Intelligent Liquid Crystal Elastomer Actuators with High Mechanical Strength, Self‐Sensing, and Automatic Control

Self‐sensing liquid crystal elastomer (LCE) actuators with integration of sensing and actuating have attracted significant attention in health monitoring and physical rehabilitation therapy. However, the development of LCE actuators equipped with high mechanical strength, self‐sensing capabilities, and automatic control simultaneously is still a significant challenge. Here, intelligent self‐sensing LCE actuators with high mechanical strength and automatic control are demonstrated by doping carbon‐materials composite of graphite and carbon black. The proposed actuators, driven by the electrothermal effect, demonstrate a significant increase in the ratio of lifted object mass/self‐weight ratio up to 1168 times, which is then tested in activities such as football kicking and arm swinging of a doll. The self‐sensing LCE actuators possess a 30% actuation strain and have real‐time sensing capability during Joule heating. They can be programmed by control circuits based on conditional logic judgments of the target. In addition, a smart glove equipped with self‐sensing LCE actuators, which have high mechanical strength and an automatic control system for assisting finger movement, is also demonstrated. The proposed LCE actuator shows great potential for applications in assisting finger movement, aiding in joint and knee recovery, and other smart physical rehabilitation therapy applications.


Introduction
Active smart materials that can react to environmental stimuli have garnered significant attention in the fields of actuators, soft robotics, and artificial muscles, resulting in a variety of fields including biomimetic devices, medical rehabilitation, and robotics. [1]As one of the most promising smart soft materials, liquid crystal elastomers (LCEs) can be actuated by various external stimuli, including heat, [2] light, [3] electricity, [4] magnetic field, [5] and humidity, [6] due to their intrinsic properties of fast responsiveness, large(>400%) and reversible deformations, and programmability. [7,8]s one of the simplest and most convenient driven methods, electrical stimulation [9,10] offers the advantages of easy control and integration in practical applications of electrically responsive LCE actuators. [11]Therefore, intelligent soft robots featured with the integration of sensing and actuating are more easily achieved through electrical signals, where the ability of perceptual movement based on the integration of sensing and actuating through biology mimic is one of the ultimate goals of robotics. [12]ecently, several works about self-sensing LCE actuators with the integration of sensing and actuating have been demonstrated.Self-sensing refers to the inherent capability of a material to perceive and monitor its own state, wherein the material functions as a sensor in and of itself.The LCE actuator in this study is capable of responding to electricity and generating deformation, which is accompanied by a change in its own resistance value, thus realizing the self-sensing of the actuation process.T. A. Kent et al. reported LCE soft actuators composed of fluidic channels of liquid metal (LM) alloy, which act as Joule heaters. [13]A. Kotikia et al. proposed an innervated self-sensing LCE fiber actuator with a pure liquid metal core via 3D printing. [14]M. Yao et al. fabricated an ionotronic LCE fiber with mechanomodulation of ionic conduction by introducing ionic liquids into the LCE network. [15]W. Liao et al. reported an intelligent LCE-LM coaxial fiber actuator with actuating and sensing functions using a modified melt-spinning method. [16]H. Liu et al. proposed a multifunctional artificial muscle based on a polydopaminecoated LCE and a low melting point alloy.This design allows for shape programmability, deformation-locking, and self-sensing capability. [17]L. Zhao et al. demonstrated an artificial muscle based on LCE and a spiral metal wire to perform sensing functions by monitoring the resistance of the spiral metal wire. [18]owever, the mechanical strength of previously reported selfsensing LCE actuators is low, which largely hinders their application in fields where high mechanical strength is required such as the medical rehabilitation field.Even though the self-sensing LCE actuators possess a similar modulus that matches soft biological tissues, [19] which shows great potential for applications in health monitoring and smart physical rehabilitation therapy, the development of an LCE actuator equipped with high mechanical strength, self-sensing, and automatic control is still a significant challenge.Carbon-based materials, which possess a good electrothermal effect, electrical conductivity, [20,21] and mechanical strength with high Young's modulus due to strong C─C covalent bonds, [22][23][24] are good choices for electric-driven self-sensing LCE actuators with high mechanical strength and automatic control.
In this work, we demonstrate intelligent self-sensing LCE actuators with high mechanical strength and automatic control by doping graphite and carbon black (CB) particles composite.The electric-driven LCE actuators are measured in terms of actuation and sensing characteristics.The doping and ratio of graphite/CB for maximized lifted object mass/self-weight ratio, and then tested in football kicking and arm swinging of a doll.Furthermore, a smart glove equipped with self-sensing LCE actuators with high mechanical strength and an automatic control system for finger movement assistance is demonstrated, indicating potential practical application potential in smart physical rehabilitation therapy.

Results and Discussion
The LCE actuator is prepared using a two-stage thiol-acrylate Michael addition and photopolymerization (TAMAP).The first stage is a thiol-Michael reaction, which is an intermediate state where the LCE network is capable of orienting mesogens by applying mechanical stress.The second stage involves a photopolymerization reaction for the permanent fixation of aligned singledomain regions.By applying an external stress (e.g., hanging weight) to the sample, the polymer chains are aligned and the mesogens are oriented in the direction of the stress, temporarily forming a single-domain LCE, which is then cured using a UV lamp.The detailed fabrication process can be found in the Experimental Section.
Figure 1a shows the chemical structure of components of liquid crystal monomer RM257 (1,4-bis-[4-(3acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene), chain extender EDDET (2,2′-(ethylenedioxy)diethanethiol), crosslinker PETMP (pentaerythritol tetrakis(3-mercaptopropionate)), catalyst DPA (dipropylamine), and photoinitiator HHMP (2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone).The introduction of graphite and CB with excellent electrical and thermal conductivity into the LCE actuator as functional materials enables it to have the ability of electrical stimulus responsiveness and the ability to sense electrical signals.Figure 1b shows the preparation process of the LCE actuator.Details of the preparation are given in the Experimental Section.Herein, the conductive materials of CB and graphite are added to the LCE actuator in a two-step process.In the first step, the conductive CB and graphite particles are dispersed throughout the reaction mixture during the cross-linking and alignment process.In the second step, additional carbon black and conductive graphite are coated on the surface of the sample by immersing the LCE in a toluene dispersion mixed with conductive graphite and CB.The introduction of the conductive materials of CB and graphite in two separate steps is important to obtain LCE with high conductivity and stable cycling, which avoids reduced deformation of the LCE actuator or rapid loss of conductivity during cycling while applying a single step of dispersion or surface infiltration.The two-step addition of conductive materials enables the LCE to have high electronic conductivity and large reversible strain.Figure 1ci,ii shows the scanning electron microscope (SEM) image of the surface of the LCE actuator at different amplification scales, with spherical carbon black and flaky graphite.Figure 1di,ii exhibits SEM images of the cross-section of the LCE actuator, it can be seen that the image is relatively flat, with the carbon-based material uniformly distributed on the surface.
For conductive materials, such as graphite and CB, the inherent resistance is usually temperature-dependent, which will significantly influence the quality of sensing in the actuation of the LCE actuator driven by the electrothermal effect.To minimize the temperature effect on resistance, the ratio of graphite and CB which possess opposite temperature coefficient of resistance (TCR) values [25,26] is optimized in our experiment.The resistance of the actuator can be influenced by both macro-deformation and temperature variations.To ensure precise measurement of the actuator's deformation, it is imperative to mitigate the influence of temperature on the resistance value of the conductive particles.Hence, the use of unoriented LCE enables the elimination of the impact of deformation on the resistance value.Figure 1e shows the relative resistance change ΔR/R 0 versus time, where R 0 is the original resistance of the LCE film at 25 °C, and ΔR is the resistance change of the LCE film during the actuation process.The graphite/CB mass ratios varied from 10:0 (100% graphite), 8:2 (hybrid 1), 6:4 (hybrid 2), 4:6 (hybrid 3), 2:8 (hybrid 4), and 0:10 (100% CB) while keeping the total doping concentration of graphite/CB in the LCE film fixed at 2 wt%.The dopant content does affect the mechanical properties of the LCE actuator.An increase in the dopant content of the conductive particles from 1 to 5 wt% decreases the elongation at break of the LCE actuator from 322% to 81%, and the suppleness of the material decreases.Both ends of the LCE film are connected to an electrochemical workstation to record the resistance change.It can be seen that the 100% graphite and 100% CB doped films show positive and negative TCR values, respectively, with the increased time and temperature.The relative resistance change ΔR/R 0 is 12.2% and −6.3% for 100% graphite and 100% CB doped films, respectively, when the temperature is increased from 25 to 100 °C.It is worth mentioning that hybridization reduces the temperature dependence of the resistance.With the increase of the CB ratio in the mixture of conductive materials, the TCR value decreases from the positive value of 7.3% (hybrid 1), 4.2% (hybrid 2), and 1.7% (hybrid 3) to the negative value of −3.4% (hybrid 4).It is shown that the hybrid 3 film possesses the most significant temperature self-compensation effect, where the TCR value is close to 0.
In the following experiment, the doping mass ratio of conductive material graphite/CB is selected as 4:6 (hybrid 3) for the LCE actuator due to its optimized TCR value.In addition, the resistance of the LCE actuator is 16.8 kΩ at a concentration of graphite/CB in a toluene solution of 100 g L −1 , which is used in our following experiment.Figure 1f demonstrates the actuation of LCE actuators with different shapes: circle, triangle, pentagram, square, and diamond under the stimulation of heat and light.All of these LCE actuators are parallel oriented and the orientation direction (n) is indicated by the blue double-headed arrow.When those LCE actuators are placed on a hot stage at 80 °C or illuminated under near-infrared light (NIR) of 808 nm, they all shrink along the alignment direction and extend along the perpendicular direction.In addition, their corresponding thermal images, which are captured by an infrared thermal imager (FLIR ONE), are also demonstrated and the temperature is indicated by the thermal bar.
Figure 2a shows a schematic of an LCE actuator with copper foil as electrodes, where the copper foil is applied due to low resistance loss and high stability in connection.The fabricated LCE actuator (26 mm × 8 mm × 0.8 mm) can be actuated by applying a voltage of 150 V between the original state (power off) and the shrink state (power on), as shown in Figure 2b.When the voltage is not applied, the length of the LCE actuator is 26 mm, where the LC mesogens align in parallel.In contrast, the LCE actuator shrinks to 18 mm with 8 mm shrinkage after applying a direct current (DC) voltage of 150 V, where the orientation of LC mesogens changes from the original parallel alignment to the random orientation due to the nematic-to-isotropic phase transition in increased temperature that is induced by the electrothermal effect.The total deformation of the LCE actuator is about 30%. Figure 2c  Figure 2e shows the Ashby plot of the actuating strain versus lifted object mass/self-weight ratio for our self-sensing graphite/CB doped LCE actuator and previously reported results.It can be seen that the lifted object mass/self-weight ratio of previously reported self-sensing actuators, from LCE actuators combined with liquid metal and helical metal wire (with lifted object mass/self-weight ratio of 40, [13] 200, [14] and 85 [18] times) to foam actuator (FA) (500 [27] times), photo-responsive liquid-vapor phase transition elastomer (PRPTE) (400 [28] times), dielectric elastomer (DE) (55 [29] and 200 [30] times), and hydrogel (100 [31] times), are all less than 500 times.In contrast, two inorganic materials, carbon black (Young's modulus of 80 GPa) [22] and graphite (Young's modulus of 1026 GPa), [23] were doped into the LCE actuator to enhance the mechanical strength in this work, resulting in a significant increase in the LCE actuator's lifted object mass/self-weight ratio up to 1168 times, which indicates a significant improvement in mechanical strength.
Figure 2f plots the relative resistance change ΔR/R 0 of the proposed graphite/CB doped LCE actuator versus time for more than 120 cycles to test the stability performance.The measurement is carried out by DC voltage with a high voltage of 150 V and a low voltage of 5 V. Five consecutive cycles (624 to 738 s) arbitrarily selected from the sequence are demonstrated in the inset figure.The resistance of the LCE actuator in its initial state is R 0 , and the resistance when it is thermally deformed under a DC voltage of 150 V to reach its maximum deformation is R 1 .Throughout the cyclic test, when the resistance of the LCE actuator is higher than R 0 , it will be applied a voltage of 150 V to shrink it, and in the process, the resistance of the LCE actuator will become lower.When the resistance of the LCE actuator is lower than R 1 , a low voltage of 5 V will be applied, which is not enough to deform the LCE actuator, and the resistance of the LCE actuator will increase.Specifically, when a high voltage of 150 V is applied to the LCE actuator in the initial state, the relative resistance change (ΔR/R 0 ) of the LCE actuator gradually decreases from 0 to −77%.At this time, after applying the low voltage of 5 V, the relative resistance change of the LCE actuator gradually returns to 0.
As shown in Figure 3a, the LCE actuator is attached to the outside of the doll's knee to control the movement of the leg.The doll stands still (Figure 3ai) when the control voltage is powered off.When the controlling voltage is powered on, the LCE actuator is stimulated to shrink due to the electrothermal effect, driving the doll's knee to straighten and kick the football in front of him into the goal (Figure 3aii-iv).The kicking process can be seen in Movie S2, Supporting Information.Figure 3b demonstrates the control of a doll's arm moving in reverse by installing two LCE actuators on each side of the shoulder joint of the doll.In this case, the LCE actuators on the inner and outer sides of the shoulder joint contract sequentially, causing the upward and downward movement of the arm, with LCE-1 marked by the yellow dotted box and LCE-2 by the pink dotted box.The angle between the arm and the horizontal line is defined as , where Δ is the angle change of the arm driven by the LCE actuator during the actuation process.Specifically, from 0 to 80 s, the LCE-2 contracts under the stimulation of driven voltage while the LCE-1 is not stimulated, leading to the downward movement of the arm with angle  increased from  1 = 5°to  5 = 28°a nd the angle change of 23°(Figure 3bi-v).and helical metal wire, [18] foam actuator, [27] photo-responsive liquid-vapor phase transition elastomer, [28] dielectric elastomer, [29,30] and hydrogel. [31]f) Relative resistance change ΔR/R 0 of proposed graphite/CB doped LCE actuator versus time for stability test.The measurement is carried by DC voltage with a high voltage of 150 V and a low voltage of 5 V for more than 120 times.Five consecutive cycles (624 to 738 s) arbitrarily selected from the sequence are demonstrated in the inset figure .From 80 to 100 s, the LCE-1 contracts under the stimulation of driven voltage while the LCE-2 is not activated, resulting in the upward movement of the arm with  decreasing from  5 = 28°t o  6 = 14°and an angle change of −14°(Figure 3bv,vi).Here, the negative angle change implies the upward movement of the arm.The arm-swinging process can be seen in Movie S3, Supporting Information.Compared to a single LCE actuator, the multi-LCE-actuator system can bring additional control capability with more degrees of freedom in complex actuations.
The nematic-isotropic transition temperature (T NI ) of the selfsensing LCE actuator is important in assisting finger movements.The high T NI of the LCE actuator will cause discomfort over extended periods of use.Therefore, if the self-sensing LCE actuator is integrated into a glove to assist finger movement, the T NI must be reduced to a temperature that a person can sustain (usually below 50 °C). [27]In this experiment, the T NI of the LCE actuator can be reduced by increasing the ratio of cross-linker (PETMP) while keeping the total mass of PETMP and RM257 fixed.Figure 4a plots the relationship of T NI of the LCE actuator versus the proportion of cross-linker (PETMP) at 1.5, 2, 2.5, 3, 4.5, and 5 wt%, respectively.It can be seen that the T NI of the LCE actuator gradually decreases from 65 to 42 °C as the percentage of PETMP increases from 1.5 to 5 wt%.However, the flexibility of the LCE actuator deteriorates as the proportion of crosslinker increases.Thus, the effect of the proportion of cross-linker (PETMP) on the mechanical properties of the LCE actuator also needs to be investigated.
Figure 4b plots uniaxial stress-strain curves of the LCE actuator with different doping concentrations of PETMP, where the elongation at break of the LCE actuator decreases (from 327% to 104%) as the ratio of PETMP increases (from 1.5 to 5 wt%), while the stress of the LCE actuator decreases (from 1.13 to 0.18 MPa).It can be seen that an increasing proportion of PETMP proportion leads to a lower elongation at break for the LCE actu-ator, which also indicates that the flexibility of the LCE actuator becomes worse.If the self-sensing LCE actuator is used to assist finger movements, relatively good flexibility and elasticity are required.Thus, the elongation at break of the LCE actuator should be higher than 200%.Therefore, it is necessary to analyze the effect of PETMP on the T NI and the elongation at break of the LCE actuator at the same time to find the optimal ratio of PETMP. Figure 4c shows the T NI and the elongation at break versus concentration of W PETMP .With the increase of W PETMP from 1.5 to 5 wt%, both T NI and the elongation at break decrease from 65 to 42 °C, and from 327% to 104%, respectively.It can be found that a higher ratio of PETMP can increase the cross-link density of the LCE actuator and thus provide a lower T NI , but it will lead to worse flexibility of the device.Considering the above reasons, the ratio of PETMP is chosen to be 2.5 wt% of LCE actuator, and its T NI is 47 °C, and elongation at break is 241%, which is suitable for assisting finger movement.
LCE materials possess intrinsic properties of excellent flexibility, large and reversible deformation, and a similar modulus The original alignment of LCE molecules is set to be parallel along the finger.e) An LCE actuator is attached to the outer side of the finger joint when the voltage of 150 V is powered off (i) and powered on (ii), respectively.The angle between the phalanx prima digitorum manus and the horizontal line is 0°and 28°, respectively, during straightening.f) An LCE actuator is attached to the inner side of the finger joint when the voltage of 150 V is powered off (i) and powered on (ii), respectively.The angle between the phalanx prima digitorum manus and the horizontal line are 85°and 65°, respectively, during bending.
of elasticity close to that of soft biological tissue, which greatly reduces the risk of discomfort and injury to human tissue.Compared to metal finger rehabilitation robots, the LCE material offers better softness and comfortability, reducing the possibility of secondary injuries to the patient caused by finger rehabilitation devices.Compared to pneumatic finger rehabilitation robots made of soft material, the LCE actuator possesses a larger energy density, leading to a compact design of devices with smaller sizes and weights.Therefore, the LCE actuator shows great application potential in the medical field, such as physiotherapy and medical rehabilitation, such as for fingers.
Figure 4d shows a schematic diagram of a smart glove equipped with our proposed self-sensing LCE actuator attached to the finger joint for finger-moving assistance, including bending or straightening, where the original alignment of LCE molecules is set to be parallel along the finger of the model.Figure 4e shows the cases of attaching an LCE actuator (the yellow dotted box) to the outer side of a finger joint when the applied DC voltage of 150 V is powered off (Figure 4ei) and powered on (Figure 4eii), respectively.The angle between the phalanx prima digitorum manus and the horizontal line increased from  1 = 0°to  2 = 28°.The angle change of 28°, here, the positive angle change implies finger straightening (the same as below) when the LCE actuator is heated and contracted through an electrothermal effect.The process can be seen in Movie S4, Supporting Information.In contrast, Figure 4f demonstrates the cases of attaching an LCE actuator (the pink dotted box) to the inner side of the finger joint when the driven voltage is powered off (Figure 4fi) and powered on (Figure 4fii), respectively.The angle between the phalanx prima digitorum manus and the horizontal line decreases from ′ 1 = 85°to ′ 2 = 65°, with an angle change of 20°.Here, the negative angle change implies finger bending (the same as below) when the LCE actuator is heated and contracted.The process can be seen in Movie S5, Supporting Information.Those experiments demonstrate the feasibility of using our LCE actuator to assist the finger movement of bending and straightening.
Considering that the self-sensing LCE actuator provides assistance for finger movements while simultaneously sensing the movement status, programmable training patterns are needed for patients with finger injury or stroke paralysis.Therefore, control circuits based on conditional logic judgments with smart sensing and actuation are required in the control of the smart self-sensing LCE actuator for rehabilitation programs.
Figure 5a shows the control circuits based on conditional logic judgments of self-sensing LCE actuator-based gloves designed for finger movement assistance to deal with the aforementioned situations and needs.The circuit is represented by an orange dotted line.The measuring and control signal transmission are represented by blue solid lines with double and single arrows, respectively.A power supply (GPD-4303S2230G-30-6, GWINSTEK) and a voltage amplifier module (MAX1771, MAXGAT) provide the driven voltage to the LCE actuator.A resistance measurement module (consisting of a purchased voltage measurement module of YOURCEE-10mΩ and a self-designed circuit) provides the signal of resistance change from the LCE actuator.A microcontroller (ARDUINO UNO) delivers the output control signal to the voltage source according to the measured resistance signal, which is displayed in a display as the real-time monitoring signal.When detected ΔR/R 0 reaches the target value (e.g., 100%), a high voltage (e.g., 150 V) will be applied to the LCE actuator.
Figure 5b demonstrates the straightening actuation of the model finger assisted by the LCE actuator-loaded glove at a target bending angle.Initially, the finger is in the original state where the applied voltage is powered off, with an original angle  1 between phalanx prima digitorum manus and the horizontal line of 75°and LCE actuator original resistance of 59 kΩ and an original relative resistance change ΔR/R 0 of 0 (Figure 5bi).Herein, the target bending angle  t is set to be 49°, with a corresponding target ΔR/R 0 of 103%.Then, a low voltage (5 V) is applied to the LCE actuator to monitor the value of ΔR/R 0 , thus measuring the bending angle.When the finger of the model bends to an intermediate state, for example, at an intermediate angle  2 of 65°w ith an intermediate resistance of 75 kΩ and a relative resistance change ΔR/R 0 of 27% (Figure 5bii), which does not reach the target bending angle of 49°(or target relative resistance change ΔR/R 0 of 103%), the low voltage (5 V) is kept applying.When the target ΔR/R 0 is reached, with the corresponding target angle  3 of 49°and the target relative resistance change ΔR/R 0 of 103% (Figure 5biii), a high voltage (150 V) will be applied to contract the LCE actuator, thus straightening (recovered state) the finger with a recovered angle  4 of 85°(Figure 5biv).The process can be found in Movie S6, Supporting Information.
Besides a single LCE actuator, two or more LCE actuators can be used together to achieve multi-actuations, for example, the repeated bending and straightening exercises of the finger in the set bending range.Figure 5c demonstrates the bending (Figure 5ci) and straightening (Figure 5cii) states (with the bending and straightening angles between the LCE actuator and the horizontal line of ′ 1 = 60°and ′ 2 = 72°, respectively) of the model finger with two LCE actuators attached to the outer (LCE-1, in the yellow dotted box) and inner (LCE-2, in the pink dotted box) sides of the joint.It can be seen that both bending and straightening or bi-directional movements of the finger can be automatically controlled by two LCE actuators by setting the bending and straightening angles.
Figure 5d plots the real-time relative resistance changes of LCE-1 and LCE-2 actuators.Initially (from 0 to 35 s), the LCE-2 actuator contracts and bends the finger toward ′ 1 = 60°under the driven voltage of 150 V, accompanied by a decrease in relative resistance change from 0 to −77% (at ′ 1 = 60°).At the same time, as there is still a certain space between the LCE-1 actuator and the finger, this bending does not lead to a significant resis-tance change of LCE-1 in the first 30 s. From 30 to 35 s, the LCE-1 actuator is gradually stretched with the bending of the finger and the relative resistance change increases from 0 to 158%.It is noted that a low voltage of 5 V is continuously applied to the LCE-1 actuator for monitoring.Herein, when the relative resistance change of the LCE-2 actuator reaches the critical value of −77% at 35 s with a bending angle of ′ 1 = 60°, the voltage on LCE-2 is switched from 150 V to the low voltage of 5 V in monitoring mode.At the same time, the LCE-1 actuator starts to contract under the voltage of 150 V and the finger is straightened back toward ′ 2 = 72°.When the relative resistance change of the LCE-1 actuator decreases to −54% at 66 s (at ′ 2 = 72°), the voltage on the LCE-1 actuator is switched from 150 to 5 V in monitoring mode.Simultaneously, in the range of 35 to 66 s, the relative resistance change of the LCE-2 actuator increases up to −64%.At 66 s, the voltage on the LCE-2 actuator is switched from 5 to 150 V in driven mode.The process can be found in Movie S7, Supporting Information.

Conclusion
In summary, intelligent self-sensing LCE actuators with high mechanical strength and automatic control have been demonstrated.Our proposed graphite/CB doped LCE actuator, based on the electrothermal effect, demonstrates a significant increase in the light object mass/self-weight ratio, up to 1168 times, as tested in football kicking and arm swinging of a doll.The self-sensing LCE actuators possess 30% actuation strain and real-time sensing capability during Joule heating, and they are programmable by control circuits based on conditional logic judgments of the target.In addition, a smart glove equipped with proposed selfsensing LCE actuators with high mechanical strength and an automatic control system for finger movement assistance is also demonstrated.The proposed LCE actuator shows great potential for application in finger movement assistance, joint and knee recovery assistance, and other smart physical rehabilitation therapy applications.
Preparation of the Self-Sensing LCE Actuator: In a typical synthesis process, first, the RM257 (2 g) was dissolved in toluene (0.6 mL) at 85 °C and then cooled down to room temperature.Second, the chain extender EDDET (0.64 g), crosslinker PETMP (0.086 g), and photoinitiator HHMP (0.022 g) were added to the toluene with thorough mixing.Then, the mixture was placed in an oven at 85 °C for 1 min and cooled down to room temperature.Finally, the DPA (0.003 mL) diluted with toluene (0.15 mL) was added to the mixture, forming an LC oligomer.Next, the LC oligomer was mixed with conductive graphite (0.028 g) and CB (0.042 g) particles (the mixture of LC oligomer/graphite/CB in toluene (step i) by sonicating.The mixture of LC oligomer/graphite/CB was poured into a polytetrafluoroethylene (PTFE) mold (25 mm × 10 mm × 1 mm) and then placed in a vacuum oven for 5 min to remove the air bubbles.Next, the mixture was placed at room temperature without light for 24 h for first crosslinking, followed by heating in an oven at 85 °C overnight for solvent evaporation.The pre-crosslinked LCE (step ii) film was peeled out of the molds, suspended with a 100 g weight along the long direction of 50 mm, and then cured using UV light at a wavelength of 365 nm with a power of 50 mW cm −2 for 20 min.The film stretching under UV illumination applied here was to align the LCE molecules parallel along the direction of stretching.Next, the parallel-oriented LCE (step iii) was immersed in a toluene solution of a graphite/CB mixture and sonicated for 6 h.After drying for another 6 h at room temperature to allow for solvent evaporation, the graphite/CB doped LCE (step iv) film was obtained with dimensions of 26 mm × 8 mm × 0.8 mm after flattening treatment at both ends.
Electrothermal Testing: A power supply (GPD-4303S2230G-30-6, GWINSTEK), controlled by a custom Arduino script, was used to provide a DC voltage for generating Joule heat in the LCE actuator.The resistance of the LCE actuator was measured and recorded by a circuit designed by ourselves.
Control Circuits Based on Conditional Logic Judgments: The control circuits based on conditional logic judgments of the self-sensing LCE actuator during the assisted finger movement were implemented by a custom Arduino script.This script was then used to program the voltage applied to the LCE actuator and measure the resistance at both ends of the LCE actuator.The target ΔR/R 0 was identified in the control script and a voltage sufficient to actuate the LCE actuator was applied when the target ΔR/R 0 was measured.
Statistical Analysis: For the data on the change in relative resistance of the LCE film versus time at different graphite/CB mass ratios, an electrochemical workstation (CHI660E, CH Instruments) was used to record the data.For the electrothermal test data, a custom Arduino script was used to record.For the data of the control circuits based on conditional logic judgments of the self-sensing LCE actuator, the data was evaluated similarly in the form of a custom Arduino script.To execute the logic judgments, the target of relative resistance changes was determined based on the parameters of the measurements.All statistical analyses were performed using Origin Pro software (version 9.8).
demonstrates the actuation of an LCE actuator (0.18 g) under 150 V loaded with a weight of 210.26 g, where a deformation of 22% (from 18 to 14 mm) is achieved with 1168 times loading ability (Movie S1, Supporting Information).The actuation stress of the LCE actuator is 0.32 MPa.Due to the doping of conductive particles, the LCE actuator possesses the capability of self-sensing.The relative resistance change and deformation of the LCE actuator versus time at a DC voltage of 150 V (corresponding to Figure 2b) are demonstrated in Figure 2d.The relative resistance change decreases from 0 to −60% when the deformation rate of the LCE actuator increases from 0 to 30%.It can be seen that the dynamic deformation of the LCE actuator can be monitored by the resistance change signal without additional sensing elements.The thermal images of the LCE actuator before and after actuation are shown inset.

Figure 2 .
Figure 2. Graphite/CB doped LCE actuator.a) Schematic of an LCE actuator with copper foil as electrodes.b) The photo of the fabricated LCE actuator when the voltage is powered off and on, and the corresponding schematic of mesogens in two states.c) The actuation of the LCE actuator (0.18 g) under 150 V loaded a weight of 210.26 g. d) Relative resistance change and deformation of LCE actuator versus time at a DC voltage of 150 V.The thermal images of the LCE actuator before and after actuation are shown inset.e) Ashby plot of actuation strain versus lifted object mass/self-weight ratio of self-sensing actuators including LCE actuator combined with liquid metal[13,14] and helical metal wire,[18] foam actuator,[27] photo-responsive liquid-vapor phase transition elastomer,[28] dielectric elastomer,[29,30] and hydrogel.[31]f) Relative resistance change ΔR/R 0 of proposed graphite/CB doped LCE actuator versus time for stability test.The measurement is carried by DC voltage with a high voltage of 150 V and a low voltage of 5 V for more than 120 times.Five consecutive cycles (624 to 738 s) arbitrarily selected from the sequence are demonstrated in the inset figure.

Figure 3 .
Figure 3. Actuation of a doll.a) Football kicking by a single LCE actuator on the outside of the knee.The doll stands still i) when the control voltage is powered off.When the controlling voltage is powered on, the LCE actuator shrinks and drives the doll's knee to straighten and kick the football into the goal (ii-iv).b) Arm swinging by two LCE actuators on the inner and outer sides of the shoulder joint at 0 s (i), 20 s (ii), 40 s (iii), 60 s (iv), 80 s (v), and 100 s (vi), with corresponding  of 5°, 15°, 19°, 23°, 28°, and 14°in downward and upward movements.

Figure 4 .
Figure 4. Characteristics of LCE actuator and finger movement assistance.a) The relationship of T NI of LCE actuator versus the proportion of cross-linker (PETMP).b) Uniaxial stress-strain curves of LCE actuator with different doping concentrations of PETMP.c) The effect of PETMP on the T NI and the elongation at break of the LCE actuator.d) Schematic diagram of a smart glove equipped with our proposed self-sensing LCE actuator near the finger joint for finger moving assistance.The original alignment of LCE molecules is set to be parallel along the finger.e) An LCE actuator is attached to the outer side of the finger joint when the voltage of 150 V is powered off (i) and powered on (ii), respectively.The angle between the phalanx prima digitorum manus and the horizontal line is 0°and 28°, respectively, during straightening.f) An LCE actuator is attached to the inner side of the finger joint when the voltage of 150 V is powered off (i) and powered on (ii), respectively.The angle between the phalanx prima digitorum manus and the horizontal line are 85°and 65°, respectively, during bending.

Figure 5 .
Figure 5. a) Control circuits based on conditional logic judgments of self-sensing LCE actuator-based glove.b) Straightening actuation of the model finger assisted by the LCE actuator loaded glove at a target bending angle.c) Bending and straightening actuations of the model finger with two LCE actuators attached to the outer and the inner side of the joint.d) The real-time relative resistance changes of two LCE actuators.