Highly Transparent, Stretchable, and Conductive Supramolecular Ionogels Integrated with Three-Dimensional Printable, Adhesive, Healable, and Recyclable Character

Review of ionic liquid and ionogel-based biomaterials for advanced drug delivery

Ionic liquids (ILs) play a crucial role in the design of novel materials. The ionic nature of ILs provides numerous advantages in drug delivery, acting as a green solvent or active ingredient to enhance the solubility, permeability, and binding efficiency of drugs. They could also function as a structuring agent in the development of nano/micro particles for drug delivery, including micelles, vesicles, gels, emulsion, and more. This review summarize the ILs and IL-based gel structures with their advanced drug delivery applications. The first part of review focuses on the role of ILs in drug formulation and the applications of ILs in drug delivery. The second part of review offers a comprehensive overview of recent drug delivery applications of IL-based gel. It aims to offer new perspectives and attract more attention to open up new avenues in the biomedical applications of ILs and IL-based gels.

Polymerized and Colloidal Ionic Liquids─Syntheses and Applications

The breadth and importance of polymerized ionic liquids (PILs) are steadily expanding, and this review updates advances and trends in syntheses, properties, and applications over the past five to six years. We begin with an historical overview of the genesis and growth of the PIL field as a subset of materials science. The genesis of ionic liquids (ILs) over nano to meso length-scales exhibiting 0D, 1D, 2D, and 3D topologies defines colloidal ionic liquids, CILs, which compose a subclass of PILs and provide a synthetic bridge between IL monomers (ILMs) and micro to macro-scale PIL materials. The second focus of this review addresses design and syntheses of ILMs and their polymerization reactions to yield PILs and PIL-based materials. A burgeoning diversity of ILMs reflects increasing use of nonimidazolium nuclei and an expanding use of step-growth chemistries in synthesizing PIL materials. Radical chain polymerization remains a primary method of making PILs and reflects an increasing use of controlled polymerization methods. Step-growth chemistries used in creating some CILs utilize extensive cross-linking. This cross-linking is enabled by incorporating reactive functionalities in CILs and PILs, and some of these CILs and PILs may be viewed as exotic cross-linking agents. The third part of this update focuses upon some advances in key properties, including molecular weight, thermal properties, rheology, ion transport, self-healing, and stimuli-responsiveness. Glass transitions, critical solution temperatures, and liquidity are key thermal properties that tie to PIL rheology and viscoelasticity. These properties in turn modulate mechanical properties and ion transport, which are foundational in increasing applications of PILs. Cross-linking in gelation and ionogels and reversible step-growth chemistries are essential for self-healing PILs. Stimuli-responsiveness distinguishes PILs from many other classes of polymers, and it emphasizes the importance of segmentally controlling and tuning solvation in CILs and PILs. The fourth part of this review addresses development of applications, and the diverse scope of such applications supports the increasing importance of PILs in materials science. Adhesion applications are supported by ionogel properties, especially cross-linking and solvation tunable interactions with adjacent phases. Antimicrobial and antifouling applications are consequences of the cationic nature of PILs. Similarly, emulsion and dispersion applications rely on tunable solvation of functional groups and on how such groups interact with continuous phases and substrates. Catalysis is another significant application, and this is an historical tie between ILs and PILs. This component also provides a connection to diverse and porous carbon phases templated by PILs that are catalysts or serve as supports for catalysts. Devices, including sensors and actuators, also rely on solvation tuning and stimuli-responsiveness that include photo and electrochemical stimuli. We conclude our view of applications with 3D printing. The largest components of these applications are energy related and include developments for supercapacitors, batteries, fuel cells, and solar cells. We conclude with our vision of how PIL development will evolve over the next decade.

High-sensitivity and ultralow-hysteresis fluorine-rich ionogel strain sensors for multi-environment contact and contactless sensing

Information transduction via soft strain sensors under harsh conditions such as marine, oily liquid, vacuum, and extreme temperatures without excess encapsulation facilitates modern scientific and military exploration. However, most reported soft strain sensors struggle to meet these requirements, especially in complex environments. Herein, a class of fluorine-rich ionogels with tunable ultimate strain, high conductivity, and multi-environment tolerance are designed. Abundant ion-dipole and dipole-dipole interactions lead to excellent miscibility between the hydrophobic ionic liquid and the fluorinated polyacrylate matrix, as well as adhesion to diverse substrates in amphibious environments. The ionogel-based sensors, even in encapsulation-free form, exhibit stable operation with a negligible hysteresis (as low as 0.119%) and high sensitivity (gauge factor of up to 6.54) under amphibious conditions. Multi-environment sensing instances in contact and even contactless forms are also demonstrated. This study opens the door for the artificial syntheses of multi-environment tolerance ionic skins with robust sensing applications in soft electronics and robotics.

Degradable Supramolecular Eutectogel-Based Ionic Skin with Antibacterial, Adhesive, and Self-Healable Capabilities

The development of degradable, cost-effective, and eco-friendly ionic conductive gels is highly required to reduce electronic waste originating from flexible electronic devices. However, biocompatible, degradable, tough, and durable conductive gels are challenging to achieve. Herein, we develop a facile strategy for the design and synthesis of degradable tough eutectogels by integrating an electrostatically driven supramolecular network composed of branched polyacrylic acid (PAA) and monoethanolamine (MEA) into a green deep eutectic solvent with chitosan quaternary ammonium salt (CQS). The specially designed PAA/MEA/CQS eutectogels present multiple desired properties, including high transparency, widely adjustable mechanical properties, high resilience, reliable adhesiveness, excellent self-healing ability, good conductivity, remarkable anti-freezing performance, and antibacterial properties. The dynamic and reversible supramolecular interactions not only significantly enhance the mechanical properties of the PAA/MEA/CQS eutectogels but also enable fast degradation, addressing the dilemma between mechanical strength and degradability. More importantly, a biocompatible and degradable multifunctional ionic skin is successfully fabricated based on the PAA/MEA/CQS eutectogel, exhibiting high sensitivity, a wide sensing range, and a rapid response speed toward strain, pressure, and temperature. Thus, this study offers a promising strategy for fabricating degradable tough eutectogels, which show great potential as high-performance ionic skins for next-generation flexible wearable electronic devices.

A Review of Epidermal Flexible Pressure Sensing Arrays

In recent years, flexible pressure sensing arrays applied in medical monitoring, human-machine interaction, and the Internet of Things have received a lot of attention for their excellent performance. Epidermal sensing arrays can enable the sensing of physiological information, pressure, and other information such as haptics, providing new avenues for the development of wearable devices. This paper reviews the recent research progress on epidermal flexible pressure sensing arrays. Firstly, the fantastic performance materials currently used to prepare flexible pressure sensing arrays are outlined in terms of substrate layer, electrode layer, and sensitive layer. In addition, the general fabrication processes of the materials are summarized, including three-dimensional (3D) printing, screen printing, and laser engraving. Subsequently, the electrode layer structures and sensitive layer microstructures used to further improve the performance design of sensing arrays are discussed based on the limitations of the materials. Furthermore, we present recent advances in the application of fantastic-performance epidermal flexible pressure sensing arrays and their integration with back-end circuits. Finally, the potential challenges and development prospects of flexible pressure sensing arrays are discussed in a comprehensive manner.

Granular Ionogel Particle Inks for 3D Printed Tough and Stretchable Ionotronics

Ionogels have garnered great attention as promising soft conducting materials for the fabrication of flexible energy storage devices, soft actuators, and ionotronics. However, the leakage of the ionic liquids, weak mechanical strength, and poor manufacturability have greatly limited their reliability and applications. Here, we propose a new ionogel synthesis strategy by utilizing granular zwitterionic microparticles to stabilize ionic liquids. The ionic liquids swell the microparticles and physically crosslink microparticles via either electronic interaction or hydrogen bonding. Further introducing a photocurable acrylic monomer enables the fabrication of double-network (DN) ionogels with high stretchability (>600%) and ultrahigh toughness (fracture energy > 10 kJ/m2). The synthesized ionogels exhibit a wide working temperature of -60 to 90 °C. By tuning the crosslinking density of microparticles and physical crosslinking strength of ionogels, we synthesize DN ionogel inks and print them into three-dimensional (3D) motifs. Several ionogel-based ionotronics are 3D printed as demonstrations, including strain gauges, humidity sensors, and ionic skins made of capacitive touch sensor arrays. Via covalently linking ionogels with silicone elastomers, we integrate the ionogel sensors onto pneumatic soft actuators and demonstrate their capacities in sensing large deformation. As our last demonstration, multimaterial direct ink writing is harnessed to fabricate highly stretchable and durable alternating-current electroluminescent devices with arbitrary structures. Our printable granular ionogel ink represents a versatile platform for the future manufacturing of ionotronics.

A multifunctional and sustainable poly(ionic liquid)-quaternized chitosan hydrogel with thermal-triggered reversible adhesion

A quaternized chitosan (QCS)@poly(ionic liquid) (PIL) hydrogel adhesive was prepared by in-situ ultraviolet (UV)-induced copolymerization of 1-vinyl-3-butyl imidazolium bromide ([BVIm][Br]) and methacryloyloxyethyl trimethylammonium chloride (DMC) in QCS aqueous solution without using any crosslinkers, which was stably crosslinked by reversible hydrogen bonding together with ion association and exhibited excellent adhesion, plasticity, conductivity and recyclability properties. Moreover, its thermal/pH-responsive behaviors and intermolecular interaction mechanism of thermal-triggered reversible adhesion were discovered, meanwhile good biocompatibility, antibacterial properties, repeated stickiness and degradability were also proved. The results showed that the newly developed hydrogel could make various tissues, organic, inorganic or metal materials adhered tightly within 1 min; after 10 binding-peeling cycles, the adhesive strength to glass, plastic, aluminum and porcine skin still remained beyond 96 %, 98 %, 92 % and 71 % of the original, respectively. The adhesion mechanism involves ion dipole interaction, electrostatic interaction, hydrophobic interaction, coordination, cation-π interaction, H-bonding and van der Waals force. For above merits, the new tricomponent hydrogel is expected to be applied in biomedical field to achieve adjustable adhesion and on-demand peeling.

Ionogels: recent advances in design, material properties and emerging biomedical applications

Ionic liquid (IL)-based gels (ionogels) have received considerable attention due to their unique advantages in ionic conductivity and their biphasic liquid-solid phase property. In ionogels, the negligibly volatile ionic liquid is retained in the interconnected 3D pore structure. On the basis of these physical features as well as the chemical properties of well-chosen ILs, there is emerging interest in the anti-bacterial and biocompatibility aspects. In this review, the recent achievements of ionogels for biomedical applications are summarized and discussed. Following a brief introduction of the various types of ILs and their key physicochemical and biological properties, the design strategies and fabrication methods of ionogels are presented by means of different confining networks. These sophisticated ionogels with diverse functions, aimed at biomedical applications, are further classified into several active domains, including wearable strain sensors, therapeutic delivery systems, wound healing and biochemical detections. Finally, the challenges and possible strategies for the design of future ionogels by integrating materials science with a biological interface are proposed.

Recent advances in bio-inspired ionic liquid-based interfacial materials from preparation to application

Natural creatures always display unique and charming functions, such as the adhesion of mussels and the lubrication of Nepenthes, to maintain their life activities. Bio-inspired interfacial materials infused with liquid, especially for ionic liquids (ILs), have been designed and prepared to meet the emerging and rising needs of human beings. In this review, we first summarize the recent development of bio-inspired IL-based interfacial materials (BILIMs), ranging from the synthesis strategy to the design principle. Then, we discuss the advanced applications of BILIMs from anti-adhesive aspects (e.g., anti-biofouling, anti-liquid fouling, and anti-solid fouling) to adhesive aspects (e.g., biological sensor, adhesive tape, and wound dressing). Finally, the current limitations and future prospects of BILIMs are provided to feed the actual needs.

Carbon Nanotubes and Silica@polyaniline Core-Shell Particles Synergistically Enhance the Toughness and Electrical Conductivity in Hydrophobic Associated Hydrogels

Soft, conductive, and stretchable sensors are highly desirable in many applications, including artificial skin, biomonitoring patches, and so on. Recently, a combination of good electrical and mechanical properties was regarded as the most important evaluation criterion for judging whether hydrogel sensors are suitable for practical applications. Herein, we demonstrate a novel carboxylated carbon nanotube (MWCNT-COOH)-embedded P(AM/LMA)/SiO2@PANI hydrogel. The hydrogel benefits from a double-network structure (hydrogen bond cross-linking and hydrophobic connectivity network) due to the role of MWCNT-COOH and SiO2@PANI as cross-linkers, thus resulting in tough composite hydrogels. The obtained P(AM/LMA)/SiO2@PANI/MWCNT-COOH hydrogels exhibited high tensile strength (1939 kPa), super stretchability (3948.37%), and excellent strain sensitivity (gauge factor = 11.566 at 100-1100% strain). Obviously, MWCNT-COOH not only improved the electrical conductivity but also enhanced the mechanical properties of the hydrogel. Therefore, the integration of MWCNT-COOH and SiO2@PANI-based hydrogel strain sensors will display broad application in sophisticated intelligence, soft robotics, bionic prosthetics, personal health care, and other fields using inexpensive, green, and easily available biomass.

High Performance Conductive Hydrogel for Strain Sensing Applications and Digital Image Mapping

A hydrogel strain sensor can successfully transform its deformation into resistance changes, offering novel options for the Internet of Things (IoT) and artificial intelligence (AI). However, it remains challenging to prepare hydrogel sensors with superior performance (e.g., high conductivity). Here, we produced a conductive hydrogel (named PPC hydrogel) utilizing only three components, PVA (poly(vinyl alcohol)), PAAS (polyacrylate sodium), and CaCl₂, through freezing cross-linking and ion chelation. The PPC hydrogel is endowed with high electrical conductivity of approximately 5.2 S/m without the addition of highly conductive materials due to the unique ionic cluster mesh structure, thus enabling an outstanding performance of strain sensing. The PPC hydrogel also maintains electrical conductivity in frozen and underwater conditions and resists swelling in underwater environments, allowing it to be used under water for extended periods of time (more than 15 days). The PPC hydrogel-based strain sensor can be used as a flexible electrode for electrocardiogram (ECG) and electromyogram (EMG) examinations and sensitively monitor human activity as well as recognize handwriting. Moreover, we designed a python-based visualization program combined with a PPC hydrogel array to implement pressure-sensing digital image mapping for remote IoT monitoring. As a flexible sensor for biosafety, the PPC hydrogel has potential applications in the field of intelligent sensing, the IoT, and even Internet of Body systems.

Thermal Nociception of Ionic Skin: TRPV1 Ion Channel-Inspired Heat-Activated Dynamic Ionic Liquid

The artificial reproduction of the tactile sensory function of natural skin is crucial for intelligent sensing, human-computer interaction, and medical health. Thermal nociception is an essential human tactile function to avoid noxious thermal stimuli, which depends on the specific heat-activation of the TRPV1 ion channel. Inspired by the TRPV1, a dynamic ionic liquid with heat-activation characteristics is designed and prepared, which can be activated at 45 °C, which is near the physiological noxious temperature, accompanied by a steep rise in electrical response signals. Its electrical behavior can be deemed to be the extreme version of temperature sensation similar to the natural thermal nociceptor. The heat-activation mechanism is confirmed as a feasible strategy to regulate the thermal response behavior of ions, and this reported dynamic ionic liquid has an unprecedented intrinsic temperature response sensitivity of up to 156.79%/°C. In consideration of the similarity between the heat-activated dynamic ionic liquid and the TRPV1 ion channel in terms of heat-activation characteristics, electrical output signal, and ultrathermal sensitivity, an all-liquid ionic skin with the ability of thermal nociception is further fabricated, which shows considerable potential to assist patients with tactile desensitization to avoid noxious thermal stimuli.

Poly(thioctic acid): From Bottom-Up Self-Assembly to 3D-Fused Deposition Modeling Printing

Inspired by the bottom-up assembly in nature, an artificial self-assembly pattern is introduced into 3D-fused deposition modeling (FDM) printing to achieve additive manufacturing on the macroscopic scale. Thermally activated polymerization of thioctic acid (TA) enabled the bulk construction of poly(TA), and yielded unique time-dependent self-assembly. Freshly prepared poly(TA) can spontaneously and continuously transfer into higher-molecular-weight species and low-molecular-weight TA monomers. Poly(TA) and the newly formed TA further assembled into self-reinforcing materials via microscopic-phase separation. Bottom-up self-assembly patterns on different scales are fully realized by 3D FDM printing of poly(TA): thermally induced polymerization of TA (microscopic-scale assembly) to poly(TA) and 3D printing (macroscopic-scale assembly) of poly(TA) are simultaneously achieved in the 3D-printing process; after 3D printing, the poly(TA) modes show mechanically enhanced features over time, arising from the microscopic self-assembly of poly(TA) and TA. This study clearly demonstrates that micro- and macroscopic bottom-up self-assembly can be applied in 3D additive manufacturing.

Highly Adhesive, Stretchable, and Antifreezing Hydrogel with Excellent Mechanical Properties for Sensitive Motion Sensors and Temperature-/Humidity-Driven Actuators

Conductive hydrogels as flexible wearable devices have attracted considerable attention due to their mechanical flexibility and intelligent sensing. How to endow more and better performance, such as high self-adhesion, stretchability, and wide application temperature range for traditional hydrogels and flexible sensors is a challenge. Herein, a stretchable, self-adhesive, and antifreezing conductive hydrogel with multiple networks and excellent mechanical properties was prepared by a two-step method for its application in sensitive motion sensors and temperature-/humidity-driven actuators. First, quaternary chitosan (QCS) was introduced into the network of an acrylamide (AM) and 1-vinyl imidazole (VI) copolymer initiated by UV-photoinitiated radical polymerization. Then, the double-network hydrogel was immersed in a FeCl₃ solution to fabricate the P(AAm-co-VI)/QCS-Fe³⁺ ionic hydrogel with multiple physical networks. The properties of the hydrogel were controllable and adjustable. The toughness of the ionic hydrogel could reach up to 654.4 kJ/m³, the fracture strength could reach 253.1 kPa, and the compressive strength reached 8.4 MPa at an 80% compression strain. The multiple physical networks improved the mechanical properties and the quick resilience of the hydrogel. A large amount of FeCl₃ in the network greatly enhanced the ionic conductivity. Meanwhile, hydrogen bonds with water molecules inhibit the formation of ice crystals between zero water molecules and enhance the freezing resistance of P(Aam-co-VI)/QCS hydrogels. The active group on the QCS chain provided adhesiveness to various substrates for hydrogels. The P(AAm-co-VI)/QCS-Fe³⁺ hydrogel-based sensor showed high sensitivity, which can detect human movement and pulse, with a gauge factor of 2.37. Finally, due to the different dehydration rates of the P(AAm-co-VI)/QCS-Fe³⁺ and P(AAm-co-VI)/QCS hydrogel, a double-layer temperature/humidity-driven actuator was fabricated, expanding the application of conductive hydrogels.

Poly(N,N-dimethyl)acrylamide-based ion-conductive gel with transparency, self-adhesion and rapid self-healing properties for human motion detection

Flexible strain sensors have been extensively studied for their potential value in monitoring human activity and health. However, it is still challenging to develop multifunctional flexible strain sensors with simultaneously high transparency, strong self-adhesion, fast self-healing and excellent tensile properties. In this study, we used N,N-dimethylacrylamide (DMA) in the imidazolium-based ionic liquid 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] imide ([BMIM][Tf₂N]) for "one-step" UV irradiation. A poly(N,N-dimethyl)acrylamide (PDMA) ion-conductive gel was prepared by site polymerization. Based on the good compatibility between PDMA and ionic liquid, the prepared ion-conductive gel has good transparency (∼90%), excellent stretchability (1080%), strong self-adhesion (67.57 kPa), fast self-healing (2 s at room temperature) and great antibacterial activity (∼99% bacterial killing efficiency). Moreover, the strain sensor based on the PDMA ion-conductive gel has good electromechanical performance and can detect different human motions. Based on the simple and easy-to-operate preparation method and the endowed multifunctionality of the PDMA ion-conductive gel, it has broad application prospects in the field of flexible electronic devices.

Recent Progress on Self-Healable Conducting Polymers

Materials able to regenerate after damage have been the object of investigation since the ancient times. For instance, self-healing concretes, able to resist earthquakes, aging, weather, and seawater have been known since the times of ancient Rome and are still the object of research. During the last decade, there has been an increasing interest in self-healing electronic materials, for applications in electronic skin (E-skin) for health monitoring, wearable and stretchable sensors, actuators, transistors, energy harvesting, and storage devices. Self-healing materials based on conducting polymers are particularly attractive due to their tunable high conductivity, good stability, intrinsic flexibility, excellent processability and biocompatibility. Here recent developments are reviewed in the field of self-healing electronic materials based on conducting polymers, such as poly 3,4-ethylenedioxythiophene (PEDOT), polypyrrole (PPy), and polyaniline (PANI). The different types of healing, the strategies adopted to optimize electrical and mechanical properties, and the various possible healing mechanisms are introduced. Finally, the main challenges and perspectives in the field are discussed.

Highly stretchable, self-healable, and self-adhesive ionogels with efficient antibacterial performances for a highly sensitive wearable strain sensor

Gel-based strain sensors with multi-functional outstanding properties have gained considerable attention. However, conventional gel sensors suffer from unsatisfactory mechanical properties and adhesion, and also a lack of self-healing and antibacterial ability. Herein, a multi-functional ionogel has been constructed based on Ag-Lignin nanoparticles (Ag-Lignin NPs), polyurethane (PU), and ionic liquids. The obtained ionogel exhibited excellent mechanical properties (tensile strength: 3.14 MPa, elongation at break: 1241%), and was conferred self-healing ability by introducing the disulfide bonds into the main chain (the best self-healing efficiency is 97.6%). The dynamic catechol redox system based on Ag-Lignin NPs endows the ionogel with repeatable and long-lasting adhesiveness. Besides, the obtained ionogel also presented favorable antibacterial and UV absorption properties. The sensor based on the ionogel possesses good and stable sensing performance. This study proposes a bright new strategy to fabricate multi-functional ionogel-based sensors exerting broad application prospects in the field of human movement and personalized physiological health monitoring.

Ultrastretchable, Adhesive, Fast Self-Healable, and Three-Dimensional Printable Photoluminescent Ionic Skin Based on Hybrid Network Ionogels

Developing multifunctional stretchable ionic skin (I-Skin) to mimic the sensations of the human skin is of great interest and shows promising potential in wearable sensors and human-machine interfaces (HMIs). However, common ionogels prepared with small-molecule cross-linkers and single networks can hardly satisfy the requirements of adjustable mechanical properties, strong adhesion, fast self-healability, and good stability in extreme environments. Herein, an ultrastretchable (>10,000%), ultrastrong adhesive (>6.8 MPa), ultrafast self-healable (10 s), high thermally stable (-60 to 250 °C), and three-dimensional (3D)-printable photoluminescent ionogel with shape memory properties has been designed. The ionogel consists of hyperbranched polymer covalent-cross-linked poly(zwitterionic ionic liquid)-co-poly(acrylic acid) and multiple dynamic bonding cross-linked networks. The excellent performance of the ionogel-based high-stretchable strain sensor and the triboelectric nanogenerator (TENG)-based self-powered touch sensor is further demonstrated over a wide temperature range (-40 to 150 °C). More importantly, ionogel-based I-Skin can work as an HMI for human gesture recognition and real-time wireless control of robots under extreme vacuum conditions and can also self-heal immediately along with function recovery after mechanical damage.

Highly Stretchable, Fast Self-Healing, and Waterproof Fluorinated Copolymer Ionogels with Selectively Enriched Ionic Liquids for Human-Motion Detection

The development of waterproof ionogels with high stretchability and fast self-healing performance is essential for stretchable ionic conductors in sophisticated skin-inspired wearable sensors but can be rarely met in one material. Herein, a semicrystalline fluorinated copolymer ionogel (SFCI) with extremely high stretchability, underwater stability, and fast self-healability was fabricated, among which hydrophobic ionic liquids ([BMIM][TFSI]) were selectively enriched in fluoroacrylate segment domains of the fluorinated copolymer matrix through unique ion-dipole interactions. Benefiting from the reversible ion-dipole interactions between the [BMIM][TFSI] and fluoroacrylate segment domains as well as the physical cross-linking effects of semicrystalline oligoethylene glycol domains, the SFCI exhibited ultrastretchability (>6000%), fast room-temperature self-healability (>96% healing efficiency after cutting and self-healing for 30 min), and outstanding elasticity. In addition, the representative SFCI also exhibited high-temperature tolerance up to 300 °C, antifreezing performance as low as -35 °C, and high transparency (>93% visible-light transmittance). As a result, the as-obtained SFCI can readily be used as a highly stretchable ionic conductor in skin-inspired wearable sensors with waterproof performance for real-time detecting physiological human activities. These attractive features illustrate that the developed ultrastretchable and rapidly self-healable ionogels with unique waterproofness are promising candidates especially for sophisticated wearable strain sensing applications in complex and extreme environments.

Highly Electrically Conductive Flexible Ionogels by Drop-Casting Ionic Liquid/PEDOT:PSS Composite Liquids onto Hydrogel Networks

Ionogels have been extensively studied as ideal flexible and stretchable materials by virtue of the unique properties of ionic liquids, such as non-volatility, non-flammability, and negligible vapor pressure. However, the generally low ionic conductivity of the current ionogels limits their applications in the market of highly conductive, flexible, and stretchable electrical devices. Here, the fabrication of highly electrically conductive ionogels is reported by combining composite liquids consisting of 1-ethyl-3-methylimidazolium dicyanamide ([EMIM][DCA]) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with flexible negative-charged poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) hydrogel. The generated composite film exhibits high electrical conductivity up to about 38 S cm^-1 with the maximum tensile strain of 45% and fracture stress of 27 kPa. In addition, it is demonstrated that the composite film can maintain conductivity in a high level under different mechanical deformations, and can also be used as flexible sensors in a wide temperature range from -58 to 120 ℃. It is believed that the designed composite film would expand the applications of flexible conductive materials where both high conductivity and robust mechanical flexibility are required.