Highly Stretchable Amino Acid-Based Antifreezing Hydrogels for Applications in Human Motion Sensors and Flexible Energy Storage Devices

Research efforts toward developing flexible strain sensors have grown rapidly in recent years due to their promising applications in touch screens, soft robotic systems, and wearable devices for human motion monitoring. Hydrogels, being moist and soft materials, have generated great interest for their use in flexible electronics and sensing. Conventional hydrogels are brittle and show inadequate self-recovery and fatigue resistance, significantly restricting their application. Moreover, the water in the hydrogel freezes at subzero temperatures, limiting the use of devices in cold climates. The efficient adhesion of the hydrogels to diverse substrates is also essential for device applications. Creating a hydrogel with a favorable combination of properties, fast self-recovery, high stretchability, self-healing, antifreezing, and self-adhesive capabilities, alongside sufficient ionic conductivity, is challenging. We report herein physically cross-linked copolymer hydrogel AAm/ORn, based on poly(acrylamide-co-acryloyl ornithine). The optimized hydrogel showed high toughness (1.3 MJ m^–3), excellent stretchability (fracture strain: 3000%), fast self-recovery (within 35 s after tensile loading–unloading to 100% strain), and good adhesive strength to various substrates like plastic (3900 N/m²), glass (13,360 N/m²), and rubber (7870 N/m²). The ionically conducting hydrogel-based flexible resistive strain sensor demonstrated high strain sensitivity (GF value of 0.65 within the strain range of 400–800%) and can detect human limb movements. Synthesizing the hydrogel in the presence of LiCl (1 M) significantly improved the ionic conductivity (from 3.47 to 8.84 mS/cm at room temperature) and rendered the hydrogel antifreezing. The corresponding AAm/ORn/LiCl hydrogel maintained good conductivity (7.6 mS/cm) even at −15 °C. The flexible supercapacitor device fabricated with the AAm/ORn/LiCl hydrogel electrolyte, when operated at −15 °C, retained ∼50% of the specific capacitance displayed at room temperature (37.66 F/g). This work offers a simple strategy for making a high-performance hydrogel, which may be used in human motion sensors and flexible energy storage devices in cold environments.