Table 1.

Methods used to create channels in hydrogels and their advantages and disadvantages.

Method	Pro	Contra
Non-sacrificial spacer	Defined channel geometry, easy-to-use	Almost no branching possible, removal of spacers can destroy ultrastructure, resulting channels have a rather large size
Sacrificial spacer	Good technique to create interconnected channels, virtually any template structure is suitable	Materials used need to have distinct physical properties in order to be removed, e.g., melting point, and at the same time the removal should not influence the hydrogel itself
Photopolymerization-based 3D printing	A group of fast, accurate methods to generate any designed microstructure in a hydrogel, micrometer resolution	Free-radical-polymerization can potentially damage cells, specialized equipment, and materials are necessary
Planar	Template can be used repeatedly, suitable to generate highly organized perfusion channel network of defined geometry	Multi-step stacking procedure can be time-consuming