Ionic Skin Research Articles

High‐Density Ionic Skin via Direct Microlithography of Photosensitive Ionogel

High‐Density Ionic Skin via Direct Microlithography of Photosensitive Ionogel

Healable, Recyclable, and Upcyclable Gel Membranes for Efficient Carbon Dioxide Separation

AbstractIonic liquids (ILs) are prized for their selective dissolution of carbon dioxide (CO2), leading to their widespread use in ionogel membranes for gas separation. Despite their advantages, creating sustainable ionogel membranes with high IL contents poses challenges due to limited mechanical strength, leakage risks, and poor recyclability. Herein, we leverage copolymerized and supramolecularly bound ILs to develop ionogel membranes with high mechanical strength, zero leakage, and excellent self‐healing and recycling capabilities. These membranes exhibit superior ideal selectivity for gas separation compared to other reported ionogel membranes, achieving a CO2/nitrogen selectivity of 61.7 and a CO2/methane selectivity of 24.6, coupled with an acceptable CO2 permeability of 186.4 Barrer. Additionally, these gas separation ionogel membranes can be upcycled into ionic skins for sensing applications, further enhancing their utility. This research outlines a strategic approach to molecularly engineer ionogel membranes, offering a promising pathway for developing sustainable, high‐performance materials for advanced gas separation technologies.

Triboelectric and iontronic dual-responsive bioinspired ionic skin for human–like dexterous robotic manipulation

Powerful tactile perception is keystone for robots to achieve human-like dexterous manipulation. However, endowing robots with tactile perception capabilities that approach or even surpass those of humans is a challenge. Organisms can perceive complex environments rapidly, and almost all perception is related to the directional transport of ions. Inspired by this mechanism, we design a flexible dual-responsive skin (FDRS) based on ions migration for proximity and tactile somatosensation, which is consisting of single-electrode triboelectric nanogenerator and iontronic sensor, achieving surpassing-skin capabilities. The single-electrode triboelectric nanogenerator can encode the proximity information of approaching targets into a series of voltage pulses with fast response time (100 μs). The iontronic tactile unit based on the hierarchical micro-hemispherical gel layer achieves high linearity (R2 = 0.998) over a broad range of 0–700 kPa. We demonstrate the applications of FDRS for three-dimensional object recognition and robotic control. These capabilities of the multilayer integrated FDRS provide new perspectives for the future development of human-like dexterous robotic manipulation.

Healable, Recyclable, and Upcyclable Gel Membranes for Efficient Carbon Dioxide Separation.

Ionic liquids (ILs) are prized for their selective dissolution of carbon dioxide (CO2), leading to their widespread use in ionogel membranes for gas separation. Despite their advantages, creating sustainable ionogel membranes with high IL contents poses challenges due to limited mechanical strength, leakage risks, and poor recyclability. Herein, we leverage copolymerized and supramolecularly bound ILs to develop ionogel membranes with high mechanical strength, zero leakage, and excellent self-healing and recycling capabilities. These membranes exhibit superior ideal selectivity for gas separation compared to other reported ionogel membranes, achieving a CO2/nitrogen selectivity of 61.7 and a CO2/methane selectivity of 24.6, coupled with an acceptable CO2 permeability of 186.4 Barrer. Additionally, these gas separation ionogel membranes can be upcycled into ionic skins for sensing applications, further enhancing their utility. This research outlines a strategic approach to molecularly engineer ionogel membranes, offering a promising pathway for developing sustainable, high-performance materials for advanced gas separation technologies.

Ultra-stable ionogels based on hydrogen networks with broad temperature, environment, and long-term stability

Ionogels with broad temperature, environment, and long-term stability are desirable in intelligent flexible electronics. Herein, we have developed a novel ultra-stable ionogel through hydrogen bonding between the amphiphilic ionic liquid--1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([EMIM][NTf2]) and butyl acrylate (BA)-ethyl ethoxyethyl acrylate (EO) copolymer (P(BA-co-EO)). The ionogel demonstrates high decomposition voltage, strong adhesion stability, non-corrosive stability, and cyclic strain sensing stability (500 cycles at 120 % strain). Additionally, it exhibits broad temperature and environment adaptability, capable of stretching up to 7 times at −20 °C, maintaining consistent weight after 30 days of storage in extreme environments such as low/high temperatures (−20∼100 °C), high vacuum (6 × 10−4 Pa), and underwater (without additional sealed packaging), while also maintaining electrophysiological monitoring at −20 °C or 100 °C. Most importantly, the ionogel without sealing can self-adhere to the human skin for continuous and high-quality electrophysiological monitoring for 1 month under daily life conditions. We have utilized the ionogel to fabricate an ionic skin with multifunctional sensing capabilities for strain, pressure, and temperature, which has been successfully employed in human motion and pressure detection. It is believed that ionogels with long-term stability will pave the way for developing next-generation intelligent flexible electronics in future research endeavors.

Zwitterionic hydrogels with high interfacial affinity for zinc metal batteries

Zinc metal batteries are expected to be the next generation of energy storage devices due to their high safety, but their application is hampered by irreversible hydrogen evolution reaction and surface side reactions of zinc anodes. Here, a zwitterionic hydrogel electrolyte is reported based on polymerization of acrylamide and 1-(3-Sulfopropyl)-2-Vinylpyridinium betaine, and it exhibits the following advantages: the sulfonic acid group of 1-(3-Sulfopropyl)-2-Vinylpyridinium betaine can be preferentially adsorbed on metal Zinc to avoid side reactions and refine the affinity at the zinc-electrolyte interface; and the coordination of the negatively charged sulfonic acid group with Zn2+ alters the solvation structure of Zn2+, which further improves the restriction ability of side reactions. Consequently, Zn||Zn symmetric batteries can be stabilized for 1, 200 h of plating stripping at 1 mA cm−2. Zn||Cu batteries exhibit an impressive average coulombic efficiency of 99.8 % over a remarkable span of 1, 800 cycles at a current density of 15 mA cm−2. Furthermore, the Zn||MnO2/Carbon nanotube batteries have been cycled more than 2, 000 cycles at a current density of 0.5 A g−1, and the zinc hybrid capacitors for more than 18, 000 cycles at 1 A g−1. And an ionic skin (i-skin) based on acrylamide and 1-(3-Sulfopropyl)-2-Vinylpyridinium betaine of polymer hydrogel electrolyte is assembled and it can stably detect physiological signals.

Fabrication of dual dynamic crosslinking polyurethane networks towards self-healing, resist fatigue, and highly stretchable ionic skins

The limited anti-fatigue performance of self-healing polyurethane elastomers under high-strain conditions restricts their application in high-performance ionic skins. Herein, a polyurethane network (PU-DDN) synergistically reversibly crosslinked by dynamic thiourethane bonds and hydrogen bonds was synthesized successfully. This dual reversibly crosslinking strategy not only endowed the polyurethane network with an excellent fatigue resistance (showing a residual strain of only 63 % after 10 uninterrupted cyclic tensile tests at 400 % strains), but also imparted it with a high strength (6.12 MPa) and stretchability (611.42 %). Owing to the thermo-reversibility of dual dynamic networks, the damaged PU-DDN heated at 100 °C for 3h could fully restore its initial mechanical properties. Subsequently, transparent stretchable ionic skins were fabricated by mixing the PU-DDN with ionic liquids. A resulting highly-sensitive ionic skin (GF1 = 1.25, GF2 = 2.01) named PU-DDN/IL20 demonstrated exceptional durability, which could produce repeatable electrical signals even after 2000 cycles of stretching to an extensive deformation of 400 %. Meanwhile, it can also monitor human motions, showcasing its promising potential in the realm of intelligent wearable applications.

Biomass and Transparent Supramolecular Elastomers for Green Electronics Enabled by the Controlled Growth and Self-Assembly of Dynamic Polymer Networks.

Determining the optimal method for preparing supramolecular materials remains a profound challenge. This process requires a combination of renewable raw materials to create supramolecular materials with multiple functions and properties, including simple fabrication, sustainability, a dynamic nature, good toughness, and transparency. In this work, a strategy is presented for toughening supramolecular networks based on solid-phase chain extension. This toughening strategy is simple and environmentally friendly. In addition, a series of biobased elastomers are designed and prepared with adjustable performance characteristics. This strategy can significantly improve the transparency, tensile strength, and toughness of the synthesized elastomer. The synthesized biobased elastomers have great ductility, repairability, and recyclability, and they show good adhesion and dielectric properties. A biobased ionic skin is assembled from these biobased elastomers. Assembled ionic skin can sensitively detect external stimuli (such as stretching, bending, compression, or temperature changes) and monitor human movement. The conductive and dielectric layers of the biobased ionic skin are both obtained from renewable raw materials. This research provides novel molecular design approaches and material selection methods for promoting the development of green electronic devices and biobased elastomers.

Radiation controllable construction of multifunctional ionogel with a soft-rigid structure for high-performance ionic skin

Radiation controllable construction of multifunctional ionogel with a soft-rigid structure for high-performance ionic skin

A novel multifunction of wearable ionic conductive hydrogel sensor for promoting infected wound healing

Conductive hydrogels have attracted widespread attention in applications of artificial biological tissues and ionic skin. Herein, a highly ionically conductive and biocompatibility hydrogels that integrate sensing, low swelling, and antimicrobial are prepared by the composition of chitosan (CS) chains, tannic acid (TA), polyacrylic acid (PAA), and metal cations. The hydrogels with high conductivity had tensile strain repeatable adhesion, wide strain detection, biocompatibility, and motion sensing capabilities. Particularly, in the aspect of hydrogel strain sensing based on ion conduction, it has high nonliearity (GF=2.05) with a large strain (20 %-1400 %), and excellent cycle stability. In addition, owing to the low-swelling effect and dipole-dipole interaction between the existence of free catechol groups of the zwitterionic group and TA in the zwitterionic sulfobetaine methacrylate (SBMA), the hydrogels display repeatable and controllable adhesiveness. Meanwhile, the covalent cross-linking between AA and SBMA synergistically enhances the mechanical properties of the hydrogels (169 kPa, 1474 %). Furthermore, as demonstrated in the L929 mouse wound infection model, the results indicated that the prepared hydrogel group had antimicrobial activity and reduced inflammation, thereby accelerating wound healing. The biocompatible and mutli-functional conductive hydrogels provide a new method for the design of novel type ionic skin device.

Highly conductive, rapid self-healing, and anti-freezing poly(3,4-ethylenedioxythiophene)/lignosulfonate-cationic guar gum ionogels for multifunctional sensors

Soft ionic conductors exhibit immense potential for applications in soft ionotronics, including ionic skin, human–machine interface, and soft luminescent device. Nevertheless, the majority of ionogel-based soft ionic conductors are plagued by issues such as freezing, evaporation, liquid leakage, and inadequate self-healing capabilities, thereby constraining their usability in complex environments. In this study, we present a novel strategy for fabricating conductive ionogels through the proportionally mixing cationic guar gum (CGG), water, 1-butyl-3-methylimidazolium chloride (BmimCl)/glycerol eutectic-based ionic liquid, and poly(3,4-ethylenedioxythiophene)/lignosulfonate (PEDOT/LS). The resultant benefits from strong hydrogen bonding and electrostatic interactions among its constituents, endowing it with an ultrafast self-healing capability (merely 30 s) while sustaining high electrical conductivity (~16.5 mS cm−1). Moreover, it demonstrates exceptional water retention (62 % over 10 days), wide temperature tolerance (−20 to 60 °C), and injectability. A wearable sensor fabricated from this ionogel displayed remarkable sensitivity (gauge factor = 17.75) and a rapid response to variations in strain, pressure, and temperature, coupled with both long-term stability and wide working temperature range. These attributes underscore its potential for applications in healthcare devices and flexible electronics.

Moisture self-regulating ionic skins with ultra-long ambient stability for self-healing energy and sensing systems

Dehydration has been a key limiting factor for the operation of conductive hydrogels in practical application. Here, we report self-healable ionic skins that can self-regulate their internal moisture level by capturing extenral moistures via hygroscopic ion-coordinated polymer backbones through antipolyelectrolyte effect. Results show the ionic skin can maintain its mechanical and electrical functions over 16 months in the ambient environment with high stretchability (fracture stretch ∼2216 %) and conductivity (23.5 mS/cm). The moisture self-regulating capability is further demonstrated by repeated exposures to harsh environments such as 200°C heating, freezing, and vacuum drying with recovered conductivity and stretchability. Their reversible ionic and hydrogen bonds also enable self-healing feature as a sample with the fully cut-through damage can restore its conductivity after 24 h at 40 % relative humidity. Utilizing the ionic skin as a building block, self-healing flexible piezoelecret sensors have been constructed to monitor physiological signals. Together with a facile transfer-printing process, a self-powered sensing system with a self-healable supercapacitor and humidity sensor has been successfully demonstrated. These results illustrate broad-ranging possibilities for the ionic skins in applications such as energy storage, wearable sensors, and human-machine interfaces.

Multistage Microstructured Ionic Skin for Real‐Time Vital Signs Monitoring and Human‐Machine Interaction

Skin‐like electronics research aiming to mimic even surpass human‐like specific tactile cognition by operating perception‐to‐cognition‐to‐feedback of stimulus to build intelligent cognition systems for certain imperceptible or inappreciable signals was so attractive. Herein, we constructed an all‐in‐one tri‐modal pressure sensing wearable device to address the issue of power supply by integrating multistage microstructured ionic skin (MM i‐skin) and thermoelectric self‐power staffs, which exhibits high sensitivity simultaneously. The MM i‐skin with multi‐stage “interlocked” configurations achieved precise recognition of subtle signals, where the sensitivity reached up to 3.95 kPa−1, as well as response time of 46 ms, cyclic stability (over 1500 cycles), a wide detection range of 0–200 kPa. Furthermore, we developed the thermoelectricity nanogenerator, piezoelectricity nanogenerator, and piezocapacitive sensing as an integrated tri‐modal pressure sensing, denoted as P‐iskin, T‐iskin, and C‐iskin, respectively. This multifunctional ionic skin enables real‐time monitoring of weak body signals, rehab guidance, and robotic motion recognition, demonstrating potential for Internet of things (IoT) applications involving the artificial intelligence‐motivated sapiential healthcare Internet (SHI) and widely distributed human‐machine interaction (HMI).

Wireless Sensor System Based on Organohydrogel Ionic Skin for Physiological Activity Monitoring.

Supermolecular hydrogel ionic skin (i-skin) linked with smartphones has attracted widespread attention in physiological activity detection due to its good stability in complex scenarios. However, the low ionic conductivity, inferior mechanical properties, poor contact adhesion, and insufficient freeze resistance of most used hydrogels limit their practical application in flexible electronics. Herein, a novel multifunctional poly(vinyl alcohol)-based conductive organohydrogel (PCEL5.0%) with a supermolecular structure was constructed by innovatively employing sodium carboxymethyl cellulose (CMC-Na) as reinforcement material, ethylene glycol as antifreeze, and lithium chloride as a water retaining agent. Thanks to the synergistic effect of these components, the PCEL5.0% organohydrogel shows excellent performance in terms of ionic conductivity (1.61 S m-1), mechanical properties (tensile strength of 70.38 kPa and elongation at break of 537.84%), interfacial adhesion (1.06 kPa to pig skin), frost resistance (-50.4 °C), water retention (67.1% at 22% relative humidity), and remoldability. The resultant PCEL5.0%-based i-skin delivers satisfactory sensitivity (GF = 1.38) with fast response (348 ms) and high precision under different deformations and low temperature (-25 °C). Significantly, the wireless sensor system based on the PCEL5.0% organohydrogel i-skin can transmit signals from physiological activities and sign language to a smartphone by Bluetooth technology and dynamically displays the status of these movements. The organohydrogel i-skin shows great potential in diverse fields of physiological activity detection, human-computer interaction, and rehabilitation medicine.

Facile Multiple Graded Wrinkle Construction Strategy for Vastly Boosting the Sensing Performance of Ionic Skins.

The construction of surface microstructures (e.g., micropyramids and wrinkles) has been proven as the most effective means to boost the sensitivity of ionic skins (I-skins). However, the single-scale micronano patterns constructed by the common fabrication strategy generally lead to a limited pressure-response range. Here, a convenient repeated stretching/coordinating/releasing strategy is developed to controllably construct multiple graded wrinkles on the polyelectrolyte hydrogel-based I-skins for increasing their sensitivity over a broad pressure range. We find that the small wrinkles allow for high sensitivity yet small pressure detection range, while the large wrinkles can reduce structural stiffening to generate large pressure-response range but incur limited sensitivity. The multiple graded wrinkles can combine the merits of both the small and large wrinkles to simultaneously improve the sensitivity and broaden the pressure-response range. In particular, the sensing performance of multiple-wrinkle-based I-skins substantially outperforms the superposition of the sensing performance of different single-wrinkle-based I-skins. As a proof of concept, the triple-wrinkle-based I-skins can provide an extremely high sensitivity of 17,309 kPa-1 and an ultrawide pressure detection range of 0.38 Pa to 372 kPa. The approach and insight contribute to the future development of I-skins with a broader pressure-response range and higher sensitivity.

Radiation synthesis of covalently/non-covalently coupled all ionic liquid-based Ionogels with rapid self-healing, environmental tolerance for multifunctional ionic skin

As novel artificial ion-conducting materials with skin-like functionalities, Ionic skins (I-Skins) have attracted wide attention in the field of smart wearables. Nonetheless, the crucial hurdle of maintaining their properties in extreme conditions, such as cold, hot or long-term storage, persists. Herein, ionic liquid monomers, ionic liquid solvents, and ionic liquid cross-linkers are employed to facilitate covalently/non-covalently coupled all ionic liquid-based Ionogel for multimodal artificial I-skin through electron beam irradiation under mild conditions at the first time. Profiting from the covalently/non-covalently interactions in the Ionogel, the Ionogel has tunable mechanical properties, excellent environmental adaptability (−60 to 150 °C), long-term stability (90 days), fast response time (0.1 s), rapid self-healing ability (120 s recovery to 90 % of original), and favorable adhesion properties. Moreover, Ionogel sensors possess the remarkable capability to accurately perceive external stimuli, including strain, temperature, and humidity, similar to human skin, which can effectively monitor human movement behavior, as well as fluctuations in ambient temperature and humidity. It is expected that this all ionic liquid-based Ionogel sensor can fill the gap of I-Skin in extreme conditions and realize multifunctional flexible sensing, thus expanding more application scenarios.

Hybrid Hydrogen Bonding Strategy to Construct Instantaneous Self-Healing Highly Elastic Ionohydrogel for Multi-Functional Electronics.

Gels show great promise for applications in wearable electronics, biomedical devices, and energy storage systems due to their exceptional stretchability and adjustable electrical conductivity. However, the challenge lies in integrating multiple functions like elasticity, instantaneous self-healing, and a wide operating temperature range into a single gel. To address this issue, a hybrid hydrogen bonding strategy to construct gel with these desirable properties is proposed. The intricate network of hybrid strong weak hydrogen bonds within the polymer matrix enables these ionohydrogel to exhibit remarkable instantaneous self-healing, stretching up to five times their original length within seconds. Leveraging these properties, the incorporation of ionic liquids, water, and zinc salts into hybrid hydrogen bond crosslinked network enables conductivity and redox reaction, making it a versatile ionic skin for real-time monitoring of human movements and respiratory. Moreover, the ionohydrogel can be used as electrolyte in the assembly of a zinc-ion battery, ensuring a reliable power supply for wearable electronics, even in extreme conditions (-20°C and extreme deformations). This ionohydrogel electrolyte simplifies the diverse structural requirements of gels to meet the needs of various electronic applications, offering a new approach for multi-functional electronics.

Low modulus ultra-ductility ionic skin by highly entangled PAM-co-UPyMA hydrogel

This work primarily discusses the construction of physically cross-linked polyacrylamide copolymer containing 2-(3-(6-methyl-4-oxo-1,4-dihydropyrimidin-2-yl)ureido)ethyl methacrylate (PAM-co-UPyMA, short for PAU) hydrogels with topological entanglement by designing high concentrations of acrylamide (AM) monomers and introducing “bridging” flexible non-covalent bonds. This method overcomes the poor stretchable performance of chemically cross-linked hydrogels, as well as the limited improvement in mechanical properties due to the low dissipation energy of physically cross-linked hydrogel (non-covalent bonding) under an external force. This new hydrogel exhibits low modulus (0.12 MPa), a certain fracture strength (0.2 MPa), and high toughness with an elongation at a break of 5700 %, demonstrating excellent tensile strain sensitivity with a gauge factor (GF) of 16.14, capable of detecting tiny deformations of 0.2 %. It also exhibits fast strain response, long-term stability, and performs well in applications such as human joints and voice monitoring. Furthermore, this work elucidates the relationship between the microstructure of PAU hydrogels and sensing performance, proving that the rate of microstructure change serves as an essential mechanism. The less cross-linked and highly entangled PAU hydrogel strain sensor increases the cross-linked domain size under tensile load, leading to ion channel blockade with a rapid increase in resistance during deformation, thus resulting in an improved sensitivity.

Ionic organohydrogel with long-term environmental stability and multifunctionality based on PAM and sodium alginate

Hydrogels possess several advantageous characteristics, including transparency, biocompatibility, and ionic conductivity, making them promising for applications in flexible electronics, drug delivery, ionic skin, and electrolytes. Nevertheless, creating a robust conductive and antifreezing hydrogel with enduring environmental stability remains a significant challenge. This study, based on the salt-out effect of alkali metal ion salts and binary solvent systems, employs a straightforward one-pot solvent exchange method to introduce dynamic and reversible non-covalent bonds (hydrophobic association and hydrogen bonds) into a polyacrylamide-sodium alginate (PAM-SA) dual-network hydrogel with excellent flexibility and biocompatibility. By adjusting the types of ions and cryoprotective agents (CPA), a series of ion-conductive organic hydrogels with anti-freezing properties were successfully prepared. The resulting PAM/SA-Gly-KCl1M hydrogel exhibits commendable mechanical properties (with a tensile strain of up to 1465 %, a tensile strength of 166 kPa, and toughness of 1354 kJ m3), high ionic conductivity (0.59 S/m), outstanding frost resistance (–100 °C), and sustained long-term stability (maintaining 70 % of its weight for over one month in an open environment). Therefore, sensors based on organic hydrogels exhibit exceptional characteristics, including high strength, superior transparency, and dependable operation in low-temperature conditions. Its remarkable sensing sensitivity and stability and antifreeze properties facilitate operation across an extensive temperature range and contribute to an extended operational lifespan. Overall, this sensor proves conducive to developing enduring flexible electronic products in challenging environments, thereby unveiling novel prospects for the realms of smart wearable technology and health monitoring.

Self-compliant ionic skin by leveraging hierarchical hydrogen bond association

Robust interfacial compliance is essential for long-term physiological monitoring via skin-mountable ionic materials. Unfortunately, existing epidermal ionic skins are not compliant and durable enough to accommodate the time-varying deformations of convoluted skin surface, due to an imbalance in viscosity and elasticity. Here we introduce a self-compliant ionic skin that consistently works at the critical gel point state with almost equal viscosity and elasticity over a super-wide frequency range. The material is designed by leveraging hierarchical hydrogen bond association, allowing for the continuous release of polymer strands to create topological entanglements as complementary crosslinks. By embodying properties of rapid stress relaxation, softness, ionic conductivity, self-healability, flaw-insensitivity, self-adhesion, and water-resistance, this ionic skin fosters excellent interfacial compliance with cyclically deforming substrates, and facilitates the acquisition of high-fidelity electrophysiological signals with alleviated motion artifacts. The presented strategy is generalizable and could expand the applicability of epidermal ionic skins to more complex service conditions.