Hydrogels are three-dimensional (3D) polymer networks that contain a large amount of water. Owing to their tunable physicochemical properties and, for some aspects, similarity to native extracellular matrices, hydrogels are widely used in biomedical applications for the preparation of 3D scaffolds in tissue regeneration, 2 drug-delivery vehicles, soft robots and microfluidic devices. For these applications, it is often important to create hydrogels with complex and detailed structures, especially in the case of tissue scaffolds and implants with customized geometries. In recent years, 3D printing has emerged as a powerful tool for hydrogel fabrication, with extrusion-based printing technologies most commonly used. With shear-thinning inks, extrusion printing allows the fabrication of objects mimicking natural structures with controlled geometry and characteristics. Nevertheless, the extrusion technique is limited by low printing resolution (> 200 µm) and poor structural fidelity. Photopolymerization-based 3D printing, in particular stereolithography (SLA) and digital light processing (DLP), offers high resolution (30-100 µm) and good surface quality of the printed objects. Compared to SLA, DLP allows the faster fabrication of objects with complex geometries using a digital micromirror device. However, as opposed to water-free networks, the printing of hydrogels by DLP is challenging. This is attributed to a variety of factors. First, as the most widely used vat photopolymerization technique, DLP requires a liquid resin of low viscosity (< 10 Pa.s) to enable layerby-layer stacking. Consequently, existing water-borne resins for DLP printing usually have high water content (typically > 70%), which results in low mechanical strength and toughness of the printed constructs. Secondly, the solidification of the DLP resin occurs through irradiation from the underneath of the resin, and the print platform moves vertically in order to allow the resin to flow back. The separation of every newly formed layer from the bottom plate subjects the structure to certain mechanical forces. Therefore, the crosslinked layers should have sufficient mechanical strength to remain intact during this process, and preserve the shape fidelity of the printed object. As a result, an ideal DLP resin should have low viscosity and provide high mechanical resistance of the print, which is highly challenging to achieve with common water-based resins, especially for high molecular weight (MW) macromonomers. Lastly, light scattering generated from the photopolymerized layer that is not optically clear can further affect the printing resolution and shape fidelity, with the poly(ethylene glycol) (PEG) hydrogels as the typical examples. As a well-known water-soluble synthetic polymer, PEG is widely used in drug delivery and regenerative medicine, because it is generally considered non-toxic, non-immunogenic and, in solution, easily clearable from the human body when its molecular weight is below the renal filtration cutoff. Photopolymerizable derivatives of PEG have been used for the 3D printing of hydrogels in various biomedical applications, with PEG diacrylate (PEGDA) and PEG dimethacrylate (PEGDMA) as the main representatives. The SLA/DLP printing of PEG hydrogels is usually limited to low MW monomers (e.g., PEGDA 700), which generate brittle products. Although it is possible to print highquality embedded channels or simple shapes using aqueous solutions of PEG (macro)monomers with higher MW (e.g., 6000 g mol -1 ), high water content and low crosslinking degree makes fabrication of free-standing PEG hydrogels with complex geometries very difficult. In addition to the aforementioned issues, the hydrogels prepared by "with-water" 3D printing inevitably further swell under physiological conditions as a result of the difference in osmotic pressure, which drastically deteriorates their mechanical properties. Therefore, 3D printing of structurally complex PEG hydrogels with high strength and toughness remains a challenging task.