A Low-Hysteresis and Highly Stretchable Ionogel Enabled by Well Dispersed Slidable Cross-Linker for Rapid Human-Machine Interaction

Ionic conductive soft materials for mimicking human skin are a promising topic since they can be thought of as a possible basis for biomimetic sensing. In pursuit of devices with a long working range and low signal delay, conductive materials with low hysteresis and good stretchability are highly demanded. To overcome the challenges of highly stretchable conductive materials with good resilience, herein a chemical design is proposed where polyrotaxanes act as topological cross-linkers to enhance the stretchability by sliding-induced reduced stress concentration while the compatible ionic liquid is introduced as a dispersant for low hysteresis. The obtained ionogels exhibit versatile properties more than low hysteresis (residual strain = 7%) and good stretchability (550%), and also anti-fatigue, biocompatibility, and good adhesion. The low hysteresis is attributed to lower energy dissipation from the well-dispersed polyrotaxanes by compatible ionic liquids. The mechanism provides a new insight in fabricating highly stretchable and low-hysteresis slide-ring materials. Furthermore, the conductivity of the ionogels and their responses to strains and temperatures are measured. Benefiting from the good conductivity and low hysteresis, the ionogel is applied to develop a wireless communication system to realize rapid human-machine interactions.     

Effects of a Protic Ionic Liquid on the Reaction Pathway during Non-Aqueous Sol–Gel Synthesis of Silica: A Raman Spectroscopic Investigation

The reaction pathway during the formation of silica via a two-component “non-aqueou” sol-gel synthesis is studied by in situ time-resolved Raman spectroscopy. This synthetic route is followed with and without the addition of the protic ionic liquid 1-ethylimidazolium bis(trifluoromethanesulfonyl)imide (C₂HImTFSI) in order to investigate its effect on the reaction pathway. We demonstrate that Raman spectroscopy is suitable to discriminate between different silica intermediates, which are produced and consumed at different rates with respect to the point of gelation. We find that half-way to gelation monomers and shorter chains are the most abundant silica species, while the formation of silica rings strongly correlates to the sol-to-gel transition. Thus, curling up of linear chains is here proposed as a plausible mechanism for the formation of small rings. These in turn act as nucleation sites for the condensation of larger rings and thus the formation of the open and polymeric silica network. We find that the protic ionic liquid does not change the reaction pathway per se, but accelerates the cyclization process, intermediated by the faster inclusion of monomeric species.     

Cellulose‐Based Ionogels for Paper Electronics

A new class of biofriendly ionogels produced by gelation of microcellulose thin films with tailored 1‐ethyl‐3‐methylimidazolium methylphosphonate ionic liquids are demonstrated. The cellulose ionogels show promising properties for application in flexible electronics, such as transparency, flexibility, transferability, and high specific capacitances of 5 to 15 μF cm^?2. They can be laminated onto any substrate such as multilayer‐coated paper and act as high capacitance dielectrics for inorganic (spray‐coated ZnO and colloidal ZnO nanorods) and organic (poly[3‐hexylthiophene], P3HT) electrolyte‐gated field‐effect transistors (FETs), that operate at very low voltages (<2 V). Field‐effect mobilities in ionogel‐gated spray‐coated ZnO FETs reach 75 cm² V^?1 s^?1 and a typical increase of mobility with decreasing specific capacitance of the ionogel is observed. Solution‐processed, colloidal ZnO nanorods and laminated cellulose ionogels enable the fabrication of the first electrolyte‐gated, flexible circuits on paper, which operate at bending radii down to 1.1 mm.     

Soft Somatosensitive Actuators via Embedded 3D Printing

Humans possess manual dexterity, motor skills, and other physical abilities that rely on feedback provided by the somatosensory system. Herein, a method is reported for creating soft somatosensitive actuators (SSAs) via embedded 3D printing, which are innervated with multiple conductive features that simultaneously enable haptic, proprioceptive, and thermoceptive sensing. This novel manufacturing approach enables the seamless integration of multiple ionically conductive and fluidic features within elastomeric matrices to produce SSAs with the desired bioinspired sensing and actuation capabilities. Each printed sensor is composed of an ionically conductive gel that exhibits both long‐term stability and hysteresis‐free performance. As an exemplar, multiple SSAs are combined into a soft robotic gripper that provides proprioceptive and haptic feedback via embedded curvature, inflation, and contact sensors, including deep and fine touch contact sensors. The multimaterial manufacturing platform enables complex sensing motifs to be easily integrated into soft actuating systems, which is a necessary step toward closed‐loop feedback control of soft robots, machines, and haptic devices.     

Mechanically Robust,Elastic, and Healable Ionogels for Highly Sensitive Ultra-Durable Ionic Skins

The fabrication of highly durable skin-mimicking sensors remains challenging because of the unavoidable fatigue and physical damage that sensors are subjected to in practical applications. In this study, ultra-durable ionic skins (I-skins) with excellent healability and high sensitivity are fabricated by impregnating ionic liquids (ILs) into a mechanically robust poly(urea-urethane) (PU) network. The PU network is composed of crystallized poly(ε-caprolactone) and flexible poly(ethylene glycol) that are dynamically cross-linked with hindered urea bonds and hydrogen bonds. Such a design endows the resultant ionogels with high mechanical strength, good elasticity, Young's modulus similar to that of natural skin, and excellent healability. The ionogel-based I-skins exhibit a high sensitivity to a wide range of strains (0.1–300%) and pressures (0.1–20 kPa). Importantly, the I-skins show a highly reproducible electrical response over 10 000 uninterrupted strain cycles. The sensing performance of the I-skins stored in open air for 200 days is almost the same as that of the freshly prepared I-skin. The fractured I-skins can be easily healed by heating at 65 °C that restores their original ultra-durable sensing performance. The long-term durability of the I-skins is attributed to the combination of non-volatility of the ILs, excellent healability, and well-designed mechanical properties.     

Ionogels Based on Poly(methyl methacrylate) and Metal-Containing Ionic Liquids: Correlation between Structure and Mechanical and Electrical Properties

Ionogels (IGs) based on poly(methyl methacrylate) (PMMA) and the metal-containing ionic liquids (ILs) bis-1-butyl-3-methlimidazolium tetrachloridocuprate(II), tetrachloride cobaltate(II), and tetrachlorido manganate(II) have been synthesized and their mechanical and electrical properties have been correlated with their microstructure. Unlike many previous examples, the current IGs show a decreasing stability in stress-strain experiments on increasing IL fractions. The conductivities of the current IGs are lower than those observed in similar examples in the literature. Both effects are caused by a two-phase structure with micrometer-sized IL-rich domains homogeneously dispersed an IL-deficient continuous PMMA phase. This study demonstrates that the IL-polymer miscibility and the morphology of the IGs are key parameters to control the (macroscopic) properties of IGs.     

Ultrastretchable Ionogel with Extreme Environmental Resilience through Controlled Hydration Interactions

Ionic conductive gels are widely sought after for applications that require reliable ionic conduction and mechanical performance under extreme conditions, which remains a grand challenge. To address this limitation, water-induced hydration interactions are deliberately controlled within the ionic liquid (IL)-based conductive gels (ionogels) to achieve all-round performance. Specifically, the competitive interactions between IL, water and cellulose nanofibrils (CNF) are balanced to preserve the nanoscale morphology of CNF while avoiding its dissolution. As a result, both mechanical performance and ionic conductivity of the resultant ionogel are synergistically enhanced. For instance, an ultra stretchable ionogel (up to 10250 ± 412% stretchability) with both high toughness (21.8 ± 0.9 MJ m⁻³) and ionic conductivity (0.70 ± 0.06 S m⁻¹) is achieved. Furthermore, multimodal sensing functions (strain, compression, temperature, and humidity) are realized by assembling the ionogel as a skin-like membrane. Due to the low volatility of IL and its strong interaction with water, the ionogel maintains an excellent performance at either ultra-low temperature (−45 °C), high temperature (75 °C) or low humidity environment (RH < 15%), demonstrating superb anti-freezing and anti-drying performance. Overall, a simple yet versatile strategy is introduced that leads to environmentally resilient ionogels to meet the requirements of next-generation electroactive devices.     

Mechanically Robust and Highly Conductive Ionogels for Soft Ionotronics

Ionogels are promising materials for flexible electronics due to their continuous conductive phase, high thermal and chemical stability. However, a large amount of ionic liquid is required to get high conductivity, resulting in a sharp decline in the mechanical properties. Therefore, it is a great challenge to prepare ionogels with both high conductivity and mechanical properties, which is important for their practical applications. Herein, ionogels with high mechanical strength and stretchability, extraordinary ionic conductivity, excellent transparency, outstanding durability, and stability are fabricated with crosslinked polymer, ionic liquid, and lithium salt. The adoption of lithium salt can significantly improve both the mechanical strength and stretchability, which is a common dilemma in material science, and simultaneously, address the conflict between mechanical strength and ionic conductivity in ionogels. It is primarily corresponding to the microphase-separation effects induced by the lithium bonds formed between lithium ions and carbonyl groups on the polymer networks. Ionotronics including resistance-type sensors for strain and temperature sensing and triboelectric nanogenerators with stable output performance are fabricated. Moreover, ionogel-based microcircuit and sensing arrays with high resolution and accuracy are fabricated through digital light processing printing technology. The ionogels have great promise for various ionotronics in many fields.     

Altering Mechanical Properties to Improve Electrode Contacts by Organic Modification of Silica-Based Ionogel Electrolytes for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are a possible candidate to create safe, sustainable, and cost-effective batteries. Solid sodium-ion conducting organically modified ionogel electrolytes are investigated. Silica-based ionogels typically consist of an ionic liquid electrolyte (ILE) confined within a silica matrix and possess high thermal stability, good ionic conductivity, safety, and good electrochemical stability. However, they readily deteriorate when stress is applied, decreasing the electrolyte's and battery's overall performance. The mechanical characteristics of silica can be improved using organic moieties, creating Ormosils®. Silica-based ionogels with phenyl-modified silanes improve the mechanical characteristics by a reduction of their Young's modulus (from 29 to 6 MPa). This is beneficial to the charge-transfer resistance, which decreases after implementing the electrolyte in half cells, demonstrating the improved interfacial contact. Most importantly, the phenyl groups change the interacting species at the silica interface. Cationic imidazolium species pi-stacked to the phenyl groups of the silica matrix, pushing the anions to the bulk of the ILE, which affects the ionic conductivity and electrochemical stability, and might affect the quality of the SEI in half cells. In essence, the work at hand can be used as a directory to improve mechanical characteristics and modify and control functional properties of ionogel electrolytes.     

DyeIonogels: Proton‐Responsive Ionogels Based on a Dye‐Ionic Liquid Exhibiting Reversible Color Change

Transparent, ion‐conducting, and flexible ionogels based on the room temperature ionic liquid (IL) 1‐butyl‐3‐methylimidazolium bis(trifluoromethane sulfonyl)imide [Bmim][N(Tf)₂], the dye‐IL (DIL) 1‐butyl‐3‐methylimidazolium methyl orange [Bmim][MO], and poly(methylmethacrylate) (PMMA) are prepared. Upon IL incorporation the thermal stability of the PMMA matrix significantly increases from 220 to 280 °C. The ionogels have a relatively high ionic conductivity of 10^?4 S cm^?1 at 373 K. Most importantly, the ionogels exhibit a strong and reversible color change when exposed to aqueous or organic solutions containing protons or hydroxide ions. The resulting material is thus a prototype of soft multifunctional matter featuring ionic conductivity, easy processability, response to changes in the environment, and a strong readout signal, the color change, that could be used in optical data storage or environmental sensing.