Fe3+-Coordination mediated synergistic dual-network conductive hydrogel as a sensitive and highly-stretchable strain sensor with adjustable mechanical properties

Flexible strain sensors are attracting enormous attention due to their high stretchability and sensitivity that are required for wearable devices and electronic skin. However, diverse application environments require materials whose hardness can be adjusted to satisfy different demands. Herein, we developed a synergistic dual network hydrogel PANI-P(AAm-co-AA)@Fe³⁺ composed of an iron-coordinated poly(acrylamide-co-acrylic acid) network and a conductive polyaniline network with adjustable mechanical properties and high sensitivity. Through controlling the degree of protonation, the cross-linking density will be changed and the mechanical properties of PANI-P(AAm-co-AA)@Fe³⁺ can be regulated in a wide range (ultimate tensile stress: 0.0710-0.3054 MPa) (fracture strain: 145-880%). Besides, the host-guest interaction between β-cyclodextrin (β-CD) and polyaniline improves the compatibility of polyaniline in a hydrogel substance and leads to the formation of homogenous interpenetrating networks, which provide PANI-P(AAm-co-AA)@Fe³⁺ with outstanding and steady conductivity (2.03-3.67 S m^-1). Furthermore, PANI-P(AAm-co-AA)@Fe³⁺ exhibits highly linear sensitivity, a wide working region (gauge factor = 0.48 at 0-400% strain) and excellent durability (300 cycles). A strain sensor based on this hydrogel can detect not only large movements such as bending fingers and wrists but also delicate movements such as swallowing and a pulse, indicating its enormous potential in wearable devices, human health monitoring, electronic skin, human-machine interactions and so on.