Photothermal‐Driven Crawlable Soft Robot with Bionic Earthworm‐Like Bristle Structure

Remote stimuli‐responsive movable soft robots without the need for complex 3D deformation processes and specific working environments is an unsolved problem and urgent need. In this work, under the inspiration of mother nature, a novel strategy of simple combination of the bionic bristles structure and the photothermal‐driven reversible shape change liquid crystal polymer (LCP) actuator is proposed to work out this difficulty. The combination structure is designed as unique three parts with two bionic bristle units at the two ends and one LCP unit in the center with a soft connection between them. After matching the driving force and the resistance, the prepared soft robot can realize the earthworm‐like unidirectionally crawl on the paper surface upon near‐infrared light irradiation through the contracting and stretching of LCP with a maximum average speed of 4.4 mm min−1. What's more, the crawling speed of the soft robot can be regulated by varying the irradiation distance of near‐infrared light or the length of the actuators. This strategy realizes the type of remote wireless control soft robot and has potential application ability in other soft robots with various demands.


Introduction
Light-driven soft robots have been a hot topic in material science because the properties (wavelength, intensity, and polarization) of light can be optimized for specific needs with a high spatial and temporal resolution, also enabling robots' excellent remote-control performance.The light-driven soft robots mostly draw support from the reversible shape changes to provide energy for various motions, [1,2] such as swimming, [3,4] crawling, [5][6][7] rolling, [8,9] and jumping. [10]The reversible shape changes are induced by various light stimulus responses such as photochemical and photothermal effects.][13][14][15] The magnitude of the force determines whether the robot can overcome the resistance, and the force direction determines how the robot moves.Nowadays, the most used approach to adjust the direction is to construct a special 3D deformation program for the stimulusresponsive material, such as undulatory motion, [3] winding and unwinding of the asymmetric helix, [16] and shrinking and untwisting of ribbon, [17] because it is hard to obtain a unidirectional force through the simple reversible deformation.][20] What's more, even with these specially prepared functional architectures, the additional demands on moving media are also necessary, which would limit the development of these kinds of novel soft robots. [3,21,22]s there a construction or method that can well control the movement of the soft robots without special complex programs and can achieve walking on a common surface?We've looked extensively in nature and found that earthworms as a mollusk could walk on different surfaces through muscle stretching with the aid of their bristles.It is the obliquely backward bristles that can make the backward moving much more difficult than forward moving in the earthworm telescopic crawling, which ensures that the earthworm can always crawl forward.
Herein, we propose a novel strategy for realizing the wireless remote-controllable earthworm crawling-like motion of soft robots on the common surface through constructing a combination of bionic bristles and common reversible shape-change soft materials.[25][26][27][28][29] The ideal adhesion for "gecko's foot" systems is essential for these kinds of structural designs of microfibrillar tip shapes, which will also inevitably increase the difficulty of robot preparation.In contrast, the motion mechanism for our design does not require the structure design of bristle tip shapes, just that the simple solid fiber will meet the requirement.In this work, this strategy will be explored by selecting previously reported photothermalresponsive liquid crystal polymers (LCP, Figure S1, Supporting Information) [5] as reversible shape change soft materials and introducing the bionic bristles construction, in which the LCP has no special 3D deformation design like that investigated in previous research, and the motions don't need additional media for assistance. [19,22,30]

Results and Discussion
Earthworms can cooperate with body muscle contractions with the obliquely backward bristles on the abdomen (Figure 1a) to crawl.Hence, the central design for our soft robot is the combination of the obliquely backward bristles to obstruct the body backward sliding and the reversible deformation of the photothermal-driven LCP actuator.The former would enable the forward resistance (F f ) to always be smaller than the backward resistance (F b ), while the latter could offer the driving force induced by near-infrared (NIR) light.The novel strategy seems very simple; however, it is wired that it has not yet been realized.The reason, we believed, should be attributed to that the soft robot prepared by inserting the bristles directly on the bottom surface of the actuator could not crawl upon the stimulus, because the deformation of the shape-changeable body will lead to the bionic bristle structure changes and just result in the in situ contracting and stretching.Further observation of earthworms inspires us that multisegment structure should be the key point to achieve crawling, because the walking of earthworms is composed of consequently shape change of different parts of the body.Therefore, our robots consist of three parts, as shown in Figure 1b, two bionic bristle units at the two ends of the whole setup and one driven unit in the center.The bionic bristle units are named front foot and back foot respectively.The driven unit is LCP laminated with bendable polyimide (PI) tape on the top, which is longer than LCP and the two free ends are used to realize a soft connection with the bionic bristle units.A single complete motion cycle of a robot should consist of two stages: First, upon NIR light irradiation, the LCP contracts and the actuator bends, in which process, due to backward sliding, it was hindered by the bionic bristles (F f < F b ) and the driving force (F d ) of the actuator is bigger than F f , the back foot slides forward.Second, after NIR light was removed, the LCP actuator stretched and the actuator restored to its original shape, just that the front foot was propelled to slide forward due to the same reason.The overall effect is that the whole setup could accomplish forward crawling.
The bionic bristle units were prepared by ultraviolet curing technology through the process shown in Figure 2a.First, the perforated resin mold with certain distribution and inclination angle holes was obtained by 3D light printing and put into a vessel.A certain amount of polydimethylsiloxane-184 (PDMS-184) was poured into the vessel and cured at 60 °C for 8 h to obtain a PDMS mold with 3 mm in thickness.The PDMS mold was put on the ultraviolet (UV) curing tank, and then the fibers (0.2 mm in diameter) were inserted into the holes of the PDMS mold.Finally, after curing for 5 min under the light-emitting diode UV (LED-UV) source, the bionic bristle unit worked out.To further explore the influence of bristle inclination angle on resistance, F f and F b of bionic bristles with an inclination angle of 30°, 45°, 60°, 75°, and 90°were tested by an electromechanical universal testing machine (Figure S2, Supporting Information).As expected, the results in Figure 2b-d  the friction of their tilted resin bristles (0.7 mm in diameter) did not differ in the two directions of forward and backward. [31]The reason for this great contrast in the results of the experiment we believe should be due to the significant deformation of our bionic bristles during the movement.All the maximum ΔF of various length bionic bristles appear at an inclination angle of 60°, which should be due to the significant deformation quantity change of bionic bristles that happened near this angle.Therefore, the deformations of the bionic bristle with different lengths and inclination angles were simulated by the finite-element analysis (FEA), and the maximum deflections were worked out.As shown in Figure S3 (Supporting Information), the maximum deflections in forward motion decreased as the inclination angle increased, while there was a transition from negative to positive for the deflections in backward motion between 45°and 60°the inclination angle.The positive and negative means that the direction of deflection is the same as or opposite to the direction of motion, respectively.The transition could explain the reason that backward moving is hindered.The difference in deflection before and after this transition for 5 mm bionic bristles is 2.8 times bigger than for that of 3 mm bionic bristles.As a result, the maximum ΔF of 5 mm bionic bristles is 2.4 times more than that of 3 mm bionic bristles.Furthermore, the result that ΔF on the smooth glass surface (Figure S4, Supporting Information) greatly decreased compared with that on the paper surface (Figure 2b-d), due to much less friction coefficient on the smoother glass surface, would reduce the deformation quantity of bionic bristles.Earthworms could not crawl on smooth glass surfaces too, which is consistent with our explanation.The driving force of the soft robot's photothermal-driven LCP actuator, which is bigger or smaller than the resistance of bionic bristles, directly determines whether the soft robot can crawl under NIR light or not.The LCP actuator was made of the oriented LCP film (Figure S5, Supporting Information) laminated with polyimide (PI) film, which can perform photothermalinduced reversible deformation.As shown in Figure 3a and Movie S1 (Supporting Information), before being subjected to NIR light stimulation, the LCP actuator was straight, and when the NIR light turned on, it began to bend to the LCP side, because the LCP displays macroscopic contraction along the orientation direction, which leads to the bending of the actuator due to the inhibition of the PI film.After the NIR light is turned off, it comes back to the straight state.The reason for this change is that oriented LCPs can contract at the isotropic phase and stretch at the anisotropic phase reversibly.It can be proved by the 2D X-Ray diffraction (XRD) pattern of the actuator in Figure 3b that there are two bright reflection arcs that imply a parallel orientation of molecular chains of LCP when the temperature is below the phase transition point (room temperature, Figure S6, Supporting Information).Then, the diffraction pattern changes into a uniform diffraction ring as the temperature reaches the phase transition temperature (80 °C, Figure 3c), which indicates the absence of macroscopic orientation of molecular chains of LCP.The reversibility of the shape change could be verified by the reversible variation of orientation degree upon the temperature change.As shown in Figure 3d, the orientation of LCP gradually decreased with the increase in temperature.When the temperature increased to 80 °C, the orientation of LCP decreased from 87.5% to 38.4% (decreased by 56%), while the orientation of LCP gradually increased with the decrease in temperature, and the orientation of LCP increased to 86.4% (the recovery rate of LCP orientation was 98.7%) when the temperature decreased to room temperature.
For the soft robot with the three-segment construction but without bionic bristles (Figure 4a and Movie S2, Supporting Information), it was found it could only stretch and contract in place and could not crawl.It is due to the impossibility of ΔF 6 ¼ 0 when the LCP actuator deforms.Furthermore, just the introduction of bionic bristles could not ensure the robot's crawl.If the actuator's driving force is less than F f , the soft robot can't crawl forward (Figure S7a and Movie S3, Supporting Information) either.So, it is necessary to adjust the driving force (bending and restoring force) of the actuator to match the resistance of the bristle unit.
The bending (Figure 4b) and restoring force (Figure 4c) of the actuator were tested in the way shown in Figure S8 (Supporting Information).For our prepared robots, as long as both the bending force in the contract process and restoring force in the stretch process is bigger than F f , the crawling must be realized, because F b is always bigger than F f as we proved, and the feet of robot would slide when the driving force is bigger than F f .Driving force will not ever be bigger than F b , because it is not going to grow anymore while the sliding starts.The result of the actuator with one piece of LCP film showed that the biggest bending force of the actuator is 0.050 N (Figure 4b), bigger than the biggest forward resistance F f = 0.025 N (7 mm in length and 60°in inclination angles, Figure 2d) of bionic bristles, while the biggest restoring force is 0.019 N (Figure 4c), smaller than F f , which means that this actuator can just pull the "back foot" sliding forward but cannot push the front foot sliding forward; then the centroid sliding failed.Fortunately, when two pieces of parallel LCP films were used, the biggest bending and restoring force of the actuator increased to 0.085 and 0.040 N separately, both of which are bigger than the forward resistance.Then, the centroid sliding was accomplished successfully.At the same time, there is no change in deforming rate between the actuators with different pieces of LCP films observed.This exhibition demonstrates that this structure could supply an advantage to adjust the driving force through just simply changing the photothermal responsive unit amount, which is not available for the currently reported LCP actuators.As shown in Figure 5a and Movie S4 (Supporting Information), the prepared soft robot with bionic bristles could successfully realize the photothermal-driven crawling as we designed.Upon NIR light irradiation, the LCP actuator bends, due to the backward sliding hindered by the bionic bristles and the back foot sliding forward.Then, the LCP actuator was restored to its original shape after NIR light was turned off, and the front foot was propelled sliding forward.Therefore, upon the repeat NIR on/off, the soft robot achieves crawling on the paper surface.The average forward speed of the robot could reach 1 mm min À1 for 3 mm bionic bristles with 60°inclination angles (F f = 0.021 N, F b = 0.024 N, and ΔF = 0.003 N) and 10 cm NIR irradiation distance.The successful implementation of this design proves a new strategy to achieve controllable normal crawling with just the simple bionic bristle structure without the need for a special environment or shape design.Certainly, the driving force and the bionic bristle's structure should match each other to ensure that soft robot crawls, which could be simply achieved by adjusting the number of LCP units.Furthermore, the continuous bending and restoring forces of the actuator with two LCPs do not decrease after 50 reversible bending and restoring cycles (Figure 5b,c), while in the biggest bending angle of the LCP actuator, there is no obvious attenuation observed after 50 reversible bending cycles (Movie S5, Supporting Information), which proved their good stability and cycle ability.
It is essential that the crawling speed of the soft robots is fast enough and controllable.The speed could be adjusted by changing the irradiation distance and LCP actuator length.
First, a soft robot was composed using the 5 mm bionic bristles units with a 60°inclination angle and a 2 cm-long LCP actuator.The crawling speed of the robot is determined by both step frequency and stride length.The average crawling speed of the soft robot could reach 3.3 mm min À1 (Movie S6, Supporting Information) at a 5 cm irradiation distance and be reduced to 1.3 mm min À1 (Movie S7, Supporting Information) at a 10 cm irradiation distance (Figure 6a).When the irradiation distances are 5 and 10 cm, the step frequencies of the robot are 1.09 steps min À1 and 0.94 steps min À1 , respectively, and the stride length of the robot at 5 cm irradiation distance is 3.42 mm, which is 2 times bigger than that at 10 cm.As the irradiation distance further increased to 15 cm, the average crawling speed decreased to 1 mm min À1 (Movie S8, Supporting Information), while the step frequency decreased to 0.55 steps min À1 , but there was no significant change in the stride length (1.81 mm).However, the robot cannot be driven to crawl at a 20 cm irradiation distance.The reason for this phenomenon is that the robot at a shorter irradiation distance could achieve a higher temperature and a greater bending force and then a bigger crawling speed.As the illumination time increases, the temperature of the LCP of the actuator gradually increases.When the illumination time is greater than 120 s, the temperature tends to stabilize (Figure 6b).The intensities of the NIR light at different distances: 102 mW cm À2 at 5 cm irradiation distance, 76 mW cm À2 at 10 cm irradiation distance, 45 mW cm À2 at 15 cm irradiation distance, and 30 mW cm À2 at 20 cm irradiation distance.While irradiation distance was 5, 10, 15, and 20 cm, the maximum temperature of the LCP of the actuator decreased gradually from 80 to 66 °C, 58 and 46 °C.When the irradiation distance is constant, the bending force of the actuator increases with the increase of illumination time.When the illumination time is greater than 120 s, the bending force tends to stabilize, because the temperature of the actuator tends to stabilize at this time (Figure 6c).When the irradiation distance is 5 and 10 cm, the maximum bending forces achieved are 0.105, and 0.092 N, which are all larger than F f (0.030 N), because the temperatures of the LCP actuator are already higher than the liquid crystal isotropic transition temperature (T LC-iso , 64.1 °C).When the irradiation distance was raised to 15 cm, the temperature of the actuator was between the crystal melting temperature (T m , 50 °C) and T LC-iso .Thus the maximum bending force decreased (0.056 N) but was still higher than F f .When the irradiation distance was further raised to 20 cm, the temperature of the actuator dropped below T m , resulting in the maximum bending force decreased to 0.024 N, lower than F f .
Second, the effect of the length of the LCP actuator on the crawling speed (Figure 6d,e and Movie S9, Movie S7, Movie S10, Supporting Information) showed that the average crawling rate is 0.3 , 1.3, and 2 mm min À1 respectively for the soft robots with 1, 2, and 3 cm-long LCP actuators, combined with 5 mm bionic bristle units with 60°inclination angle at 10 cm irradiation distance.Their stride length and step frequency are 0.3, 1.5, 2.5 mm and 1.09 steps min À1 , 0.94 steps min À1 , 0.94 steps min À1 respectively.When the length of the actuator is 4 cm or 5 cm, the soft robot cannot crawl.The reason for this result is due to the change of the force as well.The bending force and restoring forces of the 1, 2, and 3 cm-long LCP actuators (0.126, 0.092, 0.076 and 0.057, 0,044, 0.027 N) are bigger than F b (0.030 N) and F f (0.023 N), they can drive the robot to crawl.However, the restoring forces for 4 cm or 5 cm samples (0.023 and 0.017 N) are no more than F f , so they disable drive the robot to crawl.What's more, through further optimization, the robots could reach the higher crawling speeds of 4.4 mm min À1 (Figure 6c and Movie S11, Supporting Information) with a 5 cm irradiation distance and a 3 cm-long LCP actuator.Compared to the light-driven soft robot reported (the average speed and the NIR light irradiation intensity are shown in Table S1, Supporting Information), the soft robot in our work showed almost the same speed under a much lower irradiation intensity, which means a much higher power efficiency. [3,6,32,33]

Conclusion
In summary, we proposed a novel strategy to realize the crawling of wireless remote control soft robots on common surfaces through a simple combination of the bionic bristles structure and the photothermal-driven reversible shape-change LCP actuator.The forward resistance of the obliquely backward bristle's structure is always smaller than the backward resistance when sliding on surfaces in the opposite direction, whose difference (ΔF) increases as the inclination angle rises due to the different deformations of the bristles.Except for the design of a combination of bionic bristle and LCP, it is also proved that the new multisegment structure composed of one photothermal responsive unit and two bionic bristle units with soft connections between them is the necessary design to achieve crawling.What's more,  the soft connection between the LCP actuator unit and the bionic bristle units makes it possible for the driving force and the resistance simply matched by adjusting the number of LCP films in a photothermal responsive unit.Furthermore, the crawling speed of the soft robot can be regulated by varying the irradiation distance of NIR light or the length of the actuators, upon the repeat NIR on/off; the soft robot whose driving force and the resistance were matched achieves the crawling at an average speed of 4.4 mm min À1 on the paper surface successfully without any special 3D deformation design and additional media for assistance.This achievement highlights the importance of the bionic bristle in broadening the selection of driving materials for soft robots.

Experimental Section
Materials: 4,4 0 -Dihydroxybiphenyl (9 Ding Chemistry, 99%), p-coumaric acid (Shanghai Yuanye Bio-Technology Co., Ltd, 98%), 6-chlorohexanol (Heowns Biochemistry, 95%), and phenylsuccinic acid (PSA, Aladdin, 98%) were used directly without further purification.The polyimide film (DuPont Kapton) used in this study was 50 μm thick including the acrylic adhesive layer.Polyester acrylate was purchased from SARTOMER.Photoinitiator 1173 was purchased from Tianjin Jiuri Co., Ltd.Polydimethylsiloxane (PDMS-184) was purchased from DOWSIL.Polybutylene terephthalate fibers (PBT, 0.2 mm in diameter) were purchased from DuPont.Bionic Bristle's Composite Structure Preparation: The resin mold with a certain distribution and inclination angle obtained by 3D light printing was put into a PP vessel.A certain amount of PDMS was poured into the PP vessel and cured at 60 °C for 8 h to obtain a PDMS mold with a certain thickness.The PDMS mold was put on the UV-curing tank with a depth of 2 mm, and then the polybutylene terephthalate (PBT) fibers were put into the hole of the PDMS mold.Finally, polyester acrylate and photoinitiator 1173 (1 wt% of polyester acrylate) were put into the curing tank and cured for 5 min under the LED-UV source to obtain the bionic bristle composite structure.
Liquid Crystal Polymer (LCP) Fabrication: The 4,4 0 -bis(6-hydroxy hexyloxy)biphenyl (BHHBP, 1159.6 mg, 3 mmol) and 4-(6-Hydroxy -hexyloxy) cinnamic acid (6HCA, 211.4 mg, 1 mmol), and PSA (582.5 mg, 3 mmol) were loaded into a 100-mL three-necked flask, which was equipped with nitrogen inlet and outlet, a mechanical stirrer, as well as a distillation trap connected to a vacuum line.Then, the catalysts Zn(Ac) 2 (3.9 mg, 0.2 wt%) and Sb 2 O 3 (5.9mg, 0.3 wt%) were added into the flask.The system was poured with nitrogen at room temperature for ten minutes before being preheated to 170 °C to allow the reactants to melt.Afterward, the melted mixture was mechanically stirred with a speed of 90 % 100 rad min À1 .After 4 h, the system was further heated to 185 °C with high vacuum and kept for another 4 h.Finally, the LCP sample was collected after cooling the flask to room temperature.The resultant product was dissolved in chloroform and precipitated with ice methanol before filtration.Finally, the obtained product was dried at 50 °C in vacuum for 24 h.
Actuator Fabrication: The LCP film (1 mm in thickness) was carefully stretched with a mechanical tester (MTS, America) at 51 °C (in the nematic phase) to 300% AE 10% elongation.Then, the stretched film was exposed to UV light (320-500 nm filter, 60 mW cm À2 ) at 45 °C (above T g ) to form photocrosslinking.After that, the crosslinked LCP films were stretched with a mechanical tester at 51 °C (in the nematic phase) to 200% AE 10% elongation and glued to polyimide film.
Differential Scanning Calorimetry (DSC): Thermal analysis was carried out by differential scanning calorimeter (Q2000, TA Instruments, New Castle, DE).The sample weight was about 5 mg, and the scanning rate was 10 °C min À1 .
Thermal Gravity Analysis (TG): The thermal analysis was conducted using a thermogravimetric analyzer (Q500, TA Instruments, New Castle, DE) under nitrogen flow in the temperature range between 30 and 600 °C, and the heating rate was 10 °C min À1 .
Polarized Optical Microscopy (POM): Polarized optical microscopy (POM) images of the synthesized LCNs with glass slides were taken from Leica DM2500 (Leica, Germany) with crossed polarizers.The sample chamber was under air environment.The measurements were performed from room temperature to 100 °C at a heating rate of 5 °C min À1 .All images of samples were taken after the sample temperature reached the set temperature of 1 min.
Resistance Test: Resistances were measured using a custom-built instrument on a mechanical tester (MTS, America) with a load range of 10 N. Bionic bristles were kept horizontal on different contact surfaces and the speed of motion was 2 mm min À1 .The force collected by the mechanical tester to keep samples moving at a constant speed was numerically equal to the resistance force of the samples.
Bending and Restoring Force Test: Bending and restoring forces were measured on a mechanical tester (MTS, America) with a load range of 10 N. The LCP actuator was fixed straight during the measurement.The actuator was heated by a NIR light source at a sample distance of 5 mm.It was kept straight and a preload of 0.0001 N was applied when monitoring the actuation stresses during NIR light on and off cycles.A thermocouple wire was placed on the side of the sample to obtain the real-time temperature.
Finite Element Analysis (FEA): FEA was performed by Abaqus to simulate the strain in the fiber of bionic bristle under different conditions.The fiber of bionic bristle with a height of 3 mm and diameter of 200 μm was built to simplify the calculation, and the C3D8R element type was used for calculation.Material properties of BPT fiber (elastic modulus = 3000 MPa, Poisson ratio = 0.32) were input to Abaqus.In order to obtain the strain generated by the resistance of the bristles during the slide, the upper-end face of the fiber was fixed and could not be deformed, and the lower end of the fiber could be freely deformed.A boundary loading pressure was applied to the bottom of the fiber to simulate the resistance forces of the bionic bristle.A 0.03 N horizontal pulling force and 0.04 N vertical upward support force were applied along the Y and -Y axial direction at the boundary of the fiber to simulate the backward resistance forces (F b ) and the forward resistance forces (F f ).

Figure 1 .
Figure 1.Bionic principle of soft robots.a) The earthworm and the closeup of the bristles on its abdomen.b) Schematic illustrations of motion principle of soft robot with bionic bristles under photothermal driving.

Figure 2 .
Figure 2. The preparation and characterization of the bionic bristles.a) The preparation steps of bionic bristles.b-d) F f , F b , and ΔF of the bionic bristles with different inclination angles (30°, 45°, 60°, 75°, and 90°) sliding on the paper surface.The length of bionic bristles respectively is 3, 5, and 7 mm.

Figure 3 .
Figure 3. Characterization of LCP actuator and bionic bristles.a) Schematic and photographs of LCP actuator showing reversible bending.b) 2D XRD patterns of LCP actuator at room temperature.c) 2D XRD patterns of LCP actuator at 80 °C.d) The orientation of the LCP actuator at different temperatures and orientation degrees was calculated from the 2D XRD patterns.

Figure 4 .
Figure 4.The soft robot only stretched and contracted in place due to the stretching and contracting in situ without crawling under NIR light.a) The soft robot without bionic bristles could not crawl under NIR light.b) Bending force of photothermal-driven LCP actuator with different amounts of LCP.c) Restoring force photothermal-driven LCP actuator with different amounts of LCP.

Figure 5 .
Figure 5. Soft robots are crawlable based on NIR light control.a) The number of LCPs of the driven unit of the soft robot is two, and it can crawl under NIR light.The length of bionic bristles is 5 mm.b) The continuous bending force of photothermal-driven LCP actuator with two LCPs.c) The continuous restoring force of photothermal-driven LCP actuator with two LCPs.d) Tests for bending angle and bending cycles of photothermal-driven LCP actuator.

Figure 6 .
Figure 6.Modulation of the robot's crawling speed by varying the NIR light irradiation distance and the length of the LCP actuator.a) Experimental results (gray dots) for average crawling speeds from robots (the 5 mm bionic bristles units with 60 o inclination angle) with different geometric combinations used to plot a color map as a function of irradiation distance of NIR light and length of LCP actuator.b) The irradiation temperature rise curves when the NIR light irradiation distances are 5, 10, 15, and 20 cm (T m is the crystal melting temperature and T LC-iso is the liquid crystal isotropic transition temperature).c) The bending force of the LCP actuator with a length of 2 cm when NIR light irradiation distances are 5, 10, 15, and 20 cm.d) The bending force of LCP actuator with different lengths of 1, 2, 3, 4, and 5 cm, when the NIR irradiation distance was 10 cm.e) The restoring force of LCP actuator with different lengths of 1, 2, 3, 4, and 5 cm, when the NIR irradiation distances are 10 cm.