Further experimental details are provided in Table . To investigate the macro-and microscopic changes induced by varying precursor injection sites and FRRs, we examined the fibers using SEM. At low magnification, Class I and II fibers are readily distinguishable. Class I fibers exhibit a smooth surface that is slightly rougher than that of pure alginate fibers crosslinked with the ethanolic solution of Fe(BF4)2 (Figure ). Notably, the surface of Class 1 fibers becomes progressively uniform as FRR increases (Figure ). In contrast, Class II fibers exhibit a wrinkled surface with folds that become more pronounced with increasing FRR (Figure ). The distinct morphologies observed between Class I and II fibers can be attributed to two key factors: the specific flow configurations and the selective diffusion behavior of Fe(II) ions, which readily diffuse into the alginate solution, while the alginate does not diffuse back into the Fe(II) ethanolic solution (see SI, Section S2 and Table ). In Class I fibers (with Fe(BF4)2 in the sheath flows and alginate/Htrz in the central flow), fiber growth initiates as Fe(II) ions diffuse from the sheath flow into the central flow, forming a fiber that develops from the Fe(BF4)2-alginate interface toward the center of the channel (Figure -S31, S35 and S37). At a FRR of 0.5, numerical simulations results indicate that due to their lower diffusion coefficient, Fe(II) ions do not fully diffuse to the center of the main channel, likely resulting in hollow fiber formation (Figure ). Increasing the FRR focuses the central flow, producing solid fibers with reduced diameters, as Fe(II) ions have to diffuse shorter distances to reach the center of the microfluidic channel and occupy its entire cross-section (Figure ). In Class II fibers (with alginate/Htrz in the sheath flows and Fe(BF4)2 in the central flow), the diffusion pattern promotes outward fiber growth from the alginate-Fe(BF4)2 interface (Figure -S34, S36 and S38). Our simulation results suggest that this outward growth leads to hollow fiber formation at all FRRs (Figure ). However, at higher FRRs, the flow focusing of the central flow results in thinner fiber walls, as Fe(II) ions have to diffuse longer distances to reach the main channel walls and are not able to do so over the whole length of the main channel. Upon viewing the morphology of the fibers in conjunction with the simulation results, it becomes evident that limited Fe(II) diffusion leads to highly wrinkled surfaces in Class II fibers prepared at FRRs of 4 and 9 due to their thinner walls, while Class I and II fibers prepared at a FRR of 0.5 exhibit less wrinkling and reduced folding, owing to their thicker walls formed in the widespread presence of Fe(II).