Fig. 2.

Fabrication and characterization of hollow hydrogel fibers via microfluidic coaxial bioprinting. A) Schematic illustration of the microfluidic device used to fabricate the fibers. B) Photopolymerization process of the fibers (scale bar: 500 μm). C) Macroscopic view of the fibers (scale bar: 500 μm). D) Optical microscopy and E) fluorescence microscopy images showing the distinct hollow structure (scale bar: 200 μm). F) SEM images displaying the morphology and porous microstructure of the fibers (scale bar: 200 μm and 2.5 μm [inset]). G) The effect of the external phase flow rate (15-35 mL/h) on fiber dimensions (outer diameter and ID/OD ratio) when the internal phase flow rate is kept at 15 mL/h. H) Young's modulus of hydrogel groups with varying PEGDA concentrations. Groups: PL (2.5% PEGDA), PM (5% PEGDA), and PH (7.5% PEGDA), all of which contain 10% GelMA. I) Rheological analysis showing the storage (G′) and loss (G″) modulus in different groups as a function of strain (0.01–1%). J) Comparative swelling behaviors of the hydrogels in DI water, PBS, and the culture medium. K) Comparative degradation behaviors of the hydrogels in PBS containing collagenase (1 mg/mL) and in culture medium. L) Quantification of the total length of the fibers generated from 1 mL of bioink for the slim (200/400 μm, inner/outer diameters), medium (300/500 μm), and thick (400/600 μm) groups, respectively.