Stratum Corneum-Inspired Zwitterionic Hydrogels with Intrinsic Water Retention and Anti-Freezing Properties for Intelligent Flexible Sensors

Abstract

Hydrogels, which mimic the properties of natural tissues, are essential for flexible electronics in human-machine interfaces (HMIs). However, traditional hydrogels suffer from dehydration, compromising stability and functionality. To address this issue, a stratum corneum-inspired, water-retaining hydrogel is developed using hygroscopic polymers and bound water. Three types of hydrophilic monomers (non-ionic, mono-ionic, and zwitterionic) are explored, with polyzwitterions, particularly N,N-dimethyl (acrylamidopropyl) ammonium propane sulfonate (DMAAPS), forming a quasi-hydrogel that retains the softness and flexibility of conventional hydrogels. Water acts as a plasticizer, enhancing polymer chain mobility and reducing stiffness. The DMAAPS hydrogel maintains 100% weight retention under specific humidity conditions and shows skin-like softness across a wide humidity range. The Young's modulus increases from 54 to 118 kPa as relative humidity decreases from 80% to 40%. The absence of free water confers intrinsic anti-freezing properties. A triple crosslinking mechanism and conductive polymers endow the hydrogel with stretchability (> 2000%), toughness, elasticity, self-healing, and stable sensing capabilities. The hydrogel functions as an excellent flexible sensor for real-time, sensitive detection of human motion and physiological signals. An intelligent handwriting recognition platform with high accuracy is also established using double-channel signal collection and machine learning algorithms, offering insights for next-generation durable, biomimetic, and smart HMIs.