Magnetically programmed 4D printing of silicone sealant–NdFeB soft composite actuators

Abstract Magnetically programmable soft actuators require the simultaneous control of magnetic responsiveness, rheological printability, and mechanical robustness, which remains challenging in highly filled magnetic composites. Here, we develop a silicone sealant–NdFeB composite ink and an integrated magnetic-domain direct ink writing strategy. A coaxial printhead combines a capacitor-discharge pulsed field with a measured peak magnetic flux density of approximately 5.0 T for hard-magnetic domain programming and a 5 V DC coil that generates a measured static field of approximately 35 mT near the nozzle outlet for reversible magnetization of the soft-magnetic CIP and auxiliary local rheological regulation. Herschel–Bulkley fitting yielded apparent yield stresses of 989.2 and 5.7 Pa for composites containing hydrophobic and hydrophilic fumed silica, respectively. Magnetic-field-dependent rheological measurements showed that the CIP-containing composite exhibited an approximately 14%–21% higher apparent viscosity than the CIP-free reference over the investigated field range, supporting a field-responsive contribution to material retention near the nozzle outlet. Increasing the NdFeB content from 20 to 60 wt% increased the saturation and remanent mass magnetizations from approximately 28 and 18 to 78 and 54 A m 2 kg − 1 , respectively, while the coercivity remained nearly unchanged. The calculated toughness of the representative titanate-treated specimen was approximately 213.4% higher than that of the untreated specimen. The printed structures exhibited programmed magnetic domains and achieved warping angles up to 152.3 ∘ , together with bending, flipping, gripping, and bioinspired fin-like oscillation. This integrated material–printing–actuation strategy provides a route for fabricating programmable magnetic soft actuators by direct ink writing.