Hydrogels are attractive materials for wearable sensors due to their softness, wetness, responsiveness, and biocompatibility. However, conventional hydrogels often suffer from fatigue fracture under repeated stress due to inhomogeneous networks and a lack of energy dissipation mechanisms. While progress has been made in creating highly stretchable hydrogels, fatigue resistance remains a challenge. This research focuses on addressing this limitation by developing a novel hydrogel with superior mechanical properties and long-term stability for wearable sensor applications. The study draws inspiration from slide-ring (SR) hydrogels, which utilize topological networks with mechanical interlocked units to achieve high toughness and fracture resistance. However, existing SR hydrogels, often based on polyrotaxanes of poly(ethylene glycol) (PEG) and α-cyclodextrin (α-CD), have complicated and inaccessible synthetic protocols. This paper aims to overcome these limitations by exploring a new approach to creating highly fatigue-resistant hydrogels.

The literature extensively covers the use of hydrogels in wearable sensors, implantable bioelectronics, and electronic skins. Conventional chemically crosslinked hydrogels, while soft and biocompatible, are often brittle and prone to fatigue failure under repeated deformation. To improve toughness, researchers have explored double-network hydrogels and nanocomposite hydrogels, incorporating sacrificial non-covalent bonds. However, these hydrogels still exhibit limitations in fatigue resistance. Slide-ring (SR) hydrogels, based on polyrotaxanes, have shown promise in enhancing toughness and fracture resistance by dispersing stress through mobile junctions. However, the synthesis of polyrotaxanes with precise host coverage remains challenging. This study investigates the use of bile acids (BAs), such as cholic acid (CA) and lithocholic acid (LCA), as unexplored guests for constructing topological networks in hydrogels, offering a simpler and more accessible route to creating high-performance materials.

The researchers synthesized a polymerizable pseudorotaxane crosslinker by precisely assembling acrylated β-cyclodextrin (β-CD) with a bile acid derivative (LCA-AC) through host-guest recognition. This crosslinker was photocured with acrylamide (Am) in a binary solvent system of ethylene glycol (EG)/water with choline chloride (ChCl) to create the conductive PR-Gel. The synthesis involved several steps: acrylation of β-CD and LCA to create CD-AC and LCA-AC, respectively; confirmation of pseudorotaxane formation using powder X-ray diffraction (XRD), isothermal titration calorimetry (ITC), and 2D NOESY ¹H NMR spectroscopy; and photopolymerization to form the hydrogel. The mechanical properties (tensile strength, elongation, fatigue resistance, compression resistance) were assessed using a universal tensile tester and a compression tester. Fatigue resistance was evaluated via repeated loading-unloading cycles. The self-adhesion and self-healing properties of PR-Gel were studied using lap-shear tests and visual observation of cut samples. Electrical conductivity and sensitivity were measured using an electrochemical workstation, calculating the gauge factor. 3D printing of PR-Gel was accomplished using a digital light processing (DLP) based 3D printer. Finally, the performance of 3D printed sensors was evaluated by monitoring human motion and ECG signals. Detailed characterization methods included ¹H NMR, ¹³C NMR, HRMS, HPLC, XRD, SAXS, rheological measurements, and SEM. The synthesis of LCA-AC and CD-AC, along with a water-soluble analog LCA-AC-PEG for NMR studies, are meticulously described.

The PR-Gel exhibited significantly improved mechanical properties compared to control hydrogels (MBA-Gel and CD-Gel). Specifically, it demonstrated high stretchability (up to 830%), superior fatigue resistance (minimal hysteresis after 500 cycles at 300% strain), and excellent resilience (above 97% resilience at 300% strain). The mobile junctions in the PR-Gel network effectively dispersed stress during deformation, contributing to the high fracture resistance and fatigue resistance. The PR-Gel also displayed self-adhesion to various substrates, including human skin, and self-healing ability. The material showed high ionic conductivity (0.93 S/m at room temperature and 0.54 S/m at -20°C) and a high gauge factor (8.53 at 400-500% strain), indicating high sensitivity and a wide response range. 3D-printed PR-Gel sensors demonstrated high resolution and complex geometries. These sensors could successfully monitor human motions and electrocardiogram (ECG) signals in real-time with high repeatability. SAXS analysis showed the isotropic nature of the PR-Gel even under large elongation, confirming the mobility of rotaxanes in the network. The results consistently indicated that the mobile junctions of the topological networks are responsible for the superior mechanical and electrical properties.

The findings demonstrate that the incorporation of polymerizable pseudorotaxane crosslinkers based on the host-guest interaction between β-cyclodextrin and bile acid derivatives leads to a hydrogel with exceptional properties. The unique topological networks formed by the mobile junctions effectively address the limitations of conventional hydrogels in terms of fatigue resistance. This approach offers a simple and efficient method for creating highly stretchable and tough hydrogels, overcoming the complexities of traditional polyrotaxane synthesis. The integration of 3D printing further enhances the versatility and applicability of the PR-Gel for creating customized wearable sensors with complex geometries. The high sensitivity, stability, and self-healing capabilities of the PR-Gel-based sensors make them well-suited for a wide range of applications in healthcare and beyond.

This study successfully designed and fabricated a new type of conductive polymerizable rotaxane hydrogel (PR-Gel) with remarkable mechanical and electrical properties. The unique topological networks with mobile junctions significantly improved the fatigue resistance, stretchability, and self-healing ability compared to conventional hydrogels. The 3D-printability of PR-Gel opened up possibilities for creating customized wearable sensors with high resolution and complex designs, as demonstrated by the successful detection of real-time human ECG signals. Future research could explore various bile acid derivatives and β-CD modifications to further enhance the performance of PR-Gel and investigate its biocompatibility for advanced biomedical applications.

While the PR-Gel exhibits outstanding properties, the study's scope is limited to specific bile acid derivatives and β-CD modifications. Further research is needed to explore a wider range of components and optimize the hydrogel properties for different applications. The long-term stability of the PR-Gel in various environments also needs further investigation. The adhesion strength, while sufficient for many applications, could be improved for demanding scenarios.