1 Introduction
Soft robot is an artificial soft and deformable system composed of flexible actuators and agile joints/links, which is a hot research direction in the field of robotics
[1⇓⇓~4]. Unlike traditional rigid robots, which are composed of rigid components such as motors, bearings, gears and hinges, soft robots are usually made of soft materials.It has many advantages, such as a large number of degrees of freedom, large deformation, continuous deformation and compliant contact, and has important research value in the special application fields of small object manipulation and space limited environment movement
[5⇓~7].
In the past decade, stimulus-response materials, such as shape memory polymers, liquid crystal elastomers and hydrogels, have been successfully applied in the field of soft robotics
[8⇓⇓⇓⇓~13]. This new type of soft robot can imitate some structures of organisms (such as artificial muscles, octopus tentacles or insect bodies) and achieve a variety of movement modes (crawling, rolling, swimming, grasping and jumping) under external stimulation. These new soft robots also have many novel advantages, such as miniaturization, remote control and multiple sensors, which promote the further development of human society towards intelligence
[14⇓⇓⇓⇓~19].
Liquid crystal elastomer refers to a kind of polymer that can show elasticity in isotropic phase or liquid crystal phase by connecting adjacent polymer chains to form a network structure after a certain degree of crosslinking of non-crosslinked liquid crystal polymers
[20⇓⇓⇓⇓⇓⇓⇓⇓⇓~30]. As one of the most representative smart materials, liquid crystal elastomers have both liquid crystal anisotropy and rubber elasticity, as shown in Figure 1, under external stimuli (heat, light, electricity, magnetism, pH, humidity, etc.).Its phase state or molecular structure will change, and then change the order of liquid crystal units, resulting in macroscopic deformation of the material itself. When the external stimulus is removed, the liquid crystal elastomer can return to its original shape. The physical and chemical properties, the response speed and the motion mode of the liquid crystal elastomer can be greatly optimized by adjusting and controlling the molecular structure of the liquid crystal monomer, the type of liquid crystal phase, the macromolecular crosslinking network structure, the mode and the condition of external stimulation and the like, so that the liquid crystal elastomer has a wide application prospect in the field of soft robots. Although there are still some shortcomings such as low mechanical properties and high preparation cost, liquid crystalline elastomers have become one of the most suitable materials for soft robots because of their unique two-way shape memory, large deformation, fast response speed, diverse response mechanisms and strong expansibility. This review will focus on the latest research progress of soft robots based on liquid crystal elastomers, and classify the soft robots based on liquid crystal elastomers according to different driving modes.This paper discusses the driving system of the newly developed liquid crystal elastomer soft robot, summarizes the current challenges in the field of liquid crystal elastomer soft robot, and looks forward to the future research directions.
At present, according to the different driving methods, the research of liquid crystal elastomer soft robot is mainly divided into the following categories: ① Thermally driven liquid crystal elastomer soft robot. As one of the most common energy sources, heat has the advantages of easy control and wide sources, and is first applied to the driving research of liquid crystal elastomers
[31,32]. The temperature of the liquid crystal elastomer is above the clearing point by heating to realize the isotropy-liquid crystal phase transition, thereby completing the macroscopic driving behavior. (2) Optically driven liquid crystal elastomer soft robots have been widely used in the research of soft robots because of their advantages of instantaneous, high precision, strong controllability, pollution-free and wide sources of light energy
[33⇓~35]. Through photothermal effect or photoisomerization reaction, the photoresponsive liquid crystal elastomer can be reversibly deformed, and then the soft robot is driven to realize various deformation and motion modes; (3) Electricity-driven liquid crystal elastomer soft robot. As the most commonly used energy in human society, electricity has the advantages of convenience, cleanliness, low cost and strong adjustability, and is the preferred driving energy for modern equipment
[36⇓~38]. The electric energy is partially converted into heat energy by using the Joule heating effect of the conductive material, so that the liquid crystal elastomer is heated to a temperature above the clearing point, the transformation from the liquid crystal phase to the isotropic phase is realized, the liquid crystal elastomer is subjected to reversible deformation, and the soft robot is driven to move; (4) Other types of soft robots, such as soft robots driven by magnetic field and humidity, are still in the initial stage of research compared with thermal, optical and electrical driving modes, and their motion modes and application environments are relatively single.
2 Liquid Crystal Elastomer Soft Robot Based on Thermal Driving
It was shown by de Gennes in 1975 that changes in the order of liquid crystal molecules can strongly affect the shape of monodomain liquid crystal elastomers, and that this shape change is a function of temperature
[39]. In the phase transition temperature range (from isotropic to liquid crystalline phase), a slight temperature change will cause large-scale deformation of liquid crystalline elastomers, resulting in thermo-mechanical effects. On this basis, a variety of liquid crystal elastomer soft robots based on thermal drive have been designed and manufactured.
Ware et al. Prepared a thermally driven liquid crystal elastomer soft robot through three-dimensional printing (3D printing) technology
[40]. They first polymerized liquid crystal monomer RM82 and chain extender to obtain acrylate-terminated liquid crystal oligomer as printing ink, and then edited and printed the design shape by 3D printer.A cylindrical hollow liquid crystal elastomer soft robot is obtain by fixing a complex three-dimensional structure of that liquid crystal elastomer through a cross-linking reaction initiate by ultraviolet light, and can realize radial contraction and axial expansion deformation under the stimulation of external temperature.
Zhao et al. Prepared a liquid crystal elastomer soft robot containing cinnamic acid groups, which can realize programmable complex deformation
[41]. They first stretched the liquid crystal polymer film uniaxially, and then exposed it to 365 nm ultraviolet light to induce the cinnamic acid group to form a network structure and fix the orientation of the liquid crystal unit, thus obtaining a liquid crystal elastomer material that can be reversibly deformed under thermal stimulation. They further found that local ultraviolet irradiation of liquid crystal polymers can selectively prepare driving regions with photoresponsiveness and non-driving regions without photoresponsiveness on liquid crystal elastomers.Under thermal stimulation, these two different regions will cause different contraction/stretching forces inside the liquid crystal elastomer, thus achieving complex shape deformation. Based on this, they prepared an octopus-shaped soft robot, as shown in Figure 2. Because the eight feet of the soft robot have been partially programmed for photocrosslinking, it can achieve eight different shape changes under temperature stimulation.
Ware et al. Combined four-dimensional printing (4D printing) technology with liquid crystal elastomer to prepare a thermoresponsive liquid crystal elastomer soft robot with complex deformation
[42]. They used two different liquid crystal elastomer inks to print a disk-shaped soft robot, whose outer edge is a liquid crystal elastomer with a lower response temperature, while the inner part is a liquid crystal elastomer material with a higher response temperature. Under the stimulation of low temperature, the external liquid crystal elastomer deforms so that the soft robot deforms from a flat shape to a three-dimensional truncated cone shape; When stimulated by high temperature, the liquid crystal elastomers of the two parts are deformed at the same time, so that the soft robot is further transformed into a conical shape. This kind of soft robot with step-by-step driving behavior provides a new development idea for the field of intelligent actuators.
3 Optically Driven Liquid Crystal Elastomer Soft Robot
Light energy is a powerful renewable energy source, which can meet the energy needs of most living things on the earth. Light can be transmitted over long distances and precisely controlled, so it has become an ideal driving energy source for liquid crystal elastomer soft robots
[43⇓~45]. The light-driven liquid crystal elastomer soft robot has two main working principles: one is that the azobenzene group containing a diphenylacetylene structure absorbs ultraviolet light to realize cis-trans isomerization, so that the liquid crystal elastomer is deformed, and then the soft robot is driven to move; The other is to convert part of the light energy into heat energy through the photothermal conversion material, so that the liquid crystal elastomer can realize the transformation from liquid crystal phase to isotropic phase, generate reversible deformation, and then drive the soft robot to move. At present, there are mainly three kinds of motion forms of optically driven liquid crystal elastomer soft robots: crawling, rolling and swimming. In this review, optically driven soft robots will be discussed in turn from these three different motion forms.
3.1 Creeping optical-driven soft robot
Crawling can be defined as the movement of a soft robot that generates a net thrust by deforming its body so that the anchor point generates an effective friction force in the surrounding environment
[46]. Soft robots with crawling capabilities can move in narrow spaces, cracks or tunnels, and perform special tasks in space-constrained environments that human bodies cannot enter. So far, optically driven liquid crystal elastomer soft robots have been able to achieve a variety of complex crawling actions.
Zhao et al. prepared a soft crawling robot driven by near-infrared light by doping near-infrared photothermal conversion materials into a liquid crystal elastomer monolayer film containing cinnamic acid groups
[47]. The photothermal conversion reagent used in the system is an N, N-dialkyl imidazoline-2,4,5-trione nickel complex with a 2-butyryl carbon chain, which has high photothermal conversion efficiency and good dispersibility in a liquid crystal polymer network. One side of the liquid crystalline elastomer film can be driven by light, while the other side is not responsive to light, through a non-uniform photocrosslinking reaction. Under the near-infrared stimulation, part of the light energy is converted into heat energy, which causes the liquid crystal-isotropic phase transition of the liquid crystal elastomer, and the reversible shrinkage behavior of the side with light response will lead to the reverse curling deformation of the whole liquid crystal elastomer soft robot. When the near-infrared light is scanned from the soft robot tail (rear end) to the robot head, the robot body curls downward and the robot tail moves forward. After the near-infrared light is removed, the robot will return to its original shape. At this time, the friction of the head of the robot is less than that of the tail, and the tail keeps its position unchanged, thus pushing the head of the robot forward. Based on this motion mode, the soft robot can crawl on a slope of 15 ° at a speed of 2 cm/min under near-infrared light stimulation.
In order to adjust the motion direction and deformation mode of the soft robot more easily and accurately, our research group proposed a strategy to construct a multi-stimuli-responsive liquid crystal elastomer soft robot system, which can realize different shape deformation modes and carry out multi-directional motion
[48]. The system consists of two layers of liquid crystalline elastomer films, in which the top layer is a whole unstretched liquid crystalline elastomer film and the bottom layer consists of three single-domain aligned liquid crystalline elastomer films containing different photothermal reagents (Disperse Red 1, YHD796 and Dye1002).Three different wavelengths of light (520, 808, 980 nm) are used to selectively stimulate specific areas of the soft robot with a layered structure, so that the driving system can get rid of the limitation of light scanning position/direction and realize the precise multi-directional motion control of the soft robot (Fig. 3).
Priimagi and Zeng et al. reported a soft robot based on a light-driven liquid crystal elastomer that can mimic the crawling motion of a caterpillar under spatially uniform visible light stimulation
[49]. The liquid crystal elastomer used by the soft robot is prepared by taking a liquid crystal box with multiple orientation areas as a template through photopolymerization, liquid crystal units in different areas inside the liquid crystal elastomer have different arrangement forms, and the liquid crystal elastomer film spontaneously forms an omega-shaped soft robot after annealing treatment to release internal stress. Under the stimulation of near-infrared light, the light energy is partially converted into heat energy through the photothermal reagent inside the liquid crystal elastomer, and the film is induced to change from a curved structure to a flat and stretched structure, so that the soft robot is driven to realize a caterpillar-like geometric deformation mode; In addition, the soft robot can sense ground conditions and execute different gait patterns to achieve driven crawling on a series of different substrates, such as hot gratings, paper surfaces and human skin. Our group also proposed a new type of crawling soft robot, which consists of a double-layer liquid crystal elastomer membrane and zigzag feet, and can imitate the snake-like motion
[50]. As shown in fig. 4, the crawling robot is formed by sticking two arched single-domain liquid crystal elastomer films on the outer sides of one S-shaped liquid crystal elastomer film respectively. Under the stimulation of near-infrared light, the YHD796C inside converts light energy into heat energy, which causes the single-domain liquid crystal elastomer film to bend towards the multi-domain liquid crystal elastomer film, thus realizing the reversible deformation from S-shaped motion to reverse S-shaped motion. Under the repeated irradiation of near-infrared light, the robot moves forward by asymmetric friction, which can achieve snake-like motion similar to snakes. The snake-shaped liquid crystal elastomer soft robot driven by near-infrared light can provide a new perspective for the manufacture and development of bionic soft driving devices.
3.2 Rolling type optically-driven soft robot
Since the rolling resistance is significantly lower than the sliding friction, rolling is an effective planar motion mode. Based on this feature, rolling robots can move more steadily on uneven or soft surfaces (such as sand, snow, bushes and vegetation), and can easily recover after collision. Based on these advantages, rolling robots are very suitable for use in unknown and diverse terrain, and have great application potential in reconnaissance, exploration, exploration and other industries.
White et al. Successfully prepared a rolling soft robot based on azobenzene liquid crystal elastomer, and demonstrated its rolling behavior under continuous UV irradiation
[51]. When the robot is exposed to UV light, the rolling resistance is overcome by the stress generated by the asymmetric strain between the top and bottom to achieve rolling motion. In addition, when the robot begins to roll, the membrane, which was originally in the lower part, is displaced to the upper part of the robot, regenerating asymmetric stress to achieve continuous rolling. Cheng and Zeng applied the concept of kirigami (a kind of paper-cut art) to the optically driven liquid crystal elastomer film soft robot
[52]. In the system, the liquid crystal polymer doped with the photo-thermal conversion material is oriented by the liquid crystal elastomer through a liquid crystal box and is crosslinked by ultraviolet light to form a network structure,And cutting the film into a series of strips with triangular tail structures by using a laser engraving machine, wherein the cutting direction of each strip is along the orientation direction of the liquid crystal elastomer film, so that the film can be bent and deformed under illumination. As shown in fig. 5, the cut strip is rolled into a tubular shape and reinforced with a plastic ring to obtain a rolling type soft robot, and under near-infrared illumination, the strip can be asymmetrically bent and deformed to drive the robot to realize multi-gait rolling motion in the front and rear directions; In addition, it can roll along a pre-designed trajectory and climb a slope with a slope of 6 ° in 20 s. Cai et al., inspired by animal bones, built a hybrid tension robot, in which acrylic rods were used as the compressive support of the robot, while the composite material formed by liquid crystal elastomer and carbon nanotubes served as the flexible tensile structure
[53]. The tension robot has high expandability and can realize complex motion completely by light. Under the stimulation of light, one or more liquid crystal elastomer flexible rods in the tension robot are heated to contract and deform, so that the center of gravity of the whole robot is shifted, and the robot can roll in multiple directions on various terrains along a plurality of different paths.
3.3 Swimming type optically driven soft robot
Unlike dry environments, soft robots can also do a lot in wet environments. For example, soft robots can move freely in various liquids and tissues like microorganisms to perform minimally invasive diagnostic or therapeutic tasks. However, in this environment, the viscous force is often greater than the inertia. In the past, only through the coordination of multiple driving robots can achieve a variety of complex movements. Because of its unique two-way shape memory advantage, liquid crystal elastomer has become one of the most suitable materials for swimming bionic robots
[54,55].
Huang and Lv et al. Fabricated a light-driven micro swimming robot based on azobenzene liquid crystal elastomer, which can realize a variety of complex movements such as swimming, grasping, carrying and transportation
[56]. Under the irradiation of ultraviolet and visible light, the azobenzene group can realize the interconversion between cis and trans isomers. Because the lengths of the two isomers are different, the material can realize macroscopic shrinkage, bending and other deformations. As shown in Figure 6, in this system, the azobenzene liquid crystal elastomer is used as the flagellum of the robot to generate the propulsion force for swimming. Under the irradiation of ultraviolet light, the flagellum shows periodic motion, thus pushing the robot to move forward continuously in the water. The light-driven soft robot can realize non-contact driving and complex motion control, does not need any battery, power line, motor or transmission mechanism, greatly simplifies the structure of the robot, and has great application potential in long and narrow liquid environments (such as microfluidic devices and the like).
Yu et al. developed a novel light-driven swimming robot based on azobenzene liquid crystal elastomer/polyimide film double-layer composite structure
[57]. The robot's motion can be controlled by light exposure, and the bilayer film quickly bends toward the polyimide side after exposure to UV light and recovers immediately after the light is removed. When placed on the liquid surface, the swimming robot will rely on the liquid crystal elastomer tail fin to move the liquid rhythmically like a dolphin to generate thrust, thus continuously pushing itself forward. In addition, the light-driven rotation of the swimming robot can be realized by imitating the function of the dolphin pectoral fin only by changing the length-width ratio and the irradiation position of the tail fin. This new type of soft robot combines forward motion and rotational motion, and can easily achieve multi-directional motion control at room temperature, showing its application prospects in the field of miniaturized equipment for transportation. Cai et al. Successfully prepared a light-driven swimming robot based on polydopamine-liquid crystal elastomer composite
[58]. Due to the high photothermal conversion efficiency of the coated polydopamine layer, the prepared liquid crystal elastomer can respond to the stimulation of near-infrared light within one-tenth of a second. When exposed to near-infrared light, the polydopamine layer converts light energy into heat, resulting in a gradient distribution of the temperature of the liquid crystal elastomer film, resulting in reversible local bending deformation. Based on this characteristic, the actuator can be used as the tail of the swimming robot to drive its motion. By periodically turning on and off the near-infrared light, the swimming robot can move forward at an average speed of 1 mm/s.
4 Liquid Crystal Elastomer Soft Robot Based on Electric Drive
Like most polymers, liquid crystal elastomers are electrically insulating materials. In order to realize the electric drive of liquid crystal elastomers, conductive materials can be compounded with liquid crystal elastomer to prepare electrically driven soft robots. When a certain voltage or current is applied, the Joule heating effect of the conductive filler can convert a part of electric energy into heat energy, so that the liquid crystal elastomer reaches above the phase transition temperature, the liquid crystal elastomer is deformed, and the soft robot is driven to move. In addition, current, voltage and other electrical signals can be precisely output through software programs, so as to achieve precise control of soft robots. At present, the conductive materials used in electrically driven soft robots are mainly divided into three categories: carbon materials, metal materials and conductive polymer materials. We will discuss these three categories of materials in turn.
4.1 Liquid Crystal Elastomer Soft Robot Based on Carbon Material
Carbon material is one of the most widely used conductive materials, which has a variety of nanostructures, such as zero-dimensional, one-dimensional and two-dimensional, and has a wide range of sources and low cost. At present, the commonly used carbon-based conductive materials for liquid crystal elastomer soft robots mainly include carbon nanotubes and carbon black.
4.1.1 Electrically Driven Soft Robot Based on Carbon Nanotubes
The carbon nanotube can be regarded as a closed nanotube body formed by bending a single-layer or multi-layer six-membered carbon ring graphite layer, and two hemispherical half fullerene molecules are arranged at two ends of the nanotube
[59⇓~61]. According to the number of graphite layers, carbon nanotubes can be divided into single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT). Among them, the diameter of SWCNTs is small, only a few nanometers, but under the action of Van der Waals force, SWCNTs usually aggregate into bundles with a diameter of about tens of nanometers, each bundle contains tens to hundreds of SWCNTs. The inner tube diameter of MWCNT is comparable to that of SWCNT, while the outer tube diameter varies from several nanometers to tens of nanometers due to the number of graphite layers. Carbon nanotubes (CNTs) are quasi-one-dimensional materials with micron-scale axial length and large aspect ratio. Three of the four valence electrons of carbon atoms form covalent bonds, and the remaining valence electrons share delocalized π bonds with the nature of metallic bonds, so carbon nanotubes have conductivity in the axial direction.
Ware et al. Reported a programmable electrically driven soft robot, which was prepared by filling raw materials such as liquid crystal monomers and crosslinkers into liquid crystal cells with ordered carbon nanotube coatings by capillary force
[62]. In this system, carbon nanotubes have multiple roles: enhancing mechanical, thermal, and electrical properties and acting as an alignment layer for liquid crystal elastomers. The programmable response of the soft robot to the current can be realized by controlling the orientation, the position and the layer number of the carbon nanotubes in the liquid crystal elastomer/carbon nanotube composite. They further twisted the liquid crystal elastomer/carbon nanotube composite films into fibers, which can achieve about 12% of the driving strain under 15.1 V/cm DC voltage stimulation compared with the composite films. This work extends the versatility of programmable drives, showing great potential for self-deforming structures and soft robotics applications.
Shao and Tian et al. proposed a programmable electrically-driven soft robot based on liquid crystal elastomer, which is composed of a flexible electrothermal film and a liquid crystal elastomer film
[63]. The flexible electrothermal film is formed by embedding the multi-walled carbon nanotube and the silver nanowire into a polymer film with a mutually communicated network structure, wherein the multi-walled carbon nanotube and the silver nanowire can enable the electrothermal film to have good electrical conductivity while maintaining flexibility. Under the stimulation of electrical signals, the flexible electrothermal film can generate heat energy by Joule effect, while the liquid crystal elastomer film will have a large deformation under the thermal effect. Under the stimulation of 6. 5 V voltage, the robot reaches 189 ℃ at a heating rate of 21 ℃/s, thus achieving rapid shrinkage deformation. In addition, the structured design similar to that of a printed circuit as well as the customizability enable such an electrically controlled soft robot to be programmed into a variety of shapes required for practical applications.
4.1.2 Electrically driven soft robot based on carbon black
Carbon Black (CB) is a kind of conductive material which has been discovered for a long time. It belongs to amorphous Carbon. Its conductivity is excellent, stable, controllable, and the manufacturing cost is very low. It is a commonly used conductive material
[64,65]. In carbon black particles, carbon atoms are arranged in a graphite-like shape to form carbon black quasicrystals, and the arrangement and orientation of microcrystalline layers in carbon black can be enhanced by heat treatment, so that electrons can move freely between microcrystalline layers.
Yu et al. reported a soft robot based on ultra-thin sensing and electronic control, which can sense the environment and imitate the inchworm for adaptive soft crawling
[66]. These soft robots are composed of an ultra-thin deformable heating actuator, a sensor with a single crystal silicon photodetector, and a carbon black-doped liquid crystal elastomer (LCE-CB) nanocomposite.Through the combination of sensing and driving, adaptive crawling motion can be achieved, in which the sensors sense the environment, and the actuators respond accordingly and autonomously control the motion by adjusting the deformation of the LCE-CB. Our research group has developed an electrically driven soft robot system consisting of a ring-shaped liquid crystal elastomer/carbon black composite soft robot and a conductive rail
[67]. The liquid crystal elastomer/carbon black electrically-driven soft robot consists of a liquid crystal elastomer circular ring and a plurality of U-shaped carbon black conductive areas pasted on the outer surface of the liquid crystal elastomer circular ring. Benefiting from the effective Joule heating generated by the carbon black powder and the corresponding non-uniform stress generated inside the bilayer liquid crystal elastomer/carbon black film, the loop-shaped soft robot can roll forward at a speed of 1.6 mm/s on a fixed copper conductor track under 50 V DC stimulation, as shown in Figure 7. The dynamic connection between the rolling actuator and the conductive track effectively eliminates the limitations of electrical wiring in generally complex actuation devices. This work promotes the development of electrically driven liquid crystal elastomer actuators and broadens the application prospects of soft robots in the field of electrical equipment.
4.2 Liquid Crystal Elastomer Soft Robot Based on Metal Material
Compared with carbon-based conductive materials, metal-based conductive materials have better conductivity, which endows soft robots with fast response characteristics under low voltage.
Zhao et al. Developed a soft robot controlled by electric field, which can realize a variety of bionic movements
[68]. The soft robot consists of a uniaxially oriented liquid crystal elastomer, a polyimide layer, and a serpentine nickel-chromium (Ni-Cr) thin resistive wire embedded in the middle. Under the electric signal, the Joule heat generated by the Ni-Cr thin resistance wire causes the liquid crystal elastomer to contract, thereby driving the actuator to deform. As shown in Figure 8, thanks to the ingenious structural design, the soft robot can perform a variety of movement modes, including directional movement by arching the body and straightening it upward, crawling in a confined tunnel-like tube by stretching and contracting the body, and walking on two legs like a human, while pushing the load forward.
Cai et al. Reported a new type of electrically driven soft robot, which is made of three independently controlled thin snake-shaped electrothermal copper wires sandwiched between two layers of cross-linked liquid crystal elastomer films
[69]. After the sandwich composite material is coiled and stretched into a tubular structure, the arrangement of the liquid crystal elements inside the sandwich composite material is fixed by ultraviolet light to obtain a tubular actuator. Under the control of an applied electric field, the thin serpentine electrothermal copper wire generates nearly uniform temperature distribution on the surface of the tubular actuator through Joule heating effect and thermal diffusion, and the tubular actuator can show multi-directional bending and larger uniform contraction by independently controlling the on-off of single or multiple serpentine copper wires. By further integrating batteries, microcontrollers and drivers, they have created a soft robot that can crawl on the ground and transport goods, which makes it possible for electrically driven soft robots to move from laboratory to practical application.
Liu et al. reported a perceptive soft robot composed of highly conductive liquid metal and liquid crystal elastomer, which realizes sensing, execution and control at the level of independent units
[70]. They used a simple magnetic printing technology to construct liquid metal circuits on the surface of liquid crystal elastomers, which can not only be used as a compatible resistive heater, but also be used as a liquid crystal.But also can change that heat distribution in the circuit base on the change of the resistance value induced by the external force, thereby realize the autonomous deformation feedback of the pressure or strain stimulation under the participation of an inorganic electric control system. This fully flexible liquid metal-liquid crystal elastomer soft robot has excellent intelligence, including self-perception and autonomous decision-making performance. As shown in Figure 9, the gripping force generated by the bending of the robot can lift the cherry. After lifting, there is no contact pressure between the cherry and the robot, and the current is redistributed to the liquid metal circuit in the middle. Subsequently, after losing contact pressure for 28 s, the robot opens the gripper and the cherry is automatically released. The novelty of this work is that it proves that intelligent soft robots can be obtained only through reasonable design without relying on cumbersome and complex control systems, which provides a new idea for building highly integrated intelligent soft robots.
4.3 Liquid Crystal Elastomer Soft Robot Based on Conductive Polymer Material
Conductive polymers have the advantages of high conductivity, easy processing and low cost, and have good affinity for many polymer materials
[71,72]. At present, researchers have prepared soft robots by compounding them with liquid crystal elastomers, which endow them with excellent conductivity without affecting their mechanical properties.
Greco and Mattoli et al. fabricated an electrically driven soft robot by depositing a conductive polymer layer of poly (3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT: PSS) on the surface of a liquid crystalline elastomer film
[73]. In the system, the PEDOT: PSS conductive polymer layer has similar Young's modulus and Poisson's ratio with the liquid crystal elastomer film, and can be better bonded with the liquid crystal elastomer film. Under the stimulation of 1. 3 V voltage, the PEDOT ∶ PSS conductive polymer layer generates heat to make the soft robot achieve bending motion.
J Jákli et al. combined ionic liquid with liquid crystal elastomer to prepare the first electrically driven ionic liquid crystal elastomer soft robot
[74]. In the system, the ionic liquid and a liquid crystal monomer are mixed and filled into a liquid crystal box, and then the ionic liquid crystal elastomer is formed through ultraviolet crosslinking. When poly (3,4-ethylenedioxythiophene) ∶ poly (styrenesulfonate) (PEDOT ∶ PSS) was used as the electrode, it could be bent under 2 V voltage stimulation. Specifically, when a voltage is applied to the electrodes, the positively and negatively charged ions in the liquid crystal elastomer move in opposite directions, with the larger ions (cations) occupying a larger volume on one side of the film and exerting tensile stress, and the smaller ions (anions) exerting compressive stress on the other side of the film, thereby achieving bending deformation.
5 Other types of liquid crystal elastomer soft robot
In addition to the three most common driving methods of heat, light and electricity, liquid crystal elastomer soft robots driven by other stimulation methods are gradually attracting the attention of scientists, and some soft robots based on magnetic, humidity and other response modes have been reported, which opens up the application field of liquid crystal elastomer soft robots.
Magnetic response has many advantages, such as non-contact, strong penetrability and safety.By compounding magnetic response nanoparticles with liquid crystal elastomer, magnetic response soft robots with multiple motion modes can be prepared. Sitti et al. successfully fabricated a magnetically responsive soft robot consisting of a liquid crystal elastomer and magnetic nanoparticles embedded therein
[75]. In this system, the magnetic nanoparticles inside the soft robot are magnetized in different directions, so it can achieve shape change under the interaction between the local magnetic moment of the magnetic nanoparticles and the external magnetic field. As shown in Figure 10, the soft robot can walk spontaneously under the stimulation of the external magnetic field. This work provides a new idea for the design of soft robots by combining the advantages of liquid crystal elastomers and magnetic nanoparticles.
There are a lot of humidity-changing scenes in nature, and soft robots driven by humidity can make good use of this environmental advantage and convert it into the rest of the energy for human life. At present, the main strategy is to combine humidity with other driving methods to achieve different uses. Schenning et al. successfully prepared a thermal/wet dual-response soft robot by spraying a liquid crystal mixture on a stretched polyamide 6 (PA6) substrate. The oriented PA6 can guide the liquid crystal monomer to form a well-arranged and anisotropic liquid crystal network by itself
[76]. At a constant low relative humidity (5%), the curvature of the soft robot remains constant with increasing temperature; Under more humid conditions (80% relative humidity), the elastic modulus of PA6 decreases sufficiently, resulting in the deformation of the soft robot with the increase of temperature. Priimagi and Schenning reported a new type of liquid crystal elastomer soft robot, which was copolymerized by azobenzene derivative (acrylic acid disperse red), carboxylic acid monomer and crosslinking agent
[77]. After alkali treatment, the hydrogen bonds between the carboxylic acid dimers are broken and converted into hygroscopic carboxylates. At low relative humidity, the robot was turned off even under 450 nm high-intensity light (100 mW/cm
2). At high relative humidity, the polymer chain is swollen by water, and the orientation of the mesogens in the liquid crystal elastomer decreases, resulting in that it can be quickly triggered to open at a lower light intensity of 450 nm (8 mW/cm
2). This kind of soft robot is more convenient in the control of motion speed, geometry and directionality, and it is expected to be further applied in the field of adaptive biological heuristic soft robot.
6 Conclusions and outlook
Robots play an important role in our life, which can greatly improve the efficiency of human production and work. As an extension of the field of robotics, soft robots are made of soft materials. Compared with rigid robots, soft robots have the advantages of flexibility and flexibility, which can make up for the shortcomings of rigid robots. Soft robots will have great application potential in the fields of material processing, rehabilitation and environmental detection in the future. In this review, the research on liquid crystal elastomer soft robots in recent years is summarized, and the advantages and progress of liquid crystal elastomer soft robots based on thermal, optical, electrical, magnetic, humidity and other driving modes are discussed, but there are still some problems to be solved. First of all, the liquid crystal elastomer material itself has the disadvantages of high preparation cost, poor mechanical properties and poor fatigue resistance. In addition, in order to achieve out-of-plane deformation, the liquid crystal elastomer material is usually a multi-layer composite structure, and it needs the stimulation of external frequency conversion, which hinders its practical application. The current research mainly focuses on the light-driven liquid crystal elastomer soft robot, but the light penetration is limited, which leads to the relatively single structure of the soft robot, which can only be limited to the preparation of thin films, tubes and so on.It can only meet the lower requirements of mechanical force and simple motion mode, which limits the application scenarios, while the research on liquid crystal elastomer soft robots with other driving modes is relatively small. In order to solve these problems, we should continue to find new strategies and try new driving mechanisms to expand the application environment of liquid crystal elastomer soft robot while exploring the existing system. Generally speaking, it is necessary to develop new liquid crystal elastomer soft robots through the joint efforts of scientists and engineers, so as to promote them from laboratory to practical application.
National Natural Science Foundation of China(52173109)
National Natural Science Foundation of China(51903048)
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