Agar/PAAc-Fe3+ hydrogels with pH-sensitivity and high toughness using dual physical cross-linking

As promising structural materials, various tough hydrogels have been developed recently by incorporating various kinds of bonds. An important challenge is to use dual physical cross-linking to develop both toughness and self-recovery in a single material. Here we report smart, strain-responsive hydrogels composed of a fully physically linked agarose/poly(acrylic acid)-ferric ion (agar/PAAc-Fe3+) double network (DN) with high toughness and pH-sensitivity. These hydrogels were fabricated in a one-pot reaction to generate dual physical cross-linking through, first, a hydrogen-bonded cross-linked agarose network, and, second, a physically linked PAAc-Fe3+ network via Fe3+ coordination interactions. The DN hydrogels possessed high toughness, with breaking strain of 1130%, fast self-recovery properties in ambient conditions (100% recovery in 30 min) and self-healing properties (the healed hydrogels can be manually stretched up to 700% of their original length after self-healing for 60 h from the cut-off state). In addition, the hydrogels exhibited pH-sensitivity due to the dissociation of ionic coordinate bonds between –COO− ions of the PAAc chains and Fe3+ ions. Double-layer hydrogel strips with two different concentrations of PAAc formed a “C”-shaped material when initially immersed in pH 7 solution and then soaked in a pH 3 solution. These characteristics make the hydrogels attractive candidates for tissue engineering, soft actuators and flexible electronics.