Significant Roles of Ions in Enhancing and Functionalizing Anisotropic Hydrogels

Abstract

Salt ions are multifunctional in living beings, in contrast to their limited efficiency in abiotic materials. Achieving the versatility of salt ions in synthetic materials is promising yet demanding. Here, we report that multivalent metallic ions can act multiple crucial roles in a polyacrylamide/sodium alginate (PAAm/SA) composite hydrogel system, inducing a quadruple effect that toughens and functionalizes the originally weak gel. Fixation of anisotropic structures (effect I), mechanical enhancement (effect II), conductivity improvement (effect III), as well as antifreezing and moisture retention properties (effect IV) simultaneously emerge in the gel, all of which are enabled by the ion effect. The resulting tough hydrogels exhibit excellent comprehensive properties that rival existing state-of-the-art hydrogels, promising a wide range of potential applications. As proof-of-concept demonstrations, extremely durable hydrogel-based soft electronic devices are developed, which operate stably even in harsh environments. We also prove that the ion effect can be induced by other multivalent metallic ions. This work highlights the versatility of salt ions in nonliving materials, providing a simple but enlightening idea for the development of all-around soft materials.

A High‐Density Hydrogen Bond Locking Strategy for Constructing Anisotropic High‐Strength Hydrogel‐Based Meniscus Substitute

“…Several methodologies have been proposed to achieve this goal. Mechanical training, [ 15 , 16 ] drying in confined conditions, [ 17 , 18 ] prestretching, [ 19 , 20 ] electric/magnetic field‐assisted assembly, [ 21 , 22 ] and directional freezing, [ 23 , 24 ] have been proposed for constructing anisotropic hydrogels. However, the majority of these reported methods mainly aim to replicate the uniaxial orientation characteristics of tendons, overlooking the more complex circumferential anisotropy observed in the meniscus.…”

Multifunctional Conductive Double-Network Hydrogel Sensors for Multiscale Motion Detection and Temperature Monitoring

“…Hydrogel, a type of unique solid-like soft and moist material comprising of a three-dimensional (3D) polymeric network and a large amount of water, has emerged as a promising candidate for constructing flexible sensors owing to its excellent flexibility, favorable stretchability, and similarity with biological tissues. , As a wearable sensor, more demands for hydrogels such as rapid self-healing, durability, conductivity, self-adhesiveness, and a simple preparation process have been proposed gradually . For the improvement of conductivity, the introduction of conductive materials (e.g., carbon nanotubes, metal nanowires, and graphenes) and soluble metal ions (e.g., Al 3+ , Zn 2+ , Fe 3+ , Na + , and Ca 2+ ) into hydrogel are feasible strategies. − Many efforts have been devoted to improve the mechanical properties of hydrogels by incorporating energy-dissipation mechanisms, such as double network (DN), reinforced, and micellar-cross-linked hydrogels. − Among them, DN hydrogels consisting of two interpenetrating polymer networks, tough and brittle, have garnered considerable attention in developing mechanically robust tough hydrogels . In DN hydrogels, the sacrificial bonds in the first brittle network effectively dissipate energy, while the second tough network helps maintain the constructional completeness during deformation, which has been proven effective in enhancing both stiffness and toughness .…”