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(a) Photograph of a bi-layered hydrogel consisting of polyanionic (blue) and polycationic (white) hydrogels. (b,c) Photographs of a bi-layered hydrogel bending towards the polycationic side in 0.2 M (b) and 0.05 M (c) NaCl solution. The bending angle (θ) is defined as shown. (d) The reversible actuation of hydrogel shuttling between 0.2 and 0.05 M NaCl solutions (reproduced from [178], copyright 2016 RSC).
