Assessment of 4D magnetic printing capability of UV composite resin containing recycled Nd–Fe–B nanocrystalline particles

Abstract The demand for rare-earth permanent magnets and supply risks have intensified the search for alternatives based on recycling strategies. In this work, the 4D printing of magnetic composites incorporating recycled Nd–Fe–B powders was evaluated. Two recycled powders produced via hydrogenation–disproportionation–desorption–recombination (HDDR) and dynamic HDDR (dHDDR) routes were incorporated into a photopolymer resin and compared with a Nd–Fe–B powder. To establish whether recycled Nd–Fe–B powders can support magnetic programming in 4D-printed composites, magnetization patterns were encoded during fabrication using a digital light processing–based printing setup with a magnetic field. The magnetization encoding resolution was quantified by printing multidomain structures with in-plane and out-of-plane magnetization. While the commercial powder enabled submillimeter magnetic domains as small as 0.69 ± 0.05 mm, the recycled HDDR and dHDDR powders achieved minimum domain sizes of 1.70 ± 0.26 mm and 2.67 ± 0.11 mm, respectively, primarily due to their larger particle sizes. Despite this reduced spatial resolution, both recycled powders enabled magnetic programming at the millimeter scale. To demonstrate functional performance, robotically controllable single-domain printed rectangles were fabricated and actuated under external magnetic fields. Notably, the rectangles printed with embedded recycled HDDR powder and post-magnetization achieved the highest locomotion speeds, whereas the dHDDR-based rectangles exhibited lower speeds consistent with their reduced magnetic strength. These results demonstrate that recycled Nd–Fe–B powders are viable materials for 4D magnetic printing, offering a sustainable alternative for the fabrication of magnetically actuated soft devices and robotic systems.