Three-dimensional printed ionic conductive hydrogels with tunable mechanical properties for wearable strain sensors.

Flexible sensors have garnered significant research interest as essential components of wearable devices. However, achieving flexible sensors with superior mechanical properties, high sensing performance, and a broad strain range remains challenging. In this study, an ion-conducting N-(2-Amino-2-oxoethyl) acrylamide/carboxymethyl chitosan/methyl cellulose hydrogel was analyzed, and its process parameters were optimized to enable high-fidelity hydrogel printing. Furthermore, a dual-ion synergistic enhancement strategy was proposed, wherein Fe3+ and Li+ were introduced via solution immersion to tailor a "stiff-tough-ductile" balanced network, significantly enhancing the mechanical properties of the hydrogel network. Experimental results showed that the dual-ion-treated hydrogels exhibited outstanding mechanical properties, with a modulus of elasticity of up to 266 kPa, toughness reaching 1245 kJ/m3, elongation at break of up to 437 %, and remarkable resistance to swelling. Additionally, the three-dimensional-printed reticulated hydrogel exhibited high sensitivity, a strain factor of 2.57, a broad detection range (0-300 %), and excellent fatigue durability over 100 cycles at 60 % strain. Integrating hydrogels into flexible sensors enables the high-precision detection of both subtle and large-scale human body movements, thereby advancing the application of conductive hydrogels in wearable electronics and offering new insights for the design and development of flexible sensors.