The PEG hydrogels were prepared by incubating the 3D printed specimens in PBS pH 7.4 at 37 °C for 24 h to reach equilibrium swelling. Their mechanical properties were studied by performing tensile and compression tests. As shown in Figures , an important increase in elasticity was observed when raising the MW of PEG for resins prepared with BAPO-PEG and BAPO/NVP. The Young's modulus (Et) decreased from more than 30 MPa to less than 1 MPa while the elongation at break (εbreak,t) increased from about 5% to around 70%, when PEG MW was increased from 700 to 10,000 g.mol -1 . This is associated with the higher flexibility of long PEG chains as well as with the lower crosslinking density and thus higher water uptake of hydrogels (Figure ). Although the 3D printed hydrogels prepared with BAPO-PEG and BAPO/NVP showed similar Young's modulus for the same chain length of PEGDMA, the tensile strength of the objects printed with BAPO-PEG was higher in the MW range of 700 to 4000 g mol -1 . For example, the maximal tensile stress (σmax,t) of the PEGDMA 2k hydrogel prepared with BAPO-PEG was 1.7 MPa vs. 0.9 MPa for BAPO/NVP (Figures and). This is likely due to the star-shape structure of the polymeric photoinitiator with long PEG chain (2000 g.mol -1 per BAPO group) that can enhance the photocrosslinking and thereby strengthen the hydrogel networks despite the low concentration. When the polymer networks were more flexible (MW 6000 -8000 g mol -1 ), this effect became unnoticeable.