Ultrasensitive Wearable Soft Strain Sensors of Conductive, Self-healing, and Elastic Hydrogels with Synergistic “Soft and Hard” Hybrid Networks

TLDR

Robust, stretchable, and strain‑sensitive hydrogels are of great interest for wearable strain sensors, yet achieving excellent mechanical properties, self‑healing, and high sensitivity remains challenging. This work aims to fabricate conductive, elastic, self‑healing hydrogels by interconnecting a soft polymer network with a hard dynamic Fe³⁺‑cross‑linked cellulose nanocrystal network inspired by biological tissues. The dynamic Fe³⁺–CNC coordination bonds act as sacrificial bonds that dissipate energy while the homogeneous polymer network ensures smooth stress transfer, yielding high strength, toughness, stretchability, and self‑recovery. The resulting hydrogels self‑heal within 5 min without stimuli, exhibit tunable, stable electromechanical responses, and function as wearable strain sensors for finger motion, breathing, and pulse, integrating mechanical robustness, self‑healing, and sensitivity.

Abstract

Robust, stretchable, and strain-sensitive hydrogels have recently attracted immense research interest because of their potential application in wearable strain sensors. The integration of the synergistic characteristics of decent mechanical properties, reliable self-healing capability, and high sensing sensitivity for fabricating conductive, elastic, self-healing, and strain-sensitive hydrogels is still a great challenge. Inspired by the mechanically excellent and self-healing biological soft tissues with hierarchical network structures, herein, functional network hydrogels are fabricated by the interconnection between a "soft" homogeneous polymer network and a "hard" dynamic ferric (Fe3+) cross-linked cellulose nanocrystals (CNCs-Fe3+) network. Under stress, the dynamic CNCs-Fe3+ coordination bonds act as sacrificial bonds to efficiently dissipate energy, while the homogeneous polymer network leads to a smooth stress-transfer, which enables the hydrogels to achieve unusual mechanical properties, such as excellent mechanical strength, robust toughness, and stretchability, as well as good self-recovery property. The hydrogels demonstrate autonomously self-healing capability in only 5 min without the need of any stimuli or healing agents, ascribing to the reorganization of CNCs and Fe3+ via ionic coordination. Furthermore, the resulted hydrogels display tunable electromechanical behavior with sensitive, stable, and repeatable variations in resistance upon mechanical deformations. Based on the tunable electromechanical behavior, the hydrogels can act as a wearable strain sensor to monitor finger joint motions, breathing, and even the slight blood pulse. This strategy of building synergistic "soft and hard" structures is successful to integrate the decent mechanical properties, reliable self-healing capability, and high sensing sensitivity together for assembling a high-performance, flexible, and wearable strain sensor.