Ultra-strong and tough cellulose-based conductive hydrogels via orientation inspired by noodles pre-stretching

Due to the unsatisfactory mechanical properties of natural polymer-based conductive hydrogels, their applications are limited. Shaanxi Biangbiang noodles can be toughened by applying external mechanical forces through stretching and beating movements; this process provides inspiration for the preparation of high-strength hydrogels. In this paper, we propose a strategy for the preparation of ultrastrong and ultratough conductive hydrogels by directional prestretching and solvent exchange. Neatly arranged fiber bundles containing many intermolecular hydrogen bonds and metal ion coordination bonds are successfully constructed inside the prepared hydrogels. The hydrogel has exceptional mechanical properties, with a fracture stress exceeding 50 MPa, fracture strain approaching 105 %, fracture toughness exceeding 30 MJ m-3, and high conductivity reaching 11.738 ± 0.06 mS m-1. Impressively, the hydrogel can maintain its high mechanical properties after being frozen at an ultralow temperature of -80 °C for 7 days. Compared with other tough hydrogels, natural tendons and synthetic rubbers, the hydrogel exhibits excellent mechanical properties. The cellulose-based conductive hydrogel prepared in this study can be applied to robotic soft tissues (such as the Achilles tendon) that require high strength and are operated in extreme environments.

Mechanically Excellent, Notch-Insensitive, and Highly Conductive Double-Network Hydrogel for Flexible Strain Sensor

A novel double-network conductive hydrogel based on lithium acetate/gelatin/polyacrylamide (PAAM) was synthesized by heating-cooling and subsequent γ-ray radiation-induced polymerization and cross-linking. Owing to the hydrogen bonding interaction between lithium acetate, physical cross-linked gelatin, and chemical cross-linked PAAM, the resultant hydrogel exhibited high tensile strength (1260 kPa), high ionic conductivity (35.2 mS cm-1), notch-insensitivity (tensile strength 415 kPa, elongation at break 872% with transverse notch), and extensive strain monitoring range (0.15-800%) under optimum conditions. The lithium acetate/gelatin/polyacrylamide hydrogel strain sensor attached to the skin can sensitively monitor the subtle movements of the human body. The strain sensor based on the resultant hydrogel with transverse notch can still work for 1200 cycles, due to that the covalent-cross-linked PAAm chain bridges the cracks and stabilizes the deformation, while the physical-cross-linked gelatin was unzipped to make the blunting of notch. The conductive hydrogel with high-sensitivity and high stability is expected to be used as materials for the preparation of flexible strain sensors in the future.

Tough Hydrogels for Load-Bearing Applications

Tough hydrogels have emerged as a promising class of materials to target load-bearing applications, where the material has to resist multiple cycles of extreme mechanical impact. A variety of chemical interactions and network architectures are used to enhance the mechanical properties and fracture mechanics of hydrogels making them stiffer and tougher. In recent years, the mechanical properties of tough, high-performance hydrogels have been benchmarked, however, this is often incomplete as important variables like water content are largely ignored. In this review, the aim is to clarify the reported mechanical properties of state-of-the-art tough hydrogels by providing a comprehensive library of fracture and mechanical property data. First, common methods for mechanical characterization of such high-performance hydrogels are introduced. Then, various modes of energy dissipation to obtain tough hydrogels are discussed and used to categorize the individual datasets helping to asses the material's (fracture) mechanical properties. Finally, current applications are considered, tough high-performance hydrogels are compared with existing materials, and promising future opportunities are discussed.

High Multi-Environmental Mechanical Stability and Adhesive Transparent Ionic Conductive Hydrogels Used as Smart Wearable Devices

Ionic conductive hydrogels used as flexible wearable sensor devices have attracted considerable attention because of their easy preparation, biocompatibility, and macro/micro mechanosensitive properties. However, developing an integrated conductive hydrogel that combines high mechanical stability, strong adhesion, and excellent mechanosensitive properties to meet practical requirements remains a great challenge owing to the incompatibility of properties. Herein, we prepare a multifunctional ionic conductive hydrogel by introducing high-modulus bacterial cellulose (BC) to form the skeleton of double networks, which exhibit great mechanical properties in both tensile (83.4 kPa, 1235.9% strain) and compressive (207.2 kPa, 79.9% strain) stress-strain tests. Besides, the fabricated hydrogels containing high-concentration Ca²⁺ show excellent anti-freezing (high ionic conductivities of 1.92 and 0.36 S/m at room temperature and -35 ∘C, respectively) properties. Furthermore, the sensing mechanism based on the conductive units and applied voltage are investigated to the benefit of the practical applications of prepared hydrogels. Therefore, the designed and fabricated hydrogels provide a novel strategy and can serve as candidates in the fields of sensors, ionic skins, and soft robots.