After assessing the physicochemical properties of Class I and II [Fe(Htrz)₂(trz)]BF₄ particles, we further explored the capabilities of the 3D coaxial flow-focusing microfluidic device for the one-pot production of SCO composite fibers, achieving in-situ formation of both the SCO particles and the fiber matrix. We selected sodium alginate as a polymer matrix precursor due to its natural origin and capacity to polymerize into fibers upon interaction with cations. SCO composite fibers generation was achieved by flowing an aqueous solution of sodium alginate (0.75% w/v) and Htrz (2 M) with an ethanolic solution of Fe(BF4)2 (0.4 M) through the main reaction channel. As in the synthesis of Class I and II particles, the device configuration allows for the production of two distinct types of fibers -Class I and Class II fibers (Figure and S8)depending on the injection position of the SCO composite fiber precursors. Here, due to the nearly null diffusion coefficient of alginate (see Table ) and the distinct viscosities of the two precursor solutions, we expect the less viscous precursor solution (i.e. the ethanolic solution of Fe(BF4)2) to occupy a smaller cross-section of the main microfluidic channel. Numerical simulations of the flow and mass transport of these solutions in our device confirmed this hypothesis (see Simulation section S2). Interestingly, the simulations revealed that for FRRs of 0.5, 4 and 9, the size of the focused stream depends on which precursor solution is injected through the central inlet 1, and thus, on whether Class I or Class II fibers are fabricated. Specifically, the simulations results show that for the same FRR, a more focused central stream is achieved when fabricating Class II fibers, as the less viscous ethanolic precursor solution is introduced through inlet 1 (Figure ). In sharp contrast, a less focused central stream occurs during the fabrication of Class I fibers, where the more viscous aqueous solution of sodium alginate and Htrz is introduced through inlet 1 (Figure ). To investigate how these changes in flow and mass transport during the 3D flow-focusing of the precursor solutions influence the formation of SCO composite fibers and their resulting magnetic properties, we conducted a series of experiments varying the FRR (0.5, 4 and 9) while keeping the TFR constant at 750 μL/min for each fiber type (see Table for further experimental details).