Highly tough, stretchable and resilient hydrogels strengthened with molecular springs and their application as a wearable, flexible sensor

Numerous mechanically strong synthetic hydrogels have been developed in recent years; however, few of them are both tough and resilient like living tissues such as muscles. The intrinsically contradictory requirements for toughness and resilience make it a big challenge to design a gel with both high toughness and high resilience. To solve the problem here helical peptide chains are introduced into hydrogel networks by crosslinking the gel with peptide crosslinkers. The resulting hydrogel networks have a reduced inhomogeneity because of the low concentration and large size of the peptide crosslinkers. In addition, under stress the helical chains can be stretched to elongated ones and the intramolecular hydrogen bonds stabilizing the helical structures will be broken, providing a novel mechanism for energy dissipation. Therefore, the peptide-crosslinked hydrogels present significantly improved mechanical strength and extensibility. Unlike the previously used mechanisms for energy dissipation, here the intramolecular hydrogen bonds and hence the helical structure reform instantly when unloaded, leading to a small hysteresis loop and high resilience (>94%). The helical peptide chains in the network act like molecule-sized springs, absorbing and storing mechanical energy when deformed but releasing it when the stress is removed. Therefore, high toughness and resilience are achieved simultaneously. Wearable, flexible strain/pressure sensors were successfully fabricated using the gels. Thanks to the high resilience of the gels, the sensors are highly reliable with unprecedentedly stable baseline and highly reproducible signal.