To explain the Htrz impurities in the fibers at different FRRs, we analyzed each case individually. In Class I fiber prepared with a FRR of 0.5, the presence of crystalline Htrz is clear (Figure and). We attribute this impurity to the shortage of Fe(II) ions provided to the system coupled with their limited diffusion from the sheath to the central flow, as discussed in the morphological analysis of the fiber and supported by our simulations for Class I fibers (Figures and). This results in unreacted Htrz that crystallizes within the fiber. At higher FRRs of 4 and 9, the excess of Fe(II) ions supplied to the microfluidic device fully diffuse from the sheath to the central flow, enabling complete reaction with Htrz and leading to a diffractogram free of Htrz impurities. In contrast, Class II fibers show the opposite trend.