Movie S2.

Fabrication of hydrogel layer on both sides of the paper scaffold. Both sides of the paper substrate were modified with PSMa, PLL, and calcium chloride. After complete alginate gelation [1% (wt/vol)] on one side with red food-coloring reagent, the other side was exposed to another type of alginate solution [1% (wt/vol)] with yellow food-coloring reagent. Double-layered alginate hydrogels were formed.