Magnetic Reprogramming of Self‐Assembled Hard‐Magnetic Cilia

Abstract Artificial magnetic cilia are hair‐like structures that can respond to magnetic fields. Using hard magnetic materials in magnetic cilia makes possible programming and reprogramming of the magnetization state and corresponding actuation behaviors. Hard‐magnetic cilia are fabricated through self‐assembly by solvent casting of a slurry of NdFeB microparticles dispersed in a solution of a thermoplastic polyurethane elastomer. These hard‐magnetic cilia are capable of attractive and repulsive responses to magnetic fields, determined by the remanent magnetization of the NdFeB microparticles. An array of cilia can be magnetically reprogrammed through immobilization in ice, applying a damped alternating magnetic field first to reduce the remanent magnetization, and then remagnetizing the cilia with a large field. This demagnetization process significantly improves the reprogrammability of the cilia array. Different responses to magnetic fields can be programmed, including spatially nonuniform behaviors. Reprogrammed hard‐magnetic cilia exhibit unique behaviors in rotating magnetic fields. After reprogramming the magnetization direction along the width of the cilia, rotation and torsion cause them to slowly coil and then quickly uncoil in a snapping behavior. Modeling the magnetic and elastic torques during actuation provides additional insights and aids the design of magnetic cilia actuators based on hard magnets.