A Flytrap-Inspired Bistable Origami-Based Gripper for Rapid Active Debris Removal

Space debris is considered an increasingly serious threat to on‐orbit spacecrafts. There are several potential solutions to this problem, including active debris removal. Flexible robots have shown promising adaptability and dexterity in soft manipulation owing to their inherent compliance. This compliance allows them to interact safely and efficiently during space missions such as active debris removal. Herein, inspired by the bistable structure and energy‐release mechanism of the Venus flytrap, a bistable origami‐based gripper is developed. The flexible gripper, which can rapidly achieve stable state switching, is in the form of a biomimetic flytrap leaf curvature and is actuated using a shape memory alloy actuator. Subsequently, a flytrap bristle‐like locking structure is used to ensure locking via the action of a dielectric elastomer actuator to alleviate the vibration instability of the flexible robot under rapid actuation. The experimental results showed that the flexible gripper can achieve effective capture within approximately 300 ms. In addition, it exhibits good adaptability and mechanical robustness with targets having complex shapes and sizes, indicating its potential applications in the space capture and sampling fields.


Introduction
The rapid increase in the amount of space debris has limited the use of space orbit resources, posing a serious threat to the reliability and safety of in-service spacecraft. [1,2] For example, the collision between Iridium 33 and the Cosmos 2251 satellite has significantly changed the low Earth orbit environment. [1] In this regard, the active capture of space debris is an effective solution. [3] Currently, space debris smaller than the detection limit (%10 cm) of space objects accounts for approximately 99.98% of the total number of space objects. [4] Such targets tend to be faster and pose a more serious threat to spacecraft, particularly microand nanosatellites. Therefore, reliable and affordable systems for space debris removal are essential to guarantee safe and sustainable access to Earth's orbits. [5] And capturing small-scale space debris may become a key space mission in the future. Existing capture methods include contact and noncontact capture, [6] where contact capture has been validated in real space. The main forms of contact capture are rigid capture (e.g., rigid manipulator capture [7] ) and flexible capture (e.g., net robot, [8] rope robot, [9] and harpoon robot capture [10] ). For rigid capture, the inherent system stiffness can easily damage the capture targets and increase launch mass and cost. Flexible capture overcomes these drawbacks and has been verified in flight demonstrations. [11] For example, the European Space Agency (ESA) initiated the e. Deorbit project and achieved the first flight demonstration of the net capture of space debris. [12] In another project called ROGER, the ESA introduced a tether-gripper mechanism. The mechanism is typically designed to precisely capture a specific part of a target, which results in more complicated requirements than net capture. [13] Harpoon capture is an attractive method because it is compatible with differently shaped targets, has a long allowable distance, and does not require a grappling point. [6] However, the risk of generating new space debris is relatively high because the harpoon must penetrate the target in this case. [10] Moreover, although flexible capture systems are generally lighter and better adaptable to target shapes than rigid capture systems, they suffer from low reliability and difficult control. [14] Therefore, it is urgent to develop a fast nondestructive flexible capture system that is easy to control and allows reusability.
Space debris is a noncooperative target, is typically at high speed, has a complex shape, and possesses uncertain motion, [15] which pose a huge challenge to capture systems. A flexible gripper system must satisfy several key performance indicators to accommodate the capture requirements of space debris. 1) Rapidity: the capture time should be as short as possible to keep up with the fast and sophisticated targets. 2) Shape adaptability: space debris is typically irregular in shape and size.

DOI: 10.1002/aisy.202200468
Space debris is considered an increasingly serious threat to on-orbit spacecrafts. There are several potential solutions to this problem, including active debris removal. Flexible robots have shown promising adaptability and dexterity in soft manipulation owing to their inherent compliance. This compliance allows them to interact safely and efficiently during space missions such as active debris removal. Herein, inspired by the bistable structure and energy-release mechanism of the Venus flytrap, a bistable origami-based gripper is developed. The flexible gripper, which can rapidly achieve stable state switching, is in the form of a biomimetic flytrap leaf curvature and is actuated using a shape memory alloy actuator. Subsequently, a flytrap bristle-like locking structure is used to ensure locking via the action of a dielectric elastomer actuator to alleviate the vibration instability of the flexible robot under rapid actuation. The experimental results showed that the flexible gripper can achieve effective capture within approximately 300 ms. In addition, it exhibits good adaptability and mechanical robustness with targets having complex shapes and sizes, indicating its potential applications in the space capture and sampling fields.
Therefore, grippers should be properly sized and possess adequate deformability to adapt to targets. 3) Locking force: a captured target is likely to flip with the gripper because of contact collision, [16] which may result in the loss of the target. Therefore, the gripper should have a sufficient locking force or self-locking mechanism to prevent the target loss.
Many organisms in nature have developed and refined rapid and accurate capture methods during the evolutionary process. In-depth studies of these biological capture mechanisms have promoted the design of robots. [5,17] For example, as an insectivorous plant, the Venus flytrap can be closed rapidly and then locked using tip bristles. Experiments have demonstrated that during the capture process, the flytrap leaf can complete approximately 60% of its total displacement within approximately 100 ms, which is closely related to its bistable structure. [18] Moreover, the soft leaves can adapt to the shapes of different insects, providing a promising approach for space debris capture. [5] Kim et al. developed a biomimetic flytrap robot with a bistable unsymmetrically laminated structure. [19] The robot was driven by shape memory alloy (SMA) springs to achieve fast closure. Zhang et al. designed a bioinspired flytrap robot with cylindrical shell-like leaves. [20] The robot can generate a rapid snapping motion using a noncontact electromagnetic driving method. In addition, materials such as light-responsive liquid crystal elastomers, ionic polymer metal composites, and responsive hydrogels have been used to create robots based on biomimetic flytrap leaves to achieve similar behaviors. [21][22][23] However, most of the current flytrap-like robots are small and struggle to accommodate the size of space debris. Their designs always focus on behavioral bionics, and less attention has been paid to dynamic capture applications. In our study, a flytrapinspired bistable origami-based flexible gripper was developed. An illustration of the space capture mechanism of the gripper system is shown in Figure 1. Owing to a spring origami structure, the gripper can realize a bistable mechanism similar to that of a flytrap. Additionally, an integrated SMA spring actuator (SMASA) simulates the slow storage and rapid release mechanism of the elastic energy of the flytrap, which provides power for the rapid state switching of the gripper. Furthermore, a bristle-like locking structure is employed to effectively lock and prevent the loss of the captured target. In general, this flexible gripper has the advantages of being lightweight, fast-actuated, and escape-proof. Additionally, it can be used repeatedly without damaging the captured targets. Therefore, this study provides a promising candidate for future space capture applications.

System Overview
The excellent adaptability of soft robots has been demonstrated for several unstructured applications. To implement a robotic system for space debris capture inspired by the Venus flytrap, we constructed a bistable origami-based flexible gripper. The robot consists of a dielectric elastomer actuator (DEA), SMA spring actuator, and their controllers ( Figure 2a).
The rapid closure of Venus flytrap leaves relies on two main factors: its bistable structure as well as its energy storage and release mechanism. [18,24] In the designed flexible gripper, the frame creases form a bistable structure by imitating the surface curvature of the Venus flytrap leaf. The SMASA, comprising a bidirectional SMA spring, electromagnetic limiters, and a temperature control system, was designed to imitate the mechanism of slow energy storage and rapid release of the Venus flytrap. Under the restrictions of the electromagnetic limiters, the SMA spring can continuously store energy as the temperature of the internal ceramic heating tube increases. When the www.advancedsciencenews.com www.advintellsyst.com electromagnetic limiters are disconnected, the SMA spring can rapidly extend to close the gripper, thereby confining the target within the gripper envelope space. Typically, a bionic tip-locking structure can be used for near-simultaneous locking. However, if the gripper collides violently with the target, the lock often fails. In this case, the DEA actuates the gripper to complete precise locking action. Note that this connection between SMASA and DEA is conducive to improving the success rate of the capture process. Generally, the SMASA has the characteristics of fast actuation but low accuracy. The DEA is suitable for fine-tuning the position. It has high precision and fast response under small displacement actuation, [3,25,26] which compensates for the shortcomings of the SMASA. Therefore, the combination of these two actuators can ensure timely closing and locking of the gripper. Overall, the maximum allowable capture size of the gripper system was approximately 100 Â 100 Â 100 mm, which is allowed to exceed 100 mm in the moving direction. The DEA was 520 mm long and 60 mm wide. The SMA spring has a minimum length of 12 mm and can be extended to approximately 40 mm after heating under unconstrained conditions. The phase transition temperature of the SMA spring was 60°C (Baohong Metal Co., Ltd., China). The body mass of the flexible gripper system is approximately 35 g ( Figure S3, Supporting Information).

Materials Preparation
A 0.5 mm-thick polyvinyl chloride (PVC) sheet was mechanically cut to obtain a flexible frame. An approximately elliptical hole was designed to reduce the stress concentration in the DEA.
A DE film (VHB4910, 3 M company) was prestretched via pure shear to 10 times its original area (%100 μm) using a purposely built prestretch device ( Figure S3, Supporting Information). Compliant electrodes were composed of carbon black ink in a silicone elastomer matrix from MG Chemicals Co., Ltd. The PVC strips were cropped from a 0.1 mm-thick rectangular sheet.

Fabrication
The fabrication process is illustrated in Figure 2. First, one side of the frame was bonded to a prestretched DE membrane. Subsequently, the frame was released using scissors. Second, the compliant electrodes were painted on both surfaces of the DE membrane at the corresponding frame holes with a 10 μm thickness. These steps were repeated on the other side of the frame to obtain an unfolded double-layer DE membrane actuator. Subsequently, liquid polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning) was cast onto the double-layer DEA to a thickness of approximately 30 μm using a film coater. Subsequently, it was cured in an oven at 60°C for 5 h to enable the DEA packaging. Finally, a flytrap-like actuator was obtained by folding the PVC strips and copper tape along the frame creases.

Design of the Capture Structure
The rapid capture mechanism of the Venus flytrap provides a feasible solution for designing flexible capture structures. Flytrap leaves are dual-gradient structures, [21] whose surface curvature exhibits completely opposite forms during the stable www.advancedsciencenews.com www.advintellsyst.com opening and closing states. [27] In the open state, leaves always curved outward (convex), i.e., they show a positive curvature in the xand y-directions. However, in the closed state, leaves always curved inward (concave) and exhibited a negative curvature in the xand y-directions (Figure 3a). Owing to this structure, the flytrap can achieve fast stable-state switching because it facilitates the rapid release of elastic energy accumulated in the leaves. Inspired by the bistable structure of the Venus flytrap leaves, we designed a bionic bistable flexible gripper. The frame core of the gripper is a symmetrically connected Miura origami structure with a consistent surface curvature and a dual-gradient structure similar to that of Venus flytrap leaves (Figure 3a-e). Studies have proven that Miura origami structures with bistable properties must satisfy the following property where c M and c B are the extensional and rotational spring stiffness, respectively. [28] For the designed origami structure, the formula (1) is calculated as follows Therefore, the flexible gripper is a bistable system (for the specific proof, see Section S1, Supporting Information). Furthermore, we deduced the relationship between force and geometry during the folding of the origami structure. The geometry of the Miura pattern during folding has been studied previously, [29] and can be used to study the relationship between force and displacement. After the deduction process, the relationship between the force on both sides of the frame core and deformation can be determined using Equation (3).
where F 0 ðlÞ is the applied force under the corresponding deformation l, F 0 is the prestress applied to the frame core, φ is the projection angle between the two ridges, b is half the width of the frame core, and α 1 , α 2 are the dihedral angles at creases 1 and 2, respectively. The other parameters are shown in Figure 3d-e (for the specific derivation process, see Section S2, Supporting Information). The stable state switching of the Venus flytrap was caused by the rapid release of the elastic energy accumulated in the leaves.  Inspired by this energy mechanism, an SMASA was designed to trigger the flexible frame. Researchers have demonstrated that the strain field of the outer surface appears as an extension in the x-direction when the flytrap leaves are closed. [18] Therefore, the SMASA drives the frame by connecting the short arms at both ends of the frame core to achieve flytrap leaf-like behavior. Typically, the SMA spring cannot expand when the electromagnetic limiters are tightly closed. The elastic energy of the gripper system continuously accumulates as the temperature of the SMA spring increases. During this process, two short arms extrude both sides of the frame core such that it presents a positive curvature in the xand y-directions (Figure 3b-i).
Here, the frame is in an open state. When the temperature of the SMA spring exceeds its phase transition temperature, capture can commence by lifting the electromagnetic limiters. Subsequently, the elastic potential energy of the gripper system is rapidly released and converted into kinetic energy, causing the curvature of the frame core to become negative in the xand ydirections (Figure 3b-ii). Here, the frame is in a closed state. Under the action of the SMASA, the functions of the applied force and displacement distance of the frame core can be used to characterize the structural bistability of the gripper because their directions are the same. [28] Figure 3f shows a clear energy barrier between the opening and closing states, which demonstrates the bistable nature of the flexible gripper. The gripper allowed fast state switching using a small compression distance (approximately 2.6 mm). Furthermore, a function of the frame opening angle θ and distance between the two short arms can be used to intuitively represent the state of the flexible gripper. The opening angle θ is defined as the included angle between the center point of the plane frame and the midpoint at the end of the DEA. As shown in Figure 3g, when the distance between the short arms increases at a uniform speed, the flexible frame can complete 60% of the total displacement in approximately 1/5 of the total capture time. Therefore, the flexible frame shows bistable characteristics similar to those of the leaves of a Venus flytrap, which enables it to achieve rapid closing action. In addition to the bistable nature of the structure, the speed of the SMASA is critical for stable state switching. Figure 3h shows the relationship between the elastic potential energy released by the SMA spring and time under the no-load and load conditions. The elastic potential energy is expressed by Equation (4).
where k is the stiffness coefficient of the SMA spring, Δx is the elongation distance of the SMA spring, x L is the maximum elongation of the SMA spring under unconstrained conditions, x L is 28 mm, and k = 0.191 N mm À1 . Figure 3h shows that the SMA spring can be quickly actuated to the specified position within 20 ms under the no-load condition. Under a load of approximately 35 g, the actuation time of SMASA was extended to 28 ms. In addition, under load conditions, the actuation function exhibited a large overshoot before reaching a balance point. We believe that this is related to the vibrations caused by the inherent nonlinearity of the soft structures. Therefore, the fast actuation provided by the SMASA ensures fast state switching of the capture system.

Design of the Locking Structure
The Venus flytrap is tolerant of insects of different shapes and sizes, however, some insects can escape owing to vigorous movement. The bristle structure of the Venus flytrap, which forms the basis of this phenomenon, inspired our design. We improved the bristle-like locking structure and tested the maximum locking force of the I-, L-, and T-shaped locking structures ( Figure S4, Supporting Information). The experimental results showed that the locking force of the T-shaped lock structure can reach 24.88 N www.advancedsciencenews.com www.advintellsyst.com which is significantly greater than that of the other two structures. Therefore, we optimized the bristle-like locking structure from I-shaped to the T-shaped (Figure 4a). This is critical for preventing a captured target from escaping owing to collisions. The displacement function of the two ends of the DEA with respect to the driving voltage is shown in Figure 4b, where the displacement increases with an increase in the driving voltage. Simultaneously, the change in displacement accelerated with an increase in driving voltage. When the applied voltage was larger than 4.8 kV, the gripper could be effectively locked (Figure 4b-c). Furthermore, the flexible gripper body exhibited a high degree of mechanical robustness (Figure 4d-e), allowing www.advancedsciencenews.com www.advintellsyst.com the application of rolled storage and even resistance to spikes. And the driving performance of the DEA remained unchanged after more than 100 repeated tests.

Results
Microsatellite-scale demonstrations can be used to rapidly verify several active debris removal technologies. [30] A robotic testbed for space debris capture was constructed to evaluate the performance of the flexible gripper (Figure 5a). The flexible gripper was integrated with a 6-DoFs rigid robotic arm to capture moving targets. For a target that must be captured, the suspension method was used to minimize the influence of gravity on the motion state. Space target capture is typically achieved in three steps: 1) during the target moving and approaching process, the robot searches and tracks the target's position. When the target enters the capturable area, the flexible gripper approaches the target by controlling the rigid robotic arm. 2) Subsequently, the SMAAS is actuated, and the flexible gripper closes rapidly and envelopes the captured target. The tip-locking structure of the DEA effectively locks the target under the action of momentum. 3) In the case of locking failure, such as a strong collision between the flexible gripper and the target, DEA actuation can be used to achieve controlled locking. As the driving voltage increases, the tip-locking structure achieves an effective locking. A control block diagram of the capture system is shown in Figure S5, Supporting Information.
Based on the constructed robotic platform, we conducted capture experiments on moving targets, such as model cube satellites, model asteroids, model Mars rovers, model space stations, and model space shuttles. The speed of the moving targets was set to 100 mm s À1 . The experiments showed that, the flexible gripper could capture and lock a model cube satellite within 300 ms (Figure 5b). And the gripper exhibited good adaptability to targets with complex shapes ( Figure 5 and Table S1, Supporting Information). After 112 capture experiments, the capture success rate of the gripper for targets of different shapes reached 82.14%. However, the capture success rate decreased with an increase in the speed of the dynamic target, which was caused by the vibration of the flexible system itself and severe impact on the capture process. Specifically, if a valid envelope for the target is not achieved in the first step, the gripper must be reopened in preparation for the next capture. We verified two methods of reopening the flexible gripper, namely, the wiredriven method and liquid nitrogen cooling method (shown in Figure S6, Supporting Information). The process of unlocking and reopening the flexible fixture is shown in Figure S7, Supporting Information. The experimental results showed that the time to reopen the gripper is less than 0.5 s using both two methods.

Conclusions
In this study, we designed and fabricated a bionic Venus flytrap flexible gripper for space debris capture. Based on the leaf structure of a Venus flytrap and its slow elastic energy storage and rapid release mechanism, the bionic gripper can achieve rapid closing behaviors similar to those of flytraps using the SMASA and DEA. Typically, the SMASA can actuate the gripper to close quickly within approximately 300 ms. If the lock fails because of a slight collision, the DEA can actuate the gripper to achieve accurate locking and unlocking. Experiments and theoretical analysis revealed the bistable properties of the flexible gripper. Furthermore, the feasibility of capturing moving targets with complex shapes in a simulated space environment was demonstrated. The results showed that the gripper can successfully capture a model asteroid, model Mars rover, model space station, and model space shuttle in approximately 300 ms on average. Therefore, the flexible gripper shows good adaptability to targets with various complex shapes and has significant potential for noncooperative space debris capture.

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