High-magnification SEM images support these observations and provide valuable insights into SCO particle formation on the alginate matrix. It should be noted that the following analysis of particle size and shape will remain qualitative, as attempts to dissolve the fiber matrix to isolate the SCO particles consistently resulted in SCO particle dissolution, preventing a quantitative assessment. In Class I fibers, high magnification images (Figure ) reveal particulates with undefined shapes homogeneously distributed on the fiber surface as well as in the core of the fiber (Figure and). However, our simulations predict that, due to the rapid outward diffusion of Htrz from the central to the sheath flow, SCO particles can also form outside the fiber matrix through reaction with Fe(BF4)2 (Figure ), which go into the waste solution after fiber isolation. Particle size on the fiber matrix progressively decreases with increasing FRR. This behavior is supported by numerical simulations, which reveal that the width of the RD zone at the end of the reaction channel decreases from approximately 400 µm at a FRR of 0.5 to around 250 µm at a FRR of 9 (Figure -S41), due to flow focusing. Such decrease in the RD zone limits mixing, and particle growth at higher FRRs, a phenomenon that we also observed during the microfluidic synthesis of covalent organic frameworks and layered double hydroxides. In Class II fibers, high magnification images confirm the formation of hollow fibers with SCO particles displaying undefined shapes (Figure , 2j, 2l, S10 and S12). Notably, the SCO particles are homogeneously generated on the inner and within the walls of the alginate fiber, with no particles observed on the outer walls (Figure and). This distribution aligns with the flow configuration and the numerical simulation results, which predicted particle formation predominantly within fiber matrix and on the inner surface of the hollow fiber (Figure ). In contrast to Class I fibers, the size of the SCO nanoparticles within the polymer matrix in Class II fibers shows minimal variation between FRRs. The invariability in particle size can be attributed to several factors, including the limited presence of Fe(II) in the main channel as the FRR increases and the width of the RD zone decreases. At a FRR of 0.5, the Fe(II) injected into the main channel (1:2.5 Fe:Htrz) is slightly above the theoretical stoichiometric amount (1:3 Fe:Htrz) required to form [Fe(Htrz)2(trz)]BF4 (Table ). However, Fe(II) is also consumed by sodium alginate in the fiber formation process, likely leading to an iron concentration below the stoichiometric threshold. Additionally, while the diffusion coefficient of Fe(II) is relatively small (Table ), the flow configuration allows outer diffusion of Fe(II) to reach the channel walls, resulting in a large RD zone (Figure and).