Yield Stress Fluid as Ball Joints for Magnetic Miniature Robots

Small-scale ball joints are essential elements for micromanipulators and miniature robots. However, the fabrication of mechanical ball joints at sub-millimeter scale presents major challenges, due to the requirements of high-precision spherical shape and high surface smoothness for low friction. Here, for the first time, we present the concept of using yield stress fluids as simplified ball joints. We show the joints exhibit stable performance and are easy to fabricate and assemble. Four joints are applied in a crawling miniature robot to connect four Nickel legs. Owing to the yield stress property of the fluid, the legs can freely rotate around the joint, but do not translate to detach from the robot body. Force measurement results show that the maximum axial holding force of the fluid joint is ~ 0.007 mN, which matches with the rheological modelling results of ~ 0.014 mN. In 20 times crawling tests, the robots equipped with the fluid joints achieved a success rate of 100%, while the ones without fluid joints yielded a much lower success rate of only 15% due to the leg detachment. The robot reaches a crawling speed of 1.6 mm/s, a maximal payload ratio over 2, and can be wirelessly controlled by the external magnetic field to follow complicated trajectories. The fluid joints exhibit reliable performance and require uncomplicated fabrication processes, showing a wide range of applications in small-scale robotics.