Programmable Design and Fabrication of 3D Variable–Stiffness Structure Based on Patterned Graphene‐Heating Network

The 3D variable–stiffness structure can realize shape programming, reconstruction, adaptation, and locking, and therefore, it has a wide design creation space. Accurate local stiffness control is of considerable significance to the design and application of 3D variable–stiffness structures although it is challenging. Herein, a 3D variable–stiffness structure realization scheme based on a patterned heating network is introduced. The laser‐engraving and 3D‐printing technologies are combined to obtain a 3D variable–stiffness structure composed of a patterned graphene‐heating network (PGHN) and polylactic acid (PLA). The proposed scheme uses PGHN to accurately control the local stiffness of 3D PLA and realize programmable design and fabrication of 3D variable–stiffness structures. The “torsional structure,” “hexagonal structure,” and “spring” cases are used to elaborate the designability, excellent deformation and reconstruction capacity, and reasonable load bearing capacity of the PGHN/PLA variable–stiffness structure. A pneumatic disc, which is used as a reference for studies on shape control of PGHN/PLA variable–stiffness structures, is designed. Also, a pneumatic robot is designed based on the local stiffness control and shape‐locking function of PGHN/PLA to achieve multimode motion control using a single air source. The PGHN/PLA variable–stiffness structure has potential applications in multimode robots, wearable devices, and deployable structures.

stiffness structure has more creative space and potential application value.
The study of complex variable-stiffness structures can be explained from the perspective of dimensions. A 1D variablestiffness structure primarily includes wires and pipes, such as a flexible silicone tube filled with cryogenic liquid metal, to produce a variable-stiffness fiber. [5,16] Alternatively, conductive shape memory polymers are made into thin tubes with variablestiffness, and permanent magnets are attached to them to control their shape using external magnetic fields. [26] The study of 2D variable-stiffness structure includes a variable-stiffness fabric made of 1D variable-stiffness and other fabrics, [14,16] composite variable-stiffness structure based on liquid metal and flexible silica gel material, [3,27] and "jamming skins" that employ vacuum-powered. [4,28] The 2D variable-stiffness structure with an integrated drive unit can perfectly combine shape programming, reconstruction, adaptation, and locking capabilities with the infinite curvature of a 2D plane. [3,4,29,30] It can be used in multimodal vehicles, morphing drones, amphibious robots, removable exoskeletons, wearable devices, and other applications. [3,4,12,14] However, because of the continuity of the 2D plane itself, a 2D structure with a variable stiffness cannot be changed into any 3D shape, which hinders its applicability.
At present, three types of realization schemes for 3D variablestiffness structures exist. First, it is the method of transforming 2D variable stiffness structure into 3D variable stiffness structure. [3,14] Second, the 3D variable-stiffness structure is assembled using variable-stiffness elements according to the target requirements. [15,31] This scheme can be used to easily and quickly design and assemble 3D variable-stiffness structures. However, owing to the limitations of the structural shape, size, and deformation ability of variable-stiffness elements, the design of 3D variable-stiffness structures is limited. Third is a preparation scheme for variable-stiffness structures based on the 3D-printing technology. [32][33][34] This method can be used to fabricate 3D variable-stiffness structures of arbitrary shapes. However, according to existing reports, the rigid structures of this method are all composed of variable-stiffness materials, and the overall stiffness of the structure is simultaneously controlled by the external temperature field; [32][33][34] thus, the implementation of local stiffness and multimode controls for 3D variable-stiffness structures is difficult.
In this study, we present a scheme for 3D programmable variable-stiffness structures based on patterned graphene-heating networks (PGHNs), as shown in Figure 1. Polylactic acid (PLA) with thermal plasticity was selected as the variable-stiffness material, and 3D-PLA was prepared using the fused deposition modeling (FDM) 3D-printing technology. A laser engraver was used to process graphene films into PGHN, which was then bonded to the target-heating position of 3D-PLA, and PGHN/ PLA variable-stiffness structures were formed. This scheme can use a PGHN to accurately heat any position of the 3D-PLA, that is, local stiffness control. Therefore, we realized a programmable design and fabrication of 3D variable-stiffness structures. We then demonstrated the designability, excellent deformation, and reconstruction capacity, and reasonable bearing capacity of the PGHN/PLA variable-stiffness structure using the cases of "origami structure," "torsional structure," "hexagonal structure," and "spring." Additionally, we designed a pneumatic disc to demonstrate morphological control of the PGHN/PLA variablestiffness structure. Finally, we designed a pneumatic robot, named R-VS. The robot realized a two-way motion control under a single air source based on the local stiffness control and shape-locking function of the PGHN/PLA variable-stiffness structure. In conclusion, the proposed PGHN/PLA 3D variable-stiffness structure has shape designability, reconstruction, locking, and excellent load-bearing capacity, while the local stiffness control, programmable design, and fabrication of 3D variable-stiffness Figure 1. The polylactic acid (PLA) structure was prepared using fused deposition modeling (FDM) 3D-printing technology, the patterned grapheneheating network (PGHN) was prepared using laser engraver. The PGHN was then bonded to the target-heating position of PLA structure, and PGHN/PLA variable-stiffness structures were formed. Left side of the figure shown in the designability, excellent deformation and reconstruction capacity, and reasonable load bearing capacity of the PGHN/PLA variable-stiffness structure. Right side of the figure shown in potential applications of the PGHN/PLA variable-stiffness structure.  Figure 2A shows the prepared flake PGHN/PLA sample, which was used to study the local variable-stiffness control of the PLA structure. The thickness of the PLA structure was 1 mm, and the width of the graphene strip was 4 mm. The detailed dimensions are shown in Figure S1, Supporting Information. We applied a current of 0.12 A to the graphene strip. The heating curves of the graphene stripe position and deformation force under cyclic bending are shown in Figure 2B; in addition, the test method is shown in Figure S2, Supporting Information. The results show that the force required to bend the PLA structure decreases with an increasing heating temperature; the force gradually approaches zero when heated for 100 s. After the applied current was turned off, the force required to bend the PLA structure increased with a decreasing temperature. We extracted the bending force, as shown in Figure 2B, converted it into the bending modulus of the PLA structure, and plotted the curves of the bending modulus in terms of the heating temperature, as shown in Figure 2C. The results show that the bending modulus of the PLA structure gradually decreases with the increase of temperature, when the temperature was 80°C (thermal deformation temperature of PLA [20] ), the bending modulus decreased from 3.25 GPa at room temperature to 0.05 GPa, which is 1.5% of the original value. Figure 2D shows the temperature distribution in the stable heating state; the heating temperature was centrally distributed on the graphene strip. Consequently, the PLA structure can be heated in position by applying a current to the PGHN, and the stiffness at this position can be significantly changed. Movie S1, Supporting Information, shows the deformation ability of the flake PGHN/PLA sample when it is heated to temperatures higher than 80°C; it could produce a bending deformation in the full angle range of À180°-180°, as shown in Figure 2E. In addition, the designed clamp had a certain gap owing to the measurement of the cyclic bending force during the testing process, as illustrated in Figure S2A, Supporting Information. Therefore, after cooling and hardening, the slight shape changes of the PLA structure reduced the actual loading displacement during the measurement process, resulting in the bending modulus not returning to its initial state, as presented in Figure 2B. We adjusted the starting point of the displacement loading and tested the specimen at room temperature again; the results showed that the bending modulus was completely restored, as shown in Figure S2B, Supporting Information. In conclusion, by attaching the PGHN to the PLA structure, the PLA structure can be subjected to localization heating and phase transformation, thus realizing local stiffness control and designable shape reconstruction.
Additionally, we explored the heating curves of graphene strips at widths of 2, 3, and 4 mm under different applied currents, as shown in Figure S3, Supporting Information. Furthermore, we draw the curves of stable heating temperature versus the applying currents, as shown in Figure 2F. When the strip width of the www.advancedsciencenews.com www.advintellsyst.com PGHN is 2 mm, applied currents greater than 0.08 A are burned, the PLA structure can be reshaped by applying a current in the range 0.07-0.08 A. Similarly, when the strip width of PGHN is 3 or 4 mm, the PLA structure can be reshaped by applying a current in the range of 0.10-0.13 and 0.11-0.14 A, respectively. However, for the 2 mm width graphene strip, the controllable range of its current is very narrow, it is easy to burn; the 3 mm width graphene strip is also prone to fracture during actual deformation operation. Therefore, in the subsequent experiments, we selected 4 mm width graphene strip to control the local stiffness of the PLA structure.

Programmable Design of 3D Variable-Stiffness Structures
The FDM-3D-printing technology can realize the rapid prototyping of an arbitrary 3D PLA structure, and the laser-engraving technology can quickly and conveniently process 2D graphene films. In theory, programmable fabrication of arbitrary 3D variablestiffness structures can be realized by combining these two techniques. Next, we will show several cases that are discussed. First, we fabricated three collapsible PGHN/PLA variablestiffness structures, as shown in Figure 3. The PGHN in "origami structure" is a classic fold line pattern, the stiffness distribution of the PLA plate can be changed according to the design using the graphene strips on the upper and lower sides as positive and negative electrodes and applying current. Thus, the folding deformation shown in Figure 3 was realized. The variablestiffness pattern of the "torsional structure" is at the junction of the vertebral body and column plate. When the stiffness of the structure decreases after heating by PGHN, the upper and lower vertebral bodies can reverse torsion around the central axis of the structure, and the height decreases. We considered the convex and concave of each angle in the "hexagonal structure" as the variablestiffness pattern; a shrinkage deformation can occur when the stiffness decreases after heating by PGHN.
The structures presented in Figure 3 effectively prove the designability of the 3D PGHN/PLA variable-stiffness structures. However, the following problems in the actual design and fabrication process must be addressed: 1) The current of the thin graphene strip in the trunk was extremely large. For example, a current greater than 1.4 A must be applied to the upper and lower graphene strips for each graphene strip in the "origami structure" shown in Figure 3 to reach the effective current of 0.12 A. This amplitude is considerably greater than the upper limit of 0.14 A, and the PGHN will be burned. We proposed a scheme based on a conductive copper network to disperse the current and solve this problem. We used a thermoplastic polyurethanes (TPU) film to hot-press the cut copper mesh to the graphene pattern, which must flow through a large current; the preparation process is shown in Figure S5, Supporting Information. In the first step of hot-pressing, the A side of the copper network and TUP film are firmly bonded together, and the B side is exposed; in the second step of hot-pressing, the TUP thermoplastic film is firmly bonded Figure 3. Three collapsible PGHN/PLA variable-stiffness structures, including an origami structure, torsional structure, and hexagonal structure; detailed dimensions are shown in Figure S4, Supporting Information. The thermal image was captured under the condition of stable heating temperature (applied current = 0.12 A). The reconstructed shape was obtained by manual operation in the softened state of the variable-stiffness network.
www.advancedsciencenews.com www.advintellsyst.com to the graphene film through the holes in the copper mesh, and the B side of the copper mesh is in close contact with the graphene film; thus, it can conduct current. As the electrical conductivity of the copper network is considerably higher than that of the graphene film, the current travels through the copper network. In addition, because the heating rate of the copper network is considerably lower than that of the graphene film, the burning of PGHN can be avoided while guaranteeing a high conductivity. The selected copper network was a 300 mesh ultrathin copper network, which had a significant flexibility; hence, the hot-pressing process could be used to achieve strong bonding when the circuit was on.
2) The excessive current problem at the intersection of graphene strips was considerable. In Figure 3, the crossover position of graphene strips in the "origami structure" has the problem of excessive current. In this case, the hot-pressing process of the copper network was relatively complicated. Therefore, the width of the graphene film should be increased in the design to reduce the current density and avoid the burning of PGHN, as shown in Figure S4A, Supporting Information.
3) The heating problem at a nontargeted variable-stiffness position appeared. As shown in the "torsional structure" in Figure 3, the design of the variable-stiffness pattern is at the junction of the vertebral body and column plate; however, the thin graphene film strip is required to pass through the column plate and conduct current.
If the width of the graphene strip is the same as that of the variable-stiffness patterns, it will be heated by a higher applied current, resulting in a change in the stiffness at the nontargeted location. To address this problem, the graphene strip width at this location should be increased as much as possible to reduce the current density and avoid heating at nontargeted variablestiffness locations, as shown in Figure S4B, Supporting Information. In addition, graphene strips at the junction of the vertebral body and the column plate also heated the vertebral body, causing it to be deformed. For this heating spread to nontargeted location problem, the structure thickness has to be increased to avoid the undesirable deformation if conditions permit. Therefore, we designed the three bulges on the surface of the vertebral body in "torsion structure" to solve this problem.
4) The problem of excessive applied voltage caused by extremely long graphene strips appears, as shown in the "hexagonal structure" in Figure 3; indeed, its PGHN is a closed-loop heating network composed of a graphene strip. If the graphene strip is broken at one point, with the starting and ending points as the positive and negative points, respectively, the total length is 912 mm. Under this condition, if a current of 0.12 A is applied to the PGHN, a voltage greater than 100 V is required, which imposes higher requirements on the power supply, and there will be security risks in the actual operation. We adopted the multielectrode processing method, as shown in Figure S4C, Supporting Information, to effectively reduce the applied voltage amplitude and solve this problem. And, 5) the problem of length difference of graphene strips between electrodes. If there are multiple conductive paths of graphene strips between two electrodes, and the lengths of each path are different, this will lead to different current flowing through the graphene strips of each path, and then the temperature of some areas is too high or too low. The "origami structure" in Figure 2 has such a problem. For this issue, because the current has a certain controllable range ( Figure 2F), it allows certain length difference range of graphene strips between electrodes. However, the controllable range of current is limited, so it is necessary to avoid excessive length difference in structural design. Therefore, we summarized the problems and solutions in the design and preparation steps and provided design specifications for PGHN/PLA variable-stiffness structures.
The collapsible variable-stiffness structure still has many constraints on its own deformation, and can only produce deformation according to a specific mode. In contrast, the hollowed-out structure has fewer constraints, which can effectively improve the designability of variable-stiffness structures. Figure 4 shows three types of hollowed-out PGHN/PLA variable-stiffness structures. Among them, the PLA structure of the "helical structure" and PGHN are the same spiral involute patterns. Movie S2, Supporting Information, shows the process of its shape changing from 2D to 3D space under the action of gravity after its stiffness was changed by heating. Moreover, the thickness of the prepared PLA helical structure was 2 mm to achieve higher strengths. Because PGHN corresponds to its shape, heat primarily diffuses along the thickness direction. Therefore, the stiffness control of the "helical structure" can still be carried out under the action of a current of 0.12 A. The "mesh structure" shown in Figure 4 is a 5 Â 5 square array, and the PGHN is the corresponding grid pattern. After heating, the "mesh structure" was similar to a soft fabric, which can be changed into a variety of 3D shapes, as presented in Figure S7A, Supporting Information. In addition, although the thickness of the "mesh structure" is only 1 mm, the reconstructed "turtle structure" can still bear more than 1 kg of heavy objects, as shown in Figure S7B, Supporting Information. Figure 4 demonstrates that the "disk structure" can be transformed into a bulge after heating, and its bearing capacity exceeds 1 kg, as illustrated in Figure S7C, Supporting Information. In conclusion, the three hollowed-out PGHN/PLA variable-stiffness structures exhibited a flexible deformation and excellent load-bearing capacity.
Additionally, we prepared a PGHN/PLA variable-stiffness spring and adjusted its length based on its shape reconfiguration characteristics, as shown in Figure S8A, Supporting Information. We compressed the spring from the original 60 mm to a minimum of 45 mm and stretched it to a maximum of 80 mm, as shown in Figure S8B, Supporting Information. The maximum change rate in the length was 77.8%. The test results showed that the elastic coefficient of the spring after shape reconstruction was the same, and the maximum deviation was only 3.26%, as shown in Figure S8D, Supporting Information. It can be seen that the PGHN/PLA variable-stiffness spring can achieve a large proportion of length adjustment without changing the elastic coefficient, which is of great significance for the application of springs under different length requirements.

Shape Control of PGHN/PLA Variable-Stiffness Structures
Shape control of variable-stiffness structures in some applications is indispensable. Therefore, shape control of PGHN/ PLA variable-stiffness structures was realized using the excellent tensile properties and shape recovery ability of silicone elastomer. The PGHN/PLA variable-stiffness pneumatic disc shown in Figure 5A was considered as an example. The pneumatic disc was composed of a PLA variable-stiffness structure, PGHN, and www.advancedsciencenews.com www.advintellsyst.com elastomeric film. The material of the elastomeric film is Ecoflex-00-50, which is prepared by injection molding process. Its deformation was pneumatically driven and controlled, and it contained a PLA chassis with an inflatable port and annular elastomeric sealing ring. The shape-control process of the PGHN/PLA variable-stiffness pneumatic disc is shown in Figure 5B and Movie S3, Supporting Information. 1) The structure is flat in its initial state. We applied a current of 0.12 A to the PGHN, and the PLA structure underwent phase transition softening when the temperature rose to 80°C. 2) Loading an air pressure of 0.1 MPa into the structure produces tension deformation. 3) The loading pressure is maintained constant and the power supply applied to the PGHN is turned off. The shape of the PLA variable-stiffness structure is locked when the pressure source is closed after cooling. At this time, it can carry a weight greater than 1 kg. And, 4) a current of 0.12 A is reapplied to the PGHN, and the PLA structure exhibits a phase transition and softens. At this time, the PGHN/PLA variable-stiffness pneumatic disc is restored to its initial state under the action of the restoring force of the elastomeric film. The previous results effectively verify that the pneumatic structure based on silicone elastomer can be used as a shape control scheme for PGHN/PLA variable stiffness structures.

Robot Motion Direction Control Based on Local Stiffness Regulation
To realize multimode motion and direction control of pneumatic robots, it is often necessary to set multiple air sources or add logic switching components, such as pneumatic valves. [35][36][37] This requirement not only challenges the design and fabrication of robots but also complicates the structure and affects the load capacity. Therefore, we developed a multimode motion pneumatic robot based on the local stiffness control and shape-locking capabilities of PGHN/PLA variable-stiffness structures.
First, we designed a PGHN/PLA variable-stiffness pneumatic joint, which was composed of a PLA structure, PGHN, and an elastomeric chamber, as shown in Figure 6A; the size parameters are shown in Figure S11, Supporting Information. The original stiffness of the PLA structure was maintained without applying a current to the PGHN. At this time, the elastomeric chamber could expand at a pressure of 0.1 MPa; however, the PLA structure did not undergo any bending deformation. When a current of 0.12 A was applied to the PGHN, the stiffness of the PLA structure at the heating location decreased. At this time, the elastomeric chamber expanded under the action of an air pressure of 0.1 MPa and derived the variable-stiffness joint to produce a bending deformation of 45°. Movie S4, Supporting Information, shows the repeated inflating and deflating processes of the PGHN/PLA variable-stiffness pneumatic joint under heated and nonheated conditions, showing the shape-locking ability in the nonheated state and reciprocating motion ability in the heated state. Thus, the local stiffness control can be used to control the opening and closing of the motion joints.
Furthermore, a single airway was used to connect different motion joints of the robot, and a single air pressure source was used to input the power. The motion mode of the robot Figure 4. Three hollowed-out PGHN/PLA variable-stiffness structures, including a helical, mesh, and disk structures; detailed dimensions are shown in Figure S6, Supporting Information. The thermal image was captured under the condition of stable heating temperature (applied current = 0.12 A). The reconstructed shape was obtained by manual operation in the softened state of the variable-stiffness network.
www.advancedsciencenews.com www.advintellsyst.com was changed by controlling the stiffness of the variable-stiffness joint. Therefore, we designed a two-way pneumatic robot, called R-VS, based on the local stiffness control, as shown in Figure 6B. It consists of a PLA body, a PGHN, and an elastomeric chamber; its size parameters and optical photographs are shown in Figure S12, Supporting Information. The PLA body contains two variable-stiffness pneumatic joints, each with a front foot, and the two ends of the PLA fuselage serve as the hind foot. The motion process of R-VS is as follows. 1) The shape of joint B is locked, joint A is softened by heating, and an air pressure of 0.1 MPa is injected into the airway to bend joint A. During this process, hind foot A and front foot B of the robot become suspended; front foot A and hind foot B are in contact with the ground, and their force state is shown in Figure 6C. The front foot A of R-VS exerts a downward force (gravity mg þ , the downward force F Az produced by the bending process of joint A) and a backward force on the ground F Ax , while the ground generates an upward reaction force F An and forward friction force F Af on front foot A. Hind foot B of R-VS exerts a downward (gravity mg þ , the downward force F Bz produced by the bending process of joint A) force and a forward force F Bx on the ground. Meanwhile, the ground produces an upward reaction force F Bf and backward friction force F Bx on hind foot B. When F Af > F Ax and F Bf < F Bx , front foot A will not move backward, and hind foot B will move forward by some distance. 2) When the pressure source is closed, joint A regains its shape under the restorative force of the elastomeric chamber and gravity of R-VS. During this process, front foot A of R-VS exerts a downward force (gravity mg À , the upward force F Ax generated by the joint A shape restoration process) and a forward force on the ground F Ax , while the ground exerts an upward reaction force F An and backward friction force F Af on front foot A. Moreover, hind foot B of R-VS exerts a downward force (gravity mg, the upward force F Bx generated by the joint A shape restoration process) and a backward force F Bx on the ground. Meanwhile, the ground exerts an upward reaction force F Bn and forward friction force F Bf on hind foot B. When F Af < F Ax and F Bf > F Bx , front foot A will move forward some distance, and hind foot B will not move backward.
During the aforementioned motion process, R-VS can move in the direction of B ! A under a cyclic inflating and deflating loading. Similarly, when the shape of joint A is locked and joint B is softened by heating, R-VS can move in the direction of A ! B www.advancedsciencenews.com www.advintellsyst.com under a cyclic inflating and deflating loading. In summary, we achieved multimode motion control of pneumatic robots under a single air source based on the local stiffness control and shapelocking function of the PGHN/PLA structure. In addition, to improve the directional motion ability of R-VS, we glued an inverted down fiber static electricity cloth (IDF-SEC) on its front and hind feet, as shown in Figure 6B. Figure S13, Supporting Information, shows the surface microstructure of IDF-SEC. Because the surface fibers are arranged in a directional incline, the friction coefficient is directional. We tested the friction coefficient of IDF-SEC under different loads, including the forward friction coefficient in the same direction as the fiber inclination and reverse friction coefficient in the opposite direction of the fiber inclination. The test principle and results are shown in Figures S14, Supporting Information, and 5B, respectively. By fitting, the forward and reverse friction coefficients of IDF-SEC are 0.098 and 0.154, respectively; the latter is 1.57 times the former, which is a significant difference. IDF-SEC was bonded on front foot A and hind foot B to make the fiber tilt direction consistent with the direction A ! B; IDF-SEC was bonded on hind foot A and front foot B to make the fiber tilt direction opposite to the direction A ! B. This reduces the www.advancedsciencenews.com www.advintellsyst.com friction for moving each foot forward and increases the friction for moving backward, thus improving the directional motion ability of R-VS. Movie S5, Supporting Information, and Figure 6D show the motion direction control ability of R-VS. We measured the rightward and leftward movement speed of the robot R-VS as 0.29 and 0.54 cm s À1 , respectively. We believe that the difference of bidirectional motion speed is due to the interference of air tube.

Conclusion
In this study, we developed a 3D variable-stiffness structure implementation scheme based on a patterned heating network.
In particular, the 3D-printing and laser-engraving technologies were combined to obtain a PGHN/PLA variable-stiffness structure. This scheme can use the PGHN to accurately control the local stiffness of 3D-PLA and realize programmable design and fabrication of 3D variable-stiffness structures.
We studied the application of the proposed scheme for the stiffness control of PLA structures using PGHN under different graphene strip widths. The results showed that the applied currents of PLA structural remodeling were 0.07-0.08, 0.10-0.13, and 0.11-0.14 A when the graphene strip widths were 2, 3, and 4 mm, respectively. Therefore, we fabricated three collapsible PGHN/PLA variable-stiffness structures, which were the "origami structure," "torsional structure," and "hexagonal structure," and four hollowed-out PGHN/PLA variable-stiffness structures, which were the "helical structure," "mesh structure," "disk structure," and "spring." These cases completely demonstrated the designability, excellent deformation and reconstruction capacity, and reasonable bearing capacity of the PGHN/ PLA variable-stiffness structures. We summarized the problems and solutions in the design and preparation steps and provided design specifications for the PGHN/PLA variable-stiffness structures. Additionally, we presented a shape-state control scheme of the PGHN/PLA variable-stiffness pneumatic disc; the presented scheme can be used as a reference for the shape control of PGHN/PLA variable-stiffness structures.
Finally, we designed a pneumatic robot, called R-VS, based on the PGHN/PLA variable-stiffness structure. R-VS realizes twoway motion control using a single air source based on the local stiffness control and shape-locking function of the PGHN/PLA variable-stiffness structure. Indeed, the PGHN/PLA variablestiffness structure can be used for the integrated design and manufacture of the robot's fuselage. The motion joints of the robot can be independently controlled using the local stiffness control and a deformation locking function; thus, multimode motion control can be achieved using a single air source. This scheme is of considerable significance in the research of integrated design and fabrication, multimode motion control, and structure miniaturization of pneumatic robots.
The proposed PGHN/PLA 3D variable-stiffness structure has shape programming, reconstruction, locking, and excellent loadbearing capacities. In addition, the local stiffness control, programmable design, and fabrication of 3D variable-stiffness structures can be effectively realized. The proposed structure can be applied in multimode robots, soft robots, wearable devices, and deployable structures.

Experimental Section
Materials and Equipment: PLA was purchased from Cixi Lanbo Printing Consumables Co., Ltd., with a wire diameter of 1.75 mm, printing temperature of 190-220°C, and bending modulus of 3.3 GPa. Graphene films with a thickness of 80 μm and square resistance of 10 Ω were purchased from Suqian Sengu Nano Technology Co., Ltd. The silicone glue (V-1510) was purchased from Le Lai Adhesive Co., Ltd. Ecoflex-00-50 was purchased from Smooth-On, Inc. Copper mesh (300 mesh) was purchased from Anping County Kangmeilong Metal Mesh Co., Ltd. TPU film (XJU150) was purchased from Shanghai Xingxia Macromolecule Products Co., Ltd.; the thickness was 50 μm and operating temperature was 145-170°C. The inverted fiber static electric cloth was purchased from Dongguan Hongwei Electronics Co., Ltd., the surface fiber was nylon. An FDM 3D printer (ZD-210) was purchased from Shenzhen Zhandong Industrial Co. Ltd. A laser-engraving machine (L3Proe Max) was purchased from Dongguan Xinjia Laser Technology Co., Ltd. A hot-stamping machine (CM1001) was purchased from Cooldiy Digital Graphic Co., Ltd. A programmable direct current (DC) power supply (TH6402B) was purchased from Changzhou Tonghui Electronic Co., Ltd. The data acquisition card USB 8AD bipolar was obtained from Wuhan YAV Electronics Technology Co., Ltd. An infrared thermal imager Ui3293 was obtained from UniTrend Technology Co., Ltd. The force sensor (ZNLBS) was obtained from Shenzhen Huaheng Metrology Co., Ltd.
Fabrication of PGHN/PLA 3D Variable-Stiffness: The 3D PLA was prepared using an FDM 3D printer, and PGHN was prepared using a laserengraving mechanism. Subsequently, the two were bonded using the silica gel glue, and the PGHN/PLA 3D variable stiffness was obtained after a complete curing for 24 h. The fabrication process of the variable-stiffness structure containing the copper mesh electrode is illustrated in Figure S5, Supporting Information. First, a hot-stamping machine was used to press the copper network and TPU film, a temperature of 150°C for 10 s, to obtain the TPU/copper network. Second, a TPU/copper mesh with a laser-engraving machine was cut into the electrode pattern. Subsequently, the TPU/copper mesh was pressed at the target position of PGHN, temperature of 150°C for 10 s, to obtain the TPU/copper mesh/PGHN. Finally, the TPU/copper mesh/PGHN was bonded on the 3D-PLA using the silica gel glue, and the PGHN/PLA variable-stiffness structure containing a copper mesh electrode was obtained after a complete curing for 24 h.
Fabrication of PGHN/PLA Pneumatic Disc: Both the chassis and variable-stiffness structure of the pneumatic disc were made of PLA, which was prepared using an FDM 3D printer. The PGHN was prepared using a laser-engraving mechanism. The elastic film and elastic gasket were prepared using an injection molding process. Finally, the silicone glue was used to bond the three layers, and screws and nuts were used to clamp and fix.
Fabrication of Robot R-VS: The body of the robot was fabricated using an FDM 3D printer. The PGHN was prepared using a laser-engraving mechanism. The elastic chamber was prepared using injection molding. The inverted down-fiber static electric cloth uses scissors for cutting. Finally, all parts were glued together using the silicone glue.
Test of Heating Curves and Bending Modulus: A programmable DC power supply was used to apply DC current to the PGHN, as shown in Figure S2, Supporting Information. Simultaneously, the electric moving stage was used to drive the fixture, which drove the sample to undergo a bending deformation. The force required to bend the sample was measured using a force sensor, heating curves were obtained using a K-type thermocouple temperature sensor, and data were collected using a data acquisition card. During the test, the moving speed of the fixture was 50 mm min À1 . The bending modulus was calculated as E = FL 3 /3Iδ, where F is the force required by the sample to produce a bending deformation, L is the length of the sample, δ is the displacement (deflection), and I is the crosssectional moment of inertia of the sample.
Elastic Coefficient Test of PGHN/PLA Variable-Stiffness Spring: As shown in Figure S92, Supporting Information, a programmable DC power supply was used to apply DC current to the PGHN. Simultaneously, the electric moving stage was used to drive the fixture, which drove the spring compression or tensile deformation. A force sensor was used to measure the www.advancedsciencenews.com www.advintellsyst.com deformation force of the test spring, and the measured data were collected using a data acquisition card. During the test, the moving speed of the fixture was 50 mm min À1 . The elastic coefficient of the spring is expressed as k = F/x, where F is the force required to compress or stretch the spring, and x is the displacement of the spring. Friction Coefficient Test of the IDF-SEC: As shown in Figure S14, Supporting Information, an FDM 3D printer was used to prepare a small car with a dead weight of 5.14 g. The small car was an IDF-SEC at the bottom and weighed inside. We horizontally placed the small car on the cardboard; one side was connected to the force sensor with a rope, and the force sensor was fixed on the electric moving stage. In the test process, the electric moving stage drove the small car to move horizontally. The traction force measured by the force sensor was called the friction force F f , and the moving speed of the small car was 50 mm min À1 . The friction coefficient is expressed as μ = F G /F f , where F G is the total gravity of the small car.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.