Thermoresponsive ionogels with switchable adhesion in air and aqueous environments induced by LCST phase behavior

Moisture-responsive ultralow-hysteresis polymer ionogels for adhesion-switchable strain sensing

Adhesion-switchable ultralow-hysteresis polymer ionogels are highly demanded in soft electronics to avoid debonding damage and signal distortion, yet the design and fabrication of such ionogels are challenging. Herein, we propose a novel method to design switchable adhesive ionogels by using binary ionic solvents with two opposite-affinity ionic components. The obtained ionogels exhibit moisture-induced phase separation, facilitating switchable adhesion with a high detaching efficiency (>99%). Moreover, before and after phase separation, the viscoelastic behavior of the ionogels is maintained in the rubbery plateau region within common frequency ranges with ultralow mechanical hysteresis (∼3%) under large strain, enabling accurate and stable strain and pressure sensing. Accordingly, the ionogel films can be used as functional elements in a smart clamp to realize flytrap-like selective activation, based on high sensitivity to the vibration intensity from the targeted prey. This work may inspire future research on the development of advanced soft electronics.

Ionogel Adhesives: From Structural Design to Emerging Applications

Adhesives are indispensable in both daily household applications and advanced industrial settings, where they must deliver exceptional bonding performance. Ionogel adhesives, which feature a supporting polymer network infused with ionic liquid (IL), have emerged as promising candidates due to their unique structural and functional properties. The presence of ionic species within ionogels promotes non-covalent interactions-such as ionic bonds, ion-dipole interactions, and hydrogen bonding-that enhance both cohesion within the material and adhesion to various substrates. These characteristics make ionogels ideal for applications that require robust adhesive performance, especially in demanding environments. Despite the growing interest in ionogel adhesives, a comprehensive review of the latest advancements in this area is lacking. This paper aims to fill this gap by categorizing ionogel adhesives based on their composition and discussing strategies to enhance their adhesive properties. Additionally, novel ionogel adhesives designed for specific applications are highlighted. Finally, the current state of research is summarized, and offers insights into the challenges and future opportunities for the development of ionogel adhesives.

Versatile adhesive skin enhances robotic interactions with the environment

Electronic skins endow robots with sensory functions but often lack the multifunctionality of natural skin, such as switchable adhesion. Current smart adhesives based on elastomers have limited adhesion tunability, which hinders their effective use for both carrying heavy loads and performing dexterous manipulations. Here, we report a versatile, one-size-fits-all robotic adhesive skin using shape memory polymers with tunable rubber-to-glass phase transitions. The adhesion strength of our adhesive skin can be changed from minimal (~1 kilopascal) for sensing and handling ultralightweight objects to ultrastrong (>1 megapascal) for picking up and lifting heavy objects. Our versatile adhesive skin is expected to greatly enhance the ability of intelligent robots to interact with their environment.

Multiresponsive Ionogel with Switchable Adhesion Triggered by Phase Separation

The rapid development of wearable devices and soft robotics has an urgent demand for polymer conductors with a switchable adhesion property. Herein, we report a supramolecular ionogel (SIG) that can reversibly switch between adhesion and debonding to various substrates. The on/off switchable adhesion of SIG is attributed to phase separation induced by the aggregation of polymer chains and the formation of a lubricating layer, which impairs the contact between polymer chains and substrates, thus weakening interfacial interaction. The phase separation ionogel (PSIG) is highly sensitive to humidity, leading to the debonded PSIG instantly transforming into the adhesion-hydrated ionogel (HIG) owing to the disruption of phase structure. Based on the switchable adhesion property, this multiresponsive ionogel shows potential applications as a fire alarm and intelligent conductive tape. This work provides a simple method for developing a switchable adhesion ionic polymer conductor and broadens the application of the ionogel in flexible devices.

Large-scale semi-embedded AgNW-based stretchable transparent electrodes via superwettability-induced transfer of AgNWs/ionic liquid onto gradient PDMS

Stretchable transparent electrodes (STEs) composed of highly conductive silver nanowires (AgNWs) and mechanically stretchable polydimethylsiloxane (PDMS) are compelling materials for the development of flexible electronic devices. So far, the fabrication processes of the AgNWs/PDMS-based STEs suffer from low efficiency on a small scale, apart from the limitations and challenges posed by the continuous fabrication processes that are required to exploit these fascinating materials in an industrial context. Here, we have addressed these limitations by using gradient PDMS with a tunable modulus as the stretchable substrate, on which ionic liquid-mediated aligned semi-embedded AgNWs are formed via a scalable superwettability-induced transfer strategy. We have demonstrated the importance of the gradient PDMS in achieving STEs with improved optoelectrical properties, mechanical stretchability, and long-term stability. Furthermore, we have shown the applications of the designed STEs for electro-heating and electroluminescent devices. This promising fabrication process is an industrially relevant alternative to current processes that are either complicated and less effective (for example, stamping transfer) or not compatible with continuous, large-scale production (for example, spin-coating or electrospinning).

LCST ion gels fabricating "all-in-one" smart windows: thermotropic, electrochromic and power-generating

Smart windows always respond to single stimuli, which cannot satisfy various needs in practical applications. Smart windows that integrate thermotropic, electrochromic and power-generating functions in one device is highly challenging yet important in satisfying on-demand light modulation and energy efficiency in practical applications. Herein, a thermoresponsive lower critical solution temperature (LCST) ion gel was fabricated via a facile in situ polymerization of butyl acrylate in a conventional ionic liquid to explore "all in one" smart windows. The ion gel-assembled smart windows are thermotropic and electrochromic with a reliable adjustment of light transparency as well as power-generating, enabled by the ionic Soret effect of ionic liquids. Additionally, the ion gels demonstrated self-defensive robust mechanical properties, thermal insulating and antifogging properties. With such an interdisciplinary and comprehensive study of the ion gels, the LCST ion gels could fulfil the requirements of genius windows with high energy-saving potential and exceptional climate adaptability, such as shut-down of light transmission in summer, daily solar energy collection, and colour changes on demand. It conceptually updates smart windows from an energy saving to an energy supplier in buildings. It is the first time to explore the "all in one" smart windows based on integrated multifunctional ionic liquids, which could greatly bridge the gap between the materials and buildings to accelerate practical applications of smart windows.

Preparation of tough and stiff ionogels via phase separation

Ionogels have the advantages of thermal stability, non-volatility, ionic conductivity and environmental friendliness, and they can be used in the field of flexible electronics and soft robotics. However, their poor mechanical strength and complex preparation methods limit their practical application. Herein, we propose a simple strategy to improve the performance of ionogels by adjusting their phase separation behavior. In a polymer-ionic liquid (IL) binary system with an upper critical solution temperature (UCST) and Berghmans' point, the phase separation behavior will be frozen below the temperature corresponding to the Berghmans' point, and thus, the degree of phase separation can be adjusted by controlling the cooling rate. We found that a polyacrylamide (PAM)-IL binary system possessed a UCST and Berghmans' point and the resulting ionogels had excellent mechanical properties. Their tensile strength, tensile modulus, compressive strength and compressive modulus reached 31.1 MPa, 319.8 MPa, 122 MPa and 1.7 GPa, respectively, while these properties of the other ionogels were generally less than 10 MPa. Furthermore, they were highly transparent, stretchable, stable and multifunctional.

Ionic Liquids: An Ideal Solvent for Tuning the UCST Phase Behavior of Polymer Gels

Liquid-liquid phase separation (LLPS) is a common stimulus-responsive phenomenon widely studied and applied in constructing intelligent systems such as microfluidic valves, smart windows, and biosensors. However, LLPS in an aqueous solution has limited applicability confined to a narrow temperature range within 0-100 °C. In addition, for easy exploitation of thermoresponsive behavior, phase separation must be stable and accurately predictable under varying conditions. This study proposes a gel system exhibiting UCST phase behavior using ionic liquids (ILs) and hydrophilic polymers, whose phase transition temperature can be linearly tuned within a wide range (from subzero to over 100 °C) by varying the mixing ratio of ILs in their blends. Similar to the mixing of ILs with structurally similar cations, mixing ILs containing different anions proved to be an effective ideal random mixing method based on experimental results and molecular dynamics simulations. This mixing mechanism of ILs accounts for the linear regulation of the UCST of the ionogels when the mixing ratio of ILs in their blends varies. Moreover, the unique feature of ILs was further demonstrated using other hydrophilic polymer networks and multiple combinations of ILs, suggesting the generality of this strategy for UCST regulation in the ionogels.

Unveiling the Rational Development of Stimuli-Responsive Silk Fibroin-Based Ionogel Formulations

We present an approach for the rational development of stimuli-responsive ionogels which can be formulated for precise control of multiple unique ionogel features and fill niche pharmaceutical applications. Ionogels are captivating materials, exhibiting self-healing characteristics, tunable mechanical and structural properties, high thermal stability, and electroconductivity. However, the majority of ionogels developed require complex chemistry, exhibit high viscosity, poor biocompatibility, and low biodegradability. In our work, we overcome these limitations. We employ a facile production process and strategically integrate silk fibroin, the biocompatible ionic liquids (ILs) choline acetate ([Cho][OAc]), choline dihydrogen phosphate ([Cho][DHP]), and choline chloride ([Cho][Cl]), traditional pharmaceutical excipients, and the model antiepileptic drug phenobarbital. In the absence of ILs, we failed to observe gel formation; yet in the presence of ILs, thermoresponsive ionogels formed. Systems were assessed via visual tests, transmission electron microscopy, confocal reflection microscopy, dynamic light scattering, zeta potential and rheology measurements. We formed diverse ionogels of strengths ranging between 18 and 642 Pa. Under 25 °C storage, formulations containing polyvinylpyrrolidone (PVP) showed an ionogel formation period ranging over 14 days, increasing in the order of [Cho][DHP], [Cho][OAc], and [Cho][Cl]. Formulations lacking PVP showed an ionogel formation period ranging over 32 days, increasing in the order of [Cho][OAc], [Cho][DHP] and [Cho][Cl]. By heating from 25 to 60 °C, immediately following preparation, thermoresponsive ionogels formed below 41 °C in the absence of PVP. Based on our experimental results and density functional theory calculations, we attribute ionogel formation to macromolecular crowding and confinement effects, further enhanced upon PVP inclusion. Holistically, applying our rational development strategy enables the production of ionogels of tunable physicochemical and rheological properties, enhanced drug solubility, and structural and energetic stability. We believe our rational development approach will advance the design of biomaterials and smart platforms for diverse drug delivery applications.