Gradient adhesion modification of polyacrylamide/alginate–calcium tough hydrogels

Strong hydrogel adhesion requires the synergy of adhesion and cohesion. Gradient adhesive–tough hydrogels can balance adhesion and cohesion, however, their construction is still a challenging task. Here, we used ethylenediaminetetraacetic acid (EDTA) on-side coordination-induced diffusion chelating Ca2+ to form an adhesive surface in a polyacrylamide/alginate–calcium (PAAm/Alg–Ca2+) tough hydrogel as a facile method for the construction of gradient adhesive–tough hydrogels. The adhesion energy of a gradient adhesive–tough hydrogel to skin is increased by 128% compared with PAAm/Alg–Ca2+ tough hydrogels and the elongation at break is two times higher than that of PAAm/Alg hydrogels. In addition, gradient adhesive–tough hydrogels also exhibit wide linear sensitivity (the gauge factor (GF) = 0.196 (0% < ε < 400%); GF = 0.260 (400% < ε < 650%)) as a wearable strain sensor to monitor human motions. This work provides a versatile strategy for the design of gradient adhesive–tough hydrogels and also provides a practical model for the development of wearable strain sensors.