“…Hydrogels are three-dimensional networks made of lightly cross-linked polymer chains, which are capable of imbibing vast amount of water. Based on the structure and composition of the polymer chains, the swelling ratio of hydrogel networks might be affected by various external stimuli such as pH [1][2][3][4][5][6][7][8], temperature [9][10][11][12][13][14][15][16], light [17,18], mechanical loads [19] and electrical fields [20][21][22]. Smart hydrogels have been found to be a suitable material for a wide range of biomedical [23,24], drug delivery [5,23], sensor and actuator [22,25,26], soft robotic [14,27,28] and tissue engineering [28][29][30] applications.…”