Flexible and recoverable ion-conductive hydrogels with cross-linked triple network for highly sensitive wearable Motion-monitoring sensors

The practical applications of hydrogel-based sensors are limited by their low mechanical strength, insufficient sensitivity and machining difficulties caused by electrically conductive fillers. It is of great practical importance for facile preparation of hydrogel sensor combined with excellent mechanical properties and conductive sensitivity. In this work, a series of ion-conductive hybrid hydrogels based on in-situ polymerized polyacrylamide with double and triple network structures have been fabricated via both chemical and physical cross-linking. The triple network structured hydrogels achieve the tensile strength of 2.105 MPa and compression strength of 57.56 MPa, which are much higher than those of hydrogels with double network due to the established energy dissipation gradient. The reversible non-covalent cross-linking in the network also leads a relatively fast recovery in tensile cycle tests and double compression tests, enabling the excellent stability of strain sensing in the durability test. The hydrogels show the tensile resistance sensitivity factor of 0.9352 at 0–120% strain and a high pressure sensitivity factor of 5.134 kPa−1 with the pressure ranged from 0 to 6 kPa. The hydrogels are also assembled as flexible sensors for human motion monitoring, and the signals of motions can be distinguished by different resistance variation values and certain curve shape, showing the potential of applications on flexible device and human-machine interface.