Environmentally Adaptive Polymer Hydrogels: Maintaining Wet‐Soft Features in Extreme Conditions

Abstract

Hydrogels have been widely explored to adapt to different application circumstances. As typical wet‐soft materials, the high‐water content of hydrogels is beneficial to their wide biomedical applications. Moreover, hydrogels have been displaying considerable application potential in some high‐tech areas, like brain‐computer interface, intelligent actuator, flexible sensor, etc. However, traditional hydrogel is susceptive to freezing below zero, dehydration, performance swelling‐induced deformation, and suffers from mechanical damage in extremely mechanical environments, which result in the loss of wet‐soft peculiarities (e.g., flexibility, structure integrity, transparency), greatly limiting their applications. Therefore, reducing the freezing point, improving the dehydration/solution resistance, and designing mechanical adaptability are effective strategies to endow hydrogels with the extreme environmental adaptability, thus broadening their application fields. This review systematically summarizes research advances of environmentally adaptive hydrogels (EAHs), comprising anti‐freezing, dehydration‐resistant, acid/base/swelling deformation‐resistant, and mechanical environment adaptive hydrogels (MEAHs). Firstly, fabrication methods are presented, including the deep eutectic solvent/ionic liquid substituent, the addition of salts, organogel, polymer network modification, and double network (DN) complex/nanocomposite strategy, etc. Meanwhile, the features of different approaches are overviewed. The mechanisms, properties, and applications (e.g., intelligent actuator, wound dressing, flexible sensor) of EAHs are demonstrated. Finally, the issues and future perspectives for EAHs’ researches are demonstrated.

Functionalization of Natural-Derived Biogels for Soft Bioelectronics

“…13 One effective method to alter the free water component is by introducing hygroscopic substances such as salts, organic solvents, and their combinations into regular biogels to make free water molecules tightly bound up, which contributes to the water retention capacity enhancement and freeze point decrease of biogels. 21 The incorporation of high-content salts (e.g., LiCl, NaCl, KCl, CaCl 2 , etc.) could bind the free water molecules into hydration water via ionic hydration, while some pure salts offer limited water retention capacity and antifreezing capability.…”

“…Incorporating salts together with organic solvents into biogels offers a potential method to synergistically achieve good ambient stability, antifreezing capacity, and high conductivity. Besides, substituting water with other novel solvents such as nonvolatile ionic liquids and deep eutectic solvents is another promising strategy to alter the free water content in biogels . Owing to their unique environmental and chemical stability, low volatility, and good conductivity, biogels based on such novel solvents could be correspondingly equipped with long-term stability, extreme temperature adaptability and high conductivity.…”

Low-hysteresis and highly linear sensors based on environmentally stable, adhesive, and antibacterial hydrogels

“…On the other hand, some drawbacks of ionic hydrogels, such as their tendency to dry at room and high temperatures, freezing at subzero temperatures, and swelling under water, significantly affect the sensing performance of ionic hydrogel-based strain sensors. 11 It has been reported that incorporating hydrophobic monomers into the polymer matrix is an effective approach to reducing the swelling of ionic hydrogels. 12 Moreover, adding inorganic salts ( e.g.…”

Recent Progress of Anti-Freezing, Anti-Drying, and Anti-Swelling Conductive Hydrogels and Their Applications

“…Furthermore, conventional conductive hydrogels may exhibit substantial volumetric expansion due to the absorption of significant quantities of water from humid surroundings, reducing their mechanical and conductive properties [27]. This limits their applications under extreme conditions [28][29][30]; therefore, the development of hydrogels with environmental tolerance is imperative to broadening their application scope in flexible sensing and biomedical areas.…”