1. Introduction

Microfluidic chips have revolutionized fluidic manipulation and control at small volumes, finding applications in fields such as diseases diagnosis and cell cultures 1-3. Hydrogels, as the backbone of microfluidics, offer advantages over materials such as PDMS or glass due to their biological relevance on biocompatibility, physical stiffness, degradation and mass transport properties4,5. These ideal features make them promising for applications in tissue engineering 6, biomedical research 7,8, and food industry9,10.
Nevertheless, constructing heterogeneous architectures inside hydrogels remains a challenge 11, which limits their potential for mimicking the complex and multilayer structures of organs in vivo 5. Sacrificial templates are most commonly used to construct structures inside hydrogel-based microfluidics6, where a 3D degradable template is first encapsulated into the hydrogel and then removed to obtain fluidic channels (Figure 1A). However, the sacrificial templates, usually comprised of soft materials such as sodium alginate 12and gelatin 6, are mechanically weak and easy to distort during fabrication. Thus, the formed channels are often simple, inaccurate and deviated away from their designed morphology. Moreover, the chips integrally casted by homogeneous material lack the possibility to design multilayers with different materials, making it impossible to mimic the heterogeneous organs in vivo 13,14. Similarly, other hydrogel-based microfluidic preparation techniques, such as 3D printing 15, light-controlled degradation16, and direct writing 17, also failed to construct heterogeneous and accurate structures in hydrogels due to complications in handling, poor in resolution or restriction to specific materials.