Straightforward preparation of a tough and stretchable ion gel.

Ion gels, polymer networks swollen by ionic liquids, are expected to be applied to wearable devices that are tolerant to repeated stretching. High strength and excellent stretchability was achieved due to the numerous physical cross-links with abundant polymer chain entanglements in addition to a small number of immobile chemical cross-links, even though the ion gel was prepared by a facile methodology.

Long-Term Physical Stability of Amorphous Solid Dispersions: Comparison of Detection Powers of Common Evaluation Methods for Spray-Dried and Hot-Melt Extruded Formulations.

Although physical stability can be a critical issue during the development of amorphous solid dispersions (ASDs), there are no established protocols to predict/detect their physical stability. In this study, we have prepared fenofibrate ASDs using two representative manufacturing methods, hot-melt extrusion and spray-drying, to investigate their physical stability for one year. Intentionally unstable ASDs were designed to compare the detection power of each evaluation method, including X-ray powder diffraction (XRPD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and dissolution study. Each method did not provide the same judgment results on physical stability in some cases because of their different evaluation principles and sensitivity, which has been well-comprehended only for one-component glass. This study revealed that the detection powers of each evaluation method significantly depended on the manufacturing methods. DSC was an effective method to detect a small amount of crystals for both types of ASDs in a quantitative manner. Although the sensitivity of XRPD was always lower compared to that of DSC, interpretation of the data was the easiest. SEM was very effective for observing the crystallization of the small amount of drug for hot-melt extruded products, as the drug crystal vividly appeared on the large grains. The dissolution performance of spray-dried products could change even without any indication of physical change including crystallization. The advantage/disadvantage and complemental roles of each evaluation method are discussed for deeper understanding on the physical stability data of ASDs.

Ionic Liquid Interface as a Cell Scaffold.

In sharp contrast to conventional solid/hydrogel platforms, water-immiscible liquids, such as perfluorocarbons and silicones, allow the adhesion of mammalian cells via protein nanolayers (PNLs) formed at the interface. However, fluorocarbons and silicones, which are typically used for liquid cell culture, possess only narrow ranges of physicochemical parameters and have not allowed for a wide variety of cell culturing environments. In this paper, it is proposed that water-immiscible ionic liquids (ILs) are a new family of liquid substrates with tunable physicochemical properties and high solvation capabilities. Tetraalkylphosphonium-based ILs are identified as non-cytotoxic ILs, whereon human mesenchymal stem cells are successfully cultured. By reducing the cation charge distribution, or ionicity, via alkyl chain elongation, the interface allows cell spreading with matured focal contacts. High-speed atomic force microscopy observations of the PNL formation process suggest that the cation charge distribution significantly altered the protein adsorption dynamics, which are associated with the degree of protein denaturation and the PNL mechanics. Moreover, by exploiting dissolution capability of ILs, an ion-gel cell scaffold is fabricated. This enables to further identify the significant contribution of bulk subphase mechanics to cellular mechanosensing in liquid-based culture scaffolds.

Extremely Tough, Stretchable Gel Electrolytes with Strong Interpolymer Hydrogen Bonding Prepared Using Concentrated Electrolytes to Stabilize Lithium-Metal Anodes.

Extremely tough and stretchable gel electrolytes, which can be prepared by leveraging the strong interpolymer hydrogen bonding in concentrated lithium (Li)-salt electrolytes, are reported. These electrolytes can be realized by optimizing the competitive hydrogen-bonding interactions between polymer chains, solvent molecules, Li cations, and counteranions. Free polar solvent molecules, which typically impede interpolymer hydrogen bonding, are scarce in concentrated electrolytes; this feature can be exploited to prepare hydrogen-bonded gel electrolytes with unprecedented toughness. In contrast, free solvent molecules are abundant in electrolytes with typical concentrations, yielding considerably weaker gel electrolytes. The tough gel electrolyte can be used an artificial protective layer for Li-metal anodes, as it considerably enhances the cycling stability of a Li symmetric cell through uniform Li deposition/dissolution. Additionally, employing the gel electrolyte as the protecting layer significantly improves the cycling performance of the Li||LiNi0.6 Co0.2 Mn0.2 O2 full cell.

Adaptive Ion-Gel: Stimuli-Responsive, and Self-Healing Ion Gels.

Ion gels are an emerging class of polymer gels in which a three-dimensional polymer network swells with an ionic liquid. Ion gels have drawn considerable attention in various fields such as energy and biotechnology owing to their excellent properties including nonvolatility, nonflammability, high ionic conductivity, and high thermal and electrochemical stability. Since the first report on ion gels (published ∼30 years ago), diverse functional ion gels exhibiting impressive physicochemical properties have been reported. In this review, recent developments in functional ion gels that can modulate their physical properties in response to environmental conditions are outlined. Stimuli-responsive ion gels that can adaptively undergo phase transitions in response to thermal and light stimuli are initially discussed, followed by an evaluation of diverse self-healing ion gels that can spontaneously mend mechanical damage through judiciously designed ion-gel networks.

Controlling mechanical properties of ultrahigh molecular weight ion gels by chemical structure of ionic liquids and monomers.

A new class of ion gels, termed ultrahigh molecular weight (UHMW) gels, formed by physical entanglement of ultrahigh molecular weight polymers in ionic liquids, are synthesised using facile one step radical polymerisation with significantly low initiator conditions, and exhibit superior mechanical characteristics such as stretchability, recyclability, and room temperature self-healing ability. In this study, UHMW gels are synthesised using various combinations of monomer and IL structures, and the effect of their chemical structures on the physicochemical properties of UHMW gels are thoroughly investigated. UHMW polymers are prepared in situ for all combinations of ILs and monomers used in this study, indicating the wide applicability of this fabrication strategy. The structure-property relationships between chemical structures and mechanical properties of UHMW gels are investigated in detail. Furthermore, the differences in self-healing efficiency of UHMW gels depending on the chemical structure is discussed in terms of individual polymer conformation and polymer-polymer interaction based on molecular dynamics simulations.

Highly stretchable and self-healable polymer gels from physical entanglements of ultrahigh-molecular weight polymers.

Highly stretchable and self-healing polymer gels formed solely by physical entanglements of ultrahigh-molecular weight (UHMW) polymers were fabricated through a facile one-step process. Radical polymerization of vinyl monomers in ionic liquids under very low initiator concentration conditions produced UHMW polymers of more than 106 g/mol with nearly 100% yield, resulting in the formation of physically entangled transparent polymer gels. The UHMW gels showed excellent properties, such as high stretchability, high ionic conductivity, and recyclability. Furthermore, the UHMW gel exhibited room temperature self-healing ability without any external stimuli. The tensile experiments and molecular dynamics simulations indicate that the nonequilibrium state of the fractured surfaces and microscopic interactions between the polymer chains and solvents play a vital role in the self-healing ability. This study provides a physical approach for fabricating stretchable and self-healing polymer gels based on UHMW polymers.

Lithium-Ion-Conducting Ceramics-Coated Separator for Stable Operation of Lithium Metal-Based Rechargeable Batteries.

Lithium metal anode is regarded as the ultimate negative electrode material due to its high theoretical capacity and low electrochemical potential. However, the significantly high reactivity of Li metal limits the practical application of Li metal batteries. To improve the stability of the interface between Li metal and an electrolyte, a facile and scalable blade coating method was used to cover the commercial polyethylene membrane separator with an inorganic/organic composite solid electrolyte layer containing lithium-ion-conducting ceramic fillers. The coated separator suppressed the interfacial resistance between the Li metal and the electrolyte and consequently prolonged the cycling stability of deposition/dissolution processes in Li/Li symmetric cells. Furthermore, the effect of the coating layer on the discharge/charge cycling performance of lithium-oxygen batteries was investigated.

Design of azobenzene-bearing hydrogel with photoswitchable mechanics driven by photo-induced phase transition for in vitro disease modeling.

Mechanics of the extracellular matrix (ECM) exhibit changes during many biological events. During disease progression, such as cancer, matrix stiffening or softening occurs due to crosslinking of the collagen matrix or matrix degradation through cell-secreted enzymes. Engineered hydrogels have emerged as a prime in vitro model to mimic such dynamic mechanics during disease progression. Although there have been a variety of engineered hydrogels, few can offer both stiffening and softening properties under the same working principle. In addition, to model individual disease progression, it is desirable to control the kinetics of mechanical changes. To this end, we describe a photoresponsive hydrogel that undergoes stiffness changes by the photo-induced phase transition. The hydrogel was composed of a copolymer of azobenzene acrylate monomer (AzoAA) and N,N-dimethyl acrylamide (DMA). By tuning the amount of azobenzene, the phase transition behavior of this polymer occurs solely by light irradiation, because of the photoisomerization of azobenzene. This phase behavior was confirmed at 37 °C by turbidity measurements. In addition, the crosslinked poly(AzoAA-r-DMA) gel undergoes reversible swelling-deswelling upon photoisomerization by ultraviolet or visible light. Furthermore, the poly(AzoAA-r-DMA) sheet gels exhibited modulus changes at different isomerization states of azobenzene. When MCF-7 cells were cultured on the gels, stiffening at different timepoints induced varied responses in the gene expression levels of E-cadherin. Not only did this suggest an adaptive behavior of the cells against changes in mechanics during disease progression, this also demonstrated our material's potential towards in vitro disease modeling. STATEMENT OF SIGNIFICANCE: During disease progression such as cancer, cellular microenvironment called extracellular matrix (ECM) undergoes stiffness changes. Hydrogels, which are swollen network of crosslinked polymers, have been used to model such dynamic mechanical environment of the ECM. However, few could offer both stiffening and softening properties under the same working principle. Herein, we fabricated a novel photoresponsive hydrogel with switchable mechanics, activated by photo-induced structural change of the polymer chains within the hydrogel. When breast cancer cells were cultured on our dynamic hydrogels, gene expression and morphological observation suggested that cells react to changes in stiffness by a transient response, as opposed to a sustained one. The photoresponsive hydrogel offers possibility for use as a patient-specific model of diseases.

Preparation of biocompatible hydrogels reinforced by different nanosheets.

The impact of inorganic nanosheets with various chemical compositions and properties at different concentrations on the rheological properties and the gelation formation of a thermo-responsive hydrogel was investigated. F127 Pluronic triblock copolymers, with the structure (EO)99(PO)65(EO)99 (EO: ethylene oxide and PO propylene oxide respectively), functionalized by dimethacrylate (F127-DMA) at a concentration of 25% was used in this study. After careful characterization by complementary techniques: transmission electron microscopy (TEM), atomic force microscopy (AFM), and X-ray diffraction of nanosheets derived from the peeling of layered materials (montmorillonite, organoclays and hexaniobate), the nanosheets were seen to be suitably dispersed in the hydrogels. The inclusion of hydrophobic nanosheets (i.e. those treated with the grafting of surfactants onto their surface: organoclays and hexaniobate) leads to a depression of the gelation temperature while the nanocomposites exhibit an enhancement of their elastic properties, as determined by rheological measurements. In contrast, the inclusion of hydrophilic nanosheet derived from raw montmorillonite engenders an opposite trend. The whole nanocomposites whose gelation temperature can be tuned by both the nature and concentration of the nanosheets were successfully photopolymerized allowing the formation of a 3D structure containing a large content of water. The results obtained in this study open new perspectives for possible uses of hydrogel-based nanocomposites as embedding matrixes for bio-organisms.

Recent progress in self-healable ion gels.

Ion gels, soft materials that contain ionic liquids (ILs), are promising gel electrolytes for use in electrochemical devices. Due to the recent surge in demand for flexible and wearable devices, highly durable ion gels have attracted significant amounts of attention. In this review, we address recent advances in the development of ion gels that can heal themselves when mechanically damaged. Light- and thermally induced healing of ion gels are discussed as stimuli-responsive healing strategies, after which self-healable ion gels based on supramolecular and dynamic covalent chemistry are addressed. Tough, highly stretchable, and self-healable ion gels have recently been fabricated through the judicious design of polymer nanostructures in ILs in which polymer chains and IL cations and anions interact. The applications of self-healable ion gels to electrochemical devices are also briefly discussed.

Fabrication of Self-Oscillating Micelles with a Built-In Oxidizing Agent.

Various biological behaviors are fueled by "respiration", which is an example of catabolism. So far, we have reported various self-oscillating soft materials exhibiting bioinspired dynamic movements. These autonomous polymer systems are driven by the Belousov-Zhabotinsky (BZ) reaction, which is analogous to the tricarboxylic acid (TCA) cycle that is an integral part of respiration. However, in the BZ reaction, the external addition of an oxidizing agent is necessary to initiate the oxidation process, which is realized by intracellular moieties such as ubiquinone in living systems. Herein, we realized self-oscillating micelles that are driven without the external addition of an oxidizing agent. This was achieved by embedding the oxidizing source into the structure of the self-oscillating AB diblock copolymers. This strategy introduces a new function equivalent to intracellular oxidizing moieties, and is useful for the design of completely autonomous bioinspired materials.

Reactivity Control of Polymer Functional Groups by Altering the Structure of Thermoresponsive Triblock Copolymers.

A thermoresponsive ABA triblock copolymer bearing an aldehyde group on the thermoresponsive A segments was synthesized. The polymer formed a micellar assembly due to the hydrophobic interactions of the thermoresponsive segment above the lower critical solution temperature (LCST). In contrast, the ABA polymer assembly decomposed upon lowering the temperature below the LCST. Using this structural change, the reactivity of the aldehyde group toward primary amines of albumin and poly(allylamine) was investigated. When the ABA polymer assembly and reactant were mixed above the LCST, Schiff base formation was suppressed because of the aldehyde group being protected by the hydrophobic thermoresponsive core. In contrast, Schiff base formation between the ABA triblock copolymer and the primary amine moiety on the molecules was confirmed below the LCST. The reactivity of the aldehyde functional group can therefore be controlled by altering the structure of the thermoresponsive ABA polymer.

Viscoelastic change of block copolymer ion gels in a photo-switchable azobenzene ionic liquid triggered by light.

A photo-switchable ionic liquid solvent bearing an azobenzene moiety induced a viscoelastic change of block copolymer ion gels by light. ABA triblock copolymers having poly(phenethyl methacrylate) and poly(benzyl methacrylate) as A blocks exhibited opposite photoinduced rheological responses although they had only a tiny structural difference in the alkyl chain length.

Chemomechanical Motion of a Self-Oscillating Gel in a Protic Ionic Liquid.

An autonomous swelling-deswelling oscillation of polymer gels in a hydrated protic ionic liquid (PIL) as a proton source for the Belousov-Zhabotinsky (BZ) reaction is presented. Methylammonium hydrogen sulfate ([maH+ ][HSO4 - ]) was employed as the PIL because it provides stable redox oscillation in the BZ reaction. Due to the significantly higher pKa for [maH+ ][HSO4 - ] than those for conventional proton sources for the BZ reaction, chemomechanical oscillation can be expected under weaker acidic conditions. The self-oscillating polymer was designed as a ternary random copolymer of N-isopropylacrylamide, N-(3-aminopropyl)methacrylamide, and the Ru(bpy)3 moiety as a catalyst for the BZ reaction. The copolymer exhibited spontaneous soluble-insoluble oscillation in hydrated [maH+ ][HSO4 - ] containing NaBrO3 and malonic acid. Macroscopic swelling-deswelling oscillation of the porous bulk gel prepared by covalently connecting microgel particles was also observed.

Photohealable ion gels based on the reversible dimerisation of anthracene.

We report a photohealable ion gel based on the photodimerisation of anthracene as a dynamic covalent bond. A tetra-arm poly(ethylene glycol) terminally functionalised with anthracene was synthesised and combined with an ionic liquid to form an ion gel. The photodimerisation reaction was utilised to realise photohealing of the ion gels.

Photo/thermoresponsive ABC triblock copolymer-based ion gels: photoinduced structural transitions.

A photo/thermoresponsive ABC triblock copolymer-based ion gel exhibiting photoinduced structural transitions accompanied by significant rheological changes is newly developed. The ABC triblock copolymer comprises an ionic liquid (IL)-phobic A block, an IL-philic B block, and a photo/thermoresponsive C block containing azobenzene moieties. The IL-phobic A block forms a rigid micellar core in an IL over a wide temperature range and the photo/thermoresponsive C block undergoes upper critical solution temperature (UCST)-type phase transition in ILs. In concentrated polymer solution, the ABC triblock copolymer can form a percolated micellar network at low temperatures through aggregation of A and C blocks as physical crosslinks, bridged by IL-philic B blocks. In contrast, the ion gel undergoes structural transition to jammed micelles at high temperatures due to the disassembly of the thermoresponsive C block, resulting in significant softening of the ion gel. Importantly, the temperature dependences of the viscoelastic properties of the ion gel differ drastically depending on photo-irradiation conditions as the photoinduced isomerization of azobenzene moieties in the C block modulates the affinity between the polymer chain and IL. Utilizing this feature, photoinduced softening/hardening of the ion gel is realized at constant temperature. This study provides a promising strategy to control the rheological properties of nonvolatile soft materials via contactless light irradiation that could be exploited in various applications such as photoresponsive soft actuators and photo-healable soft materials.

Self-Healing Micellar Ion Gels Based on Multiple Hydrogen Bonding.

Ion gels, composed of macromolecular networks filled by ionic liquids (ILs), are promising candidate soft solid electrolytes for use in wearable/flexible electronic devices. In this context, the introduction of a self-healing function would significantly improve the long-term durability of ion gels subject to mechanical loading. Nevertheless, compared to hydrogels and organogels, the self-healing of ion gels has barely investigated been because of there being insufficient understanding of the interactions between polymers and ILs. Herein, a new class of supramolecular micellar ion gel composed of a diblock copolymer and a hydrophobic IL, which exhibits self-healing at room temperature, is presented. The diblock copolymer has an IL-phobic block and a hydrogen-bonding block with hydrogen-bond-accepting and donating units. By combining the IL and the diblock copolymer, micellar ion gels are prepared in which the IL phobic blocks form a jammed micelle core, whereas coronal chains interact with each other via multiple hydrogen bonds. These hydrogen bonds between the coronal chains in the IL endow the ion gel with a high level of mechanical strength as well as rapid self-healing at room temperature without the need for any external stimuli such as light or elevated temperatures.

Block copolymer self-assembly in ionic liquids.

Ionic liquids (ILs), solely composed of cations and anions, are regarded as a novel class of promising liquids, potentially applicable to energy devices, reaction media, separation materials, etc. ILs have also attracted great attention as new media for molecular self-assembly, capable of producing novel soft materials with unique features never observed for conventional soft materials containing organic and aqueous solvents. In this review, we focus on recent developments in block copolymer (BCP) self-assembly in ILs. Self-assembled structures formed by dilute and concentrated BCP solutions in ILs are discussed in detail. Ion gels formed by BCP self-assembly have received special interest because they exhibit excellent physical properties of tunable viscoelasticity and solution processability without impairing the intrinsic properties of ILs, such as nonvolatility, nonflammability, and high ionic conductivity. Applications of ion gels based on BCP self-assembly for electric double layer capacitors, lithium-ion batteries, and electroactive soft actuators are also addressed.

Macroscopic Adhesion of Thermoreversible ABC Triblock Copolymer-Based Hydrogels Via Boronic Acid-Sugar Complexation.

Two complementary thermoreversible ABC triblock copolymers containing either phenylboronic acids with low pKa values or galactosyl groups in the hydrophilic B blocks are synthesized by sequential reversible addition-fragmentation chain transfer polymerization and subsequent modification of the functional groups. Both ABC triblock copolymers undergo reversible sol-to-gel transitions upon temperature change and form physically cross-linked hydrogels under physiological conditions. Furthermore, the spontaneous adhesion of these thermoreversible hydrogels via the formation of boronic esters between the phenylboronic acid and galactosyl groups under physiological conditions is realized for the first time.