However, for PEGDMA 10k, the BAPO-PEG hydrogels turned weaker than BAPO/NVP hydrogels (Figure ). This phenomenon was more remarkable in the compression tests (Figures and). BAPO/NVP hydrogels based on PEGDMA 10k showed much higher compressive toughness (Tc) than BAPO-PEG hydrogels using the same macromonomer (0.29 vs. 0.05 MJ•m -3 , Table ). This can be attributed to the relatively low methacrylation degree of PEGDMA 10k (71% vs. 80% -90% for others) and thus fewer crosslinking sites. In the case of BAPO/NVP hydrogels, the addition of NVP can promote the photopolymerization and strengthen the network. For the hydrogels based on BAPO-PEG (Figure ), the swelling ratio increased from 150% to about 500% when the PEG MW increased from 2000 to 10,000 g mol -1 . This was confirmed by the change in size of the swollen samples (Figures and). The swelling ratios of the 3D printed hydrogels using BAPO/NVP were in general similar to those of BAPO-PEG hydrogels. To compare our approach with molding or conventional DLP printing from PEG aqueous solutions, we prepared PEG hydrogels using a water soluble photoinitiator, lithium phenyl-2,4,6trimethylbenzoylphosphinate (LAP) bearing the monoacylphosphine oxide moiety (Figure ). By molding with PEGDMA 6k at 70% water content via photo-crosslinking, the equilibrium swelling ratio of the obtained sample was 570%, which was much higher than that via "water-free" 3D printing (~360%). As a result, the former hydrogel was very weak with Young's modulus of only 0.4 MPa, while the value for "water-free" 3D printed hydrogel was 1.2 MPa (Table , Figure ). We also printed PEGDMA 10k hydrogels with 80% water content by DLP at room temperature. To enhance the printability, NVP (8.0 wt%) was added to the resin. As expected, the swelling ratio of the 3D printed hydrogels was much higher than that obtained by "water-free" 3D printing (400-500%, Table ) and reached 1400% (Figure ), which means that the equilibrium water content of the hydrogel was as high as 93%. Clearly, the "water-free" approach may prevent the over-swelling of the hydrogels that can negatively impact the mechanical strength. This might be due to the much denser polymer networks formed by the direct crosslinking of the PEG macromonomers in the absence of large amounts of water molecules. In addition, the "water-free" 3D printing avoids the aforementioned light scattering and layer separation issues that are commonly encountered in SLA/DLP printed PEG hydrogels, resulting in low-resolution prints (Figure , inset).