Magnetoactive small-scale soft robots have wide applications in minimally invasive medicine, [1] targeted cargo delivery, [2] and biomedical engineering, [3] because of their prominent ability in swift locomotion, fast response, and remote control. [4] Recently, hard-magnetic soft robots have realized complex deformations. [5] The interactions between the external magnetic field and the programmed magnetic domains of the materials induce body torques, leading to the deformation of the structure to align the magnetization with the external magnetic field. [6] Bending is a basic deformation mode for these shape-transforming structures. However, the homogeneous bending of the whole structure is not energy efficient, with every volume element deforming and storing energy. [7] A design strategy is needed to improve the performance of the magnetic soft robots.For actuation, generating a large folding angle locally is always easier and more energy efficient than homogeneous bending. A successful example can be found in nature. Arthropod (e.g., crab, shrimp) refers to an invertebrate animal with an exoskeleton, a segmented body, and pairs of jointed appendages. [8] Compared to mollusks (e.g., snail, octopus), arthropods have multiple jointed limbs to perform complex movements. For instance, a snail moves its body by squirming (Figure 1a), while a crab moves faster through the movement of the limbs and can further grab objects with claws (Figure 1b). The crab claw functions through the exquisite structure of the limb joint: the driving force is generated by muscle contraction and transmitted through the joint to open or close the claw (Figure 1c). [8b] The joints connect skeleton systems and enable a large folding angle. Joints are also ubiquitous in more advanced mammals, and are widely used in hard robots nowadays. [9] It would be of great interest to introduce joints into the design of magnetic soft robots.In this article, we have developed a series of bioinspired magnetic arthropod millirobots. The basic deforming element is a magnetic beam with a joint. The joint can turn the bending deformation into folding deformation, with the joint region deforming locally. As shown in Figure 1d, the jointed beam deforms into a V-shape (folding mode) instead of a U-shape (bending mode) for the beam without a joint. Obviously, the strain energy is stored more intensively and locally around the sharp angle in the V-shaped sample. Figure 1e indicates that the folding mode is more energy efficient than the bending mode since less strain energy is required for the folding mode to achieve the same bending angle as the bending mode.Toward biomedical applications, hydrogels are preferable as the matrix material of magnetic robots for their good
