Ionic Conductive Hydrogel Research Articles

High-performance ionic conductive double-network hydrogel enabling a long-term flexible strain sensor

Hydrogel sensors have attracted much attention for their promising applications in artificial skin and personal health monitoring. However, developing conductive hydrogel sensors with good self-healing, mechanical strength, fatigue resistance, self-adhesion, high conductivity and sensitivity remains a significant challenge. Herein, a multifunctional ionic conductive double-network (DN) hydrogel was prepared by mixing natural biomolecule, chitosan (CS) and hyaluronic acid (HA) in water. The synergistic physical interactions endowed the hydrogel with rapid self-healing capability and good mechanical flexibility. Benefiting from the abundant hydroxyl groups of CS and HA, the hydrogel exhibited robust adhesion capability to diverse materials surfaces, the maximum peel strength for poly-(tetrafluoroethylene) PTFE was 49.4 kPa and the highest adhesion strength for glass was 38.7 kPa. Moreover, the hydrogel showed high sensing sensitivity [gauge factor (GF) = 4.42], wide strain range (800%), and long-term durability (>300 cycles) to tensile strain, which can be constructed to strain sensor for monitoring large deformation movements and small deformations of human body. It is anticipated that the developed hydrogels provide useful insight and guideline into design and development of high-performance wearable strain sensor and broaden the application field in electronic skin, human-machine interfaces, and personalized health assessment.

Highly conductive and anti-freezing cellulose hydrogel for flexible sensors

Ionic conducting hydrogels (ICHs) are emerging materials for multi-functional sensors in the fields of healthcare monitoring and flexible electronics. However, there is a long-standing dilemma between ionic conductivity and mechanical properties of the ICHs. In this work, ionic conductive, flexible, transparent, and anti-freezing hydrogels are fabricated by dissolving cotton linter pulp in ZnCl2/CaCl2 solution and cross-linking with epichlorohydrin (ECH). The presence of inorganic salt imparts the hydrogel with high ionic conductivity and low-temperature tolerance. While the introduction of ECH as the second network gives the hydrogel with desirable mechanical performance. By tailoring the ECH addition, the tensile strength, compressive strength, elongation at break, and conductivity of the hydrogel could reach 0.82 MPa, 2.80 MPa, 260 %, and 5.48 S m−1, respectively. The prepared ICHs are fabricated into sensors for detecting full-range human body motions, and they demonstrate fast response and durable sensitivity to both tensile strain and compressive deformation. Moreover, flexible sensors can work at subzero temperatures. This work provides a new idea for the preparation of cellulose-based hydrogels with good ionic conductivity and mechanical properties under extreme conditions.

Reconfigurable Touch Panel Based on a Conductive Thixotropic Supramolecular Hydrogel.

Touch panels based on ionic conductive hydrogels perform excellent flexibility and biocompatibility, becoming promising candidates for the next-generation human-machine interface. However, these ionic hydrogels are usually composed of cross-linked polymeric networks that are difficult to be recycled or reconfigured, resulting in environmental issues. Herein, we designed a lithium ion-triggered gelation strategy to provide a conductive molecular hydrogel with thixotropy, which can be mechanically recycled or reconfigured at room temperature. In this hydrogel, lithium ions function as ionic bridges to construct supramolecular nanoassemblies and charge carriers to impart ionic conductivity. With polymer additives, the mechanical accommodability of the hydrogel was improved to meet the requirements of the daily use of touch panels. When this molecular hydrogel was fabricated into a surface capacitive touch panel, real-time sensing and reliable touch locating abilities were achieved. Remarkably, this touch panel can be reconfigured into 1D, 2D, and 3D device structures by a simple stirring-remolding method under ambient conditions. This work brings new insight into enriching the functionalities of hydrogel-based ionotronics with a supramolecular approach.

Stretchable, transparent, self-adhesive, anti-freezing and ionic conductive nanocomposite hydrogels for flexible strain sensors

Conductive hydrogels have attracted considerable attentions due to their great potential in the field of flexible strain sensors. However, the low stretchability, poor adhesiveness as well as the lack of freezing-resistant capacity of conventional conductive hydrogels greatly limited their practical applications. Herein, a stretchable, transparent, self-adhesive and anti-freezing conductive nanocomposite hydrogel (PAHS gel) was fabricated via a one-step in situ free-radical polymerization of sulfobetaine methacrylate (SBMA), acrylic acid (AA) and 2-hydroxyethyl methacrylate (HEMA) in the presence of alumina nanoparticles (Al2O3 NPs) and lithium chloride (LiCl) as the inorganic cross-linkers and conductive substance, respectively. The obtained PAHS gels displayed high transparency (higher than 85 % at 550 nm), excellent stretchability (up to 800 %) and good ionic conductivity (2.25 S/m) and could keep flexible and conductive at the temperature at −18 ℃. Furthermore, the various types of functional groups on the polymer chains endow the PAHS gels with strong self-adhesiveness to different substrates such as glass, rubber, skin, etc. In addition, the PAHS gels also revealed superior strain sensitivity (GF = 2.69) in the strain of 0 ∼ 100 %, which can be assembled into wearable strain sensors to monitor various human activity. Based on these combined merits, it is believed that this newly developed conductive nanocomposite hydrogel would have prospective applications in the field of wearable strain sensors and other flexible electronics.

Construction and characterization of highly stretchable ionic conductive hydrogels for flexible sensors with good anti-freezing performance

As an ideal material for flexible sensors, electrically conductive hydrogels (ECHs) has been faced with water loss and easy freezing, which affects the conductive and tensile properties. Polyacrylamide/sodium alginate/LiCl (PAM/SA/LiCl) hydrogel was constructed to enhance the anti-freezing property without deficiencies of tensile and electrical performance. A semi-interpenetrating network (semi-IPN) was formed by PAM and SA through hydrogen bonding, which performed excellent mechanical property (fracture strain 2100%, fracture stress 110 KPa). LiCl contributed to the high ionic conductivity (up to 21.7 S/m) and sensitivity (Gauge Factor (GF) = 17.45). Double hydrogen bonding of SA and PAM, hydration of LiCl and ion interaction between SA and lithium ion improved the anti-freezing performance and mechanical property of the hydrogel together. The proportion of non-freezing water in system was proved to be significantly increased. The comprehensive properties of the hydrogels were evaluated and PAM/SA/3M LiCl hydrogel kept conductive without obvious loss of stretchability at −30 °C. The PAM/SA/LiCl hydrogel integrates remarkable toughness, conductivity, sensitivity and anti-freezing property, which is a suitable choice in smart wearable devices, soft robots and medical monitoring.

Highly Stretchable and Biocompatible Wrinkled Nanoclay-Composite Hydrogel With Enhanced Sensing Capability for Precise Detection of Myocardial Infarction.

It is challenging to balance high biocompability with good mechanical-electrical sensing performance, especially when triggering inflammatory stress response after in vivo implantation. Herein, a bioinspired wrinkle-reinforced adaptive nanoclay-interlocked soft strain-sensor based on a highly stretchable and elastic ionic-conductive hydrogel is reported. This novel nanoclay-composite hydrogel exhibits excellent tensile properties and high sensing capacity with steady and reliable sensing performance due to the structural-mechanical-electrical integrity of the nanoclay crosslinked and nano-reinforced interpenetrating network. The incorporation of amphiphilic ions provides the hydrogel with significant protein resistance, reducing its non-specific adsorption to proteins upon implantation, improving its biosafety as an implanted device, and maintaining the authenticity of the sensing results. Based on the revealed sensing enhanced mechanism based on hierarchical ordered structures as a proof-of-concept application, this hydrogel sensor is demonstrated to be able toaccurately localize the region where myocardial infarction occurs and may become a novel strategy for real-time monitoring of pathological changes in heart disease.

Ultrahigh conductivity and antifreezing zwitterionic sulfobetaine hydrogel electrolyte for low-temperature resistance flexible supercapacitors

Recently, ionic conductive hydrogels have been developed in the field of energy-storage devices owing to their unique properties, such as high ionic conductivity, mechanical flexibility, liquid-leakage-free operation, and stability.

Conductive hydrogels for tissue repair.

Conductive hydrogels (CHs) combine the biomimetic properties of hydrogels with the physiological and electrochemical properties of conductive materials, and have attracted extensive attention in the past few years. In addition, CHs have high conductivity and electrochemical redox properties and can be used to detect electrical signals generated in biological systems and conduct electrical stimulation to regulate the activities and functions of cells including cell migration, cell proliferation, and cell differentiation. These properties give CHs unique advantages in tissue repair. However, the current review of CHs is mostly focused on their applications as biosensors. Therefore, this article reviewed the new progress of CHs in tissue repair including nerve tissue regeneration, muscle tissue regeneration, skin tissue regeneration and bone tissue regeneration in the past five years. We first introduced the design and synthesis of different types of CHs such as carbon-based CHs, conductive polymer-based CHs, metal-based CHs, ionic CHs, and composite CHs, and the types and mechanisms of tissue repair promoted by CHs including anti-bacterial, antioxidant and anti-inflammatory properties, stimulus response and intelligent delivery, real-time monitoring, and promoted cell proliferation and tissue repair related pathway activation, which provides a useful reference for further preparation of bio-safer and more efficient CHs used in tissue regeneration.

Utilizing cellulose-based conducting hydrogels in iontronics

This review highlights how the inherent properties of cellulose have been employed to improve the properties of ionic conductive hydrogels (IHCs) and their application in fabricating iontronics.

Nanostructured ionic hydrogel with integrated conductivity, stretchability and thermal responsiveness for a high-performance strain and temperature sensor.

Ionic conductive hydrogels are promising candidates for fabricating wearable sensors for human motion detection and disease diagnosis, and electronic skin. However, most of the existing ionic conductive hydrogel-based sensors primarily respond to a single-strain stimulus. Only a few ionic conductive hydrogels can respond to multiple physiological signals. Although some studies have explored multi-stimulus sensors, such as those detecting strain and temperature, the ability to identify the type of stimulus remains a challenge, which limits their applications. Herein, a multi-responsive nanostructured ionic conductive hydrogel was successfully developed by crosslinking the thermally sensitive poly(N-isopropylacrylamide-co-ionic liquid) conductive nanogel (PNI NG) with a poly(sulfobetaine methacrylate-co-ionic liquid) (PSI) network. The resultant hydrogel (PNI NG@PSI) was endowed with good mechanical stretchability (300%), resilience and fatigue resistance, and excellent conductivity (2.4 S m-1). Furthermore, the hydrogel exhibited a sensitive and stable electrical signal response and has a potential application in human motion detection. Moreover, the introduction of a nanostructured thermally responsive PNIPAAm network also endowed it with a sensitive and unique thermal-sensing ability to timely and accurately record temperature changes in the range of 30-45 °C, holding promise for application as a wearable temperature sensor to detect fever or inflammation in the human body. In particular, as a dual strain-temperature sensor, the hydrogel demonstrated an excellent capability of distinguishing the type of stimulus from superposed strain-temperature stimuli via electrical signals. Therefore, the implementation of the proposed hydrogel in wearable multi-signal sensors provides a new strategy for different applications, such as health monitoring and human-machine interactions.

One-Pot Freezing-Thawing Preparation of Hydroxyethyl Cellulose Reinforced Polyvinyl Alcohol Based Ionic Hydrogel

Hydroxyethyl cellulose, polyvinyl alcohol (PVA) and dimethyl sulfoxide (DMSO) were used as raw materials. Ion-conducting hydrogels with different hydroxyethyl cellulose contents were prepared by blending and freezing-thawing methods. Hydrogels were characterized by Fourier transform infrared spectrometer, universal material testing machine, LCR digital bridge and scanning electron microscope (SEM). The results show that when the content of hydroxyethyl cellulose is 1.5 wt%, the mechanical properties of the hydrogel are improved, the elongation at break reaches 340%, and the tensile strength reaches 239 KPa. The relative resistance of the hydrogel will grows with the increase of tensile strain. The ionic conductive hydrogel was assembled into a strain sensor to monitor the angle change of finger bending. The hydrogel can also be assembled to form a self-powered battery, and the voltage of the self-powered battery can reach 0.839 V. This indicates that the hydrogel has potential applications in flexible sensors.

Stretchable Surface Electrode Arrays Using an Alginate/PEDOT:PSS-Based Conductive Hydrogel for Conformal Brain Interfacing.

An electrocorticogram (ECoG) is the electrical activity obtainable from the cerebral cortex and an informative source with considerable potential for future advanced applications in various brain-interfacing technologies. Considerable effort has been devoted to developing biocompatible, conformal, soft, and conductive interfacial materials for bridging devices and brain tissue; however, the implementation of brain-adaptive materials with optimized electrical and mechanical characteristics remains challenging. Herein, we present surface electrode arrays using the soft tough ionic conductive hydrogel (STICH). The newly proposed STICH features brain-adaptive softness with Young's modulus of ~9.46 kPa, which is sufficient to form a conformal interface with the cortex. Additionally, the STICH has high toughness of ~36.85 kJ/mm3, highlighting its robustness for maintaining the solid structure during interfacing with wet brain tissue. The stretchable metal electrodes with a wavy pattern printed on the elastomer were coated with the STICH as an interfacial layer, resulting in an improvement of the impedance from 60 kΩ to 10 kΩ at 1 kHz after coating. Acute in vivo experiments for ECoG monitoring were performed in anesthetized rodents, thereby successfully realizing conformal interfacing to the animal's cortex and the sensitive recording of electrical activity using the STICH-coated electrodes, which exhibited a higher visual-evoked potential (VEP) amplitude than that of the control device.

Dual thermo-responsive multifunctional ionic conductive hydrogel by salt modulation strategy for multilevel encryption and visual monitoring

Despite the current progress in thermo-responsive materials, it is still a significant challenge to develop superior multiple temperature-sensitive ionic conductive hydrogels with programmable regulation capability for interactive wearable iontronics. Here, a dual thermo-responsive ionic conductive hydrogel (DTICH) is constructed based on a salt modulation strategy, which cleverly integrates low-temperature sensitive sodium dodecyl sulfate (SDS) and high-temperature sensitive poly(N-isopropylacrylamide) PNIPAm-based polymer. Owing to the common ion effect and salting-out effect, the resultant DTICH hydrogel visually presents wide and tunable dual temperature response behaviors of 0-18℃ (Tk) and 37-80℃ (LCST) by controlling NaCl concentration for the first time. Meanwhile, benefiting from the synergy of NaCl and thermo-sensitive materials, the hydrogel is concurrently endowed with excellent stretchability (>900 %), adhesion (adhesion strength >30 kPa), frost resisting (-40℃) and multilevel encryption functions. More importantly, the DTICH hydrogel can ingeniously perceive mechanical force and external temperature via the changes of relative resistance and transparency, which can be applied as human–computer interfaces to interactively monitor human motions and temperatures in real time. Therefore, our study establishes a brand new and effective route for the rational design and fabrication of high-level visual interactive devices and smart wearable iontronics.

High Multi-Environmental Mechanical Stability and Adhesive Transparent Ionic Conductive Hydrogels Used as Smart Wearable Devices

Ionic conductive hydrogels used as flexible wearable sensor devices have attracted considerable attention because of their easy preparation, biocompatibility, and macro/micro mechanosensitive properties. However, developing an integrated conductive hydrogel that combines high mechanical stability, strong adhesion, and excellent mechanosensitive properties to meet practical requirements remains a great challenge owing to the incompatibility of properties. Herein, we prepare a multifunctional ionic conductive hydrogel by introducing high-modulus bacterial cellulose (BC) to form the skeleton of double networks, which exhibit great mechanical properties in both tensile (83.4 kPa, 1235.9% strain) and compressive (207.2 kPa, 79.9% strain) stress-strain tests. Besides, the fabricated hydrogels containing high-concentration Ca2+ show excellent anti-freezing (high ionic conductivities of 1.92 and 0.36 S/m at room temperature and -35 ∘C, respectively) properties. Furthermore, the sensing mechanism based on the conductive units and applied voltage are investigated to the benefit of the practical applications of prepared hydrogels. Therefore, the designed and fabricated hydrogels provide a novel strategy and can serve as candidates in the fields of sensors, ionic skins, and soft robots.

Preparation and Properties of Cellulose Nanofiber-Reinforced Ionic Conductive Hydrogels Sensor

Preparation and Properties of Cellulose Nanofiber-Reinforced Ionic Conductive Hydrogels Sensor

Multiply cross-linked poly(vinyl alcohol)/cellulose nanofiber composite ionic conductive hydrogels for strain sensors

Building multiple chemical crosslinks is an effective strategy to improve mechanical properties and to diversify final application of polysaccharide nanoparticles reinforced poly(vinyl alcohol) (PVA) physical hydrogels. In this work, PVA/cellulose nanofibers (CNFs) were used as composite substrate to fabricate ionic conductive hydrogels for strain sensor. Three types of characteristic crosslinks, including chemical crosslinking via boronic ester covalent bonds only, and with additional metal coordination bonding, as well as coexistence of physical crosslinks via PVA crystallites and aforementioned two kinds of chemical crosslinks, were constructed. The sample with triple crosslinks has superior mechanical strength and resistance to fatigue, and the polydopamine/Fe3+ ratio act as key to tune final performance because double-network structure prefers to form as Fe3+ is superfluous, while dual-crosslink one forms in the case of insufficient Fe3+. As-optimized ionic conductive hydrogel is suitable as strain sensor for probing human motions. This work provides an interesting insight into the network structure and property regulation for PVA/CNF composite hydrogels with multiple crosslinks.

Highly stretchable, self-healing, and degradable ionic conductive cellulose hydrogel for human motion monitoring

Self-healing biomass-based conductive hydrogels are applied as flexible strain sensors for wearable devices and human movement monitoring. Cellulose is the most abundant biomass-based materials and exhibits excellent toughness, dispersion and degradability. In this paper, nanocellulose crystals (NCCs) prepared from sisal, used as reinforcing fillers were coated with tannic acid (TA) to prepare inexpensive bio-nanocomposite hydrogels that also included polyvinyl alcohol, okra polysaccharide (OP), and borax. These hydrogels exhibit excellent self-healing and mechanical properties with the maximum elongation, toughness, and self-healing efficiency (9 min) of 1426.2 %, 264.4 kJ/m3, and 62.1 %, respectively. A fabricated hydrogel strain sensor was successfully used to detect and monitor various human movements such as wrist bending, elbow bending, and slight changes in facial expression. In addition, this sensor possessed excellent durability and good working stability after repeated circulation. The nanocomposite hydrogel synthesized in this work utilized natural polysaccharide to manufacture flexible functional materials with good application prospects in the field of flexible sensors.

Gelatin/polyacrylamide ionic conductive hydrogel with skin temperature-triggered adhesion for human motion sensing and body heat harvesting

Flexible wearable devices based on hydrogels have attracted more and more attention, however, the preparation of hydrogels with high toughness, strong adhesion, on-demand debonding is still a challenge. Here, we report a gelatin/polyacrylamide double network ionic conductive hydrogel with skin temperature-triggered adhesion and low temperature-triggered detachment. The hydrogel has excellent stretchability, compressibility and conductivity. The strain sensor assembled by the hydrogel can realize the sensing of human movement, swallowing water and other behaviors. The thermoelectric generator assembled by the hydrogel has a Seebeck coefficient of 10.4 mV/K and output power of 2.26 μW/cm2 at 10 K temperature difference, showing the application potential in low-grade body heat harvesting. This work provides an intelligent hydrogel with skin temperature-triggered adhesion for flexible wearable devices such as human motion sensors and portable power supplies.

Blue and white light modulation of a flexible electroluminescent device based on phosphors.

Flexibility, certain mechanical strength, and color modulation are significant elements for flexible optoelectronic devices. However, it is laborious to fabricate a flexible electroluminescent device with balanceable flexibility and color modulation. Here, we mix a conductive nonopaque hydrogel and phosphors to fabricate a flexible alternating current electroluminescence (ACEL) device with color modulation ability. This device realizes flexible strain based on polydimethylsiloxane and carboxymethyl cellulose/polyvinyl alcohol ionic conductive hydrogel. The color modulation ability is achieved by varying the voltage frequency applied on the electroluminescent phosphors. The color modulation could realize blue and white light modulation. Our electroluminescent device exhibits great potential in artificial flexible optoelectronics.

Hydrogel electrolyte based on sodium polyacrylate/KOH hydrogel reinforced with bacterial cellulose aerogel for flexible supercapacitors

Hydrogels are particularly promising electrolyte materials for constructing flexible and safe quasi-solid state supercapacitor due to their good flexibility, ionic conductivity and intrinsic safety. However, it remains a challenge to simultaneously obtain excellent mechanical robustness, high ionic conductivity and outstanding flexibility, particularly in sub-zero temperatures. Hence, we introduced a novel bacterial cellulose (BC) aerogel reinforced sodium polyacrylate/potassium hydroxide (PANa/KOH) hydrogel as a hydrogel electrolyte for flexible alkaline supercapacitor. The PANa/BC/KOH hydrogel electrolyte showed outstanding mechanical property (stress of 1.1 MPa), excellent anti-swelling performance and high ionic conductivity of 1.91 S m−1 at 25 °C, 1.35 S m−1 at −25 °C, exhibiting a superior anti-freezing performance. Benefiting from these intriguing integrated characteristics, the asymmetric supercapacitor (SC-PANa/BC/KOH) assembled PANa/BC/KOH hydrogel electrolyte and CoNi LDH@CC//AC@CC electrodes delivered a superior energy density of 25.63 W h kg−1 at a power density of 1200 W kg−1 and maintained a good retention of 92.05 % after 10,000 cycles. Meanwhile, the SC-PANa/BC/KOH exhibited excellent flexibility and safety, which is well adapted to various extreme conditions, such as weight-loading, bending and hammering. This work provided new sights into design of strong, anti-freezing, anti-swelling and high ionic conductive hydrogel electrolyte, paving the way for developing extremely safe wearable storage devices.