Processing-induced magnetic softening in high-loading printable composites for additive manufacturing

Abstract Additive manufacturing (AM) of magnetic composites has traditionally been limited by trade-offs between filler loading, filament quality, processability, and functional performance. Yet, processing-driven advances in AM can redefine this design space, enabling unprecedented filler loadings, broader material compatibility, and scalable fabrication beyond conventional flexible metastructures. Here, we introduce a versatile feedstock-engineering strategy that fabricates magnetic composite filaments with high filler contents while maintaining homogeneity, printability, and performance. By systematically comparing three polymer matrices with two classes of soft magnetic powders, we report the critical interplay among matrix chemistry, filler morphology, and processing conditions, achieving magnetic filaments at high loadings (up to 61 wt% in rigid and 52 wt% in flexible systems), well above commercial standards (~ 40 wt%). Polyethylene terephthalate yields rigid, defect-free filaments at high loadings, whereas thermoplastic polyurethane uniquely enables mechanically flexible, magnetically functional filaments. Structural analyses confirm uniform filler dispersion and preservation of the amorphous or crystalline characters of the powders, while magnetic characterization uncovers an unexpected processing-induced magnetic softening: extrusion reduces coercivity relative to raw powders. This counterintuitive softening transforms extrusion from a passive fabrication step into a functional design tool. Proof-of-concept flexible demonstrations, including flexible actuators and magnetic grippers, exhibit magnetic field-responsive deformation and flux-guided manipulation, validating multifunctionality. The results establish a new design principle, processing-induced property enhancement, which enables AM of magnetically active yet mechanically compliant and reconfigurable systems for soft robotics and electromagnetic control. This scalable, polymer-independent method expands material and functional options for engineering multifunctional composites with structural integrity, mechanical flexibility, and enhanced functionality.