An oriented Fe3+-regulated lignin-based hydrogel with desired softness, conductivity, stretchability, and asymmetric adhesiveness towards anti-interference pressure sensors

Thermal and light-driven soft actuators based on a conductive polypyrrole nanofibers integrated poly(N-isopropylacrylamide) hydrogel with intelligent response

The development of soft hydrogel actuators with outstanding mechanical properties, fast actuation speed, and available quantification of self-sensing actuation remains a challenging endeavor. In this work, dopamine-decorated polypyrrole nanofibers (DAPPy) were introduced into the polyethylene glycol diacrylate (PEGDA)-crosslinked poly(N-isopropyl acrylamide) network to generate a stretchable, NIR-responsive, and strain sensitive DAPPy/PNIPAM hydrogel layer. Besides, this active layer was combined with the passive ligninsulfonate sodium/polyacrylamide (LS/PAAM) to give DAPPy/PNIPAM//LS/PAAM bilayer hydrogel actuator, which exhibits ultrafast thermo-responsive actuation (19°/s) and underwater grasping and lifting performance. Moreover, the DAPPy/PNIPAM layer has excellent electrical conductivity (0.29 S/m) and thermal conversion ability (10.8 °C/min), which enable such a conductive hydrogel to act as a highly sensitive strain and temperature sensor with real-time resistance change in response to tensile strain (gauge factor up to 3.4), applied pressure, temperature, and remote NIR light irradiation. More importantly, the bilayer hydrogel actuator can integrate both actuation and self-sensing functions through the bending angle-surface temperature-relative resistance change relationship of the photothermal process. With excellent mechanical actuation and self-sensing ability, the resulting bilayer hydrogel showed a promising application potential as soft biomimetic actuating materials and soft intelligent actuators.

Mussel-inspired chitin nanofiber adherable hydrogel sensor with interpenetrating network and great fatigue resistance for motion and acoustics monitoring

A method for grafting dopamine onto TEMPO-oxidized chitin nanofibers (TOChN) was developed, achieving a surface grafting rate of 54 % through the EDC/NHS reaction. This process resulted in the formation of dopamine-grafted TOChN (TOChN-DA). Subsequently, an adherent, highly sensitive, fatigue-resistant conductive PAM/TOChN-PDA/Fe3+ (PTPF) hydrogel was successfully synthesized based on the composition of polyacrylamide (PAM) and TOChN-DA, which exhibited good cell compatibility, a tensile strength of 89.42 kPa, and a high adhesion strength of 62.56 kPa with 1.2 wt% TOChN-DA. Notably, the PTPF hydrogel showed stable adherence to various surfaces, such as rubber, copper, and human skin. Specifically, the addition of FeCl3 contributed to a multifunctional design in the PTPF interpenetrating network (IPN) hydrogel, endowing it with conductivity, cohesion, and antioxidant properties, which facilitated sensitive motion and acoustics monitoring. Moreover, the PTPF hydrogel demonstrated exceptional fatigue resistance and sensing stability, maintaining performance at 50 % strain over 1000 cycles. These attributes render the PTPF hydrogel a promising candidate for advanced biosensors in medical and athletic applications.

Lignin reinforced tough, adhesive, and recoverable protein organohydrogels for wearable strain sensing under sub-zero temperatures

Protein-based hydrogels with promising biocompatibility and biodegradability have attracted considerable interest in areas of epidermal sensing, whereas, which are still difficult to synchronously possess high mechanical strength, self-adhesion, and recoverability. Hence, the bio-polymer lignosulfonate-reinforced gluten organohydrogels (GOHLx) are fabricated through green and simple food-making processes and the following solvent exchange with glycerol/water binary solution. Ascribing to the uniform distribution of lignosulfonate in gluten networks, as well as the noncovalent interactions (e.g., H-bond) between them, the resultant GOHLx exhibit favorable conductivity (∼14.3 × 10-4 S m-1), toughness (∼711.0 kJ m-3), self-adhesion (a maximal lap-shear strength of ∼33.5 kPa), high sensitivity (GF up to ∼3.04), and durability (∼3000 cycles) toward shape deformation, which are suitable for the detection of both drastic (e.g., elbow and wrist bending) and subtle (e.g., swallowing and speaking) human movements even under -20 °C. Furthermore, the GOHLx is also biocompatible, degradable, and recoverable (by a simple kneading process). Thus, this work may pave a simple, green, and cheap way to prepare all-biomass-based, tough, sticky, and recoverable protein-based organohydrogels for epidermal strain sensing even in harsh environments.

Rapid fabricated in-situ polymerized lignin hydrogel sensor with highly adjustable mechanical properties

Conductive hydrogels have been widely used as sensors owing to their tissue-like properties. However, the synthesis of conductive hydrogels with highly adjustable mechanical properties and multiple functions remains difficult to achieve yet highly needed. In this study, lignin hydrogel characterized by frost resistance, UV resistance, high conductivity, and highly adjustable mechanical properties without forming by-products was prepared through a rapid in-situ polymerization of acrylic acid/zinc chloride (AA/ZnCl2) aqueous solution containing lignin extract induced by the reversible quinone-catechol redox of the ZnCl2-lignin system at room temperature. Results revealed that the PAA/ZnCl2/lignin hydrogel exhibited mechanical properties with tensile stress (ranging from 0.08 to 3.28 MPa), adhesion to multiple surfaces (up to 62.05 J m-2), excellent frost resistance (-70-20 °C), UV resistance, and conductivity (0.967 S m-1), which further endow the hydrogel as potential strain and temperature sensor with wide monitor range (0-300 %), fatigue resistance, and quick response (70 ms for 150 % strain). This study proposed and developed a green, simple, economical, and efficient processing method for a hydrogel sensor in flexible wearable devices and man-machine interaction fields.

Lignin derivatives-based hydrogels for biomedical applications

Recently, numerous studies have been conducted on renewable polymers derived from different natural sources, exploring their suitability for diverse biomedical applications. Lignin as one of the main components of lignocellulosic has garnered significant attention as a promising alternative to petroleum-based polymers. This interest is primarily due to its cost-effectiveness, biocompatibility, eco-friendly nature, as well as its antioxidant and antimicrobial properties. These characteristics could be more beneficial when incorporating lignin into the formulation of value-added products. Although lignin has a chemical structure that is suitable for various applications, these characteristics require modifications to guarantee that the resultant materials display the desired biological, chemical, and physical properties when applied in the creation of biodegradable hydrogels, particularly for biomedical purposes. This study delineates the recent modification approaches that have been employed in the creation of lignin-based hydrogels. These strategies encompass both chemical and physical interactions with other polymers. Additionally, this review encompasses an examination of the current applications of lignin hydrogels, spanning their use as scaffolds for tissue engineering, carriers for pharmaceuticals, materials for wound dressings and biosensors, and elements in flexible and wearable electronics. Finally, we delve into the challenges and constraints associated with these materials, discuss the necessary steps required to attain the appropriate properties for the development of innovative lignin-based hydrogels, and derive conclusions based on the presented findings.

Conductive and Eco-friendly Biomaterials-based Hydrogels for Noninvasive Epidermal Sensors: A Review

As noninvasive wearable electronic devices, epidermal sensors enable continuous, real-time, and remote monitoring of various human physiological parameters. Conductive biomaterials-based hydrogels as sensor matrix materials have good biocompatibility, biodegradability, and efficient stimulus response capabilities and are widely applied in motion monitoring, healthcare, and human-machine interaction. However, biomass hydrogel-based epidermal sensing devices still need excellent mechanical properties, prolonged stability, multifunctionality, and extensive practicality. Therefore, this paper reviews the common biomass hydrogel materials for epidermal sensing (proteins, polysaccharides, polyphenols, etc.) and the various types of noninvasive sensing devices (strain/pressure sensors, temperature sensors, glucose sensors, electrocardiograms, etc.). Moreover, this review focuses on the strategies of scholars to enhance sensor properties, such as strength, conductivity, stability, adhesion, and self-healing ability. This work will guide the preparation and optimization of high-performance biomaterials-based hydrogel epidermal sensors.

Ultrastretchable and highly conductive hydrogels based on Fe3+- lignin nanoparticles for subzero wearable strain sensor

Conductive hydrogels have attracted considerable interest for potential applications in soft robotics, electronic skin and human monitoring. However, insufficient mechanical characteristics, low adhesion and unsatisfactory electrical conductivity severely restrict future application possibilities of hydrogels. Herein, lignin nanoparticles (LNPs)-Fe3+-ammonium persulfate (APS) catalytic system was introduced to assemble Poly(2-hydroxyethyl methacrylate)/LNPs/Ca2+ (PHEMA/LNPs/Ca) hydrogels. Due to the abundant metal coordination and hydrogen bonds, the composite hydrogel displayed ultrahigh stretchable capacity (3769 %), adhesion properties (248 kPa for skin) and self-healing performance. Importantly, hydrogel sensors possess with high durability, strain sensitivity (GF = 8.75), fast response time and freeze resistance (-20 °C) that could be employed to monitor motion signals in low-temperature regime. Therefore, the LNPs-Fe3+ catalytic system has great potential in preparing hydrogel for various applications such as human-computer interaction, artificial intelligence, personal healthcare and subzero wearable devices. At the same time, incorporation of natural macromolecules into polymer hydrogels is tremendous research significance for investigating high-value utilization of lignin.

A Janus supramolecular hydrogel prepared by one-pot method for wound dressing

Despite the adhesive hydrogels have gained progress and popularity, it is still an enormous challenge to develop a smart adhesion hydrogel for clinical medicine, which is an asymmetric adhesion hydrogel with on-demand detachment. Motivated by the thermal phase transition mechanism of gelatin, we have synthesized a Janus supramolecular hydrogel dressing with skin temperature-triggered adhesion by a simple one-pot process. This hydrogel has asymmetric and controllable adhesion, which not only can become the external objects barrier but also can achieve repeated adhesion and on-demand detachment triggered by temperature in tens of seconds. This hydrogel presents great mechanical performance (compressive strain of 65 %, 1.38 MPa) owing to the presence of supramolecular interactions in the hydrogel. Additionally, this hydrogel exhibits excellent antibacterial activity and biocompatibility. The synergistic effect of modified gelatin and ionic liquid greatly facilitates wound healing of full-thickness skin with high wound healing efficiency (98.45 %). Therefore, thanks to all these advantages, the Janus supramolecular hydrogel can be applied for wound management and treatment, which has huge potential in healing skin wounds.

Preparation of lignin-based hydrogels, their properties and applications

Polymers obtained from biomass are a concerning alternative to petro-based polymers because of their low cost of manufacturing, biocompatibility, ecofriendly and biodegradability. Lignin as the second richest and the only polyaromatics bio-polymer in plant which has been most studied for the numerous applications in different fields. But, in the past decade, the exploitation of lignin for the preparation of new smart materials with improved properties has been broadly sought, because lignin valorization plays one of the primary challenging issues of the pulp and paper industry and lignocellulosic biorefinery. Although, well suited chemical structure of lignin comprises of many functional hydrophilic and active groups, such as phenolic hydroxyls, carboxyls and methoxyls, which provides a great potential to be applied in the preparation of biodegradable hydrogels. In this review, lignin hydrogel is covered with preparation strategies, properties and applications. This review reports some important properties, such as mechanical, adhesive, self-healing, conductive, antibacterial and antifreezing properties were then discussed. Furthermore, herein also reviewed the current applications of lignin hydrogel, including dye adsorption, smart materials for stimuli sensitive, wearable electronics for biomedical applications and flexible supercapacitors. Overall, this review covers recent progresses regarding lignin-based hydrogel and constitutes a timely review of this promising material.

High Strength, Conductivity, and Bacteriostasis of the P(AM-co-AA)/Chitosan Quaternary Ammonium Salt Composite Hydrogel through Ionic Crosslinking and Hydrogen Bonding

Traditional hydrogels with a single-crosslinked network structure suffer from poor stretchability, low sensitivity, and easy contamination, which seriously affect their practical application in the strain sensor field. To overcome these shortcomings, herein, a multiphysical crosslinking strategy (ionic crosslinking and hydrogen bonding) was designed to prepare a hydrogel strain sensor based on chitosan quaternary ammonium salt (HACC)-modified P(AM-co-AA) (acrylamide-co-acrylic acid copolymer) hydrogels. The ionic crosslinking for the double-network P(AM-co-AA)/HACC hydrogels was achieved by an immersion method with Fe³⁺ as crosslinking sites, which crosslinked with the amino group (-NH₂) on HACC and the carboxyl group (-COOH) on P(AM-co-AA) and enabled the hydrogels to recover and reorganize rapidly, resulting in a hydrogel-based strain sensor with excellent tensile stress (3 MPa), elongation (1390%), elastic modulus (0.42 MPa), and toughness (25 MJ/m³). In addition, the prepared hydrogel exhibited high electrical conductivity (21.6 mS/cm) and sensitivity (GF = 5.02 at 0-20% strain, GF = 6.84 at 20-100% strain, and GF = 10.27 at 100-480% strain). Furthermore, the introduction of HACC endowed the hydrogel with excellent antibacterial properties (up to 99.5%) and excellent antibacterial activity against bacteria of three forms, bacilli, cocci, and spores. The flexible, conductive, and antibacterial hydrogel can be applied as a strain sensor for real-time detection of human motions such as joint movement, speech, and respiration, which exhibits a promising application prospect in wearable devices, soft robotic systems, and other fields.

A sweat-pH-enabled strongly adhesive hydrogel for self-powered e-skin applications

On-skin hydrogel electrodes are poorly conformable in sweaty scenarios due to low electrode-skin adhesion resulting from the sweat film formed on the skin surface, which seriously hinders practical applications. In this study, we fabricated a tough adhesive cellulose-nanofibril/poly(acrylic acid) (CNF/PAA) hydrogel with tight hydrogen-bond (H-bond) networks based on a common monomer and a biomass resource. Furthermore, inherent H-bonded network structures can be disrupted through judicious engineering using excess hydronium ions produced through sweating, which facilitate the transition to protonation and modulate the release of active groups (i.e., hydroxyl and carboxyl groups) accompanied by a pH drop. The lower pH enhances adhesive performance, especially on skin, with a 9.7-fold higher interfacial toughness (453.47 vs. 46.74 J m^-2), an 8.6-fold higher shear strength (600.14 vs. 69.71 kPa), and a 10.4-fold higher tensile strength (556.44 vs. 53.67 kPa) observed at pH 4.5 compared to the corresponding values at pH 7.5. Our prepared hydrogel electrode remains conformable on sweaty skin when assembled as a self-powered electronic skin (e-skin) and enables electrophysiological signals to be reliably collected with high signal-to-noise ratios when exercising. The strategy presented here promotes the design of high-performance adhesive hydrogels that may serve to record continuous electrophysiological signals under real-life conditions (beyond sweating) for various intelligent monitoring systems.

Fabrication of lignin reinforced hybrid hydrogels with antimicrobial and self-adhesion for strain sensors

Ionic conductive hydrogels prepared from various biological macromolecules are ideal materials for the manufacture of human motion sensors from the perspective of resource regeneration and environmental sustainability. However, it is still challenging to prepare hydrogels with both high toughness and self-healing ability. In this study, lignin-based β-CD-PVA (LCP) self-healing conductive hydrogels with high tensile properties were prepared by one-step method using alkali lignin as a plasticizer. Compared with PVA hydrogel, the maximum storage modulus and elongation were increased by 2.5 and 20.0 times, respectively. Uniform distribution of lignin can increase the fluidity and distance of polymer molecular chains, thus improving the viscoelastic and tensile properties of the LCP self-healing hydrogel. LCP hydrogels can maintain self-healing ability in both high (45 °C) and low temperature (0 °C) environments, and the self-healing ability is not affected by pH. Moreover, it also has good conductivity, anti-bacterial, thermostability, and anti-UV property, which has a good application prospect in the field of 3D printing and wearable electronic devices, which expands the efficient utilization of lignin in biorefinery.

Conductive Hydrogels Based on Industrial Lignin: Opportunities and Challenges

The development of green materials, especially the preparation of high-performance conductive hydrogels from biodegradable biomass materials, is of great importance and has received worldwide attention. As an aromatic polymer found in many natural biomass resources, lignin has the advantage of being renewable, biodegradable, non-toxic, widely available, and inexpensive. The unique physicochemical properties of lignin, such as the presence of hydroxyl, carboxyl, and sulfonate groups, make it promising for use in composite conductive hydrogels. In this review, the source, structure, and reaction characteristics of industrial lignin are provided. Description of the preparation method (physical and chemical strategies) of lignin-based conductive hydrogel is elaborated along with their several important properties, such as electrical conductivity, mechanical properties, and porous structure. Furthermore, we provide insights into the latest research advances in industrial lignin conductive hydrogels, including biosensors, strain sensors, flexible energy storage devices, and other emerging applications. Finally, the prospects and challenges for the development of lignin-conductive hydrogels are presented.

Lignin-containing hydrogels with anti-freezing, excellent water retention and super-flexibility for sensor and supercapacitor applications

We developed a highly conductive and flexible, anti-freezing sulfonated lignin (SL)-containing polyacrylic acid (PAA) (SL-g-PAA-Ni) hydrogel, with a high concentration of NiCl₂. Ni²⁺ contributes multi-functions to the preparation of the hydrogel and its final properties, such as fast polymerization reaction as a result of the presence of redox pairs of Ni³⁺/Ni²⁺ and hydroquinone/quinone, and anti-freezing properties of the hydrogel due to the salt effects of NiCl₂ so that at -20 °C the hydrogel shows similar properties to those at the room temperature. Thanks to the effective coordinations of Ni²⁺ with catecholic groups and carboxylic groups, as well as the rich hydrogen bonding capacity, the resultant hydrogel possesses excellent mechanical properties. High ionic conductivity (6.85 S·m^-1) of the hydrogel is obtained due to the supply of high concentration of Ni²⁺. Moreover, the ionic solvation effect of NiCl₂ in the hydrogel imparts excellent water retention ability, with water retention of ~93 % after 21-day storage. The SL-g-PAA-Ni hydrogel can accurately detect various human motions at -20 °C. The supercapacitor assembled from SL-g-PAA-Al hydrogel at -20 °C manifests a high specific capacitance of 252 F·g^-1, with maximum energy density of 26.97 Wh·kg^-1, power density of 2667 W·kg^-1, and capacitance retention of 96.7 % after 3000 consecutive charge-discharge cycles.

Recent advances in lignosulfonate filled hydrogel for flexible wearable electronics: A mini review

With the rapid development of flexible wearable devices, various polymer hydrogels have gained immense progress due to their adjustable mechanical properties, high conductivity, super sensitivity, good biocompatibility and adaptable wearability. Lignosulfonate (LS), generating from the sulfite pulping industry, was emerged as a promising filler in polymer hydrogels with great potential for multifunctional wearable electronics. Herein, we comprehensively review the latest research progress associated with LS-based hydrogels. Firstly, the function mechanism of lignosulfonate in diverse polymer hydrogels was introduced in detail. Then, the rational design strategies of LS filled multifunctional hydrogels was summarized as toughening filler, adhesive agent, conductive filler dispersant, UV protectant and catalysts. Finally, the future development of LS filled hydrogel for flexible wearable electronics was proposed.

Design of asymmetric-adhesion lignin-reinforced hydrogels based on disulfide bond crosslinking for strain sensing application

Soft and elastic polymer hydrogel materials are booming in the fields of wearable biomimetic skin, sensors, robotics, and bioelectrodes. Currently, many researchers are exploring new chemistries for the preparation of hydrogels to improve their performance. In the present study, we design and develop a strategy to prepare lignin reinforced hydrogels based on disulfide bond crosslinking mechanisms, and resultant hydrogels exhibit excellent stretchability, with tensile strain of up to 1085.4%, and high adhesion (with the highest T-peel strength of up to 432.2 N/m to pigskin). The underlying mechanism is based on the disulfide bonds that act as crosslinkers in the as-prepared hydrogel, and they can be easily cleaved and re-formed under mild conditions. Thanks to the presence of lignin, the as-obtained hydrogels also have excellent UV shielding effect. When assembled into a strain sensor, they can output stable and sensitive sensing signals, with gauge factor (GF) of 2.72 (strain: 0-72.8%). Furthermore, a simple and effective strategy to construct asymmetric adhesive hydrogels was adopted, which is based on directional soaking of the top portion of the hydrogel in a high-concentrated calcium chloride solution. The asymmetric hydrogel strain sensor transmits accurate and stable signals without the interference of various contaminants.

Preparation of Hemicellulose Nanoparticle-Containing Ionic Hydrogels with High Strength, Self-Healing, and UV Resistance and Their Applications as Strain Sensors and Asymmetric Pressure Sensors

Smart functional fillers can significantly enhance the comprehensive properties of ionic hydrogels, such as their mechanical properties, which are key features of hydrogels in wearable sensor applications. As a plant-derived natural polymer, hemicellulose can serve as smart functional fillers. In this study, tannic acid-modified hemicellulose nanoparticles (TA@HC) and Fe³⁺ were used in the preparation of PAA/TA@HC/Fe³⁺ hydrogels. The addition of TA@HC and Fe³⁺ in the sodium persulfate (SPS) and acrylic acid (AA) polymerization system resulted in a fast gelation process that was completed within a short time (as short as 30 s) at room temperature. The catechol-rich TA and Fe³⁺ system allows for quick activation of SPS to produce free radicals, generating abundant hydroxyl groups in a short period of time, which was responsible for the fast gelation. Furthermore, due to the TA@HC effect and the dynamic catechol (TA)-Fe³⁺ redox system, the PAA/TA@HC/Fe³⁺ hydrogel exhibited excellent mechanical properties with an exceptionally high strain (as high as 5600%), adhesiveness, rapid and efficient self-healing ability, and reproducible self-adhesion onto various substrates. More importantly, asymmetric adhesive PAA/TA@HC/Fe³⁺ hydrogels were prepared by selective Fe³⁺ coating of the upper hydrogel surface to render the top surface nonadhesive so that the same hydrogel with different adhesiveness between the upper and bottom surfaces was obtained. The asymmetric adhesive hydrogel design permits the adhesive side to fit comfortably to the skin and the nonadhesive side showing anti-interference against various different pollutant materials, accurately serving as a pressure sensor.

Design of asymmetric-adhesion lignin reinforced hydrogels with anti-interference for strain sensing and moist air induced electricity generator

Flexible hydrogels with integration of excellent mechanical and electrical properties are well suited for applications as wearable electronic sensors, and others. Self-adhesion is an important feature of wearable sensors. However, the usual isotropic- adhesion hydrogels have the drawback of poor anti-interference, which negatively affects their applications. In this study, we developed asymmetric-adhesion and tough lignin reinforced hydrogels in a facile two-step process: 1) PAA hydrogels, with lignin as the binder and conductive filler, were first prepared; 2) the asymmetric-adhesion property was imparted to lignin reinforced hydrogel by simple soaking of the top portion of the hydrogel in CaCl₂ solution. The as-obtained asymmetric-adhesion lignin reinforced hydrogel was assembled into a wearable sensor, which shows excellent anti-interference and accurate and stable collections of sensing signals, with its gauge factor (GF) of 2.51 (in the strain range of 0-51.5%). In addition, the tough hydrogel is capable of generating electricity upon moist air sweeping through it, showing excellent energy conversion capabilities, with open-circuit voltage of as high as 306.6 mV. These results provided new prospects for the application of polyelectrolyte hydrogel materials in the fields of wet-to-electric conversion and wearable electronic sensors.

High lignin containing hydrogels with excellent conducting, self-healing, antibacterial, dye adsorbing, sensing, moist-induced power generating and supercapacitance properties

Herein, we design a dynamic redox system of using high contents of lignosulfonate (LS) and Al³⁺ to prepare poly acrylic acid (PAA) (LS-g-PAA-Al) hydrogels. The presence of high LS and Al³⁺ contents, in combination with the effective Al³⁺ complexes formed, renders the resultant hydrogel with some unique attributes, including excellent ionic conductivity (as high as 7.38 S·m^-1) and antibacterial activity; furthermore, a very fast gelation (in 1 min) was obtained. As a flexible strain sensor, the LS-g-PAA-Al hydrogel with high conductivity demonstrates superior sensitivity in human movement detection. In addition, the rich anionic hydrophilic groups, such as sulfonic groups, phenolic hydroxyl groups, in the hydrogels impart the resultant hydrogels with excellent adsorption capacity for cationic dyes: when using Rhodamine B (RB) as a model cationic dye, the adsorption capacity of the resultant hydrogel reaches 334.64 mg·g^-1; as a moist-induced power generator, it generates maximum 150.5 mV open circuit voltage with moist air flow. When the hydrogel electrolyte is assembled into a supercapacitor assembly, it shows high specific capacitance of 245.4 F·g^-1, with the maximum energy density of 21.8 Wh·kg^-1, power density of 2.37 kW·kg^-1, and capacitance retention of 95.1% after 5000 consecutive charge-discharge cycles.

Nanolignin filled conductive hydrogel with improved mechanical, anti-freezing, UV-shielding and transparent properties for strain sensing application

Herein, we innovatively synthesized an ionic conductive PVA/LNP hydrogel with integrated excellent mechanical, anti-freezing, moisturizing, transparent and UV-shielding performances via incorporating nanolignin (also called lignin nanoparticle, LNP) and aluminum chloride (AlCl₃) into polyvinyl alcohol (PVA) matrix containing ethylene glycol/water (EG/H₂O) binary solvent. The rigid porous network structure was well constructed by the hydrogen bond interactions among the evenly distributed LNP and PVA chains, thus providing abundant ion transport channels, which attributed to the outstanding ionic conductivity (up to 1.35 × 10^-2 S/m, at -24 °C) with improved mechanical strength and flexibility. The tensile strength and elongation at break of PVA/LNP hydrogel were greatly increased from 574.6 kPa and 363.7% to 1241.4 kPa and 589% at the addition of 0.35% LNP, respectively. In addition, the UV-resistance ability was 95% at 365 nm, while the transparency was 74% at 550 nm. The binary solvent of EG and H₂O ensured long-term moisturizing capability (10 days) of the hydrogel at 35 °C and 60 RH%, as well as possessing superior anti-freezing performance over the temperature range of -62.6 to 24 °C. As a result, the fabricated PVA/LNP hydrogel was successfully used as strain sensor for detecting diverse human motions and electrophysiological signals.

Design of Fe3+-Rich, High-Conductivity Lignin Hydrogels for Supercapacitor and Sensor Applications

Preparation of natural polymer-based highly conductive hydrogels with tunable mechanical properties for applications in flexible electronics is still challenging. Herein, we report a facile method to prepare lignin-based Fe³⁺-rich, high-conductivity hydrogels via the following two-step process: (1) lignin hydrogels are prepared by cross-linking sulfonated lignin with poly(ethylene glycol) diglycidyl ether (PEGDGE) and (2) Fe³⁺ ions are impregnated into the lignin hydrogel by simply soaking in FeCl₃. Benefiting from Fe³⁺ ion complexation with catechol groups and other functional groups in lignin, the resultant hydrogels exhibit unique properties, such as high conductivity (as high as 6.69 S·m^-1) and excellent mechanical and hydrophobic properties. As a strain sensor, the as-prepared lignin hydrogel shows high sensitivity when detecting various human motions. With the flow of moist air, the Fe³⁺-rich lignin hydrogel generates an output voltage of 162.8 mV. The assembled supercapacitor of the hydrogel electrolyte demonstrates a high specific capacitance of 301.8 F·g^-1, with a maximum energy density of 26.73 Wh·kg^-1, a power density of 2.38 kW·kg^-1, and a capacitance retention of 94.1% after 10 000 consecutive charge-discharge cycles. These results support the conclusion that lignin-based Fe³⁺-rich, high-conductivity hydrogels have promising applications in different fields, including sensors and supercapacitors, rendering a new platform for the value-added utilization of lignin.

Low-temperature adaptive conductive hydrogel based on ice structuring proteins/CaCl2 anti-freeze system as wearable strain and temperature sensor

Conductive hydrogels as wearable devices meet the basic demands of mechanical flexibility and smart sensing. However, achieving anti-freeze property in conductive hydrogels is still challengeable. Here, a novel anti-freezing system based on ice structuring proteins and CaCl₂ was introduced to enable a conductive hydrogel with low-temperature adaptability. Both formation of ice nuclei and ice growth of the hydrogel at sub-zero temperature could be inhibited. Supported by the anti-freeze system, the hydrogel revealed good flexibility (890% at -20 °C), recovery and conductivity (0.50 S/m at -20 °C) at both room temperature and sub-zero temperature. The low-temperature adaptability enabled the hydrogel to be used as strain and temperature sensors at both room temperature and sub-zero temperature. The anti-freeze system in this work is expected to open up a new avenue to promote the conductive hydrogel with low-temperature adaptability.