4D Printing of Ionic Liquid-Based Acrylate-Spin Crossover Composite Resins

Abstract Significant advances have been made in recent years in the development of liquid resins for lithography-based 3D printing. Nevertheless, the incorporation of filler materials, including active and inactive nanoparticles, remains a major challenge due to issues related to resin stability, particle dispersion, and printability. Herein, we demonstrate the key role of ionic liquids in stabilizing and homogenizing spin-crossover (SCO) particle-filled composite resins and investigate how resin composition affects both the spin-crossover behavior and the mechanical properties of 4D-printed structures fabricated by 405 nm digital light processing (DLP). While the selected ionic liquid does not alter the intrinsic spin-crossover properties of the particles, the composition of the base resin, comprising phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide (BAPO) as photoinitiator, neopentyl glycol diacrylate (NPGDA) as cross-linker, and varying ratios of hydroxyethyl methacrylate (HEMA) and hydroxyethyl acrylate (HEA), significantly influences the particles’ physicochemical environment and provides an effective means to tune the mechanical performance of the printed architectures. These results highlight the potential of ionic liquid-based SCO composite resins as promising platforms for the fabrication of responsive 4D-printed devices.