INTRODUCTION

In the fields of unmanned aerial vehicles (UAVs), aerial transportation and manipulation (1–6) have drawn more attention because they notably extend the capabilities of UAVs (7), where UAV grasping plays a crucial role (8, 9). Numerous studies on UAV grasping (10–14) have been reported to date, including rigid grippers designed to mimic avian claws (15–20), achieving grasping by establishing precise dynamic models of the entire system (19, 21) or installing a series of sensors on the gripper (20). However, these approaches require complex controllers and heavy rigid grippers. Some designs use clever mechanical structures to ensure reliable object grasping while reducing the weight of the overall systems. Nevertheless, they are limited in terms of the shapes and sizes of objects they can grasp and also lessen the likelihood that the UAV will successfully grasp an object (22, 23). In brief, rigid grippers depend on accurate models, sensors’ feedback, and high-precision control of UAVs, making UAV grasping challenging. Even worse, most rigid grippers are heavy, have a single grasping mode, are limited in the shape and size of the object to be grasped, and lack self-adaptation.

In recent years, with the rise of soft robotics and soft materials (24–28), numerous soft grippers have been reported (29), with some specifically applied to UAV grasping (30–37). These grippers are primarily based on pneumatic bending actuators, providing lightweight solutions for UAV grasping. Because of the inherent softness of these grippers, they offer advantages in grasping delicate objects such as glass, live animals, and cardboard. In addition, the softness reduces the complexity of gripper control. However, the shapes of these grippers remain resemble rigid grippers, lacking the ability to achieve shape-adaptive grasping. Potted plants, for example, are difficult to grasp. To enhance adaptability of grasping, a form of entangled grasping has been proposed for designing soft grippers with straight elongated circular tube actuators (38–42), which provides an interesting grasping form and has strong shape-adaptive ability. However, this type of grasping tends to be random, with less success in grasping regular objects like spheres (39). Moreover, these grippers are only capable of grasping lightweight objects and are prone to damage (38). These shortcomings prevent their potential applications in UAVs. In addition, bulky setups with 16 pneumatic pumps are required to generate high pressure (>100 kPa) for gripper actuation (39) because each actuator in the gripper requires its own air supply, further hindering their potential application in UAVs because of UAVs’ limited load capacity.

Smooth tendrils are usually used as inspiration for these soft grippers with entangled grasping forms (38–42). In nature, tendril plants need external support to grow vertically and to get more sunlight (43). Most tendril plants have smooth surfaces (i.e., tendril without surface structures) and, as they grow, can quickly come into contact with nearby stems and branches, relying on winding deformation. They eventually have a strong grasp on these stems and branches. In Darwin’s famous paper on the habits of climbing plants, these types of tendrils were described as excellent grapnels, tightly clutching branches like birds perching on them (44). The aforementioned entangled grasping forms of grippers (38–42) draw inspiration from these tendrils. However, these tendrils quite fail to attach themselves to a brick wall because of their smooth surface, which cannot support their weight on the wall. We refer to these tendrils (tendril without surface structures) as tendril climbers. In addition to these smooth tendrils, there is another kind of climbing plant with hook-like structures on its surface (i.e., tendril with surface structures). These tendrils’ hooks can interlock with objects for support, enabling them to climb and support their weight (45, 46). Though they also exhibit some winding shapes, the growth of these tendrils is solely dependent on hooks. Consequently, these tendrils can climb up the walls of tall buildings and are excellent climbers (44). We refer to this type of tendril (tendril with surface structures) as hook climbers. Tendril climber has stronger adaptability, while hook climber has stronger grasping ability due to their different surfaces.

Inspired by tendril plants, we propose a class of self-adaptive soft self-contained grippers that enable UAV to grasp various objects with different sizes and shapes (movie S1) in multiple environments. We design two types of elongated soft eccentric circular tube actuators: one with a smooth surface (i.e., without surface structures), similar to tendril climbers; the other with surface structures, similar to hook climbers. On this basis, we fabricate two types of U-shaped soft eccentric circular tube actuators (UCTAs) by fixing the ends of the elongated soft eccentric circular tube actuators, respectively. We add an appropriate amount of low–boiling point liquid and soft resistance wires into the UCTAs. When voltages are applied to the soft resistance wires, the low–boiling point liquid undergoes a phase transition from liquid to gas. The liquid-gas phase transition mechanism (47, 48) can generate high pressure (>200 kPa), which can make UCTAs to generate winding deformations. Hence, by eliminating the requirement for multiple bulky pumps, the mechanism opens the door to the application of UCTAs in UAV manipulation/grasping. Two types of multiple UCTAs are separately cross-arranged to construct two types of soft grippers, forming self-contained systems that can be directly driven by voltage. Similar to tendrils wrapping around branches, both types of grippers can electrically curl to adapt to the shape of the object, achieving entangled grasping. The gripper without surface structures has a large curve after deformation, allowing for better adaptability to the object’s shape, similar to tendril climbers. This makes it ideal for delicate grasping jobs like picking up flowers. Unlike the gripper without surface structures, the curvature of the gripper with surface structures reduces during deformation, resulting in a decrease in adaptability, but an increase in load capacity of the gripper due to that the protruding surface structures can interlock with the object, similar to hook climbers. This kind of gripper is suitable for strong grasping tasks, such as picking up heavy stones. Because of the grippers’ self-adaptability, UAV manipulation/grasping can be achieved by the grippers without precise positioning or complex grasping planning, reducing the positioning precision required during UAV object grasping and extending the UAV’s capability to grasp objects with various sizes and shapes. The actuators’ U-shaped bending allows the UAV to grasp objects directly using simple ways such as hanging or hooking. In addition, the soft grippers can adjust the grasping position by making direct contact with objects. All these features of the soft grippers play a significant role in UAV manipulation due to the fact that UAV itself is difficult to control. Successful UAV grasping of various objects on the ground (movie S1), as well as in challenging scenarios such as tree branches (movie S2) and lakes (movie S3), and UAVs collaborative lifting objects (movie S4) demonstrate that the bioinspired self-adaptive soft self-contained grippers pave the way for UAV to achieve powerful manipulation with low positioning accuracy, no complex grasping planning, self-adaptability, and multiple environments.