Stretchable, self-adhesive, conductive, anti-freezing sodium polyacrylate-based composite hydrogels for wearable flexible strain sensors

Design, application, and recycling of zinc alginate/guar gum hydrogel-based fibers

Extreme cold events are quite common, highlighting the urgent need for flexible wearable electronic devices capable of diagnosing human health in low-temperature environments. Using a wet spinning strategy, we successfully developed sodium zinc alginate/guar gum(SZA/GG) hydrogel fibers with excellent environmental resistance, antimicrobial properties, and electrical conductivity. Building on this, we developed a flexible wearable sensing device that operates stably at low temperatures and exhibits a sensitivity of 0.585 within the range of -20 °C to -40 °C, demonstrating excellent response performance. When evaluating the physical state of outdoor athletes, the amplitude and fluctuation range of electrical resistance provide valuable information about the monitored environment and the risk of frostbite for the individual. However, like any device, it eventually reaches its usage limit. To address the issue of recycling hydrogel fiber waste, we propose recycling and carbonizing the discarded devices to use as a biomass carbon source for fabricating button-type supercapacitors. After 10,000 charge-discharge cycles, the capacitance retention rate reached 92.53 %, demonstrating the potential of these supercapacitors and offering a new approach to reducing resource waste.

Conductive, sensitivity, flexibility, anti-freezing and anti-drying silica/carbon nanotubes/sodium ions modified sodium alginate hydrogels for wearable strain sensing applications

The biocompatibility and salient gelling feature of alginate via forming the interpenetrating network structure has received extensive interests for different applications. Traditional alginate hydrogels freeze at low temperature and evaporate easily at room temperature, leading to reduced performance. Consequently, it is crucial to develop methods to prevent alginate hydrogel from freezing at subzero temperature and dehydration at normal temperature to maintain the performance stability. Utilizing polyacrylic acid, sodium alginate, and acrylamide-hydroxyethyl methacrylate copolymers as flexible matrix materials, this study develops a wearable silica (SiO2)/carbon nanotubes (CNT)/sodium ions (SiO2/CNT/Na+) modified sodium alginate hydrogel strain sensor characterized by high sensitivity, flexibility, and anti-freezing and anti-drying properties. The hydrogel doped with NaCl (50 mg), CNT (10 mg) and M-SiO2 (200 mg) shows excellent mechanical and electrical properties, the tensile strength is 436 KPa, the break elongation is 426 %, the elastic modulus is 99 KPa, and the toughness is 897 kJ/m3. The modified sodium alginate hydrogel used as strain sensor shows fast response time (∼100 ms), high sensitivity factor and excellent stability. The strain sensor exhibits excellent flexibility, ductility, self-adhesion, anti-freezing and anti-drying properties, significantly enhancing its strain sensing application field.

A self-healing, long-lasting adhesive, lignin-based polyvinyl alcohol organo-hydrogel for strain-sensing applications

Hydrogel-based flexible sensors have garnered considerable interest in the fields of soft electronics, robotics, and human-machine interfaces. For better practical applications, integrating multiple properties-such as self-adhesive, anti-freeze, anti-volatile, self-healing, and antibacterial-into a single gel for flexible sensors remains a challenge. In this paper, a multifunctional lignin-based polyvinyl alcohol gel, containing dynamic covalent bonds, hydrogen bonds, and coordination bonds, is constructed by a simple one-pot method, in which ethylene glycol/water chosen as a binary solvent and KI as a conductive medium. The resulting organogel exhibits self-healing, long-lasting adhesion, UV shielding, antibacterial properties, excellent frost resistance (-20 °C), and volatile resistance properties. In addition, the organogel-based sensor demonstrates satisfactory sensitivity in detecting joint movements and facial expressions. This study provides a new strategy for developing a versatile flexible sensor through the introduction of renewable and bio-based lignin, promising applications in the fields of wearable electronics.

Porphyrin-Regulated Heterostructured Hydrogel Ionic Diode with a High Rectification Ratio and Output Voltage

Nanochannel ionic diodes require extremely complex and expensive fabrication processes. Polyelectrolyte ionic diodes attracted widespread attention among ionic rectification systems due to their simplicity of development and the ability to break the size limits of the nanochannel. However, enhancement of their rectification ratio is still in the exploratory stage. In this study, chitosan (CS) hydrogels and sodium polyacrylate (PAAs) hydrogels were prepared as the substrates for the heterostructured ionic diodes. 5,10,15,20-Tetrakis(4-aminophenyl)-21H,23H-porphyrin (TAPP) was selected to regulate the rectification ratio of ionic diodes. By adding 0.05 wt % TAPP to the CS hydrogel, the rectification ratio of the ionic diode can be increased to 10, which is 4 times larger than that of the undoped ionic diode. In contrast, the rectification ratio of the ionic diodes with TAPP added in the PAAs hydrogel decreases to 2. In addition, the ionic diode composed of the TAPP-doped CS hydrogel and PAAs hydrogel has the characteristics of a high open-circuit voltage. The open-circuit voltage of the 10 mm × 10 mm × 4 mm heterojunction hydrogel reached 370 mV. The ionic diodes can be used as a self-powered power supply device.

Self-Healable, Adhesive, Anti-Drying, Freezing-Tolerant, and Transparent Conductive Organohydrogel as Flexible Strain Sensor, Triboelectric Nanogenerator, and Skin Barrier

Conductive hydrogels have attracted tremendous interest in the construction of flexible strain sensors and triboelectric nanogenerators (TENGs) owing to their good stretchability and adjustable properties. Nevertheless, how to simultaneously achieve high transparency, self-healing, adhesion, antibacterial, anti-freezing, anti-drying, and biocompatibility properties through a simple method remains a challenge. Herein, a transparent, freezing-tolerant, and multifunctional organohydrogel (PAOAM-PDO) as electrode for strain sensors and TENGs was constructed through a free radical polymerization in the 1,3-propanediol (PDO)/water binary solvent system, in which oxide sodium alginate, aminated gelatin, acrylic acid, and AlCl3 were used as raw materials. The obtained PAOAM-PDO exhibited good transparency (>90%), self-healing, adhesiveness, antibacterial property, good conductivity (1.13 S/m), and long-term environmental stability. The introduction of PDO endowed PAOAM-PDO with freezing resistance with a low freezing point of -60 °C, and PAOAM-PDO could serve as a protective skin barrier to prevent frostbite at low temperature. PAOAM-PDO could be assembled as strain sensors to monitor heterogeneous human movements with high strain sensitivity (gauge factor of 7.05, strain = 233%). Meanwhile, PAOAM-PDO could be further fabricated as a TENG with a "sandwich" structure in single electrode mode. Moreover, the resulting TENG achieved electrical outputs with simple hand tapping and served as a self-powered device to light light-emitting diodes. This work displays a feasible strategy to build environment-tolerant and multifunctional organohydrogels, which possess potential applications in the wearable electronics and self-powered devices.

Stretchable Organohydrogel with Adhesion, Self-Healing, and Environment-Tolerance for Wearable Strain Sensors

Stretchable hydrogels as landmark soft materials have been efficiently utilized in the field of wearable sensing devices. However, these soft hydrogels mostly cannot integrate transparency, stretchability, adhesiveness, self-healing, and environmental adaptability into one system. Herein, a fully physically cross-linked poly(hydroxyethyl acrylamide)-gelatin dual-network organohydrogel is prepared in a phytic acid-glycerol binary solvent via a rapid ultraviolet light initiation. The introduction of gelatin as the second network endows the organohydrogel with desirable mechanical performance (high stretchability up to 1240%). The presence of phytic acid not only synergizes with glycerol to impart environment-tolerance to the organohydrogel (from -20 to 60 °C) but also increases the conductivity. Moreover, the organohydrogel demonstrates a durable adhesive performance toward diverse substrates, a high self-healing efficiency through heat treatment, and favorable optical transparency (transmittance of 90%). Furthermore, the organohydrogel achieves high sensitivity (gauge factor of 2.18 at 100% strain) and rapid response time (80 ms) and could detect both tiny (a low detection limit of 0.25% strain) and large deformations. Therefore, the assembled organohydrogel-based wearable sensors are capable of monitoring human joint motions, facial expression, and voice signals. This work proposes a facile route for multifunctional organohydrogel transducers and promises the practical application of flexible wearable electronics in complex scenarios.