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Figure 3: Tunable actuation of programmed hydrogels. (a) In the SA hydrogel, the G-blocks and MG-blocks on polymer chains form ionic cross-links through Ca2+ (b), resulting in tight polymer chains in the CaCl2 solution (c). (e) Partial ionic cross-links are unlocked as certain amounts of Ca2+ ions are replaced by a large amount of Na+ ions, resulting in looser polymer chains in the NaCl solution (f). (d) shows the helical structure of the hydrogel sheet with microchannels facing inward in the 0.1 M CaCl2 solution, which changes its helical rotation oppositely with microchannels facing outward in the 0.1 M NaCl solution (g). (h) The secondly deformed helix can retain its shape in water or recovers to its primary helix with microchannels facing inward again after being immersed in the 0.1 M CaCl2 solution (i). (j) The images of dynamic deformation of hydrogel sheets in a mixed solution of 0.001 M NaCl and 0.001 M CaCl2 for 24 h and the corresponding curve (k). (l) Increasing the concentration of NaCl changes the length of the hydrogel sheet due to the swelling ratio changes in various NaCl solutions (0.0001 M, 0.001 M, 0.01 M, and 0.1 M). (m, n) The EDS demonstrates the change of Na+ and Ca2+ ions in various conditions. The scale bars are 0.5 cm.