Mechanical design and analytic solution for unfolding deformation of locomotive ferromagnetic robots

Magnetically actuated robots are attracting much interest due to the advantages of fast response, remote manipulation and enabling operations in enclosed spaces. Recent advances in fabricating ferromagnetic polymeric matrices embedded with hard magnetic fillers provide routes to multimodal locomotion for soft-bodied robots. One limitation of these matrix-based robot designs is that it requires low volume fraction of hard magnetic fillers to achieve soft and compliant robot body such that moderate magnetic fields are sufficient for actuation. However, low volume fraction of functional magnetic fillers leads to magnetically weak soft robots that are difficult to actuate. Here, we propose a compliant and high-performance robot design operating at magnetic fields down to 1 mT by utilizing a high-quality ferromagnetic film and mechanics-guided three-dimensional (3D) assembly technique. A parylene coating is deposited to keep the assembled arch shape, allowing releasing and actuating the structure as a freestanding robot. The robot would unfold and fold periodically under cyclic magnetic fields, driving the robot in a desired direction. To illustrate the versatile applicability of this approach, robots in two different representative geometries are presented, one in traditional straight configuration and the other in serpentine configuration. Through theoretical analysis and finite element analysis, fundamental results are offered for the proposed robot design, including concise solutions to the unfolding deformation, the effects of coating thickness on spring back, the maximum strain in the hard ferromagnetic film and a comparison of unfolding deformation of both designs. The results clearly show the effect of geometry/material parameters, external magnetic field and prestrain in assembly process, providing essential design guidelines to compliant and fast-moving magnetic robots via the proposed method.