Ionic Conductive Hydrogel Research Articles

Nanocellulose‐reinforced, anti‐freezing, highly conductive ionic organic hydrogels for flexible electronic devices

AbstractCurrently, flexible electronic devices based on ionic conductive hydrogels are receiving widespread attention in the field of human health detection. In this paper, a facile one‐pot method is proposed for the preparation of ionic organic hydrogels, in which polyvinyl alcohol (PVA), cellulose nanofiber (CNF), and graphene oxide (GO) are dissolved in dimethyl sulfoxide‐water (DMSO/H2O), and the organic hydrogel is obtained by a freezing–thawing process. The ionic organic hydrogel with excellent properties is then prepared by soaking the hydrogel in a calcium chloride (CaCl2) solution using a salting‐out strategy. The ionic organic hydrogel possesses good tensile (283%) and strength (0.3 MPa), super electrical conductivity (7.72 S/m), and high strain sensitivity [gauge factor (GF) up to 5.22]. Meanwhile, it has excellent anti‐freezing and moisturizing properties. In addition, the ionic organic hydrogels can be used in flexible strain sensors and triboelectric nanogenerators to realize real‐time monitoring of human motion, traceless writing, and energy conversion. It is foreseen that the prepared ionic organic hydrogels provide a feasible method for realizing the long‐term use of wearable electronic devices in extreme environments and daily life.

Stretchable, self-healing, adhesive and anti-freezing ionic conductive cellulose-based hydrogels for flexible supercapacitors and sensors

Stretchable, self-healing, adhesive and anti-freezing ionic conductive cellulose-based hydrogels for flexible supercapacitors and sensors

Poly(ionic liquid) functionalization: A general strategy for strong, tough, ionic conductive, and multifunctional polysaccharide hydrogels toward sensors

Poly(ionic liquid) functionalization: A general strategy for strong, tough, ionic conductive, and multifunctional polysaccharide hydrogels toward sensors

Ionic Potential Relaxation Effect in a Hydrogel Enabling Synapse-Like Information Processing.

The next-generation brain-like intelligence based on neuromorphic architectures emphasizes learning the ionic language of the brain, aiming for efficient brain-like computation and seamless human-computer interaction. Ionic neuromorphic devices, with ions serving as information carriers, provide possibilities to achieve this goal. Soft and biocompatible ionic conductive hydrogels are an ideal substrate for constructing ionic neuromorphic devices, but it remains a challenge to modulate the ion transport behavior in hydrogels to mimic neuroelectric signals. Here, we describe an ionic potential relaxation effect in a hydrogel device prepared by sandwiching a layer of polycationic hydrogel (CH) between two layers of neutral hydrogel (NH), allowing this device to simulate various electrical signal patterns observed in biological synapses, including short- and long-term plasticity patterns. Theoretical and experimental results show that the selective permeation and hysteretic diffusion of ions caused by the anion selectivity of the CH layer are responsible for potential relaxation. Such an effect allows us with hydrogels to enable synapse-like information processing functions, including tactile perception, learning, memory, and neuromorphic computing. Additionally, the hydrogel device can operate stably even under 180° bending and 50% tensile strain, expanding the pathway for implementing advanced brain-like intelligent systems.

Wood Cell Wall Nanoengineering toward Anisotropic, Strong, and Flexible Cellulosic Hydrogel Sensors.

Achieving highly ionic conductive hydrogels from natural wood remains challenging owing to their insufficient surface area and low number of active sites on the cell wall. This study proposes a viable strategy to design a strong and anisotropic wood-based hydrogel through cell wall nanoengineering. By manipulating the microstructure of the wood cell wall, a flexible cellulosic hydrogel is achieved through Schiff base bonding via the polyacrylamide and cellulose molecular chains. This results in excellent flexibility and mechanical properties of the wood hydrogel with tensile strengths of 22.3 and 6.1 MPa in the longitudinal and transverse directions, respectively. Moreover, confining aqueous salt electrolytes within the porous structure gives anisotropic ionic conductivities (19.5 and 6.02 S/m in the longitudinal and transverse directions, respectively). The wood-based hydrogel sensor has a favorable sensitivity and a stable working performance at a low temperature of -25 °C in monitoring human motions, thereby demonstrating great potential applications in wearable sensor devices.

Ultra-stretchable, robust, self-healable conductive hydrogels enabled by the synergistic effects of hydrogen bonds and ionic coordination bonds toward high-performance e-skins

Ionic conductive hydrogels as electronic skins (e-skins) have showcased pivotal potentials in the realm of human health monitoring and human–machine interfaces. However, developing intelligent hydrogel with remarkable stretchability and mechanical elasticity is yet challenging. Herein, high-performance ion-conducting hydrogels with unprecedented mechanical properties and good transparent, conductive, self-healing properties are constructed via free-radical polymerization of acrylic acid (AA) within the polyvinyl alcohol (PVA) network, and the subsequent freeze–thaw (F-T) treatment and Zr4+ ion immersion crosslinking technique. Profiting from the synergy of multiple intermolecular/intramolecular hydrogen bonds between different polymers and coordination bonds of carboxyl-Zr4+, the resultant PVA/PAA/Zr4+ hydrogel exhibits high transparency (∼97 %), superior conductivity (0.6089 S/m), a long elongation at break (2053 %), robust self-healing efficiency (96.3 %), high deformation-tolerate property, and notable mechanical repeatability. These multifaceted attributes render the hydrogel to serve as an ionic conductor for capacitive/resistive −type strain sensor. The hydrogel-based resistive-type strain sensor features a wide detection range (0.5 %-800 %), rapid response time (150 ms), long-term stability, and consistent output stability, making it promising in monitoring diverse human motions and seamless human–machine interactions. Additionally, the hydrogel sensor demonstrates excellent linear response in ultra-wide range (0–900 % strain), high sensitivity, and excellent cycling stability in a capacitive mode, enabling to be used for accurately detecting the weights of objects. As such, this work advances the development of ionic conductive hydrogel as flexible and wearable electronic devices.

Attapulgite-Reinforced Cellulose Hydrogels with High Conductivity and Antifreezing Property for Flexible Sensors.

Ionic conductive cellulose hydrogels are some of the most promising candidates for flexible sensors. However, it is difficult to simultaneously prepare cellulose hydrogels with high mechanical strength, good ionic conductivity, and antifreeze performance. In this work, a natural clay (attapulgite)-reinforced cellulose hydrogel was fabricated. Through a one-pot method, cellulose and attapulgite were dispersed in a concentrated ZnCl2 solution. The obtained hydrogel exhibited a dual network of hydrogen bonds and Zn2+-induced ionic interactions. Attapulgite serves as an inorganic filler that can regulate the hydrogen-bonding density among cellulose molecules and provides abundant channels for fast ion transport. By optimizing the attapulgite loading, a mechanically strong (compressive strength up to 1.10 MPa), tough (fracture energy up to 0.36 MJ m-3), highly ionic conductive (4.15 S m-1), and freezing-tolerant hydrogel was prepared. These hydrogels can be used for sensitive and stable human motion sensing, demonstrating their great potential for healthcare applications.

Molecular Bridging Induced Anti-salting-out Effect Enabling High Ionic Conductive ZnSO4-based Hydrogel for Quasi-solid-state Zinc Ion Batteries.

Hydrogel electrolytes (HEs) hold great promise in tackling severe issues emerging in aqueous zinc-ion batteries, but the prevalent salting-out effect of kosmotropic salt causes low ionic conductivity and electrochemical instability. Herein, a subtle molecular bridging strategy is proposed to enhance the compatibility between PVA and ZnSO4 from the perspective of hydrogen-bonding microenvironment re-construction. By introducing urea containing both an H-bond acceptor and donor, the broken H-bonds between PVA and H2O, initiated by the SO42--driven H2O polarization, could be re-united via intense intermolecular hydrogen bonds, thus leading to greatly increased carrying capacity of ZnSO4. The urea-modified PVA-ZnSO4 HEs featuring a high ionic conductivity up to 31.2 mS cm-1 successfully solves the sluggish ionic transport dilemma at the solid-solid interface. Moreover, an organic solid-electrolyte-interphase can be derived from the in-situ electro-polymerization of urea to prohibit H2O-involved side reactions, thereby prominently improving the reversibility of Zn chemistry. Consequently, Zn anodes witness an impressive lifespan extension from 50 h to 2200 h at 0.1 mA cm-2 while the Zn-I2 full battery maintains a remarkable Coulombic efficiency (>99.7%) even after 8000 cycles. The anti-salting-out strategy proposed in this work provides an insightful concept for addressing the phase separation issue of functional HEs.

PNIPAAm-based temperature responsive ionic conductive hydrogels for flexible strain and temperature sensing

Conductive hydrogels have received much attention in the field of flexible wearable sensors due to their outstanding flexibility, conductivity, sensitivity and excellent compatibility. However, most conductive hydrogels mainly focus on strain sensors to detect human motion and lack other features such as temperature response. Herein, we prepared a strain and temperature dual responsive ionic conductive hydrogel (PPPNV) with an interpenetrating network structure by introducing a covalent crosslinked network of N-isopropylacrylamide (NIPAAm) and 1-vinyl-3-butylimidazolium bromide (VBIMBr) into the skeleton of the hydrogel composed of polyvinylalcohol (PVA) and polyvinylpyrrolidone (PVP). The PPPNV hydrogel exhibited excellent anti-freezing properties (−37.34 °C) and water retention with high stretchability (∼930 %) and excellent adhesion. As a wearable strain sensor, the PPPNV hydrogel has good responsiveness and stability to a wide range of deformations and exhibits high strain sensitivity (GF=2.6) as well as fast response time. It can detect large and subtle body movements with good signal stability. As wearable temperature sensors, PPPNV hydrogels can detect human physiological signals and respond to temperature changes, and the volumetric phase transition temperature (VPTT) can be easily controlled by adjusting the molar ratio of NIPAAm to VBIMBr. In addition, a bilayer temperature-sensitive hydrogel was prepared with the temperature responsive hydrogel by two-step synthesis, which shows great promising applications in temperature actuators.

Rapid photo-crosslinking, highly conductive, and anti-freezing acrylamide-based hydrogels applied for ECG sensors

In recent years, ionic conductive hydrogels have attracted significant attention in the wearable sensors field due to their simplified preparation process, cost- effectiveness, and high conductivity. However, a large amount of water in traditional hydrogels inevitably condense into ice at low temperatures and make them lose various properties in their practical applications. Hence, this study proposes a method to rapidly synthesized a water/glycerol binary anti-freezing hydrogel only in 10 s through photo-initiated free-radical polymerization. The introduction of LiCl into the crosslinked network composed of acrylamide (AAm) and poly (ethylene glycol) diacrylate (PEGDA) regulated the ionic conductivity of the hydrogel. The hydrogel exhibits a fracture strain of 1062.1 %, a tensile strength of 31 kPa, and an adhesive strength of 6.49 kPa. The synergistic effects of glycerol and LiCl imparted excellent anti-freezing properties (−60 °C). The three-dimensional network structure of the hydrogel facilitates ion transport, exhibiting superior conductivity (30.25 S/m), maintaining high conductivity capability of 1.46 S/m even at a low temperature of −55 °C. Notably, the hydrogel demonstrates prolonged durability and stability in real-time human motion monitoring applications while concurrently exhibiting remarkable sensitivity as an ECG sensor. Hence, this as-prepared hydrogel can adapt to extreme environments and deliver comprehensive performance as an ECG sensor.

Body temperature-triggered adhesive ionic conductive hydrogels for bioelectrical signal monitoring

Bioadhesive ion-conductive hydrogels are essential in bioelectrical signal monitoring. However, highly adhesive hydrogels may cause discomfort, such as skin redness and pain when peeled off, which hinders their application. Here, we design a body temperature-triggered adhesive ionic conductive hydrogel based on biocompatible polyacrylamide (PAAM), gelatin, and sodium alginate (SA). Due to the temperature sensitivity of gelatin, the adhesion of the developed PAAM/gelatin/SA (PGS) hydrogel is rapidly enhanced upon exposure to human body temperature, and it decreases quickly within a short period (10 s) after cooling, thus enabling painless removal. At the same time, the hydrogel exhibits a Young’s modulus (∼265 kPa) similar to that of skin tissue and achieves a conformal contact to the skin. Benefiting from the low electrode-epidermal impedance, the developed hydrogel has a decent signal-to-noise ratio of about 25 dB, allowing high-fidelity recording of various bioelectrical signals and accurate recognition of movements. The temperature-triggered adhesion-changing conductive hydrogel presented in this work shows great potential for biomedical applications.

Recyclable Organic Ionic Conductive Hydrogels for Flexible Wearable Sensors with Excellent Environmental Resistance

Recyclable Organic Ionic Conductive Hydrogels for Flexible Wearable Sensors with Excellent Environmental Resistance

Preparation and performance study of sodium alginate/bamboo fiber/gelatin ionic conductive self-healing hydrogel

This study has been successfully developed the Sodium alginate/Bamboo fiber /Gelatin（SA/BF/Gel）composite conductive hydrogel with adhesive and self-healing properties. Through in-depth research, the influence of Gel content on the tensile, adhesive, self-healing properties, and conductivity of the SA/BF/Gel composite conductive hydrogel was discussed. The sensing performance and sensing mechanism of the material were also investigated, along with a preliminary exploration of its potential applications. An attempt was made to apply the SA/BF/Gel composite conductive hydrogel to 3D printing technology, establishing a connection between the rheological properties of the hydrogel and its printing structure. The addition of Gel significantly improved the flexibility of the hydrogel, with a conductivity of up to 3.12 S/m at a Gel content of 1.5 %. When employed as a sensor, the material exhibited high sensitivity (GF = 2.21) and excellent cyclic stability, rendering it suitable for a wide range of applications in real-time monitoring of bending movements of fingers and wrists, as well as dynamic contact and variations in contact forces on the hydrogel surface. The SA/BF/Gel composite conductive hydrogel has the potential to be utilized in a multitude of applications, including the development of smart wearable devices, the monitoring of individual human beings, and the integration of human beings and machines. Furthermore, the research findings associated with this hydrogel will provide a strong foundation for the advancement of materials science and the integration of smart technologies.

Environmentally stable and rapidly polymerized tin-tannin catalytic system hydroxyethyl cellulose hydrogel for wireless wearable sensing

In recent years, flexible sensors constructed mainly from hydrogels have played an indispensable role in several fields. However, the traditional hydrogel preparation process involves complex and time-consuming steps and the freezing or volatilization of water in the water gel in extreme environments greatly limits the further use of the sensor. Therefore, an ionic conductive hydrogel (SnHTD) was designed, which was composed of tannic acid (TA), metal ions Sn2+, hydroxyethyl cellulose (HEC), and acrylamide (AM) in a deep eutectic solvent (DES) and water binary solvent. It is worth noting that the gel time is shortened to less than 3 min by introducing the Sn-TA redox system. The addition of DES makes the hydrogel have a wide temperature tolerance range (−20 to 60 °C) and the ability to store for a long time (30 days). The introduction of HEC increased the tensile stress of hydrogel from 140.17 kPa to 219.89 kPa. Additionally, the hydrogel also has high conductivity, repeatable adhesion and UV shielding properties. In general, this research opens up a new way for room temperature polymerization of environmentally resistant hydrogel materials and effectively meets the growing demand for wireless wearable sensing.

High stretchable and self-adhesive multifunctional hydrogel for wearable and flexible sensors

Ionic conductive hydrogel has recently garnered significant research attention due to its potential applications in the field of wearable and flexible electronics. Nonetheless, the integration of multifunctional and synergistic advantages, including reliable electronic properties, high swelling capacity, exceptional mechanical characteristics, and self-adhesive properties, presents an ongoing challenge. In this study, we have developed an ionic conductive hydrogel through the co-polymerization of 4-Acryloylmorpholine (ACMO) and sodium acrylate using UV curing technology. The hydrogel exhibits excellent mechanical properties, high conductivity, superior swelling capacity, and remarkable self-adhesive attributes. The hydrogel serves as a highly sensitive strain sensor, enabling precise monitoring of both substantial and subtle human motions. Furthermore, the hydrogel demonstrates the capability to adhere to human skin, functioning as a human-machine interface for the detection of physiological signals, including electromyogram (EMG) signals, with low interfacial impedance. This work is anticipated to yield a new class of stretchable and conductive materials with diverse potential applications, ranging from flexible sensors and wearable bio-electronics to contributions in the field of artificial intelligence.

Attapulgite‐Reinforced Robust and Ionic Conductive Composite Hydrogels for Digital Light Processing 3D Printing

AbstractIonic conductive hydrogels are widely used in many applications such as electrochemical energy storage, flexible electronic devices, and catalyst transistors due to their excellent ionic conductivity as well as chemical stability. However, the fragile mechanical properties and the lack of shaping methods severely limit their further applications. Herein, an ionic conductive composite hydrogel with reinforced mechanical properties is demonstrated that can be rapidly 3D printed using digital light processing technology. By using both γ‐methacryloxypropyltrimethoxysilane moderately modified attapulgite rigid particle and polyvinyl alcohol (PVA) semicrystal dual‐network reinforcement, the mechanically robust and highly conductive N,N,N‐trimethylethanaminium‐chloride‐based hydrogels are obtained, demonstrating a 5 times higher tensile strength than the initial one due to the turning and orienting of the attapulgite as well as the robust PVA secondary network. Furthermore, encapsulation strategy is used to avoid the dehydration of hydrogel, and strain sensors that exceed the strain limit of the hydrogel are fabricated through structural design. This work provides a reference for attapulgite‐reinforced hydrogel in biosensing.

Engineering versatile bi-network ionic conductive hydrogels wearable sensors via on demand graft modification for real-time human movement monitoring

With the rapid development of the field of flexible electronics, it is necessary to develop more new flexible sensor materials with a high degree of flexibility, stretchability, biosafety, and good electrical conductivity to better achieve wearable with the human body. Cellulose hydrogels are promising building blocks for flexible sensors due to their renewability and excellent mechanical strength properties. In this study, we demonstrate an imidazolium ionic cellulose/polyacrylic acid hydrogel (ILHPMC/PAA) with high flexibility, antimicrobial properties and electrical conductivity. Firstly, epoxidized imidazole ionic liquids were synthesized using methyl imidazole as raw material, and imidazole ionic cellulose was prepared by grafting modified hydroxypropyl methyl cellulose (HPMC). Imidazolium ionic cellulose hydrogels were prepared by one-step cross-linking and forming a copolymeric bi-network with acrylic acid according to the principle of free radical polymerisation. This hydrogel has high flexibility, stretchability, biocompatibility, and desired electrical conductivity due to the introduction of imidazole ions, and the sensing sensitivity factor (GF) reaches 6.83. The tensile strength can be up to 1.2 MPa, and it also has excellent antibacterial properties, with an antibacterial zone diameter of up to 29.25 mm. This provides insights into the development of cellulose-based sustainable sensing hydrogel sensors for wearable ionic conductive sensing.

Poly(vinyl alcohol)/polyacrylamide double‐network ionic conductive hydrogel strain sensor with high sensitivity and high elongation at break

AbstractAs a soft material with biocompatibility and stimulation response, ionic conductive hydrogel‐based wearable strain sensors show great potential across a wide spectrum of engineering disciplines, but their mechanical toughness is limited in practical applications. In this study, freeze‐thawing techniques were utilized to fabricate double‐network hydrogels of poly(vinyl alcohol)/polyacrylamide (PVA/PAM) with both covalent and physical cross‐linking networks. These double‐network hydrogels demonstrate excellent mechanical performance, with an elongation at break of 2253% and tensile strength of 268.2 kPa. Simultaneously, they also display a high sensitivity (Gage factor, GF = 2.32 at 0%–200% strain), achieve a rapid response time of 368 ms without the addition of extra conductive fillers or ions, stable signal transmission even after multiple cycles, and fast response to human motion detection.

Dually-crosslinked ionic conductive hydrogels reinforced through biopolymer gellan gum for flexible sensors to monitor human activities

Human-machine interactions, monitoring of health equipment, and gentle robots all depend considerably on flexible strain sensors. However, making strain sensors have better mechanical behavior and an extensive sensing range remains an urgent difficulty. In this study, poly acrylamide-co-butyl acrylate with gellan gum (poly(AAm-co-BA)@GG) hydrophobic association networks and intermolecular hydrogen bonding interactions are used to fabricate dual cross-linked hydrogels for wearable resistive-type strain sensors. This could be an acceptable way to minimize the limitations in hydrogels previously identified. The robust fracture strength (870 kPa) and exceptional stretchability (1297 %) of the hydrogel arise from the collaborative action of intermolecular hydrogen bonding and hydrophobic associations. It also demonstrates exceptional resilience to repeated cycles of uninterrupted stretching and relaxation, retaining its structural integrity. The response and restoration times are 110 and 120 ms respectively. Furthermore, a wide sensing range (0–900 %), notable sensitivity across various strain levels, and an impressive gauge factor (GF) of 31.51 with high durability were observed by the dual cross-linked (DC) hydrogel-based strain sensors. The measured conductivity of the hydrogel was 0.32 S/m which is due to the incorporation of NaCl. Therefore, the hydrogels can be tailored to function as wearable strain sensors that can detect subtle human gestures like speech patterns, distinguish between distinct words, and recognize vibrations of the larynx during drinking, as well as large joint motions like wrist, finger, and elbow. Furthermore, these hydrogels are capable of reliably distinguishing and reproducing various printed text. These findings imply that any electronic device that demands strain-sensing functionality might make use of these developed materials.

Ureido-Ionic Liquid Mediated Conductive Hydrogel: Superior Integrated Properties for Advanced Biosensing Applications.

Ionic conductive hydrogels (ICHs) have recently gained prominence in biosensing, indicating their potential to redefine future biomedical applications. However, the integration of these hydrogels into sensor technologies and their long-term efficacy in practical applications pose substantial challenges, including a synergy of features, such as mechanical adaptability, conductive sensitivity, self-adhesion, self-regeneration, and microbial resistance. To address these challenges, this study introduces a novel hydrogel system using an imidazolium salt with a ureido backbone (UL) as the primary monomer. Fabricated via a straightforward one-pot copolymerization process that includes betaine sulfonate methacrylate (SBMA) and acrylamide (AM), the hydrogel demonstrates multifunctional properties. The innovation of this hydrogel is attributed to its robust mechanical attributes, outstanding strain responsiveness, effective water retention, and advanced self-regenerative and healing capabilities, which collectively lead to its superior performance in various applications. Moreover, this hydrogel exhibited broad-spectrum antibacterial activity. Its potential for biomechanical monitoring, especially in tandem with contact and noncontact electrocardiogram (ECG) devices, represents a noteworthy advancement in precise real-time cardiac monitoring in clinical environments. In addition, the conductive properties of the hydrogel make it an ideal substrate for electrophoretic patches aimed at treating infected wounds and consequently enhancing the healing process.