Hydrogels, known for their biocompatibility and ability to swell in water, are widely used in various applications, including flexible sensors and robotics. However, traditional hydrogels often lack the high mechanical properties necessary for demanding applications. Slide-ring gels, with their movable cross-links, offer a solution. These materials, usually based on polyrotaxanes, exhibit excellent mechanical properties due to the “pulley effect” of ring structures, allowing for even stress distribution. However, the limited water solubility of polyrotaxanes and the complex synthesis have hindered their use. This research addresses these limitations by utilizing Hy-α-CD, which offers good water solubility and biocompatibility, to create a new type of slide-ring supramolecular hydrogel.

The authors reviewed existing literature on hydrogels and their applications, highlighting the limitations of traditional hydrogels in terms of mechanical strength and stretchability. They discussed various methods employed to improve hydrogel properties, such as dual network formation and ionic crosslinking, but noted the challenges in achieving high strength, toughness, and fatigue resistance simultaneously. The advantages of slide-ring gels (polyrotaxane gels) were emphasized, focusing on the unique "pulley effect" of movable crosslinks leading to superior mechanical properties. However, the challenges of low water solubility and complex synthesis of conventional polyrotaxanes were also discussed as major limitations.

The researchers synthesized a polypseudorotaxane by mixing ACA-PEG20000-ACA and Hy-α-CD in water. This was then converted to capped polyrotaxane via photo-initiated polymerization with acrylamide. The capped polyrotaxane was further cross-linked using 1,4-butanediol diglycidyl ether in a sodium hydroxide solution to form the slide-ring supramolecular hydrogel. The synthesis of ACA-PEG20000-ACA involved a three-step reaction. Characterization techniques included 2D ROESY spectroscopy to confirm the inclusion of PEG chains within Hy-α-CD cavities, solid-state NMR to verify the presence of cyclodextrin units after dialysis and freeze-drying, and analysis of polymerization efficiency. Scanning electron microscopy (SEM) was used to visualize the hydrogel's porous structure. Fourier-transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) were performed to characterize the chemical composition and thermal stability. Rheological tests determined the hydrogel's mechanical properties (storage modulus, G', and loss modulus, G''). Tensile tests measured the hydrogel's stretchability, strength, and toughness. The impact of CaCl2 addition on the hydrogel's properties, specifically ionic conductivity and mechanical strength, was investigated. Finally, the hydrogel's suitability as a wearable strain sensor was evaluated through adhesion, conductivity, and sensitivity tests. The sensor's performance was assessed by measuring its resistance change under various strains and during dynamic movements like wrist bending.

The synthesized Hy-α-CD/ACA-PEG20000-ACA hydrogel exhibited exceptional stretchability (2540% elongation), high fracture energy (17.4 MJ/m³), and a relatively high elastic modulus (35.67 kPa). The hydrogel demonstrated rapid recovery after stretching. The addition of Ca²⁺ ions significantly enhanced the hydrogel's ionic conductivity. The Ca²⁺-doped hydrogel showed excellent adhesion to various surfaces, including human skin. When used as a strain sensor, the hydrogel exhibited high sensitivity and a rapid response to strain changes. The sensor showed good stability over multiple stretching cycles (300 cycles) and effectively monitored dynamic human movements, such as wrist bending. The sensor could also operate a touch screen.

The results demonstrate the successful fabrication of a highly stretchable and conductive hydrogel with excellent mechanical properties. The unique slide-ring structure, combined with the use of Hy-α-CD, addressed the limitations of traditional polyrotaxane-based hydrogels, achieving both high stretchability and water solubility. The enhancement of ionic conductivity through Ca²⁺ doping is crucial for sensor applications. The excellent adhesion and responsiveness make the hydrogel a promising material for wearable strain sensors. The demonstrated ability to control a touch screen further broadens the hydrogel's potential applications in human-computer interaction.

This study successfully synthesized a novel slide-ring supramolecular hydrogel with exceptional stretchability, toughness, and rapid recovery. The incorporation of Ca²⁺ ions enhanced ionic conductivity, enabling its use as a highly sensitive and stable wearable strain sensor for monitoring human motion. The hydrogel’s ability to operate a touch screen opens up new possibilities in human-computer interaction. Future research could focus on exploring other dopants for further improving conductivity or exploring different polymer backbones for enhanced properties.

While the hydrogel demonstrates impressive properties, further investigation into long-term stability and biocompatibility in vivo is necessary for broader biomedical applications. The adhesion strength of the hydrogel, although sufficient for the demonstrated applications, could be quantified more rigorously. The study primarily focused on wrist movements; testing the sensor's performance with other types of human motion would further validate its versatility. The mechanism of the enhanced conductivity upon CaCl2 addition could be further investigated at the molecular level.