Fig. 1. Generation of hollow hydrogel microfibers with folded channels. (a) Scheme of the formation of hollow fiber with double channels. (b) The 3D structure of three fibers, in which green fluorescent microspheres represent area of the alginate hydrogel shell, and red microspheres represent hollow channel. The scale bar is 500 μm. (c) Channel shapes of microfibers at different flow conditions of two Q_i with constant Q_o (200 μl min⁻¹). (d) The relationship between the period (λ) of folded channels and Q_o when Q_i/Q_o is constant.