrument (Agilent 34970A, Keysight Technologies (China) Co., Ltd., Beijing, China) was employed. 5.4. Thermal Conductivity Measurement. The thermal conductivity of LMC was measured by hot disk TPS 2500s.
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The authors declare no competing financial interest. All data will be made available on request.
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ACKNOWLEDGMENTS The authors acknowledge the assistance of Dr. Shijin Dong, Beijing DREAM Ink Technologies Co. Ltd, for the partial material tests. They also appreciate the help from Siyao Qiu and Prof. Zhijie Qiu from Central Academy of Fine Arts in designing the octopus pattern and editing the video. This work was partially supported by National Natural Science Foundation of China under Key Project # 91748206 and Grant No. 51476181, the Ministry of Higher Education Equipment Development Fund, Dean’s Research Funding of the Chinese Academy of Sciences, and the Frontier Project of the Chinese Academy of Sciences.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b03466. LMC expansion like a balloon in response to heat (AVI) LMC transformation between planar shape and 3D structure reversibly (AVI) Sequential LMC swelling with selective heating (AVI) A cylinder shape LMC hunches and recovers when heated and cooled (AVI) Two-dimensional LMC-made octopus inflating itself in response to selective heating (AVI) Inching behavior of a soft robot made of LMC (AVI) LMC gripper bending and grabbing the object (AVI) Three-legged robot warping on a hot plate (AVI) Theoretical analysis of the expansion of LMEE matrix; microstructure change of composite when stretching (Figure S1); expansion mechanism of LMC (Figure S2); microstructure of LMC with different stirring times (Figure S3); deformation of LMC in response to heat (Figure S4); LMC ball deformation after repeated heating and cooling accompanied with surface color change (Figure S5); expansion properties of LMC (Figure S6); demonstration of various deformation behaviors (Figure S7) (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Tel: 010-62798963 (J.L.). *E-mail:
[email protected] (Z.H.). ORCID
Hongzhang Wang: 0000-0003-2733-9461 Youyou Yao: 0000-0002-3449-4016 Yuntao Cui: 0000-0002-3273-9163 Wei Rao: 0000-0002-4168-0298 Shuting Liang: 0000-0002-7772-0757 Jing Liu: 0000-0002-0844-5296 Author Contributions ⊥
H.W. and Y.Y. contributed equally.
Author Contributions
H.W. and Y.Y. performed all experiments and wrote the manuscript. J.L. and Z.H. supervised the project and wrote a part of the manuscript. X.W. performed some experiments and wrote the manuscript. L.S. and X.Y. wrote a part of the manuscript and gave suggestions in performing the experiments. Y.C. and P.Z. performed a part of the tests and processed the data. W.R. wrote a part of the manuscript, and R.G. performed a part of experiments. S.L. wrote a part of the manuscript. W.W. performed a part of the experiments and processed the data. All authors discussed the results. 2318
DOI: 10.1021/acsomega.8b03466 ACS Omega 2019, 4, 2311−2319
ACS Omega
Article
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