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.