Soft‐Hard Magnetic Transformable Coaxial Fiber with Multimodal Deformations

Abstract Magnetic soft materials have been extensively utilized in the development of soft robots. However, they usually fail to achieve programmable soft‐hard magnetic transformations, limiting multimodal magnetoactive deformation and locomotion. Herein, a coaxial magnetic fiber (CMF) consisting of a porous polydimethylsiloxane (p‐PDMS) sheath and a core of sodium alginate sol (SAS) mixed with neodymium‐iron‐boron (NdFeB) particles (NdFeB@SAS) is reported, fabricated using intermittent coaxial 3D printing. The CMF can reversibly transition between a hard magnetic state (coercivity H cm 6900 Oe and remanence M r 61 emu g −1 ) and a soft magnetic state ( H cm 450 Oe and M r 16 emu g −1 ) through the solvent exchange strategy, achieving a ≈15‐fold change in coercivity and a ≈4‐fold increase in remanence. Three CMF‐based samples are constructed and programmed with varying soft‐hard magnetic profiles, exhibiting multimodal magnetoactive deformation. Moreover, under a fluctuating magnetic field, a bionic butterfly robot can flap its wings over a vertical branch without falling. A magnetic brush is also constructed to paint origami objects using magnetic actuation. These demonstrations underscore that the responsiveness of the CMF to global magnetic fields can be locally tailored, allowing for the coexistence of attraction and repulsion in various regions of the CMF, providing a novel approach for developing soft robots.