A Soft, Centimeter‐Scaled, Thin‐Cable‐Crawling Robot for Narrow Space Inspection

Cables are critical in engineering structures for load‐bearing, electronic connection, and mechanical transmission. Various cable‐crawling robots (CCRs) have been developed to perform scheduled inspection or convey supplies. Most existing CCRs are often actuated by motors and used in large‐scaled engineering structures. The heavy bodies of these CCRs can cause damage or even casualties once slippage or drop occurs. A small and lightweight CCR that can crawl on thin cables is highly demanded for safety inspection in narrow and confined inner spaces of engineering structures. Herein, a soft CCR (weight, 2.1 g; length, 43 mm) is developed by utilizing multilayered dielectric elastomer actuators. Compared with existing solutions, this CCR achieves crawling on thin cables (diameter: <1 mm) while crawling fastest (horizontal: 0.72 body length per second). The CCR is also capable of transporting objects (horizontal: 3.69 times its own weight; vertical: 0.76 times its own weight), climbing upward on a vertical cable, and locomoting across the water–air interface. The CCR is also demonstrated to crawl on a slack cable and circular/spiral cables. Finally, the soft robot, equipped with an endoscope, demonstrates inspections on a tensegrity structure as well as in an airplane wing model with a preplaced cable.


Introduction
Cables are widely used in engineering structures, for example, satellites, cable-suspended parallel robots, high-voltage transmission systems, and cable-stayed bridges, serving as mechanical transmission, electronic connections, or load-bearing.A robot that can crawl on such cables to perform scheduled inspection or convey supplies is highly demanded.Several cable-crawling robots (CCRs) have been developed for inspecting high-voltage transmission lines, [1] suspension bridges, [2] and plantation growth. [3]These CCRs, often actuated by motors, are of large size (>10 cm) and only suitable for applications in open and large spaces.The heavy bodies of the CCRs could lead to damage or even cause casualties once slippage or drop occurs.
There are many engineering structures with narrow and confined inner spaces that are also requiring scheduled inspection with high safety factor, for example, gas pipelines and airplane wings.Limited by the overall internal size, the cables contained inside these structures or preplaced for inspection are generally very thin in diameter, even several hundred microns.Existing CCRs cannot crawl on these thin cables.This motivates us to develop a miniaturized (centimeter-scaled) soft CCR that can crawl on slender cables for inspection purpose.The main challenges are as follows: first, a new actuating solution needs to be applied, which has the advantages of lightweight and high-power output; second, a new anchoring structure needs to be proposed to achieve reliable anchoring and release for extremely thin cables.Pneumatic actuators are used in some robots that crawl on the outer surfaces of pipes, [4] but such actuators are difficult to miniaturize to achieve the crawling on thin cables.Light-responsive artificial muscles [5] are designed as robots that can crawl along a human hair; however, this material is not suitable for confined spaces.Some other smart materials are also being used to actuate small and lightweight robots.Ionic polymer metal composites [6] yield bending deformation at low voltages (<5 V), but its low operating frequency (only a few Hz) and small output force fail to realize a high-efficient actuation in thin cable crawling.Liquid crystal elastomers [7] and shape-memory alloys [8] deform under thermal stimulation, and their operating frequencies are determined by the period of heating and cooling, so they are also difficult to be utilized to achieve fast crawling of CCR.
Dielectric elastomer actuators (DEAs), composed of dielectric elastomer (DE) membranes and compliant electrodes, can deform significantly under an electric field.In many reports, carbon nanotubes (CNTs) are used as compliant electrodes of DEA for the self-clearing characteristics, which endows the actuator with good tolerance of electrical defects and significantly prolongs the service life of actuation. [9]Besides, multiple layers of DOI: 10.1002/aisy.202300828Cables are critical in engineering structures for load-bearing, electronic connection, and mechanical transmission.Various cable-crawling robots (CCRs) have been developed to perform scheduled inspection or convey supplies.Most existing CCRs are often actuated by motors and used in large-scaled engineering structures.The heavy bodies of these CCRs can cause damage or even casualties once slippage or drop occurs.A small and lightweight CCR that can crawl on thin cables is highly demanded for safety inspection in narrow and confined inner spaces of engineering structures.Herein, a soft CCR (weight, 2.1 g; length, 43 mm) is developed by utilizing multilayered dielectric elastomer actuators.Compared with existing solutions, this CCR achieves crawling on thin cables (diameter: <1 mm) while crawling fastest (horizontal: 0.72 body length per second).The CCR is also capable of transporting objects (horizontal: 3.69 times its own weight; vertical: 0.76 times its own weight), climbing upward on a vertical cable, and locomoting across the water-air interface.The CCR is also demonstrated to crawl on a slack cable and circular/spiral cables.Finally, the soft robot, equipped with an endoscope, demonstrates inspections on a tensegrity structure as well as in an airplane wing model with a preplaced cable.
DE can be bonded via CNTs into a monolithic actuator unit and the mechanical output of the actuator is greatly leveraged.When a DEA is actuated at its resonant frequency, its power density can be leveraged to 563 W kg À1 , [10] which is comparable to biological muscles.The resonant actuation of DEA evidently facilitates a variety of soft robots, such as insect-sized crawling robots, [11] pipeline inspection robot, [12] flapping wing robots, [13] frequencycontrolled flow valves, [14] and wearable soft haptic devices. [15]ompared to existing soft actuation strategy, the high power density and small size of multilayered DEA make it an ideal candidate for actuating miniaturized soft CCR.
The article is organized as follows.First, the structure design and the working principle of the soft CCR constituted of three multilayered DEAs are described in detail.Second, the mechanical properties of DEAs to achieve either elongation or anchoring functions of the soft CCR are characterized.Next, the frequency and phase of the voltage applied on the DEAs are tuned to maximize the crawling speed of the robot.Then, the crawling performances of the soft CCR are tested on slender cables with diameter ranging from several hundred microns to 1 mm.Finally, the soft CCR equipped with an endoscope is demonstrated to perform inspections on a tensegrity structure and inside an airplane wing model, showing its potential applications.

Structure Design
The structure of the soft CCR is illustrated in Figure 1a.The robot consists of three multilayered DEA units: one is an elongation unit in the middle as the robotic body and the other two are anchoring units as the front and rear feet.The detailed fabrication process of the DEA units is described in the Supporting Information.The locomotion on a cable is enabled by the collaborative operations of the stretch of the elongation unit and the alternative clamp and release of the two anchoring units on the cable.Figure 1b illustrates the explosive diagrams of an anchoring unit in detail.The prestretched DEA anchors the cable reliably with an attached hook and acrylic frame.After the voltage is loaded, the DEA elongates to reduce the clamping force of the hook on the cable, thereby releasing the cable and allowing the anchoring unit to slide along the cable.After removing the voltage, the DEA contracts and restores the anchoring state to the cable.A thin silicone mat is utilized to create sufficient friction.Figure 1c illustrates an axial cross section of the elongation unit.This unit consists of an elongation DEA and two concentric thin steel mandrels that are fixed at each end, which improves the bending stiffness of the elongation unit to ensure it can be stretched only.We experimentally confirmed that additional friction and mass due to the steel mandrels have acceptable reduction on the performance of the elongation DEA (Figure S5, Supporting Information).The two anchoring units and the elongation unit are assembled by glue to form a soft CCR (Figure 1d) whose overall length is 43 mm and the mass is 2.1 g.The soft CCR is equipped with hooks of different sizes so that it can crawl on cables of different diameters.Figure 1e shows the soft CCR crawling along a curved cable.

DEA Characterization
The actuation performance of the DEAs can be characterized by three factors: 1) The maximum displacement: the maximum displacement of the elongation unit determines the step size of the robot.In addition, the deformation response of the anchoring unit determines if the robot can timely switch between clamping and releasing states.2) The blocked force: the blocked force of the anchoring DEA is crucial for providing an adequate friction toward a stable anchoring.The friction force is positively related to the clamping force of the hook on the cable, and the latter is generated by the tensile stress inside the prestretched anchoring DEA.The blocked force can be understood as the force that actuates the DEA to elongate itself after being applied with voltage.The blocked force generated by the anchoring DEA can counteract the prestretching force, thus reduce the clamping force on the cable and achieve a change in friction.Figure S6 (Supporting Information) can help to understand how the blocked force contributes the friction force.3) The resonant frequency: the power density of DEA can be maximized when the DEA is dynamically actuated at its resonant state, which consequently promotes the robot's crawling speed.In this section, the theoretical derivation [16] on axial deformation and blocked force of DEA are used to predict the performance of our DEAs.A finite element method (FEM) proposed in our previous work [17] is used as well to evaluate the deformation and blocked forces of DEAs.The results are plotted and compared in Figure 2a,b.
The applied voltage was increased in steps of 100 V until the electrical breakdown of DE material.A laser displacement sensor was used to measure the deformations of three anchoring DEAs and three elongation DEAs.The results are plotted in Figure 2a.It can be observed that the mean value and standard deviation of the maximum axial displacements of the anchoring DEAs are 0.5 and 0.031 mm, respectively.For the elongation DEAs, these data are 1.08 and 0.038 mm, respectively.Both the analytical results of the anchoring DEA and the elongation DEA underestimate the experimental and the FEM results because the decrease in DE thickness becomes non-negligible with the increase of voltage.
A force gauge was used to record the blocked forces of the DEAs by confining the end deformation.The mean value and standard deviation of the maximum blocked forces of the anchoring DEAs are 87.72 and 5.891 mN, respectively.For the elongation DEAs, these data are 98.35 and 3.03 mN, respectively.The experimental results are in good agreement with the FEM and analytical results (Figure 2b).The frequency sweep tests (1000 V peak-to-peak amplitude and 500 V DC bias) were performed on the anchoring and elongation DEAs, and the results are plotted in Figure 2c.The frequency responses show that the three samples of the anchoring DEA are consistent, resonating at 300 Hz and harmonically resonating at 150 Hz.Similarly, the frequency responses of the three samples of the elongation DEAs agree with each other, with the resonant frequency occurring at 120 Hz.Fatigue tests were also conducted on the DEAs.A sinusoidal voltage with an amplitude of 1000 V at 50 Hz was continuously applied to an elongation DEA sample for 30 000 cycles (Figure 2d).Although the displacement of the actuator drifted during the fatigue test due to the creep of DE, the amplitude slightly reduced from 0.46 to 0.45 mm.This ensures the robot maintains a steady crawling speed in a long running time.The resonant frequency of the elongation DEA changed after assembled in the robot, and frequency sweep tests on the three samples of the elongation DEAs show that the resonant frequency drops to 50 Hz with the amplitude of 0.8 mm (Figure 2e).The frequency response of the anchoring DEA is considered to be consistent with that in Figure 2c because the structure of the anchoring DEA does not change significantly.This guided us to determine the frequency corresponding to the robot's fastest crawling around 50 Hz.

Locomotion Principle
The locomotion of our soft CCR is inspired by an inchworm crawling on tree branches.The left subgraph of Figure 3a shows the voltage loading sequence for the three units of the robot, with the square wave voltage being applied to the two anchoring units and the sinusoidal wave voltage for the elongation unit.The right subgraph of Figure 3a illustrates four typical states in a crawling cycle corresponding to the voltage loading sequence: 1) powering on the anchoring unit 1 to release its clamp on the cable; 2) holding the anchoring unit 1 powered and powering on the elongation unit to push the anchoring unit 1 to slide forward; 3) powering on the anchoring unit 2 and powering off the anchoring unit 1 while holding the powering status of the elongation unit; and 4) holding the anchoring unit 2 powered and powering off the elongation unit to pull the anchoring unit 2 to slide forward to complete the forward crawling step.The next cycle begins by switching the powering states of the two anchoring units.The robot's crawling process along a thin cable (diameter: 0.17 mm) at 1 Hz can be found in Movie S1 (Supporting Information).Moreover, a corresponding FEM simulation of the robot's crawling process was performed, and the several states during one crawling cycle are collected in Figure S7 (Supporting Information).The anchoring units were powered by square wave voltage to ensure a fast anchoring or a release from the cable.Both square and sinusoidal wave voltages were tested for the elongation unit, and the results revealed that the robot crawled faster under sinusoidal wave voltage than that of the square wave voltage at the identical frequency and amplitude (Movie S1, Supporting Information).The sinusoidal wave voltage offers a smoother collaborative operation between the elongation DEA and the anchoring DEAs, thus is employed for the elongation unit in the subsequent experiments.
The clamping friction of a single anchoring unit against a thin cable was characterized (Figure 3b).A Teflon cable with a diameter of 0.17 mm through the hook of the anchoring unit was pulled along the track at a constant speed by a force gauge.When a voltage of 1100 V was applied, the anchoring DEA deformed and the hook released the cable; the friction was negligible.When the voltage was removed, the DEA recovered its original length and the hook clamped on the cable.The friction decreased slightly after reaching the maximum of 36.14 mN and fluctuated in a small range with an average of 30.6 mN, which was enough to overcome the gravity of the robot (2.1 g Â 9.8 mN g À1 = 20.58mN) as it crawls vertically.After applying the voltage to the DEA again, the friction vanished instantaneously.

Tuning Locomotion Speed
To ensure the robot crawls at a maximum speed and avoid electrical breakdown, the voltage amplitudes for the anchoring units and the elongation unit are set to 1100 and 1000 V, respectively.Figure 3c shows the crawling speed as a function of the powering frequency.The crawling speed increases first and then decreases as the increase of frequency.The maximum speed, 0.63 body length per second (BL s À1 ), is obtained at 50 Hz.This is consistent with the resonant actuation of the elongation DEA measured in previous subsection (Figure 2e).The axial displacement responses of the elongation and anchoring units were measured under the AC voltages at 50 Hz, and their real-time deformations were recorded synchronously with the applied voltages (Figure 3d).It is found that the phase delay of the elongation unit to the voltage is larger than that of the anchoring unit due to the difference in the geometry.The phase delay of the former is 1.57rad, which is 0.31 rad larger than that of the latter (1.26 rad).This was compensated by advancing the phase of the voltage applied on the elongation unit by 0.31 rad, leading to an increase of the crawling speed of the robot from 0.63 to 0.72 BL s À1 (Figure 3e, Movie S1, Supporting Information).

Crawling Performance
As the robot can crawl quickly on a Teflon cable with a diameter of 0.17 mm, in this section, the crawling capabilities of the robot on cables of different materials and different diameters were tested.Various crawling modes were demonstrated, as well as some representative application examples.By altering the size of the hook on the anchoring unit, this robot can fit cables with diameter ranging from 0.17 to 1.4 mm.The friction of the anchoring unit was tested on different cables, including Teflon cable, steel cable, carbon fiber cable, and braided cable.The frictions at the clamping position were simplified as the products of the normal clamp forces and the constant sliding friction coefficients fitted by experiments (Figure S8 and S9, Movie S1, Supporting Information).
Figure 4a shows a comparison of fastest horizontal speeds with respect to body lengths for existed CCRs, and the specific parameters are recorded in Table S1 (Supporting Information).The currently developed soft CCR features the fastest crawling speed (0.72 BL s À1 ) and a small body length (43 mm).
The crawling capabilities of the robot under different scenarios were investigated.A bidirectional crawling of the soft CCR along a 0.17 mm-diameter Teflon cable is depicted in Figure 4b.With a phase delay (0.31 rad in Figure 3d) compensated, the robot crawled forward for 11.93 cm within 3.86 s.Then, the phase was reversed (3.45 rad) and the robot took 4 s to crawl back to its starting location (Movie S2, Supporting Information).The difference between forward and backward crawling speed can be attributed to the anisotropic friction in two directions.Figure 4c shows the robot crawled upward along a vertically oriented Teflon cable (diameter: 0.3 mm).The robot spent 60 s crawling 13.4 cm vertically under a voltage at 10 Hz (Movie S2, Supporting Information).The vertical crawling was slower than the horizontal one due to the gravity and the swing of the cable.
The robot's ability to transport cargoes was also demonstrated.A container with 10 small nuts inside (total mass of 7.75 g and 3.69 times the robot's weight) was hung on a cable, and the robot pushed it along the cable for 14.5 cm in 16.8 s (Figure 4d, Movie S3, Supporting Information).In the vertical direction, the robot lifted four nuts weighing 1.6 g (0.76 times the robot's weight) by 7.2 cm in 48 s (Figure 4e, Movie S3, Supporting Information).A cable crawling across the water-air interface is presented with a switch of actuating frequency in Figure 4f (Movie S4, Supporting Information).Along a 0.17 mm-diameter and 15.3°-inclined Teflon cable, the robot climbed to the water surface at a speed of 1.07 BL s À1 under an AC voltage at 50 Hz.After switching the frequency to 10 Hz, the robot spent 41.4 s crossing the water-air interface.The switching is necessary due to the swing of the cable at 50 Hz is attenuated by the water, but significant in the air.The latter is suppressed by switching the frequency to 10 Hz to benefit the stable anchoring of the robot.As shown in Movie S4 (Supporting Information), the robot crossed the water-air interface reversibly and noiselessly, offering the potential for stealthy underwater robot.
The robot was also demonstrated to crawl along a slack Teflon cable with a diameter of 0.17 mm (Figure 4g, Movie S2, Supporting Information) and circular rings (steel, diameter of the cable: 0.5 mm) with different diameters (Movie S5, Supporting Information).The robot managed to crawl at a speed of 0.43 BL s À1 ; even the diameter of the circular ring was only 1.16 times the robotic length (Figure 4h).To verify the adaptability of the robot to the 3D track, a 1 m-long steel cable was bent into the spiral shape (Figure 4i), and the robot completed a spiral crawling within 126 s (Movie S5, Supporting Information).

Inspection Application
The proposed soft CCR, equipped with an endoscope (resolution: 400 Â 400 pixels), was demonstrated to perform inspections in two scenarios, open and enclosed.Figure 5a shows the robot climbing the braided cable (diameter: 1 mm) in a tensegrity structure.The robot spent 80 s crawling 26.4 cm upward along a 70°-inclined cable; meanwhile, the endoscope recorded the view on the cable track in real time (Movie S6, Supporting Information).Figure 5b-d demonstrates the robot for inspecting the interior of an airplane wing model.In the wing model (Figure 5c), a 1.4 mm-diameter-curved steel cable passing  S1, Supporting Information).b) A soft CCR crawling bidirectionally along a thin Teflon cable (diameter: 0.17 mm).c) A soft CCR crawling upward along a thin vertical Teflon cable (diameter: 0.3 mm).d) A soft CCR delivering objects up to 3.69 times its own weight (a container with ten small nuts) along a thin cable.e) A soft CCR lifting four nuts up to 0.76 times its own weight along a thin cable.f ) A soft CCR crawling across the water-air interface.g) A soft CCR crawling along a slack Teflon cable (diameter: 0.17 mm) past the lowest point and then crawling up against the gravity.h) The robot crawling at a speed of 0.43 BL s À1 when the diameter of the circular ring was only 1.16 times the robotic length.i) A soft CCR crawling along a steel cable bent into a spiral shape.
through the holes on the ribs was preplaced for the robot to climb on.It took 140 s for the robot to perform an inspection of different parts inside the airplane wing (Movie S6, Supporting Information).Figure 5d collects a few screenshots captured during the inspection and the corresponding real-time endoscopic images.It should be noted that in Figure 3e, Figure 4b-g, and Figure 5d, the actual cable is too thin to be clearly seen.We improve the visual clarity by emphasizing cables to facilitate reader understanding.

Conclusion
Existing bulky CCRs cannot crawl on thin cables to complete inspection tasks in narrow and confined spaces of engineering structures.In this work, a soft CCR that can crawl on thin cables has been proposed, and its anchoring to the cable and body stretching are realized by three multilayered DEAs.The miniaturized soft CCR has almost the smallest length and weight (43 mm, 2.1 g) of all current CCRs.Our robot's horizontal crawling speed relative to the body length is currently the fastest (0.72 BL s À1 ).The robot also crawled on a variety of thin cables with different diameters (from 0.17 to 1.4 mm) and different materials, including Teflon cable, steel cable, carbon fiber cable, and braided cable.The crawling capabilities of the soft CCR under different scenarios were demonstrated.A bidirectional crawling of the robot was achieved by reversing the voltage phase on a DEA.The robot could also crawl along a vertical cable, slack cable, and spiral steel cable.It was capable of delivering objects up to 3.69 times its own weight horizontally and 0.76 times its own weight vertically, as well as crossing the water-air interface.The soft CCR could crawl along a circular ring track with a diameter of only 1.16 times its body length.In addition, two potential nondestructive inspection applications in open or enclosed space of the robot have been demonstrated.The crawling capability of the new centimeter-scaled robot proposed in this work provides a potential solution for performing inspection tasks in hazardous or difficult-to-access spaces, especially in narrow and confined internal spaces of engineering structures.
For future work, we will improve from the following two aspects: first, we hope to realize the miniaturization and integration design of power supply and control system to further realize an untethered robot; second, we will improve the structure of this CCR to make it suitable for networked cables with branches.Current robot can only crawl along a single cable, and we hope that the future CCR can switch between cables.

Experimental Section
Materials for DEA: The DEA materials were composed of platinumcatalyzed silicone (Ecoflex-0030, Smooth-on, USA) and polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning, USA) at a mass ratio of 5:4.Ecoflex-0030 was produced by mixing part A and part B at a mass ratio of 1:1.In Sylgard 184, the mass of the crosslinker was 1/40 that of the monomer.For the electrodes, the initial CNT aqueous suspension (0.15 wt%, XFNANO Materials Tech., China) was diluted 100 times with deionized water, and then 0.9 mL of the solution was diluted again to 50 mL with deionized water.The CNT solution was distributed uniformly (average areal density of 2.12 Â 10 À9 g mm À2 ) on a PTFE filter (Delvstlab, China) through filtration.An epoxy conductive silver adhesive (8331S, MG Chemicals, Canada) was used to glue the DEA and metal end caps.A Teflon-coated conductive copper cable (Shenzhen Hualiansheng Cable and Cable Co., Ltd.) with a diameter of 0.17 mm was used to power the DEA.
Water-Proof Treatment: For the soft CCR crossing the water-air interface, plastic end caps were used instead of metal end caps for electrical insulation.A silicone rubber (K-706 from Kafuter, China) was employed to seal the bonded part between the DEAs and the plastic end caps.
Assembly: The anchoring unit was formed by bonding the DEA and the acrylic frame together with glue.The acrylic frame was obtained by laser cutting a 1 mm-thick acrylic sheet to get several small parts and gluing them together.The dimensions of the acrylic frame can be found in Figure S3 (Supporting Information).
Characterization of DEA and Robot: A signal generator (DG4202, Rigol, China) was used to provide the original voltage signal, which was amplified by a high-voltage amplifier (AMP-20B20, Matsusada, Japan).A point laser displacement sensor (IL-065, Keyence, Japan) was used for the deformation measurements, fatigue test, and frequency sweep test of the DEAs (Figure S10, Supporting Information), and a data read card (USB-6341, National Instruments, USA) was used to record the deformation of the actuator.A force gauge (ZNLBS-IIX, Chino Sensor, China) was used to record the blocked force of the actuator (Figure S11, Supporting Information).Three high-voltage amplifiers (AMP-20B20, Matsusada Japan; 20/20C, Trek, USA; 610E, Trek, USA) were used to power two anchoring units and one elongation unit.

Figure 1 .
Figure 1.The structure of the soft CCR.a) The soft CCR is composed of three DEA units: two are anchoring units and the other is elongation unit.b) Detailed illustration of the anchoring unit.c) Axial cross section of the elongation unit.d) Photograph of the anchoring unit, the elongation unit, and assembled robot.e) Photograph of the soft CCR crawling along a curved cable.

Figure 2 .
Figure 2. DEA characterization.a) Axial displacement of anchoring and elongation DEAs.b) Blocked force of anchoring and elongation DEAs.c) Frequency sweep of anchoring and elongation DEAs in axial deformation.d) Fatigue test of an elongation DEA over 30 000 cycles.e) Frequency response of elongation DEAs after being assembled into the soft CCRs.

Figure 3 .
Figure 3.The crawling principle and crawling speed tuning of the soft CCR.a) Left: The actuating strategy of each unit in the robot.Right: Illustration of several states in a crawling cycle corresponding to the left image.b) Change of the friction of the anchoring unit against a thin Teflon cable (diameter: 0.17 mm) before and after applying a voltage of 1100 V.The insets illustrate that a force gauge is used for determining the clamping friction of the anchoring unit against the cable.c) Crawling speed of the robot under the AC voltages at different frequencies.d) Axial displacement of the elongation unit (left subgraph) and the anchoring unit (right subgraph) under the AC voltages at 50 Hz; the phase delay of the elongation unit is 1.57rad, which is 0.31 rad larger than that of the anchoring unit.e) After compensating the phase delay of the deformation in (d), the robot reached a maximum speed of 0.72 BL s À1 .

Figure 4 .
Figure 4. Crawling demonstrations under different scenarios.a) Comparison of different CCRs based on two figures-of-merit: the crawling speed and the robotic length.The proposed soft CCR features the fastest crawling speed of 0.72 BL s À1 and a small body length of 43 mm.The data were obtained from the indicated references (TableS1, Supporting Information).b) A soft CCR crawling bidirectionally along a thin Teflon cable (diameter: 0.17 mm).c) A soft CCR crawling upward along a thin vertical Teflon cable (diameter: 0.3 mm).d) A soft CCR delivering objects up to 3.69 times its own weight (a container with ten small nuts) along a thin cable.e) A soft CCR lifting four nuts up to 0.76 times its own weight along a thin cable.f ) A soft CCR crawling across the water-air interface.g) A soft CCR crawling along a slack Teflon cable (diameter: 0.17 mm) past the lowest point and then crawling up against the gravity.h) The robot crawling at a speed of 0.43 BL s À1 when the diameter of the circular ring was only 1.16 times the robotic length.i) A soft CCR crawling along a steel cable bent into a spiral shape.

Figure 5 .
Figure 5. Inspection of a tensegrity structure and an airplane wing model by the soft CCR equipped with an endoscope.a) The soft CCR crawls along a braided cable (diameter: 1 mm) in a tensegrity structure; the texture of the cable is recorded by the endoscope in real time.b) An endoscope is attached to the bottom of the soft CCR to perform internal inspection of an airplane wing model.c) The structure of the airplane wing model.d) Screenshots of the robot crawling along the cable inside the airplane wing model and corresponding real-time images captured by the endoscope.