On-the-fly synthesis of freestanding spin-crossover architectures with tunable magnetic properties

Spin-crossover (SCO) molecular-based switches displaying magnetic bistability have shown promise across a range of applications since their discovery, including sensors, memory storage devices, actuators, or displays. Yet, limited processability remains a substantial barrier to their real-world implementation. Efforts to overcome this limitation by integrating SCO materials into polymer matrices have often been constrained by complex, costly, and time-consuming multi-step methods, which tend to produce inhomogeneous particle distributions within the matrix. Herein, we demonstrate how three-dimensional (3D) flow-focusing chemistry provides unprecedented control for the direct fabrication of SCO composite materials, effectively addressing key challenges in processability, scalability, and cost. By using a continuous 3D coaxial flow-focusing microfluidic device, we simultaneously synthesize [Fe(Htrz)2(trz)](BF4) and achieve its homogeneous incorporation into alginate fibers in a seamless continuous manner. The versatility of the microfluidic device allows for precise manipulation of the reaction-diffusion (RD) zone, resulting in SCO composite fibers with tunable physicochemical and magnetic properties. Additionally, we demonstrate the ability to isolate these fibers as freestanding architectures and highlight the potential for printing them with defined shapes. Finally, we show that the 3D control of the RD zone granted by continuous flow microfluidic devices offers precise spatiotemporal control over the distribution of SCO complexes within the fibers, effectively encoding SCO materials into them. SCO-encoded fibers can seamlessly combine adaptability and functionality, offering innovative solutions for application-specific customization.