Physically Cross-Linked Silk Fibroin-Based Tough Hydrogel Electrolyte with Exceptional Water Retention and Freezing Tolerance

Abstract

Flexible ionic conductive hydrogel is attracting significant interest as it could be one of the crucial components for multifunctional ionotronic devices. However, their features of inevitably drying out without package and freezing at subzero temperatures may greatly limit the applications of conventional hydrogels in specific situations. Here, we present an ionic conductive hydrogel with water retention and freezing tolerance that consists of silk fibroin, ionic liquid, water, and inorganic salt. It is discovered that the ionic liquid serves multiple purposes to prevent water evaporation, decrease the freezing point, provide the essential conductivity of the hydrogel, etc. As a binary mixed solvent, the ionic liquid/water mixture enhances both water retention and freezing tolerance of the hydrogel electrolyte. Based on the silk fibroin (SF)/1-ethyl-3-methylimidazolium acetate (EMImAc)/H2O/KCl hydrogel electrolyte, the flexible fiberlike supercapacitor could still function well at a temperature as low as −50 °C and after being stored in the open air for a long time. It is anticipated that this hydrogel will prove useful in developing new applications operating under harsh environments.

Rapid and scalable fabrication of ultra‐stretchable, anti‐freezing conductive gels by cononsolvency effect

“…Tremendous improvements have been made for anti‐freezing hydrogels with the aforementioned strategies; however, there remains a common issue for a part of reported works: the anti‐freezing mixtures or conductive components are introduced into the gel matrix after the gels synthesized 14,19 . This process requires long times, from hours to days, because of the slow diffusion of anti‐freezing solutions and conductive ions into a bulk gel matrix; this limits the large‐scale and rapid fabrication of anti‐freezing gels 13,25,26 . Therefore, it is highly desirable to develop new strategies for producing conductive anti‐freezing gels remaining mechanically robust under low temperatures.…”

Poly(vinyl alcohol) Hydrogels with Integrated Toughness, Conductivity, and Freezing Tolerance Based on Ionic Liquid/Water Binary Solvent Systems

“…Figure 3A shows that the freezing points of all PVA/IL/H 2 O hydrogels were much lower than those of pure water and the ILs (for instance, −20 °C for pure EMImAc) due to the strong interaction between ILs and H 2 O. 39,52 The freezing point of The results showed that when EMImAc/H 2 O = 4/5, the PVA/ EMImAc/H 2 O hydrogel exhibited the best anti-freezing performance (Figure 3B). Therefore, the EMImAc/H 2 O binary solvent system (EMImAc/H 2 O = 4/5) was taken as the best solvent and all PVA hydrogels discussed in the remaining sections of the article used this solvent system.…”

“…36−38 In our laboratory, an ionic conductive regenerated silk fibroin (RSF) hydrogel with low-temperature workability has been successfully prepared by using an ionic liquid (IL)/water (EMImAc/H 2 O) binary solvent system. 39,40 However, when silk was dissolved in other ILs, it was difficult to produce other RSF/IL/H 2 O hydrogels with the same properties as the original RSF/EMImAc/H 2 O, mainly because of the insufficient solubility of silk in the other ILs. In addition, other proteins or polysaccharides that can form physically crosslinked hydrogels in water were found to be hard to form hydrogels in the IL/H 2 O binary solvent system like RSF.…”

“…However, conventional hydrogels composed of purely hydrophilic systems inevitably freeze below subzero temperatures and thus become rigid, fragile, and nonconductive or dry out in open air, which severely limits their application in flexible devices . Recently, some anti-freezing hydrogels based on the colligative properties of salt solutions or the polyol/water binary solvent systems, such as ethylene glycol (EG), glycerol (GC), sorbitol, or a mixture of them, have been developed to inhibit ice crystallization or decrease the ice point of water. − Nonetheless, these hydrogels have either sophisticated fabrication processes or complicated components, as well as other limitations to some degree, such as a reduction in the ionic conductivity due to the addition of cryoprotectants. − In our laboratory, an ionic conductive regenerated silk fibroin (RSF) hydrogel with low-temperature workability has been successfully prepared by using an ionic liquid (IL)/water (EMImAc/H 2 O) binary solvent system. , However, when silk was dissolved in other ILs, it was difficult to produce other RSF/IL/H 2 O hydrogels with the same properties as the original RSF/EMImAc/H 2 O, mainly because of the insufficient solubility of silk in the other ILs. In addition, other proteins or polysaccharides that can form physically crosslinked hydrogels in water were found to be hard to form hydrogels in the IL/H 2 O binary solvent system like RSF.…”

Tendon-inspired anti-freezing tough gels

“…to lower the freezing point ( Xu et al., 2020 ). Inspired by the poikilotherms and plants, cosolvents ( Wu et al., 2021 ), ionic liquids ( Wang et al., 2020 ), and salts ( Morelle et al., 2018 ) have been utilized to depress the freezing point. However, these freezing point depression methods are not very effective in enhancing mechanical properties, and some methods even weaken the hydrogels ( Sadeghi and Jahani, 2012 ).…”