Multi-functional hydrogels with stiffness switchability and asymmetric adhesion for pH monitoring in aqueous environments

Traditional pH-responsive hydrogels have garnered significant interest for smart sensing applications. However, their widespread adoption is often hampered by a compromise between mechanical robustness and rapid fabrication, as well as limited functional versatility. Herein, we present a multifunctional hydrogel with broadly tunable mechanical properties, dual-mode (visual and resistive) pH sensing and unique asymmetric adhesion. The hydrogel network can be rapidly fabricated within 1 min via a straightforward UV photopolymerization. By modulating the photoinitiator concentration, the crosslinking density and polymer chain length can be systematically adjusted, thereby regulating the glass transition temperature ( T g ) and enabling hydrogels with a stiffness variation spanning three orders of magnitude in storage modulus. Furthermore, a flexible resistive sensor based on a copolymer network of hydroxyethyl methacrylate, acrylic acid and conductive sulfopropyl methacrylate salt was developed. Within a test range of pH 4.0–8.0, the sensor exhibited stable real-time response characteristics and an ultrafast response time of 100 ms. Combined with smartphone-based colorimetric quantification via an integrated pH-sensitive dye, this hydrogel platform offers comprehensive dual-channel monitoring. This work provides a novel design strategy for next-generation smart materials, showcasing great potential in wearable sensors, soft robotics and intelligent biomedical devices.