On‐the‐Fly Synthesis of Freestanding Spin‐Crossover Architectures With Tunable Magnetic Properties

Spin-crossover (SCO) molecular-based switches have shown promise across a range of applications since their discovery, including sensing, information storage, actuators, and displays. Yet limited processability remains a barrier to their real-world implementation, as traditional methods for integrating SCO materials into polymer matrices are often complex, expensive, and prone to producing uneven material distributions. Herein, we demonstrate how 3D flow-focusing chemistry enables unprecedented control for the direct fabrication of SCO composite materials, addressing key challenges in processability, scalability, and cost. By using a 3D coaxial flow-focusing microfluidic device, we simultaneously synthesize [Fe(Htrz)₂(trz)](BF₄) and achieve its homogeneous incorporation into alginate fibers in a continuous manner. The device's versatility 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.