High-Fidelity Hydrogel Thin Films Processed from Deep Eutectic Solvents

Abstract

Polyampholyte (PA) hydrogels are a fascinating class of soft materials that can exhibit high toughness while retaining self-healing characteristics. This behavior results from the random distribution of oppositely charged monomers along the polymer chains that form transient bonds with a range of bond strengths. PAs can be dissolved in aqueous salt solutions and then re-cast via immersion precipitation, making them particularly useful as surface coatings in biomedical applications. Moreover, this immersion precipitation technique allows these PA hydrogels to be fabricated into films less than 100 nm. One critical challenge to this aqueous processing method is the recrystallization of the salt upon water evaporation. Such re-crystallization can disrupt the hydrogel morphology especially in thin films. In this study, a deep eutectic solvent (DES) formed from urea and choline chloride was used to dissolve PAs made from p-styrenesulfonic acid sodium salt and 3-(methacryloylamino)propyl trimethylammonium chloride. This DES has a freezing point of 12 • C, allowing it to remain stable and liquid-like at room temperatures. Thus, these PAs can be processed in DES solutions, without this issue of re-crystallization and with simple methods such as spin coating and dip coating. These methods allow these hydrogels to be used in thin (<100 nm) film coating applications. Finally, the complete miscibility of DES in water allows a wider range of 1 phase compositions and expands the processing window of these polyampholyte materials.

Poly(vinyl alcohol)/Gelatin-Based Eutectogels for the Sensitive Strain Sensor with Recyclability and Multienvironmental Suitability

“…In recent years, eutectic gels, also known as eutectogels, prepared by using deep eutectic solvent (DES) as the disperser, were proposed as an additional type of gel. , A DES is usually composed of a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD), which has physicochemical properties, such as ionic conductivity and nonvolatility, similar to ILs. What is more, compared with ILs, the DES is nontoxic, biocompatible, and easily available . Thus, the use of DES has opened a window toward creating soft materials to meet the increasing demand for the sensing material in broad environments. , More recently, it has been demonstrated that conductive eutectogels can be used to assemble stretchable strain sensors, which attract much interest in wearable electronic devices. , …”

Processing Polyelectrolyte Complexes with Deep Eutectic Solvents

“…The evaporation of water during processing not only reduces the solvent content but also increases the salt and PEC concentration in the solution, and alters the microstructure of the PEC and its properties . When the loss of water reaches a critical level, crystallization of the salt can occur, adversely affecting the uniformity and physical properties of the PEC . Therefore, a noncrystallizing and low-volatility solvent is needed.…”

“…In terms of the phase diagram shown in Figure , a homogeneous material is obtained in the entire composition range apart from the two-phase region in the lower-left portion of the phase diagram, whereas KBr crystallization is observed at high salt concentrations in the corresponding ternary phase diagram for the PEC/water/KBr system. Similar behavior has been reported in a polyampholyte system, where the anionic and cationic charges reside on the same polymer component . In the EG/GudnSCN system, crystallization of the GudnSCN salt is prevented by its formation of a eutectic with ethylene glycol, which remains present in the system for long times because of its low volatility.…”

“…11 When the loss of water reaches a critical level, crystallization of the salt can occur, adversely affecting the uniformity and physical properties of the PEC. 12 Therefore, a noncrystallizing and lowvolatility solvent is needed. Ionic liquids, with a melting point lower than the room temperature, are obvious candidates to consider for these purposes.…”

“…Similar behavior has been reported in a polyampholyte system, where the anionic and cationic charges reside on the same polymer component. 12 In the EG/GudnSCN system, crystallization of the GudnSCN salt is prevented by its formation of a eutectic with ethylene glycol, which remains present in the system for long times because of its low volatility. The low-volatility solvent remains in the film, preventing salt crystallization and providing a film with a stable microstructure and uniform physical properties.…”