60 µL precursor solution of GelMA, GelMA/DbPE, and GelMA/TbPE was transferred into a cylindrical PDMS mold (diameter: 5 mm, height: 3 mm) and photocrosslinked by UV light irradiation (302 nm, 8 W). Then, the hydrogel pads were frozen with liquid nitrogen and cut with a razor blade to reveal their cross-section. The frozen hydrogel slabs were attached to a SEM pin mount holder with a double-sided conductive tape, followed by lyophilization. After sputtering the specimen surface with gold, pore size and morphology were characterized by SEM microscopy (Nova 230 NanoSEM, FEI company, Hillsboro, USA). All images were taken using an acceleration voltage of 5-10 kV and a secondary electron detector.