Figure.3.

Tumor-on-a-chip approach to generate hypoxia. A. A paper-based hypoxia generation system. Multiple layers of hydrogel coated paper blocked oxygen supply from the air. (reproduced with permission from ref. 62) B. A typical hypoxia control design for cell migration. The top layer was designed for hypoxia gradient generation, and the bottom layer for cell migration. (reproduced with permission from ref. 65). C. Hypoxia-on-a-chip design with a side-channel for chemical oxygen depletion by NaOH and pyrogallol reaction (reproduced with permission from ref. 69). D. Hypoxia-inducible hydrogel produced through laccase-mediated dimerization of ferulic acid molecules with oxygen consumption (reproduced with permission from ref. 72).