Self-sensing magnetic actuators of bilayer hydrogels

Hard magnetic soft robots have been widely used in biomedical engineering. In these applications, it is crucial to sense the movement of soft robots and their interaction with target objects. Here, we propose a strategy to fabricate a self-sensing bilayer actuator by combining magnetic and ionic conductive hydrogels. The magnetic hydrogel containing NdFeB particles exhibits rapid response to magnetic field and achieve bending deformation. Meanwhile, the polyacrylamide (PAAm) hydrogel with lithium chloride (LiCl) allows for the sensing of deformation. The bending behavior of the bilayer under magnetic field is well captured by theoretical and simulated models. Additionally, the bilayer strain sensor shows good sensitivity, stability and can endure a wide-range cyclic stretching (0-300%). These merits qualify the self-sensing actuator to monitor the motion signals, such as bending of fingers and grasping process of an intelligent gripper. When subject to an external magnetic field, the gripper can grab a cube and sense the resistance change simultaneously to detect the object size. This work may provide a versatile strategy to integrate actuating and self-sensing ability in soft robots.