Ionic Conductive Hydrogel Research Articles

Low-temperature strain-sensitive sensor based on cellulose-based ionic conductive hydrogels with moldable and self-healing properties

Bioelectronics based on high-performance conductive ionic hydrogels, which can create novel technological interfaces with the human body, have attracted significant interest from both academia and industry. However, it is still a challenge to fabricate hydrogel sensor with integration of good mechanical properties, fast self-healing ability and flexible strain sensitivity below 0 °C. In this paper, we present a moldable, self-healing and adhesive cellulose-based ionic conductive hydrogel with strain-sensitivity, which was prepared by forming dual-crosslinked networks using poly(vinyl alcohol) (PVA) with borax, calcium chloride (CaCl2), zinc chloride (ZnCl2) and 2,2,6,6-tetramethylpiperidine-1-oxyl oxidized cellulose nanofibril (TCNF). The hydrogel exhibited fast self-healing within 10 s, moderate modulus of 5.13 kPa, high elongation rate of 1500 % and excellent adhesion behavior on various substrates. Due to multiple hydrogen bonding and the presence of CaCl2 and ZnCl2, the hydrogel presented a reduced freezing point as low as −41.1 °C, which enabled its application as a low-temperature strain sensor. The proposed hydrogel provides a simple and facile method for fabricating multi-functional hydrogels that can be used as suitable strain sensors for applications such as wearable electronic sensor, soft robotics and electronic skins in a wide temperature range.

High-Conductivity, Low-Impedance, and High-Biological-Adaptability Ionic Conductive Hydrogels for Ear-EEG Acquisition

High-Conductivity, Low-Impedance, and High-Biological-Adaptability Ionic Conductive Hydrogels for Ear-EEG Acquisition

Recyclable Ionic Conductive Hydrogels Based on Autonomous Polymerization of α-Lipoic Acid for Flexible Wearable Electronic Sensors

Recyclable Ionic Conductive Hydrogels Based on Autonomous Polymerization of α-Lipoic Acid for Flexible Wearable Electronic Sensors

Self-sensing magnetic actuators of bilayer hydrogels

ABSTRACT Hard magnetic soft robots have been widely used in biomedical engineering. In these applications, it is crucial to sense the movement of soft robots and their interaction with target objects. Here, we propose a strategy to fabricate a self-sensing bilayer actuator by combining magnetic and ionic conductive hydrogels. The magnetic hydrogel containing NdFeB particles exhibits rapid response to magnetic field and achieve bending deformation. Meanwhile, the polyacrylamide (PAAm) hydrogel with lithium chloride (LiCl) allows for the sensing of deformation. The bending behavior of the bilayer under magnetic field is well captured by theoretical and simulated models. Additionally, the bilayer strain sensor shows good sensitivity, stability and can endure a wide-range cyclic stretching (0–300%). These merits qualify the self-sensing actuator to monitor the motion signals, such as bending of fingers and grasping process of an intelligent gripper. When subject to an external magnetic field, the gripper can grab a cube and sense the resistance change simultaneously to detect the object size. This work may provide a versatile strategy to integrate actuating and self-sensing ability in soft robots.

Conductive Hydrogel for Flexible Bioelectronic Device: Current Progress and Future Perspective

AbstractConductive hydrogels (CHs) for flexible bioelectronic devices have raised great attention due to their tunable mechanical performances, adhesion, anti‐swelling, and biocompatibility. This review summarizes the current development of conductive hydrogel‐based flexible bioelectronic devices in the aspect of classifications and applications. Firstly, the conductive hydrogels are classified into two kinds according to the types of conductivity: ionic conductive hydrogels and electronic conductive hydrogels (hydrogel based on pure conductive materials, introducing conductive micro/nano‐materials). Secondly, the applications of conductive hydrogels for bioelectronic device, like wearable devices (strain sensor, body fluid detector, serviced in extreme environment), tissue engineering (skin, heart, nerve, muscle), and other applications (bionic robot, cancer treatment), are highly illustrated. Finally, a depth outlook is given, which aims to promote the development of this field in the future.

Transparent, High Stretchable, Environmental Tolerance, and Excellent Sensitivity Hydrogel for Flexible Sensors and Capacitive Pens.

The prospect of ionic conductive hydrogels in multifunctional sensors has generated widespread scientific interest. The new generation of flexible materials should be combined with superior mechanical properties, high conductivity, transparency, sensitivity, good self-restoring fatigue properties, and other multifunctional characteristics, while the current materials are difficult to meet these requirements. Herein, we prepared poly(acrylamide-acrylic acid) (P(AM-AA))/gelatin/glycerol-Al3+ (PG1G2A) ionic conducting hydrogel by one-pot polymerization under UV light. The prepared PG1G2A ionic conductive hydrogel had high tensile strength (539.18 kPa), excellent tensile property (1412.96%), good fast self-recovery and fatigue resistance, high transparency (>80%), excellent moisturizing, and antifreezing/drying properties. In addition, the ionic conductive hydrogel-based strain sensor can respond to mechanical stimulation and generate accurate, stable, and recyclable electrical signals, with excellent sensitivity (GF 5.81). In addition, the PG1G2A hydrogel could be used as flexible wearable devices for monitoring multiple strain and subtle movements of different body parts at different temperatures. Interestingly, the PG1G2A hydrogel capacitive pen embedded in the mold can be used to write and draw on the screen of a phone or tablet. This new multifunctional ionic conducting hydrogel shows broad application prospects in E-skin, motion monitoring, and human-computer interaction in extreme environments.

Mussel-Inspired Wet-Adhesive Multifunctional Organohydrogel with Extreme Environmental Tolerance for Wearable Strain Sensor.

As a flexible artificial material, the conductive hydrogel has broad application prospects in flexible wearable electronics, soft robotics, and biomedical monitoring. However, traditional hydrogels still face many challenges, such as long-term stability, availability in extreme environments, and long-lasting adhesion to the skin surface under sweaty or humid conditions. To circumvent the above issues, one kind of ionic conductive hydrogel was prepared by a simple one-pot method that dissolved chitosan (CS), 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), tannic acid (TA), and 2-methoxy-ethyl acrylate (MEA) into dimethyl sulfoxide (DMSO)/H2O solvent. The resulting hydrogel showed excellent tensile properties (1440%), extreme environmental tolerance (-40-60 °C), adhesion (72 KPa at porcine skin), ionic conductivity (0.87 S m-1), and high-efficiency antibacterial property. Furthermore, the produced organohydrogel strain sensor exhibited high strain sensitivity (GF = 4.07), excellent signal sensing capabilities (human joint movement, microexpression, and sound signals), and long-term cyclic stability (400 cycles). Looking beyond, this work provides a simple and promising strategy for using hydrogel sensors in extreme environments for e-skin, health monitoring, and wearable electronic devices.

Preparation and Properties of Multifunctional Nanocellulose-Reinforced PVA Ionic Hydrogel

In this paper, polyvinyl alcohol (PVA), nanocellulose crystals (CNC), glycerol, and aluminum chloride hexahydrate (AlCl3·6H2O) were used to prepare PVA-CNC ionic conductive hydrogels by a one-pot freeze-thaw method. The microscopic morphology and internal structure of the hydrogel were characterized by scanning electron microscopy (SEM) and Fourier transform infrared spec-troscopy (FTIR). The mechanical properties, electrical conductivity, and sensing properties of the hydrogel were tested by a universal material testing machine and LCR digital bridge, and the anti-freezing performance and water retention performance of the hydrogel were analyzed and compared. The results show that different concentrations of nanocellulose can significantly affect the mechanical strength of the hydrogel. When the content of CNC is 2%, the hydrogel has high tensile properties (elongation at break 384.29%) and firmness (up to 1.92 MPa). The open-circuit voltage of the hydrogel-based self-powered device can reach 0.815 V. In the bending sensing test, it was monitored that the resistance of the hydrogel changes rapidly at different bending angles, indicating that the hydrogel has good strain sensitivity. The addition of glycerol makes the hydrogel have high water retention rate, good mechanical properties and certain conductivity at −20 °C. The ionic conductive hydrogels prepared in this work have certain application potential in sensors, human-computer interaction, and flexible wearable devices.

Compressible, anti-freezing, and ionic conductive cellulose/polyacrylic acid composite hydrogel prepared via AlCl3/ZnCl2 aqueous system as solvent and catalyst

From the perspective of environmental sustainability, introducing cellulose into ionic conductive hydrogel is an inevitable trend for the development of flexible conductive materials. We report a double-network cellulose/polyacrylic acid (Cel/PAA) composite hydrogel based on the dissolving of cellulose by AlCl3/ZnCl2 aqueous system. The Cel/PAA composite hydrogel consists of rigid cellulose chains and flexible polyacrylic acid, which synergistically realize the improvement of the mechanical properties. The AlCl3/ZnCl2 aqueous system not only serves as the green solvent for cellulose, but also the Al3+ and Zn2+ metal ions can be served as a catalyst to activate the initiator for polymerization of acrylic acid. Compared with pure cellulose hydrogel, the compression strain of the Cel/PAA composite hydrogel was significantly improved to 80 %, and its conductivity increased by 28.1 %. In addition, its compression stress was enhanced over 2 times than pure PAA hydrogel. The Cel/PAA composite hydrogel exhibits excellent anti-freezing (−45 °C), weight retention (90 %), and conductivity (2.70 S/m) properties, still maintaining transparency and storage stability in the extreme environment. This work presents a facile strategy to develop an ionic conductive cellulose-based composite hydrogel with good conductivity and mechanical properties, which shows potential for the application fields of flexible sensors and 3D-printing functional materials.

A transparent, self-adhesive and fully recyclable conductive PVA based hydrogel for wearable strain sensor

Hydrogel-based wearable sensors have attracted tremendous attention in the sensing electronics field due to their unique properties. However, integrating hydrogel sensors with transparency, biosafety, self-adhesiveness, recyclability and conductivity remains a great challenge. Here, we report an ionic conductive hydrogel prepared using poly (vinyl alcohol) (PVA) grafted with the 2,3-epoxypropyltrimethylammonium chloride (TMAC) through a facile freezing-thawing (F-T) process. The obtained PVA grafted with TMAC hydrogel (PVATH) exhibited relatively high transparency (∼87% transmittance) and strong self-adhesion to different substrates with high stretchability (544%) and good conductivity (0.038 S m−1). Most notably, the hydrogel can be fully recycled by a simple and green process on account of a fully physical cross-linking network. Even after three recycling cycles, the hydrogel sensor exhibited almost identical electromechanical performance with respect to the original sensor. The hydrogel was further assembled as a strain sensor for monitoring real-time human motions, including finger, wrist, elbow, knee joint and neck movement. This work provides an innovative design strategy to fabricate ionic conductive hydrogels and reduce electronic waste, which will broaden the applications of hydrogels in wearable devices.

Design of a Highly Sensitive Ionic Conductive Hydrogel Sensor based on the Kirigami Structure

Conductive hydrogels are polymers that respond to mechanical stimuli and have been widely used in wearable sensors and soft machines. Myriads applications posed high-performance requirements for hydrogels: compliance, stretchability, and high sensitivity. However, sensors based on flexible polymers often struggle to achieve a highly sensitive response. We propose a highly sensitive hydrogel strain sensor based on the Kirigami structure. The structure solves the problem of low stretchability caused by the high stiffness of double-network hydrogel. It greatly improves the stretchability of the strain sensor by customizing the cutting of a high-toughness PAAM-PAA double-network hydrogel. We further analyze the deformation and sensing characteristics through simulation and experiments and successfully apply them to monitor human motion.

Ultrastrong and Tough Urushiol-Based Ionic Conductive Double Network Hydrogels as Flexible Strain Sensors.

Ionic conductive hydrogels have attracted increasing research interest in flexible electronics. However, the limited resilience and poor fatigue resistance of current ionic hydrogels significantly restrict their practical application. Herein, an urushiol-based ionic conductive double network hydrogel (PU/PVA-Li) was developed by one-pot thermal initiation polymerization assisted with freeze-thaw cycling and subsequent LiCl soaking. Such a PU/PVA-Li hydrogel comprises a primary network of covalently crosslinked polyurushiol (PU) and a secondary network formed by physically crosslinked poly(vinyl alcohol) (PVA) through crystalline regions. The obtained PU/PVA-Li hydrogel demonstrates exceptional mechanical properties, including ultrahigh strength (up to 3.4 MPa), remarkable toughness (up to 1868.6 kJ/m3), and outstanding fatigue resistance, which can be attributed to the synergistic effect of the interpenetrating network structure and dynamic physical interactions between PU and PVA chains. Moreover, the incorporation of LiCl into the hydrogels induces polymer chain contraction via ionic coordination, further enhancing their mechanical strength and resilience, which also impart exceptional ionic conductivity (2.62 mS/m) to the hydrogels. Based on these excellent characteristics of PU/PVA-Li hydrogel, a high-performance flexible strain sensor is developed, which exhibits high sensitivity, excellent stability, and reliability. This PU/PVA-Li hydrogel sensor can be effectively utilized as a wearable electronic device for monitoring various human joint movements. This PU/PVA-Li hydrogel sensor could also demonstrate its great potential in information encryption and decryption through Morse code. This work provides a facile strategy for designing versatile, ultrastrong, and tough ionic conductive hydrogels using sustainable natural extracts and biocompatible polymers. The developed hydrogels hold great potential as promising candidate materials for future flexible intelligent electronics.

Preparation and properties of lignin-based dual network hydrogel and its application in sensing

Ionic conductive hydrogels prepared from various biological macromolecules are ideal materials for the manufacture of human motion sensors from the perspective of resource regeneration and environmental sustainability. However, it is now difficult to develop conductive hydrogels including excellent self-healing and mechanical properties, mainly due to their inherent trade-off between dynamic cross-linked healing and stable cross-linked mechanical strength. In this work, alkali lignin-Polyvinyl alcohol-polyacrylic acid double network conductive hydrogels with high mechanical strength and good self-healing properties were prepared. We formed the primary network structure by hydrogen bonding interaction between polyvinyl alcohol, alkali lignin and polyacrylic acid, and the secondary network structure by coordination interaction with polyacrylic acid through the addition of Fe3+. The added lignin acts as a dynamic linkage bridge in a porous network mediated by multiple ligand bonds, imparting superior mechanical properties to the hydrogels. The relationships between the alkali lignin and iron ion dosage and the comprehensive properties of hydrogels (adhesion, antibacterial, self-healing, electrical conductivity and mechanical properties) were studied in detail. On this basis, the hydrogels explored the role of lignin in the regulation of hydrogels properties and revealed the self-healing and conductive mechanism.

Trehalose-enhanced ionic conductive hydrogels with extreme stretchability, self-adhesive and anti-freezing abilities for both flexible strain sensor and all-solid-state supercapacitor

Flexible strain sensors and flexible supercapacitors have generated a lot of interest because of their great potential to be used in flexible electronic equipment. Ionic hydrogels are ideal for the preparation of strain sensors and supercapacitor electrolytes, so the design of a multifunctional ionic conductive hydrogel is the key to obtain both high-performance strain sensors and flexible all-solid-state supercapacitors. During our work, trehalose and LiCl were added concurrently to the P(AM-co-AA) polymer network to prepare a multifunctional ionic conductive hydrogel combining ultra stretchability (elongation at break up to 4529%), high ionic conductivity, self-adhesive and freeze-resistant moisturizing properties. The ionic conductive hydrogel was encapsulated into a strain sensor, which has excellent sensing performance, achieving a great detection range (0–4200%) and high sensitivity (GF = 3.59 in the strain range of 2200–4200%). The strain sensor was applied to human motion detection, demonstrating precise detection behavior for joint flexion and muscle movement. The ionic conductive hydrogel was assembled with flexible fabric electrodes loaded with carbon nanotubes (CNTs) and MXene to form a sandwich-type supercapacitor. The supercapacitor exhibited high areal capacitance (148.2 mF/cm2), excellent cycling performance, outstanding stability and good low temperature resistance.

Stretchable and sensitive sodium alginate ionic hydrogel fibers for flexible strain sensors

Ionic conductive hydrogel fibers based on natural polymers provide an immense focus for a new generation of electronics due to their flexibility and knittability. The feasibility of utilizing pure natural polymer-based hydrogel fibers could be drastically improved if their mechanical and transparent performances satisfy the requirements of actual practice. Herein, we report a facile fabrication strategy for significantly stretchable and sensitive sodium alginate ionic hydrogel fibers (SAIFs), by glycerol initiating physical crosslinking and by CaCl2 inducing ionic crosslinking. The obtained ionic hydrogel fibers not only show significant stretchability (tensile strength of 1.55 MPa and fracture strain of ∼161 %), but also exhibit wide-range sensing, satisfactorily stable, rapidly responsive, and multiply sensitive abilities to external stimulus. In addition, the ionic hydrogel fibers have excellent transparency (over 90 % in a wide wavelength range), and good anti-evaporation and anti-freezing properties. Furthermore, the SAIFs have been easily knitted into a textile, and successfully applied as wearable sensors to recognize human motions, by observing the output electrical signals. Our methodology for fabrication intelligent SAIFs will shed light on artificial flexible electronics and other textile-based strain sensors.

Fabricating of a custom 3D-printed setup for evaluating gel-based strain sensors

Health monitoring systems that incorporate stretchable sensors can provide real-time feedback on a person's physical activity, stress levels, and overall well-being. For instance, stretchable sensors based on conductive hydrogels have emerged as a promising technology for monitoring physiological signals such as heart rate, respiration, and muscle activity. For developing such systems, a strain machine is accounted as the first requirement for laboratories focusing on developing conductive hydrogels as strain sensors. In this article, we present a programmable, user-friendly, and versatile 3D-printed setup designed and fabricated for measuring the strain and strain sensitivity of soft electronic materials. The setup comprises a computer (or mobile), an impedance analyzer, and the strain machine that are required for developing stretchable strain sensors in scientific laboratories. We provide a very detailed build manual on how to manufacture and use the strain machine, using an ionic conductive hydrogel based on polyacrylic acid and polyvinyl alcohol as a model system. The results demonstrated the excellent performance of the strain machine in applying programmable strain regimes to the material of interest. By offering clear instructions and a build manual, our work enables scientific researchers to construct an affordable and user-friendly strain machine to boost their research progress.

Super‐Stretchable, Anti‐Freezing, Anti‐Drying Organogel Ionic Conductor for Multi‐Mode Flexible Electronics

AbstractDue to their intrinsic flexibility, tunable conductivity, multiple stimulus‐response, and self‐healing ability, ionic conductive hydrogels have drawn significant attention in flexible/wearable electronics. However, challenges remain because traditional hydrogels inevitably faced the problems of losing flexibility and conductivity because of the inner water loss when exposed to the ambient environment. Besides, the water inside the hydrogel will freeze at the water icing temperatures, making the device hard and fragile. As a promising alternative, organogels have attracted wide attention because they can, to some extent, overcome the above drawbacks. Herein, a kind of organogel ionic conductor (MOIC) by a self‐polymerization reaction is involved, which is super stretchable, anti‐drying, and anti‐freezing. Meanwhile, it can still maintain high mechanical stability after alternately loading/unloading at the strain of 600% for 600 s (1800 cycles). Using this MOIC, high‐performance triboelectric nanogenerator (TENG) is constructed (MOIC‐TENG) to harvest small mechanical energy even the MOIC electrode underwent an extremely low temperature. In addition, multifunctional flexible/wearable sensors (strain sensor, piezoresistive sensor, and tactile sensor) are realized to monitor human motions in real time, and recognize different materials by triboelectric effect. This study demonstrates a promising candidate material for flexible/wearable electronics such as electronic skin, flexible sensors, and human‐machine interfaces.

A conductive multifunctional hydrogel dressing with the synergistic effect of ROS-scavenging and electroactivity for the treatment and sensing of chronic diabetic wounds

Chronic diabetic wound with persistent inflammatory responses is still a serious threat to human health and life. Ideal wound dressings can be applied not only for covering the injury area, but also for regulating the inflammation to accelerate the wound healing and long-term monitoring of wound condition. However, there remains a challenge to design a multifunctional wound dressing for simultaneous treatment and monitoring of wound. Herein, an ionic conductive hydrogel with intrinsic reactive oxygen species (ROS)-scavenging properties and good electroactivity was developed for achieving the synergetic treatment and monitoring of diabetic wounds. In this study, we modified dextran methacrylate with phenylboronic acid (PBA) to prepare a ROS-scavenging material (DMP). Then the hydrogel was constructed by phenylboronic ester bonds induced dynamic crosslinking network, photo-crosslinked DMP and choline-based ionic liquid as the second network, and the crystallized polyvinyl alcohol as the third network, realizing good ROS-scavenging performance, high electroactivity, durable mechanical properties, and favorable biocompatibility. In vivo results showed that the hydrogel combined with electrical stimulation (ES) demonstrated good performance in promoting re-epithelization, angiogenesis and collagen deposition in chronic diabetic wound treatment by alleviating inflammation. Notably, with desirable mechanical properties and conductivity, the hydrogel could also precisely monitor movements of human body and possible tensile and compressive stresses of the wound site, providing timely alerts of excessive mechanical stress applied to the wound tissue. Thus, this “all-in-one” hydrogel exhibits great potential in constructing the next generation flexible bioelectronics for wound treatment and monitoring. Statement of significanceChronic diabetic wounds characterized by overexpressed reactive oxygen species (ROS) are still a serious threat to human health and life. However, there remains a challenge to design a multifunctional wound dressing for simultaneous wound treatment and monitoring. Herein, a flexible conductive hydrogel dressing with intrinsic ROS-scavenging properties and electroactivity was developed for the combined treatment and monitoring of the wound. The antioxidant hydrogel combined with electrical stimulation synergistically accelerated chronic diabetic wound healing by regulating oxidative stress, alleviating inflammation, promoting re-epithelization, angiogenesis and collagen deposition. Notably, with desirable mechanical properties and conductivity, the hydrogel also presented great potential in monitoring possible stresses of the wound site. The “all-in-one” bioelectronics integrating the treatment and monitoring functions present great application potential for accelerating chronic wound healing.

Acrylate Copolymer-Reinforced Hydrogel Electrolyte for Strain Sensors and Flexible Supercapacitors

Ionic conductive hydrogels with good conductivity and biocompatibility have become one of the research highlights in the field of wearable flexible sensors and supercapacitors. In this work, poly(methacrylic acid–methyl methacrylate)-reinforced poly(sodium acrylate–vinyl phosphonic acid) composite hydrogels (P(AAS-VPA)/PMMS) were designed and tested for strain sensor or supercapacitor applications. The results showed recoverability for 20 cycles of tension and compression experiments, an excellent breaking strain of 2079%, and ionic conductivity of 0.045 S·cm−1, demonstrating strong support for the application of the P(AAS-VPA)/PMMS hydrogel in strain sensors and supercapacitors. The composite hydrogel exhibited outstanding sensing and monitoring capability with high sensitivity (GF = 4.0). The supercapacitor based on the P(AAS-VPA)/PMMS composite hydrogel showed excellent capacitance performance (area capacitance 100.8 mF·cm−2 and energy density 8.96 μWh·cm−2) at ambient temperature and even −30 °C (25.3 mF·cm−2 and 2.25 μWh·cm−2). The hydrogel has stable electrochemical stability (1000 cycles, Coulomb efficiency > 97%) and exhibits electrochemical properties similar to those in the normal state under different deformations. The excellent results demonstrate the great potential of the P(AAS-VPA)/PMMS composite hydrogel in the field of strain sensors and flexible supercapacitors.

Amorphous biomineral-reinforced hydrogels with dramatically enhanced toughness for strain sensing

Ionic conductive hydrogels are promising candidates for flexible wearable strain sensors and artificial skin. However, achieving high mechanical and sensing performance concurrently remains challenging. Herein, a novel biomineral-reinforced hydrogel composed of polyacrylamide (PAM) and highly stable amorphous calcium carbonate (ACC) is reported. Benefiting from the dual ionic doping strategy (Mg2+ and PO43−), ACC nanoparticles in hybrid hydrogels show a super stable amorphous nature. The resulting mineral hydrogel displays a high stretchability (>1150% strain), a dramatically enhanced fracture toughness (9.57±1.28 vs. 0.91±0.12 kJ m−2), and a desirable linear strain sensitivity. Moreover, the novel mineral hydrogel exhibits high biocompatibility and flame retardance, making it an appealing candidate for wearable device applications.