High-strength, biocompatible and multifunctional hydrogel sensor based on dual physically cross-linked network

Hydrogel-based wearable sensors have raised great interest due to their potential application in human-machine interfaces and healthcare monitoring. However, there are still challenges in designing hydrogel-based sensors with high mechanical strength, biocompatibility, and multifunctionality. Herein, we designed sensors based on ionic conductive hydrogels composed of poly (acrylamide-co-acrylic acid) (PMAA), poly (vinyl alcohol) (PVA), and ferric chloride (FeCl3) by simple two-steps methods of copolymerization and freezing/thawing. FeCl3 was used as a cross-linking agent to coordinate with PMAA chains to construct the first network. After being subjected to the freezing/thawing treatment, PVA crystalline domains were formed to serve as knots of the second network. PVA/PMAA/Fe3+ hydrogels exhibited excellent mechanical properties (maximum tensile strength of 905 kPa, elongation at break of 816%) due to the double physically cross-linked network. Meanwhile, Fe3+ ions also contributed to the enhancement of the conductivity and sensitivity of hydrogels. Poly (acrylamide-acrylic acid) imparted hydrogels with multifunctional sensing properties. It could accurately and stably detect human body movement, physiology (temperature, sweat), pH change, and touch recognition with excellent biocompatibility. Therefore, this work provides a new route to design ionic conductive hydrogels with high-strength, biocompatibility, and the ability to respond to multiple stimuli, hopefully expanding their applications in flexible sensor devices.