Overall, we can conclude that the 3D coaxial flow-focusing microfluidic device used here is a powerful tool to manipulate the Fe(II):Htrz ratio on-the-fly. This is achieved by precisely tuning the RD zone at will, simply by altering the injection position of the precursor solutions and their flow rates. This control is not only useful for the continuous preparation of SCO composite fibers with tailored macro-and microscale features but also for generating materials with marked differences in their magnetic properties.