Conductive Hydrogel- and Organohydrogel-Based Stretchable Sensors

Lignosulfonate-enhanced dispersion and compatibility of liquid metal nanodroplets in PVA hydrogel for improved self-recovery and fatigue resistance in wearable sensors

Stretchable and resilient conductive hydrogels, incorporating flowable liquid metals (LM) into polyvinyl alcohol (PVA), have emerged as promising materials for wearable sensors due to their exceptional mechanical properties and sustainability. However, the fluidity and compatibility of LM with the hydrogel matrix limit the construction and performance of LM/PVA conductive hydrogels. This study aimed to develop a flexible, high-performance hydrogel for advanced wearable sensors by introducing LM nanoparticles encapsulated in sodium lignosulfonate (LS-LM) into the PVA matrix. The renewable natural macromolecule LS, rich in functional groups, enhanced the compatibility between LM and the PVA matrix. Moreover, LS formed a stable shell around the LM droplets, preventing rupture and leakage of LM, ensuring uniform dispersion within the hydrogel and significantly improving its durability by preventing phase separation. The optimized conductive lignosulfonate-liquid metal/polyvinyl alcohol hydrogel (LS-LM/PVA) exhibited a tensile stress of 1.60 MPa, a compressive strength of 0.53 MPa under 70 % strain, and electrical conductivity (4.87 S m-1). The hydrogel-based sensor demonstrated excellent sensitivity (GF = 2.40) and outstanding fatigue resistance (over 500 cycles). A Life Cycle Assessment (LCA) was conducted to evaluate the environmental impacts of LS-LM/PVA hydrogel production. The composite hydrogel-based sensor shows significant promise for advancing human motion tracking and information recognition.

Skin-like Heterogeneous and Self-Healing Conductive Hydrogel toward Ultrasensitive Marine Sensing

Flexible wearable electronic devices based on hydrophobic, conductive hydrogels have attracted widespread attention in the field of underwater sensing. However, traditional homogeneous hydrogels tend to compromise their conductivity and sensing performance when achieving hydrophobicity, and the high complexity of marine environments further reduces their sensing performance and service life. Here, we develop a seawater-resistant conductive hydrogel with ultrahigh sensitivity and self-healing ability by the introduction of a skin-like heterogeneous structure, consisting of a hydrophobic outer layer that protects against seawater and a conductive internal layer that senses. Based on a heterogeneous structure obtained through surface hydrophobic modification of confined nitrogen-alkylation reaction, the conductive hydrogel simultaneously achieves satisfying seawater resistance (contact angle of 123.2°), high ionic conductivity (2.86 S m-1), and excellent sensing sensitivity in seawater (GF = 6.15), harmonizing the contradiction between water resistance and sensing of traditional hydrophobic hydrogels. In addition, abundant hydrogen-bonding and dipole-dipole interactions endow the heterogeneous hydrogel with an outstanding self-healing ability, exhibiting high-efficiency self-healing behavior in seawater. Underwater strain sensors constructed with the heterogeneous hydrogel can be used for detecting human motion in simulated seawater environments and real-time signal transmission, showcasing their great potential as wearable electronic devices in the marine sensing field.

Development of a Nano-toughened multifunctional composite hydrogel based on chitosan and its applications in catalytic and flexible sensors

In this study, we developed a novel composite catalytic hydrogel, which integrates excellent mechanical properties, catalytic activity, and sensing performance. Discarded hydrogel sensors are reused as templates for in-situ generation of metal nanoparticles, and multifunctional hydrogels combining sensing and catalysis are realized. Polyacrylamide (PAM) provides a three-dimensional network structure, while octadecyl methacrylate (SMA) acts as a hydrophobic association center, enhancing the structural stability of the hydrogel. Dopamine coated with silica (PDA@SiO₂) nanoparticles act as nanoreinforcement points, further improving the mechanical strength of the hydrogel. Graphene(GN) imparts the hydrogel with good electrical conductivity and sensing capabilities. The hydrogel exhibits a strain of 1878 %, a tensile strength of 668 kPa, and toughness of 5615.2 kJ/m3, while also demonstrating excellent sensing performance, with gauge factor (GF) of 7.49 and response time of 168 ms, enabling a quick response to external strain. It effectively detects human motions, such as finger bending and joint movement. Additionally, PDA@SiO₂ acts as an active site for the synthesis of Ag NPs, facilitating the reduction of Rhodamine B at 25 °C with a catalytic rate constant of 0.504 min-1. After five catalytic cycles, the hydrogel retains over 99 % efficiency, demonstrating excellent cyclic stability and recyclability.

Rapid Mental Stress Evaluation Based on Non-Invasive, Wearable Cortisol Detection with the Self-Assembly of Nanomagnetic Beads

The rapid and timely evaluation of the mental health of emergency rescuers can effectively improve the quality of emergency rescues. However, biosensors for mental health evaluation are now facing challenges, such as the rapid and portable detection of multiple mental biomarkers. In this study, a non-invasive, flexible, wearable electrochemical biosensor was constructed based on the self-assembly of nanomagnetic beads for the rapid detection of cortisol in interstitial fluid (ISF) to assess the mental stress of emergency rescuers. Based on a one-step reduction, gold nanoparticles (AuNPs) were functionally modified on a screen-printed electrode to improve the detection of electrochemical properties. Afterwards, nanocomposites of MXene and multi-wall carbon nanotubes were coated onto the AuNPs layer through a physical deposition to enhance the electron transfer rate. The carboxylated nanomagnetic beads immobilized with a cortisol antibody were treated as sensing elements for the specific recognition of the mental stress marker, cortisol. With the rapid attraction of magnets to nanomagnetic beads, the sensing element can be rapidly replaced on the electrode uniformly, which can lead to extreme improvements in detection efficiency. The detected linear response to cortisol was 0-32 ng/mL. With the integrated reverse iontophoresis technique on a flexible printed circuit board, the ISF can be extracted non-invasively for wearable cortisol detection. The stimulating current was set to be under 1 mA for the extraction, which was within the safe and acceptable range for human bodies. Therefore, based on the positive correlation between cortisol concentration and mental stress, the mental stress of emergency rescuers can be evaluated, which will provide feedback on the psychological statuses of rescuers and effectively improve rescuer safety and rescue efficiency.

A Spiropyran-Based Hydrogel Composite for Wearable Detectors to Monitor Visible Light Intensity to Prevent Myopia

A wearable detector to monitor visible light intensity is realized by the restrained photochromism of a hydrogel composite containing light-responsive spiropyran with hydroxyl groups (SPOH). When exposed to visible light, the SPOH experiences a ring-opening to a ring-closed transition accompanied by discoloration from red to yellow. Unlike in the solution, the photochromism/discoloration rate is strongly correlated to the cross-linking points. By reducing the amount of cross-linker from 40 to 5 mg, the photochromism rate of SPOH is 300% faster. Inspired by the Chinese Jade Loong from Hongshan, the hydrogel composite is shaped into a Loong to monitor the light intensity. By increasing the amount of cross-linker in the head, body, and tail, the photochromism/discoloration rate sequentially turns slower from one region to the other. Higher light intensity is required to realize the discoloration in the hydrogel composite containing a larger amount of the cross-linker. Because the initial colors are identical, the light intensity can be easily traced by checking the discoloration of these pieces containing different amounts of cross-linker. Based on this unique and reversible photochromic capability, the present hydrogel composite can be used for monitoring the visible light intensity to prevent myopia, especially for children and students.

Rubber-like Deep Eutectic Solvent-Assisted Poly(N-acryloylglycinamide) Hydrogel for Highly Sensitive Pressure Detecting

Deep eutectic solvent (DES)-based conductive hydrogels have attracted great interest in the building of flexible electronic devices that can be used to replace conventional temperature-intolerant hydrogels and expensive ionic liquid gels. However, current DES-based conductive hydrogels obtained have limited mechanical strength, high hysteresis, and poor microdeformation sensitivity of the assembled sensors. In this work, a rubber-like conductive hydrogel based on N-acryloylglycinamide (NAGA) and DES (acetylcholine chloride/acrylamide) has been synthesized by a one-step method. The prepared conductive PNAGA-DES hydrogel has exhibited excellent mechanical strength, stability, and resilience during the long-term loading-unloading cycles, endowed with service durability. Besides, the as-prepared PNAGA-DES also possesses high transparency, high conductivity, and favorable antienvironmental disturbance, which can enhance the designability and robustness of the PNAGA-DES-based devices. Based on the remarkable properties, the PNAGA-DES hydrogel can be used for wearable pressure-strain sensors with high sensitivity of tiny strain for transferring information (gauge factor (GF) = 8.18, 0.2-2% strain) and long-term stability. Furthermore, it can also sensitively detect the large strain of human motion, showing potential application in information interaction and wearable electronics.

Cellulose nanofibers/liquid metal hydrogels with high tensile strength, environmental adaptability and electromagnetic shielding for temperature monitoring and strain sensors

Hydrogel sensors are widely recognized in the fields of flexible electronics and human motion monitoring due to their multiple properties and potential applications. However, how to prepare hydrogels with multiple excellent properties simultaneously and how to improve the compatibility of conductive fillers with hydrogel matrices remain a major challenge. Therefore, in this work, liquid metal (LM) droplets stabilized by cellulose nanofibers (CNFs) were utilized to initiate the polymerization of acrylamide monomer (Am), which was used as a conductive filler. Meanwhile, reduced graphene oxide (rGO) was introduced to bridge the LM droplets. The hydrogels were then further crosslinked in glycerol. The constructed CNF@LM/polyacrylamide/rGO/gelatin/glycerol hydrogel possesses high tensile properties (>1317 %), high environmental adaptability (-80 to 80 °C), and adhesion properties for multifunctional sensing. What's more, it offers the high sensitivity of both a strain sensor and a temperature sensor for accurate monitoring of human movement at room temperature and even in extreme environments. In addition, this hydrogel has excellent electromagnetic shielding properties and antimicrobial properties. This research opens up a new direction for the preparation of multifunctional hydrogel sensors, expanding their applications in cutting-edge fields such as temperature monitoring, wearable smart devices, e-skin and intelligent robotics.

Ultrasensitive, Fast-Response, and Stretchable Temperature Microsensor Based on a Stable Encapsulated Organohydrogel Film for Wearable Applications

Ionic conductive hydrogel-based temperature sensors have emerged as promising candidates due to their good stretchability and biocompatibility. However, the unsatisfactory sensitivity, sluggish response/recovery speed, and poor environmental stability limit their applications for accurate long-term health monitoring and robot perception, especially in extreme environments. To address these concerns, here, the stretchable temperature sensors based on a double-side elastomer-encapsulated thin-film organohydrogel (DETO) architecture are proposed with impressive performance. It is found that the water-polyol binary solvent, organohydrogel film, and sandwiched device structure play important roles in the temperature sensing performance. By modifying the composition of binary solvent and thicknesses of organohydrogel and elastomer films, the DETO microsensors realize a thickness of only 380 μm, unprecedented temperature sensitivity (37.96%/°C), fast response time (6.01 s) and recovery time (10.53 s), wide detection range (25-95.7 °C), and good stretchability (40% strain), which are superior to those of conventional hydrogel-based sensors. Furthermore, the device displays good environmental stability with negligible dehydration and prolonged operation duration. With these attributes, the wearable sensor is exploited for the real-time monitoring of various physiological signals such as human skin temperature and respiration patterns as well as temperature perception for robots.

Highly conductive and sensitive alginate hydrogel strain sensors fabricated using near-field electrohydrodynamic direct-writing process

Hydrogel flexible sensors have attracted considerable attention because of their wearability, biocompatibility, and precision signal transmission capability. However, the hydrogel strain sensors fabricated by conventional printing or hand-injection methods have difficulty balancing their mechanical strength and sensing characteristics, limiting the application of hydrogel strain sensors. Herein, polyvinyl alcohol and polyacrylamide were loosely crosslinked with sodium alginate through chemical cross-linking. Subsequently, MXene nanosheets were introduced for doping, the crosslinked hydrogel conductive network was constructed, and the hydrogel strain sensors were fabricated using the electrohydrodynamic (EHD) printing method. The ions in the EHD-printed hydrogel undergo directional movement under an externally enhanced electric field, causing the formation of more uniform and dense porous conductive networks inside the hydrogel, and high electrical conductivity (0.49 S m-1) is obtained. These hydrogel strain sensors have excellent mechanical properties (tensile strength: 0.17 MPa at 787 % strain), high sensitivity (gauge factor: 1.54 at 0-100 % strain), and low detection limits (1 % strain). Furthermore, demonstrations of real-time Morse code tapping information transmission, handwriting recognition during writing, and human physiological behavior monitoring demonstrations using the fabricated sensors indicate that the EHD-printed hydrogel strain sensor method has significant potential for wearable devices and human-computer interaction applications.

Hydrogel-based bimodal sensors for high-sensitivity independent detection of temperature and strain

Avoiding crosstalk between strain and temperature detection is crucial for bimodal hydrogel sensors, yet achieving high sensitivity for both parameters while maintaining signal decoupling remains a significant challenge. In this study, a bimodal sensor was developed by locally coating poly (3,4-ethylene dioxythiophene): polystyrene sulfonate (PEDOT: PSS) onto the hydrogel surface, creating distinct regions for strain and temperature detection. These regions form localized strain concentration zones and wrinkle structures, respectively. The localized strain concentration enhances the sensor's sensitivity from 8.5 to 18.5. Additionally, the sensor demonstrates a low detection limit (0.2 %), a wide detection range (up to 1356 %), a fast response time, and excellent cyclic stability for strain measurements. The temperature detection region, leveraging the thermoelectric effect, improves the Seebeck coefficient of the PEDOT: PSS coating from 20 to 122.86 μVK-1 through de-doping and energy band modulation. Moreover, the temperature sensing of the PEDOT: PSS coating features good cyclic stability, a rapid response time, and versatile testing capabilities. This innovative structural design effectively decouples strain and temperature signals across a broad strain range (0-600 %). These sensors hold potential applications in human health monitoring and as electronic skin for flexible robotics.

Fishing net-inspired PVA-chitosan-CNT hydrogels with high stretchability, sensitivity, and environmentally stability for textile strain sensors

Soft electronic products are being extensively investigated in diverse applications including sensors and devices, due to their superior softness, responsiveness, and biocompatibility. One-dimensional (1-D) fiber electronic devices are recognized for their lightweight, wearable, and stretchable qualities, thus emerging as critical constituents for seamless integration with the human body and attire, exhibiting great potential in wearable applications. However, wearable conductive hydrogel fibers usually face challenges in combining stretchability and excellent stability, notably in high-temperature environment. Herein, a novel stretchable conductive hydrogel fiber, namely PVA-CS-CNT (Polyvinyl Alcohol-Chitosan-Carbon Nanotube) hydrogel fiber, was successfully prepared through a straightforward low-temperature process. This hydrogel fiber not only maintains stable signal transmission at high temperatures but also exhibits significant mechanical and sensing capabilities, ensuring signal stability during repetitive cyclic stretching. Inspired by fishing net, textile sensors were fabricated by weaving PVA-CS-CNT hydrogel fibers, which offered breathability, high stability (withstanding over 500 stretch cycles), high sensitivity (detecting strains as low as 1 %), and exceptional mechanical strength (exceeding 17 MPa). The wearable sensor could not only accurately monitor human movements like stretching and bending, but also adeptly captured delicate signals such as pulses and sounds. These characteristics demonstrated the potential applications of the hydrogel fibers encompassing human motion tracking, intelligent textiles, and soft robotics.

Fabrication of anti-freezing and self-healing hydrogel sensors based on carboxymethyl guar gum and poly(ionic liquid)

As classical soft materials, conductive hydrogels have attracted wide attention in the field of strain sensors due to their unique flexibility and conductivity. However, there are still challenges in developing conductive hydrogels with comprehensive mechanical strength, self-healing ability and sensitive sensing properties. In this paper, a novel PAV/CMGG hydrogel was prepared by a simple one-pot method through the introduction of 1-vinyl-3-butylimidazolium bromide (VBIMBr), acrylic acid (AA), carboxymethyl guar gum (CMGG) and AlCl3. The coordination bond between Al3+ and -COO- groups on PAA and CMGG, the hydrogen bond between PAA and CMGG, and the electrostatic interaction between [VBIM]+ and -COO- endow the hydrogel with good mechanical properties, self-recovery ability, fatigue resistance and great self-healing properties. PAV/CMGG hydrogel had good conductivity of 2.31 S/m which could successfully light up the bulb. The hydrogel as the strain sensor had not only a wide strain sensing capability (strain ranging from 0 to 800 %), but also a high strain sensitivity (gauge factor (GF) = 28.50 for the strain ranging from 600 to 800 %). This study can provide inspiration for the construction of new high-performance flexible sensors.

Accordion-Structured Hydrogel Battery Capable of Maintaining Ion Gradients for Extended Periods

Inspired by the electric eel, biomimetic, biocompatible energy storage, and power generation technologies show promise for applications in portable and wearable electronic devices by mimicking the electric cell tandem structure of the electric eel and utilizing ionic gradients between hydrogel compartments to generate electricity. Previously, inspired by the unique morphology of the torpedo fish, an artificial flexible power source that can output a large current was introduced. This power source uses a hydrogel-infused paper hybrid to create, accordionize, and reconfigure arbitrary-sized gel films in series and parallel, and the power output of the flexible battery was significantly enhanced. However, maintaining the ionic gradient of hydrogel batteries during storage remains a challenge. Here, by borrowing the isolation properties of the accordion structure, we propose a unique paper accordion structure design to fabricate an Accordion-Structured Hydrogel Battery (ASHB). Pretreatment of hydrogel-injected paper strips improved storage stability and maintained the ionic gradient of hydrogel cells in the nonworking state, so that the cell's gradient retention time after the assembly is completed is increased by at least 30 h compared to stacking, and its per-cell operating voltage is still able to reach. The design also makes the assembly and use of flexible batteries more modular and holistic. In the future, it may be possible to power the cells with ions generated by the human body or the metabolites of living organisms, leading to the development of more efficient, sustainable, and eco-friendly power solutions.

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.

Silk Fibroin-Based Multiple-Shape-Memory Organohydrogels

Organohydrogels (OHGs) are intriguing materials due to their unique composition of both hydrophilic and hydrophobic domains. This antagonistic nature endows the OHGs with several remarkable properties, making them highly versatile for various applications. We present here a simple and inexpensive approach to fabricate silk fibroin (SF)-based OHGs with multistage switching mechanics and viscoelasticity. The continuous hydrophilic phase of the OHG precursor consists of an aqueous SF solution, while the hydrophobic droplet phase consists of a crystallizable n-octadecyl acrylate (C18A) monomer and several long-chain saturated hydrocarbons (HCs) with various chain lengths between 14 and 32 carbon atoms, namely, n-tetradecane, n-octadecane, n-docosane, n-dotriacontane, and 1-docosanol. After the addition of a C18A/HC mixture containing Irgacure photoinitiator into the continuous aqueous SF phase under stirring, a stable oil-in-water emulsion was obtained, which was then photopolymerized at 23 ± 2 °C to obtain nonswelling OHGs with multiple-shape-memory behavior. By changing the chain length and mass proportion of HCs, a series of OHGs with tunable transition temperatures could be obtained, meeting various applications. OHGs containing dimer, trimer, and quadruple combinations of in situ-formed poly(C18A) and HC microinclusions exhibit effective triple- or quintuple-shape memory whose shape-recovery temperatures could be adjusted over a wide range, e.g., between 7 and 70 °C.

Effects of Enteromorpha prolifera sulfated polysaccharide and aluminium ion addition on the multifunctional property of conductive hydrogel for wearable strain sensing

Although introducing Enteromorpha prolifera sulfated polysaccharide (SPEP) enhances the mechanical properties of hydrogels significantly, little is known about the effects of polysaccharide and ion addition on morphological and physicochemical properties of conductive hydrogel. Therefore, the Poly (acrylic acid)/SPEPn/Al3+m (PAA/SPEPn/Al3+m) hydrogels with different SPEP and Al3+ addition were synthesized by simple one-pot method. The porosity, tensile strength, and swelling ration increased, while compressive strength, elongation at break, self-healing, self-adhesion properties increased first and then decreased as SPEP addition increased from 0 % to 3.80 %. The Al3+ addition increased from 0.08 % to 0.30 %, both tensile and compressive strength increased first and then decreased, while elongation at break kept increasing. Unexpectedly, both increasing SPEP and Al3+ addition reduced the electrical conductivity, while SPEP increased the gauge factor of hydrogel. The hydrogel exhibited optimal comprehensive properties when SPEP and Al3+ addition were 2.31 % and 0.24 %, respectively. The PAA/SPEP2.31%/Al3+0.24% hydrogel showed high tensile strength (107.60 kPa), elongation at break (2426.67 %), strain self-healing rate (81.87 %), adhesion strength (21.61 kPa), and conductivity (3.60 S/m). Overall, the properties of PAA/SPEPn/Al3+m hydrogels can be regulated through tailoring SPEP and Al3+ addition, which can be used as on-demand strategy to improve the performance of PAA/SPEPn/Al3+m hydrogels for each application.

A Review of Conductive Hydrogel-Based Wearable Temperature Sensors

Conductive hydrogel has garnered significant attention as an emergent candidate for diverse wearable sensors, owing to its remarkable and tailorable properties such as flexibility, biocompatibility, and strong electrical conductivity. These attributes make it highly suitable for various wearable sensor applications (e.g., biophysical, bioelectrical, and biochemical sensors) that can monitor human health conditions and provide timely interventions. Among these applications, conductive hydrogel-based wearable temperature sensors are especially important for healthcare and disease surveillance. This review aims to provide a comprehensive overview of conductive hydrogel-based wearable temperature sensors. First, this work summarizes different types of conductive fillers-based hydrogel, highlighting their recent developments and advantages as wearable temperature sensors. Next, this work discusses the sensing characteristics of conductive hydrogel-based wearable temperature sensors, focusing on sensitivity, dynamic stability, stretchability, and signal output. Then, state-of-the-art applications are introduced, ranging from body temperature detection and wound temperature detection to disease monitoring. Finally, this work identifies the remaining challenges and prospects facing this field. By addressing these challenges with potential solutions, this review hopes to shed some light on future research and innovations in this promising field.

Flexible high electrochemical active hydrogel for wearable sensors and supercapacitor electrolytes

With the rapid advancement of flexible, portable devices, hydrogel electrolytes have gained considerable attention as potential replacements for conventional liquid electrolytes. A hydrogel electrolyte was synthesised by cross-linking acrylic acid (AA), acrylamide (AM), carboxymethyl cellulose (CMC), and zinc sulphate (ZnSO4). The formation of hydrogen bonds and chelate interactions between the P(AA-co-AM) polymer, CMC, and ZnSO4 created a robust network, enhancing the mechanical properties of the hydrogel electrolytes. Notably, the hydrogel electrolyte containing 0.6 % CMC demonstrated superior mechanical strength (compression strength of 1.22 MPa, tensile stress of 230 kPa, tensile strain of 424 %, adhesion strength of 1.98 MPa on wood). Additionally, the CMC/P(AA-co-AM) hydrogels exhibited commendable electrical performance (38 mS/cm) and a high gauge factor (2.9), enabling the precise detection of physiological activity signals through resistance measurements. The unique network structure of the hydrogel electrolyte also ensured a stable bonding interface between the electrode and the electrolyte. After 2000 charge-discharge cycles, the supercapacitor maintained good capacitance characteristics, with a capacitance retention rate of 71.21 % and a stable Coulombic efficiency of 98.85 %, demonstrating excellent cyclic stability. This study introduces a novel methodology for fabricating multifunctional all-solid-state supercapacitors and suggests that the hydrogel can significantly advance the development of wearable energy storage devices.

Gelatin/sodium alginate-based strongly adhesive, environmentally resistant, highly stable hydrogel for 3D printing to prepare multifunctional sensors and flexible supercapacitors

The increasing demand for environmentally friendly performance materials in the field of wearable electronics has brought renewable and low-cost hydrogels based on natural polymers into the research spotlight. As a biodegradable natural polymer, sodium alginate (SA) shows great promise for applications in wearable electronics. Here, we report a hydrogel with printability, adhesion, and is highly stable based on gelatin (Gel) and SA. SA improves the viscosity of the hydrogel, which can bond iron products weighing up to 20 kg due to metal coordination with the material, and the hydrogel binder is recyclable and reusable. The presence of glycerin allowed the hydrogel sensor device to maintain sensitivity after exposure to air at 25 °C for up to 35 days, and printed hydrogel samples retained their compressive resilience after exposure to air (25 °C, 55 % RH) for 30 days. Hydrogel-based supercapacitors show good stability after 58 h of charge/discharge cycling. This paper provides research ideas for the preparation of hydrogels with strong adhesion properties, as well as hydrogel 3D printing technology for the preparation of flexible sensor devices and flexible energy storage devices.

An Antibacterial, Highly Sensitive Strain Sensor Based on an Anionic Copolymer Interpenetrating with κ-Carrageenan

Polysaccharide-based hydrogels are suitable for use in the field of flexible bioelectronics due to their benign mechanical properties and biocompatibility. However, the preparation of hydrogel sensors with high performance without affecting their physicochemical properties (e.g., flexibility, toughness, self-healing, and antibacterial activity) remains a challenge and needs to be solved. Herein, a metal ion cross-linking reinforced, double network hydrogel was formed from a 2-acrylamide-2-methylpropanesulfonic acid (AMPS) copolymer interpenetrating κ-carrageenan (CAR), followed by immersing the gel in a Cu2+ ion solution to obtain an antibacterial CAR/P(AM-co-AMPS)-Cu2+ conductive hydrogel. LiCl was added as the electrolyte. The presence of the LiCl electrolyte and sulfonated molecular chain units not only gives the hydrogel good electrical conductivity (conductivity up to 2.68 S/m) but also improves the sensitivity of the hydrogel as a stress-strain sensor, with a hydrogel sensitivity GF of up to 3.76 in the 20%-100% strain range and response time of up to 280 ms. The CAR double-helical structure and sol-gel properties and the interaction of multiple noncovalent bonds between polymers provide the hydrogel with excellent self-healing, with a self-healing efficiency of 68%. In addition, the electrostatic interaction of Cu2+ with Escherichia coli cells can inhibit their growth, exhibiting good antibacterial properties with an inhibition circle diameter of 20.5 mm. This work could provide an effective strategy for antibacterial multifunctional CAR-based bionic sensors.

Low-Temperature Rapid Polymerization of Intrinsic Conducting PAD/OC Hydrogels with a Self-Adhesive and Sensitive Sensor for Outdoor Damage Repair and Detection

Intrinsic conducting hydrogels fabricated in situ at low temperatures with self-adhesive properties and excellent flexibility hold significant promise for energy applications and outdoor damage repair. However, challenges such as low polymerization rate and self adhesion, insufficient ionic conductivity, inflexibility, and poor stability under extreme cold conditions have hindered their utilization as high-performance sensors. In this study, we designed an intrinsic conducting hydrogel (PADOC) composed of acrylic acid, acryloyloxyethyltrimethylammonium chloride, N,N'-methylenebis(2-propenamide), self-fabricated oxidized curdlan (OC), and a water/glycerol binary solvent. The novel hydrogel exhibited rapid gelation (30 s) at 0 °C facilitated by the promotion of OC, without the need for external energy input. Our findings from FT-IR, NMR, XPS, XRD, EPR spectra, MS, and DSC analyses revealed that OC underwent selective oxidation via the evolved Fenton reaction at 30 °C, serving as bioaccelerators for PAD polymerization. Due to OC's reductive structure and increased solubility, the reaction activation energy of the PAD polymerization reaction significantly reduced from 103.2 to 54.4 kJ/mol. PADOC ionic hydrogels demonstrated an electrical conductivity of 1.00 S/m, 0.7% low hysteresis, 39.6 kPa self-adhesive strength, and 923% strain-at-break and kept even at -20 °C owing to dense hydrogen and ionic bonds between PAD and OC chains. Furthermore, PADOC ionic hydrogels exhibited antifatigue properties for 10 cycles (0-100%) due to electrostatic interactions and hydrogen bonding. Remarkably, using a self-designed device, the rapid polymerization of PADOC effectively repaired copper pipeline leakage under 86 kPa pressure and detected 1% strain variation as a strain sensor. This study opens a new avenue for the rapid gelation of self-adhesive and flexible intrinsic conducting hydrogels with robust sensor performance.

One-step of ionic liquid-assisted stabilization and dispersion: Exfoliated graphene and its applications in stimuli-responsive conductive hydrogels based on chitosan

Conductive hydrogels, as novel flexible biosensors, have demonstrated significant potential in areas such as soft robotics, electronic devices, and wearable technology. Graphene is a promising conductive material, but its dispersibility in aqueous solutions exists difficulties. Here, we discover that untreated graphene, after exfoliation by different ionic liquids, can disperse well in aqueous solutions. We investigate the impact of four ionic liquids with varying alkyl chain lengths ([Bmim]Cl, [Omim]Cl, [Dmim]Cl, [Hmim]Cl) on the dispersibility of grapheme, and a dual physically cross-linked network hydrogel structure is designed using acrylamide (AM), acrylic acid (AA), methyl methacrylate octadecyl ester (SMA), ionic liquid@graphene (ILs@GN), and chitosan (CS). Notably, SMA, CS, AA and AM act as dynamic cross-linking points through hydrophobic interactions and hydrogen bonding, playing a crucial role in energy dissipation. The resulting hydrogel exhibits outstanding stretchability (2250 %), remarkable toughness (1.53 MJ/m3) in tensile deformation performance, high compressive strength (1.13 MPa), rapid electrical responsiveness (response time ∼ 50 ms), high electrical conductivity (12.11 mS/cm), and excellent strain sensing capability (GF = 12.31, strain = 1000 %). These advantages make our composite hydrogel demonstrate high stability in extensive deformations, offering repeatability in pressure and strain and making it a promising candidate for multifunctional sensors and flexible electrodes.

Amphibious Multifunctional Hydrogel Flexible Haptic Sensor with Self-Compensation Mechanism

In recent years, hydrogel-based wearable flexible electronic devices have attracted much attention. However, hydrogel-based sensors are affected by structural fatigue, material aging, and water absorption and swelling, making stability and accuracy a major challenge. In this study, we present a DN-SPEZ dual-network hydrogel prepared using polyvinyl alcohol (PVA), sodium alginate (SA), ethylene glycol (EG), and ZnSO4 and propose a self-calibration compensation strategy. The strategy utilizes a metal salt solution to adjust the carrier concentration of the hydrogel to mitigate the resistance drift phenomenon to improve the stability and accuracy of hydrogel sensors in amphibious scenarios, such as land and water. The ExpGrow model was used to characterize the trend of the ∆R/R0 dynamic response curves of the hydrogels in the stress tests, and the average deviation of the fitted curves ϵ¯ was calculated to quantify the stability differences of different groups. The results showed that the stability of the uncompensated group was much lower than that of the compensated group utilizing LiCl, NaCl, KCl, MgCl2, and AlCl3 solutions (ϵ¯ in the uncompensated group in air was 276.158, 1.888, 2.971, 30.586, and 13.561 times higher than that of the compensated group in LiCl, NaCl, KCl, MgCl2, and AlCl3, respectively; ϵ¯ in the uncompensated group in seawater was 10.287 times, 1.008 times, 1.161 times, 4.986 times, 1.281 times, respectively, higher than that of the compensated group in LiCl, NaCl, KCl, MgCl2 and AlCl3). In addition, for the ranking of the compensation effect of different compensation solutions, the concentration of the compensation solution and the ionic radius and charge of the cation were found to be important factors in determining the compensation effect. Detection of events in amphibious environments such as swallowing, robotic arm grasping, Morse code, and finger-wrist bending was also performed in this study. This work provides a viable method for stability and accuracy enhancement of dual-network hydrogel sensors with strain and pressure sensing capabilities and offers solutions for sensor applications in both airborne and underwater amphibious environments.

Antifreezing, Antidrying, and Conductive Hydrogels for Electronic Skin Applications at Ultralow Temperatures

Although conductive hydrogel-based flexible electronic devices have superb flexibility and high conductivities, they tend to malfunction in dry or frigid areas. Herein, an ultralow-temperature tolerant, antidrying, and conductive composite hydrogel is designed for electronic skin applications on the basis of the synergy of double-cross-linked polymer networks, Hofmeister effect, and electrostatic interaction and fabricated by in situ free radical polymerization of 2-acrylamido-2-methyl-1-propanesulfonic acid and acrylic acid in the presence of poly(vinyl alcohol) and conductive MXene sheets, followed by impregnation with LiCl. Thanks to the synergy of LiCl and the charged polar terminal groups of the synthesized polymers, the composite hydrogel can not only bear an ultralow temperature of -80 °C without freezing but also maintain its original mass. Meanwhile, the resultant hydrogel possesses satisfactory self-regeneration ability benefiting from the moisturizing effect of LiCl. The conductive network of MXene sheets greatly improves the ionic conductivity of the hydrogel at low temperatures, exhibiting an ionic conductivity of 1.4 S m-1 at -80 °C. Furthermore, the electronic skin assembled by the multifunctional hydrogel is efficient in monitoring human motions at -80 °C. The antifreezing and antidrying features along with favorable ionic conductivity, high tensile strength, and outstanding flexibility make the composite hydrogel promising for applications in frigid and dry regions.

Self-Healing and Wide Temperature-Tolerant Cellulose-Based Eutectogels for Reversible Humidity Detection

A kind of ionic conductive gel (also named eutectogel) is developed from an inorganic salt (ZnCl2)-based deep eutectic solvent (DES). The ternary DES consists of ZnCl2, acrylic acid, and water, and cotton linter cellulose is introduced into the DES system to tailor its mechanical and conductive properties. Enabled by the extensive hydrogen bonds and ion-dipole interactions, the obtained eutectogel displays superior ionic conductivity (0.33 S/m), high stretchability (up to 2050%), large tensile strength (1.82 MPa), and wide temperature tolerance (-40 to 60 °C). In particular, the water-induced coordination interactions can tune the strength of hydrogen/ionic bonds in the eutectogels, imparting them with appealing humidity sensing ability in complex and extreme conditions.

Wearable Sensor Based on a Tough Conductive Gel for Real-Time and Remote Human Motion Monitoring

The usage of a conductive hydrogel in wearable sensors has been thoroughly researched recently. Nonetheless, hydrogel-based sensors cannot simultaneously have excellent mechanical property, high sensitivity, comfortable wearability, and rapid self-healing performance, which result in poor durability and reusability. Herein, a robust conductive hydrogel derived from one-pot polymerization and subsequent solvent replacement is developed as a wearable sensor. Owing to the reversible hydrogen bonds cross-linked between polymer chains and clay nanosheets, the resulting conductive hydrogel-based sensor exhibits outstanding flexibility, self-repairing, and fatigue resistance performances. The embedding of graphene oxide nanosheets offers an enhanced hydrogel network and easy release of wearable sensor from the target position through remote irradiation, while Li+ ions incorporated by solvent replacement endow the wearable sensor with low detection limit (sensing strain: 1%), high conductivity (4.3 S m-1) and sensitivity (gauge factor: 3.04), good freezing resistance, and water retention. Therefore, the fabricated wearable sensor is suitable to monitor small and large human motions on the site and remotely under subzero (-54 °C) or room temperature, indicating lots of promising applications in human-motion monitoring, information encryption and identification, and electronic skins.

Cold-resistant, highly stretchable ionic conductive hydrogels for intelligent motion recognition in winter sports

Conductive hydrogels have attracted much attention for their wide application in the field of flexible wearable sensors due to their outstanding flexibility, conductivity and sensing properties. However, the weak mechanical properties, lack of frost resistance and susceptibility to microbial contamination of traditional conductive hydrogels greatly limit their practical application. In this work, multifunctional polyvinyl alcohol (PVA)/carboxymethyl cellulose (CMC)/poly(acrylamide-co-1-vinyl-3-butylimidazolium bromide) (P(AAm-co-VBIMBr)) (PCPAV) ionic conductive hydrogels with high strength and good conductive, transparent, anti-freezing and antibacterial properties were constructed by introducing a network of chemically crosslinked AAm and VBIMBr copolymers into the base material of PVA and CMC by in situ free radical polymerization. Owing to the multiple interactions between the polymers, including covalent crosslinking, multiple hydrogen bonding interactions, and electrostatic interactions, the obtained ionic conductive hydrogels exhibit a high tensile strength (360.6 kPa), a large elongation at break (810.6%), good toughness, and fatigue resistance properties. The introduction of VBIMBr endows the PCPAV hydrogels with excellent transparency (∼92%), a high ionic conductivity (15.2 mS cm-1), antimicrobial activity and good flexibility and conductivity at sub-zero temperatures. Notably, the PCPAV hydrogels exhibit a wide strain range (0-800%), high strain sensitivity (GF = 3.75), fast response, long-term stability, and fantastic durability, which enable them to detect both large joint movements and minute muscle movements. Based on these advantages, it is believed that the PCPAV-based hydrogel sensors would have potential applications in health monitoring, human motion detection, soft robotics, ionic skins, human-machine interfaces, and other flexible electronic devices.

Carboxymethyl cellulose-based hydrogel with high-density crack microstructures inspired from the multi-tentacles of octopus for ultrasensitive flexible sensing microsystem

Constructing high-density contact-separation sites on conductive materials highly determines the sensitivity of flexible resistance-type sensors relying on the crack microstructures. Herein, inspired from the multiple-tentacle structures on octopus, we demonstrated a sort of novel carbonized ZIF-8@loofah (CZL) as conductive material to develop ultrasensitivity flexible sensor, in which the carbonized ZIF-8 nanoparticles (~100 nm) served as tentacles. Originating from the formation of high-density contact-separation sites, the fabricated CZL-based strain sensor delivered ultrahigh sensitivity of GFmax = 15,901, short response time of 22 ms and excellent durability over 10,000 cycles. These features enable the sensor with efficient monitoring capacity for complex human activities, such as pulse rate and phonation. Moreover, when CZL was assembled into triboelectric nanogenerator (TENG), CZL-based TENG can effectively convert the irregular biomechanical energy into electric energy, providing sustainable power supply for the continuous operation of the sensing micro-system. Our findings established a novel platform to develop high-performance self-powered sensing systems of physiological parameter of human inspired from the nature.

Ultrasensitive stretchable bimodal sensor based on novel elastomer and ionic liquid for temperature and humidity detection

In this work, we present a novel stretchable bimodal sensor that can simultaneously detect temperature and humidity changes based on poly-hydroxyethyl acrylate (PHEA) elastomer infused with 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4) ionic liquid. The sensor exhibits high transparency, stability, and biocompatibility, as well as excellent mechanical and sensing properties. The sensor can achieve a maximum strain of 761%, a sensitivity of 4.5%/°C at room temperature, a detection range from -35 to 120 °C, and a response time of 10 ms. The sensor is able to provide acute response to movement of human hand at close range and can detect temperature changes as small as 0.004 °C in the range of 20-30 °C. The sensor also responds to humidity change, showing a high sensitivity to humidity change of 4.4%/RH% under the temperature of 30 °C. The sensor can be used for various applications in wearable electronics, human-machine interfaces, and soft robotics.

Stable Flexible Electronic Devices under Harsh Conditions Enabled by Double-Network Hydrogels Containing Binary Cations

Hydrogels are increasingly used in flexible electronic devices, but the mechanical and electrochemical stabilities of hydrogel devices are often limited under specific harsh conditions. Herein, chemically/physically cross-linked double-network (DN) hydrogels containing binary cations Zn2+ and Li+ are constructed in order to address the above challenges. Double networks of chemically cross-linked polyacrylamide (PAM) and physically cross-linked κ-Carrageenan (κ-CG) are designed to account for the mechanical robustness while binary cations endow the hydrogels with excellent ionic conductivity and outstanding environmental adaptability. Excellent mechanical robustness and ionic conductivity (25 °C, 2.26 S·m-1; -25 °C, 1.54 S·m-1) have been achieved. Utilizing the DN hydrogels containing binary cations as signal-converting materials, we fabricated flexible mechanosensors. High gauge factors (resistive strain sensors, 2.4; capacitive pressure sensors, 0.82 kPa-1) and highly stable sensing ability have been achieved. Interestingly, zinc-ion hybrid supercapacitors containing the DN hydrogels containing binary cations as electrolytes have achieved an initial capacity of 52.5 mAh·g-1 at a current density of 3 A·g-1 and a capacity retention rate of 82.9% after 19,000 cycles. Proper working of the zinc-ion hybrid supercapacitors at subzero conditions and stable charge-discharge for more than 19,000 cycles at -25 °C have been demonstrated. Overall, DN hydrogels containing binary cations have provided promising materials for high-performance flexible electronic devices under harsh conditions.

Highly Stretchable, Fast Self-Healing, Self-Adhesive, and Strain-Sensitive Wearable Sensor Based on Ionic Conductive Hydrogels

Conductive hydrogels integrate the conductive performance and soft nature, which is like that of human skin. Thus, they are more suitable for the preparation of wearable human-motion sensors. Nevertheless, the integration of outstanding multiple functionalities, such as stretchability, toughness, biocompatibility, self-healing, adhesion, strain sensitivity, and durability, by a simple way is still a huge challenge. Herein, we have developed a multifunctional chitosan/oxidized hyaluronic acid/hydroxypropyl methylcellulose/poly(acrylic acid)/tannic acid/Al3+ hydrogel (CS/OHA/HPMC/PAA/TA/Al3+) by using a two-step method with hydroxypropyl methylcellulose (HPMC), acrylic acid (AA), tannic acid (TA), chitosan (CS), oxidized hyaluronic acid (OHA), and aluminum chloride hexahydrate (AlCl3·6H2O). Due to the synergistic effect of dynamic imine bonds between CS and OHA, dynamic metal coordination bonds between Al3+ and -COOH and/or TA as well as reversible hydrogen, the hydrogel showed excellent tensile property (elongation at break of 3168%) and desirable toughness (0.79 MJ/m3). The mechanical self-healing efficiency can reach 95.5% at 30 min, and the conductivity can recover in 5.2 s at room temperature without stimulation. The favorable attribute of high transparency (98.5% transmittance) facilitates the transmission of the optical signal and enables visualization of the sensor. It also shows good adhesiveness to various materials and is easy to peel off without residue. The resistance of the hydrogel-based sensors shows good electrical conductivity (2.33 S m-1), good durability, high sensing sensitivity (GF value of 4.12 under 1600% strain), low detection limit (less than 1%), and short response/recovery time (0.54/0.31 s). It adhered to human skin and monitored human movements such as the bending movements of joints, swallowing, and speaking successfully. Therefore, the obtained multifunctional conductive hydrogel has great potential applications in wearable strain sensors.

Multifunctional organohydrogel via the synergy of dialdehyde starch and glycerol for motion monitoring and sign language recognition

Conductive hydrogel which belongs to a type of soft materials has recently become promising candidate for flexible electronics application. However, it remains difficult for conductive hydrogel-based strain sensors to achieve the organic unity of large stretchability, high conductivity, self-healing, anti-freezing, anti-drying and transparency. Herein, a multifunctional conductive organohydrogel with all of the above superiorities is prepared by crosslinking polyacrylamide (PAM) with dialdehyde starch (DAS) in glycerol-water binary solvent. Attributing to the synergy of abundant hydrogen bonding and Schiff base interactions caused by introducing glycerol and dialdehyde starch, respectively, the organohydrogel achieved balanced mechanical and electrical properties. Besides, the addition of glycerol promoted the water-locking effects, making the organohydrogel retain the superior mechanical properties and conductivity even at extreme conditions. The resultant organohydrogel strain sensor exhibits desirable sensing performance with high sensitivity (GF = 6.07) over a wide strain range (0-697 %), enabling the accurate monitoring of subtle body motions even at -30 °C. On the basis, a hand gesture monitor system based on the organohydrogel sensors arrays is constructed using machine learning method, achieving a considerable sign language recognition rate of 100 %, and thus providing convenience for communications between the hearing or speaking-impaired and general person.

A 10-micrometer-thick nanomesh-reinforced gas-permeable hydrogel skin sensor for long-term electrophysiological monitoring

Hydrogel-enabled skin bioelectronics that can continuously monitor health for extended periods is crucial for early disease detection and treatment. However, it is challenging to engineer ultrathin gas-permeable hydrogel sensors that can self-adhere to the human skin for long-term daily use (>1 week). Here, we present a ~10-micrometer-thick polyurethane nanomesh-reinforced gas-permeable hydrogel sensor that can self-adhere to the human skin for continuous and high-quality electrophysiological monitoring for 8 days under daily life conditions. This research involves two key steps: (i) material design by gelatin-based thermal-dependent phase change hydrogels and (ii) robust thinness geometry achieved through nanomesh reinforcement. The resulting ultrathin hydrogels exhibit a thickness of ~10 micrometers with superior mechanical robustness, high skin adhesion, gas permeability, and anti-drying performance. To highlight the potential applications in early disease detection and treatment that leverage the collective features, we demonstrate the use of ultrathin gas-permeable hydrogels for long-term, continuous high-precision electrophysiological monitoring under daily life conditions up to 8 days.

Gum Arabic-based three-dimensional printed hydrogel for customizable sensors

Most three-dimensional (3D) printed hydrogel exhibit non-idealized rheological properties in the process of direct ink writing and complicated curing. Therefore, accurate writability and convenient curing for 3D printed hydrogel remain a challenge. In this paper, we developed a typical 3D printed hydrogel which realized direct ink writing (DIW) at temperatures similar to human body. Silicon dioxide (SiO2) and Gum Arabic (GA) formed the Bingham fluid to ensure shape stability. The rapid initiation system of potassium persulfat (KPS) and N,N,N',N' -tetramethylethylenediamine (TMEDA) allowed the 3D printed hydrogel precursor solution to transiently form a hydrophobic conjoined cross-linking network structure of acrylamide (AAM) and lauryl methacrylate (LMA) after printing, resulting in preferable mechanical properties. Hydrogel precursor solution showed better rheological properties with the nature of Bingham fluids, and achieved transient cross-linking at 30 °C for 10 s in the rheological test. A variety of 3D printed hydrogel with individual strain sensing properties are prepared as customizable sensor that could monitor significant strain signals within 0-20 % strain with high sensitivity. Moreover, they were discovered excellent temperature sensitivity over a wide operating range (0-80 °C). The 3D printing hydrogel sensors were expected to have broad application prospects in flexible wearable devices and medical monitoring.

Multi-characteristic tannic acid-reinforced polyacrylamide/sodium carboxymethyl cellulose ionic hydrogel strain sensor for human-machine interaction

Big data and cloud computing are propelling research in human-computer interface within academia. However, the potential of wearable human-machine interaction (HMI) devices utilizing multiperformance ionic hydrogels remains largely unexplored. Here, we present a motion recognition-based HMI system that enhances movement training. We engineered dual-network PAM/CMC/TA (PCT) hydrogels by reinforcing polyacrylamide (PAM) and sodium carboxymethyl cellulose (CMC) polymers with tannic acid (TA). These hydrogels possess exceptional transparency, adhesion, and remodelling features. By combining an elastic PAM backbone with tunable amounts of CMC and TA, the PCT hydrogels achieve optimal electromechanical performance. As strain sensors, they demonstrate higher sensitivity (GF = 4.03), low detection limit (0.5 %), and good linearity (0.997). Furthermore, we developed a highly accurate (97.85 %) motion recognition system using machine learning and hydrogel-based wearable sensors. This system enables contactless real-time training monitoring and wireless control of trolley operations. Our research underscores the effectiveness of PCT hydrogels for real-time HMI, thus advancing next-generation HMI systems.

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.

Recent advances in wearable iontronic sensors for healthcare applications

Iontronic sensors have garnered significant attention as wearable sensors due to their exceptional mechanical performance and the ability to maintain electrical performance under various mechanical stimuli. Iontronic sensors can respond to stimuli like mechanical stimuli, humidity, and temperature, which has led to exploration of their potential as versatile sensors. Here, a comprehensive review of the recent researches and developments on several types of iontronic sensors (e.g., pressure, strain, humidity, temperature, and multi-modal sensors), in terms of their sensing principles, constituent materials, and their healthcare-related applications is provided. The strategies for improving the sensing performance and environmental stability of iontronic sensors through various innovative ionic materials and structural designs are reviewed. This review also provides the healthcare applications of iontronic sensors that have gained increased feasibility and broader applicability due to the improved sensing performance. Lastly, outlook section discusses the current challenges and the future direction in terms of the applicability of the iontronic sensors to the healthcare.

Dual detection of human motion and glucose in sweat with polydopamine and glucose oxidase doped self-healing nanocomposite hydrogels

The detection of human motion and sweat composition are important for human health or sports training, so it is necessary to develop flexible sensors for monitoring exercise processes and sweat detection. Mussel secretion of adhesion proteins enables self-healing of byssus and adhesion to surfaces. We prepared Au nanoparticles@polydopamine (AuNPs@PDA) nanomaterials based on mussel-inspired chemistry and compounded them with polyvinyl alcohol (PVA) hydrogels to obtain PVA/AuNPs@PDA self-healing nanocomposite hydrogels. Dopamine (DA) was coated on the surface of AuNPs to obtain AuNPs based composite (AuNPs@PDA) and the AuNPs@PDA was implanted into the PVA hydrogels to obtain nanocomposite hydrogel through facile freeze-thaw cycle. Glucose oxidase (GOD) was added to the hydrogel matrix to achieve specific detection of glucose in sweat. The obtained hydrogels exhibit high deformability (573.7 %), excellent mechanical strength (550.3 KPa) and self-healing properties (85.1 %). The PVA/AuNPs@PDA hydrogel sensors exhibit quick response time (185.0 ms), wide strain sensing range (0-500 %), superior stability and anti-fatigue properties in motion detection. The detection of glucose had wide concentration detection range (1.0 μmol/L-200.0 μmol/L), low detection limits (0.9 μmol/L) and high sensitivity (24.4 μA/mM). This work proposes a reference method in dual detection of human exercise and sweat composition analysis.

Organohydrogels with cellulose nanofibers enhanced supramolecular interactions toward high performance self-adhesive sensing pads

Gel materials with tailored functions and tissue-like properties have gained significant interest in emerging applications, including tissue engineering scaffolds, flexible electronics, and soft robotics. In this work, we developed a stretchable, flexible, adhesive, and conductive organohydrogel through physical cross-linking of the poly (N-[tris (hydroxymethyl) methyl] acrylamide-co-acrylamide) (denoted as P(THMA-AM)) network in the presence of cellulose nanofiber (CNF), sodium chloride, and glycerol. The gel matrix is rich in intermolecular interactions, including hydrogen bonding and ionic interactions, which contribute to a highly compact and cohesive structure without the requirement of any chemical crosslinkers. Moreover, the plasticizing effect of glycerol can mitigate the self-entanglement of CNFs, enhancing their mobility and ultimately conferring the organohydrogel with exceptional stretchability and flexibility. The resulting organohydrogel exhibited superior mechanical properties, self-adhesion, and ionic conductivity, making it an excellent candidate for strain-sensing applications, particularly in distinguishing and monitoring human movements.

Molecular Clogging Organogels with Excellent Solvent Maintenance, Adjustable Modulus, and Advanced Mechanics for Impact Protection

Inspired by mechanically interlocking supramolecular materials, exploiting the size difference between the bulky solvent and the cross-linked network mesh, a molecular clogging (MC) effect is developed to effectively inhibit solvent migration in organogels. A bulky solvent (branched citrate ester, BCE) with a molecular size above 1.4 nm is designed and synthesized. Series of MC-Gels are prepared by in situ polymerization of crosslinked polyurea with BCE as the gel solvent. The MC-Gels are colorless, transparent, and highly homogeneous, show significantly improved stability than gels prepared with small molecule solvents. As solvent migration is strongly inhibited by molecular clogging, the solvent content of the gels can be precisely controlled, resulting in a series of MC-Gels with continuously adjustable mechanics. In particular, the modulus of MC-Gel can be regulated from 1.3 GPa to 30 kPa, with a variation of 43 000 times. The molecular clogging effect also provides MC-Gels with unique high damping (maximum damping factor of 1.9), impact resistant mechanics (high impact toughness up to 40.68 MJ m-3 ). By applying shatter protection to items including eggs and ceramic armor plates, the potential of MC-Gels as high strength, high damping soft materials for a wide range of applications is well demonstrated.

Ultrathin hierarchical hydrogel-carbon nanocomposite for highly stretchable fast-response water-proof wearable humidity sensors

Wearable humidity sensors play an important role in human health monitoring. However, challenges persist in realizing high performance wearable humidity sensors with fast response and good stretchability and durability. Here we report wearable humidity sensors employing an ultrathin micro-nano hierarchical hydrogel-carbon nanocomposite. The nanocomposite is synthesized on polydimethylsiloxane (PDMS) films via a facile two-step solvent-free approach, which creates a hierarchical architecture consisting of periodic microscale wrinkles and vapor-deposited nanoporous hydrogel-candle-soot nanocoating. The hierarchical surface topography results in a significantly enlarged specific surface area (>107 times that of planar hydrogel), which along with the ultrathin hydrogel endow the sensor with high sensitivity and a fast response/recovery (13/0.48 s) over a wide humidity range (11-96%). Owing to the wrinkle structure and interpenetrating network between the hydrogel and PDMS, the sensor is stable and durable against repeated 180° bending, 100% strain, and even scratching. Furthermore, encapsulation of the sensor imparts excellent resistance to water, sweat, and bacteria without influencing its performance. The sensor is then successfully used to monitor different human respiratory behaviors and skin humidity in real time. The reported method is convenient and cost-effective, which could bring exciting new opportunities in the fabrication of next-generation wearable humidity sensors.

Synthesis, characterization and application of oligomeric proanthocyanidin-rich dual network hydrogels

A structurally dense hydrogel, with strong hydrogen bonding networks, was formed from poly(vinyl alcohol), sodium alginate, and oligomeric proanthocyanidins, using a combination of freeze-thaw cycles and calcium ion cross-linking. The structure of the hydrogel was characterized by scanning electron microscopy and Fourier transform infrared spectroscopy. Mechanical testing and thermogravimetric analysis showed that incorporation of proanthocyanidins enhanced both the mechanical properties and the thermal stability of the hydrogel. The hydrogel was also demonstrated to have excellent ultraviolet resistance and antioxidant properties. The hydrogel was further shown that this hydrogel is also capable of generating electrochemical reactions, which strongly suggests that this hydrogel has exciting potential in many fields.

Effect of ionic conductivity of electrolyte on printed planar and vertical organic electrochemical transistors

Conducting polymers with mixed electronic/ionic transport are attracting a great deal of interest for applications in organic electrochemical transistors (OECTs). Ions play a crucial role in OECT performance. The concentration and mobility of ions in the electrolyte influence the current flow in the OECT and its transconductance. This study examines the electrochemical properties and ionic conductivity of two semi-solid electrolytes, iongels, and organogels, with diverse ionic species and properties. Our results indicate that the organogels exhibited higher ionic conductivities than the iongels. Furthermore, the geometry of OECTs plays an important role in determining their transconductance. Thus, this study employs a novel approach for fabricating vertical-configuration OECTs with significantly shorter channel lengths planar devices. This is achieved through a printing method that offers advantages, such as design versatility, scalability, expedited production time, and reduced cost relative to traditional microfabrication methods. The transconductance values obtained for the vertical OECTs were significantly (approximately 50 times) higher than those of the planar devices because of their shorter channel lengths. Finally, the impact of different gating media on the performance of both planar and vertical OECTs was studied, and devices gated by organogels demonstrated improved transconductance and switching speed (almost two times higher) than those gated by iongels.

Easy-to-Prepare Flexible Multifunctional Sensors Assembled with Anti-Swelling Hydrogels

Recent years have witnessed the development of flexible electronic materials. Flexible electronic devices based on hydrogels are promising but face the limitations of having no resistance to swelling and a lack of functional integration. Herein, we fabricated a hydrogel using a solvent replacement strategy and explored it as a flexible electronic material. This hydrogel was obtained by polymerizing 2-hydroxyethyl methacrylate (HEMA) in ethylene glycol and then immersing it in water. The synergistic effect of hydrogen bonding and hydrophobic interactions endows this hydrogel with anti-swelling properties in water, and it also exhibits enhanced mechanical properties and outstanding self-bonding properties. Moreover, the modulus of the hydrogel is tissue-adaptable. These properties allowed the hydrogel to be simply assembled with a liquid metal (LM) to create a series of structurally complex and functionally integrated flexible sensors. The hydrogel was used to assemble resistive and capacitive sensors to sense one-, two-, and three-dimensional strains and finger touches by employing specific structural designs. In addition, a multifunctional flexible sensor integrating strain sensing, temperature sensing, and conductance sensing was assembled via simple multilayer stacking to enable the simultaneous monitoring of underwater motion, water temperature, and water quality. This work demonstrates a simple strategy for assembling functionally integrated flexible electronics, which should open opportunities in next-generation electronic skins and hydrogel machines for various applications, especially underwater applications.

Highly Strain-Stable Intrinsically Stretchable Olfactory Sensors for Imperceptible Health Monitoring

Intrinsically stretchable gas sensors possess outstanding advantages in seamless conformability and high-comfort wearability for real-time detection toward skin/respiration gases, making them promising candidates for health monitoring and non-invasive disease diagnosis and therapy. However, the strain-induced deformation of the sensitive semiconductor layers possibly causes the sensing signal drift, resulting in failure in achievement of the reliable gas detection. Herein, a surprising result that the stretchable organic polymers present a universal strain-insensitive gas sensing property is shown. All the stretchable polymers with different degrees of crystallinity, including indacenodithiophene-benzothiadiazole (PIDTBT), diketo-pyrrolo-pyrrole bithiophene thienothiophene (DPPT-TT) and poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-alt-[1,2,5]thiad-iazolo [3,4-c] pyridine] (PCDTPT), show almost unchanged gas response signals in the different stretching states. This outstanding advantage enables the intrinsically stretchable devices to imperceptibly adhere on human skin and well conform to the versatile deformations such as bending, twisting, and stretching, with the highly strain-stable gas sensing property. The intrinsically stretchable PIDTBT sensor also demonstrates the excellent selectivity toward the skin-emitted trimethylamine (TMA) gas, with a theoretical limit of detection as low as 0.3 ppb. The work provides new insights into the preparation of the reliable skin-like gas sensors and highlights the potential applications in the real-time detection of skin gas and respiration gas for non-invasive medical treatment and disease diagnosis.

Plant-based Biomass/Polyvinyl Alcohol Gels for Flexible Sensors

Flexible sensors show great application potential in wearable electronics, human-computer interaction, medical health, bionic electronic skin and other fields. Compared with rigid sensors, hydrogel-based devices are more flexible and biocompatible and can easily fit the skin or be implanted into the body, making them more advantageous in the field of flexible electronics. In all designs, polyvinyl alcohol (PVA) series hydrogels exhibit high mechanical strength, excellent sensitivity and fatigue resistance, which make them promising candidates for flexible electronic sensing devices. This paper has reviewed the latest progress of PVA/plant-based biomass hydrogels in the construction of flexible sensor applications. We first briefly introduced representative plant biomass materials, including sodium alginate, phytic acid, starch, cellulose and lignin, and summarized their unique physical and chemical properties. After that, the design principles and performance indicators of hydrogel sensors are highlighted, and representative examples of PVA/plant-based biomass hydrogel applications in wearable electronics are illustrated. Finally, the future research is briefly prospected. We hope it can promote the research of novel green flexible sensors based on PVA/biomass hydrogel.

Enhanced Mechanical Properties of PVA Hydrogel by Low-Temperature Segment Self-Assembly vs. Freeze-Thaw Cycles

The rapid and effective fabrication of polyvinyl alcohol (PVA) hydrogels with good mechanical properties is of great significance yet remains a huge challenge. The preparation of PVA hydrogels via the conventional cyclic freeze-thaw method is intricate and time-intensive. In this study, a pioneering approach involving the utilization of low-temperature continuous freezing is introduced to produce a novel PVA-ethylene glycol (EG) gel. Fourier transform infrared (FTIR) spectroscopy, X-ray diffractometry (XRD) and scanning electron microscopy (SEM) confirm that with the assistance of EG, PVA molecular chains can self-assemble to generate an abundance of microcrystalline domains at low temperatures, thus improving the mechanical properties of PVA-EG gel. Remarkably, when the mass ratio of H2O/EG is 4:6, the gel's maximum tensile strength can reach 2.5 MPa, which is much higher than that of PVA gels prepared via the freeze-thaw method. The preparation process of PVA-EG gel is simple, and its properties are excellent, which will promote the wide application of PVA tough gel in many fields.

Intrinsically Stretchable Conductive Self-Healable Organogels for Strain, Pressure, Temperature, and Humidity Sensing

Intrinsically stretchable conductive self-healable organogels containing poly(lipoic acid), Al3+ ion, tannic acid, and reduced graphene oxide are produced in this report. These noncovalent networks interlocked through physical (hydrogen and coordination) bonds offered high stretchabilities and mechanical strengths as well as fast self-healing behaviors. The optimum organogel-based sensor showed outstanding pressure sensitivities (0.94 kPa-1 up to 10 and 1.07 kPa-1 for 10-50 kPa) and high strain responses (corresponding gauge factors of 1.1 and 0.4 for 0-50 and 50-100% stretching ratios). This organogel also revealed high stabilities at ambient atmosphere due to the presence of binary solvents of dimethyl sulfoxide and glycerol. Additionally, this stretchable thermistor displayed remarkable two-stage sensitivities of -2.6 and -0.4%/°C ranging over 0-30 and 30-80 °C, respectively. Besides, the signal variations of water droplet addition and removal with different temperatures were recorded by the organogel sensor to elucidate the practical applicabilities as a temperature sensor. Moreover, the organogel was utilized to demonstrate humidity sensing, where individual sensitivities of 0.89 and 0.55 were obtained in the respective relative humidity ranges of 10-30 and 40-90%. In the meanwhile, the sensor device illustrated distinct humidity signals during respiration monitoring of nose and mouth breathing detection. Accordingly, these quad-functional sensor applications in strain, pressure, temperature, and humidity detection enable this gel to act as a promising material for future multifunctional flexible electronics.

Design of stretchable and self-powered sensing device for portable and remote trace biomarkers detection

Timely and remote biomarker detection is highly desired in personalized medicine and health protection but presents great challenges in the devices reported so far. Here, we present a cost-effective, flexible and self-powered sensing device for H2S biomarker analysis in various application scenarios based on the structure of galvanic cells. The sensing mechanism is attributed to the change in electrode potential resulting from the chemical adsorption of gas molecules on the electrode surfaces. Intrinsically stretchable organohydrogels are used as solid-state electrolytes to enable stable and long-term operation of devices under stretching deformation or in various environments. The resulting open-circuit sensing device exhibits high sensitivity, low detection limit, and excellent selectivity for H2S. Its application in the non-invasive halitosis diagnosis and identification of meat spoilage is demonstrated, emerging great commercial value in portable medical electronics and food security. A wireless sensory system has also been developed for remote H2S monitoring with the participation of Bluetooth and cloud technologies. This work breaks through the shortcomings in the traditional chemiresistive sensors, offering a direction and theoretical foundation for designing wearable sensors catering to other stimulus detection requirements.

High-Strength, Freeze-Resistant, Recyclable, and Biodegradable Polyvinyl Alcohol/Glycol/Wheat Protein Complex Organohydrogel for Wearable Sensing Devices

The application of flexible wearable sensing devices based on conductive hydrogels in human motion signal monitoring has been widely studied. However, conventional conductive hydrogels contain a large amount of water, resulting in poor mechanical properties and limiting their application in harsh environments. Here, a simple one-pot method for preparing conductive hydrogels is proposed, that is, polyvinyl alcohol (PVA), wheat protein (WP), and lithium chloride (LiCl) are dissolved in an ethylene glycol (EG)/water binary solvent. The obtained PVA/EG/WP (PEW) conductive organohydrogel has good mechanical properties, and its tensile strength and elongation at break reach 1.19 MPa and 531%, respectively, which can withstand a load of more than 6000 times its own weight without breaking. The binary solvent system composed of EG/water endows the hydrogel with good frost resistance and water retention. PEW organohydrogel as a wearable strain sensor also has good strain sensitivity (GF = 2.36), which can be used to detect the movement and physiological activity signals in different parts of the human body. In addition, PEW organohydrogels exhibit good degradability, reducing the environmental footprint of the flexible sensors after disposal. This research provides a new and viable way to prepare a new generation of environmentally friendly sensors.