Rapid fabrication of xylan-based hydrogel by graft polymerization via a dynamic lignin-Fe3+ plant catechol system

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.

High stretchable and tough xylan-g-gelatin hydrogel via the synergy of chemical cross-linking and salting out for strain sensors

Stretchable and tough hydrogels have been extensively used in tissue engineering scaffolds and flexible electronics. However, it is still a significant challenge to prepare hydrogels with both tensile strength and toughness by utilizing xylan, which is abundant in nature. Herein, we present a novel hydrogel of carboxymethyl xylan(CMX) graft gelatin (G) and doped with conductive hydroxyl carbon nanotubes (OCNT). CMX and G are combined through amide bonding as well as intermolecular hydrogen bonding to form a semi-interpenetrating hydrogel network. The hydrogel was further subjected to salting-out treatment, which induced the aggregation of the CMX-g-G molecular chain and the formation of chain bundles to toughen the hydrogel, the tensile strain, tensile stress, and toughness of CMX-g-G hydrogels were 1.547 MPa, 324 %, and 2.31 MJ m-3, respectively. In addition, OCNT was used as a conductive filler to impart electrical conductivity and further improve the mechanical properties of CMX-g-G/OCNT hydrogel, and a tensile strength of 1.62 MPa was obtained. Thus, the synthesized CMX-g-G/OCNT hydrogel can be used as a reliable and sensitive strain sensor for monitoring human activity. This study opens up new horizons for the preparation of xylan-based high-performance hydrogels.

Novel Hydrogel Membranes Based on the Bacterial Polysaccharide FucoPol: Design, Characterization and Biological Properties

FucoPol, a fucose-rich polyanionic polysaccharide, was used for the first time for the preparation of hydrogel membranes (HMs) using Fe³⁺ as a crosslinking agent. This study evaluated the impact of Fe³⁺ and FucoPol concentrations on the HMs' strength. The results show that, above 1.5 g/L, Fe³⁺ concentration had a limited influence on the HMs' strength, and varying the FucoPol concentration had a more significant effect. Three different FucoPol concentrations (1.0, 1.75 and 2.5 wt.%) were combined with Fe³⁺ (1.5 g/L), resulting in HMs with a water content above 97 wt.% and an Fe³⁺ content up to 0.16 wt.%. HMs with lower FucoPol content exhibited a denser porous microstructure as the polymer concentration increased. Moreover, the low polymer content HM presented the highest swelling ratio (22.3 ± 1.8 g/g) and a lower hardness value (32.4 ± 5.8 kPa). However, improved mechanical properties (221.9 ± 10.2 kPa) along with a decrease in the swelling ratio (11.9 ± 1.6 g/g) were obtained for HMs with a higher polymer content. Furthermore, all HMs were non-cytotoxic and revealed anti-inflammatory activity. The incorporation of FucoPol as a structuring agent and bioactive ingredient in the development of HMs opens up new possibilities for its use in tissue engineering, drug delivery and wound care management.

In situ rapid synthesis of hydrogels based on a redox initiator and persistent free radicals

The development of fast and economical hydrogel manufacturing methods is crucial for expanding the application of hydrogels. However, the commonly used rapid initiation system is not conducive to the performance of hydrogels. Therefore, the research focuses on how to improve the preparation speed of hydrogels and avoid affecting the properties of hydrogels. Herein, a redox initiation system with nanoparticle-stabilized persistent free radicals was introduced to rapidly synthesize high-performance hydrogels at room temperature. A redox initiator composed of vitamin C and ammonium persulfate rapidly provides hydroxyl radicals at room temperature. Simultaneously, three-dimensional nanoparticles can stabilize free radicals and prolong their lifetime, thereby increasing the free radical concentration and accelerating the polymerization rate. And casein enabled the hydrogel to achieve impressive mechanical properties, adhesion, and electrical conductivity. This method greatly facilitates the rapid and economical synthesis of high-performance hydrogels and presents broad application prospects in the field of flexible electronics.

Room Temperature Ca2+-Initiated Free Radical Polymerization for the Preparation of Conductive, Adhesive, Anti-freezing and UV-Blocking Hydrogels for Monitoring Human Movement

In recent years, conductive hydrogels have received increasing attention as wearable electronics due to the electrochemical properties of conductive polymers combined with the softness of hydrogels. However, conventional hydrogels are complicated to prepare, require high temperature or UV radiation to trigger monomer polymerization, and are frozen at low temperatures, which seriously hinder the application of flexible wearable devices. In this paper, a conductive sensor integrating mechanical properties, adhesion, UV shielding, anti-dehydration, and anti-freeze was prepared based on Ca²⁺-initiated radical polymerization at room temperature using the synergy of sodium lignosulfonate, acrylamide (AM), and calcium chloride (CaCl₂). Metal ions can activate ammonium persulfate to generate free radicals that allow rapid gelation of AM monomers at room temperature without external stimuli. Due to ionic cross-linking and non-covalent interaction, the hydrogels have good tensile properties (1153% elongation and 168 kPa tensile strength), high toughness (758 KJ·m^-3), excellent adhesive properties (48.5 kPa), high ionic conductivity (7.2 mS·cm^-1), and UV resistance (94.4%). CaCl₂ can inhibit ice nucleation, so that the hydrogels have anti-dehydration and frost resistance properties and even at -80 °C can maintain flexibility, high conductivity, and adhesion. Assembled into a flexible sensor, it can sense various large and small movements such as compression, bending, and talking, which is a flexible sensing material with wide application prospects.

Strain-induced orientation facilitates the fabrication of highly stretchable and tough xylan-based hydrogel for strain sensors

Stretchable and tough polysaccharide-based functional hydrogels have gained popularity for various applications. However, it still remains a great challenge to simultaneously own satisfactory stretchability and toughness, particularly when incorporating renewable xylan to offer sustainability. Herein, we describe a novel stretchable and tough xylan-based conductive hydrogel utilizing the natural feature of rosin derivative. The effect of different compositions on the mechanical properties and the physicochemical properties of corresponding xylan-based hydrogels were systematically investigated. Owing to the multiple non-covalent interactions among different components to dissipate energies and the strain-induced orientation of rosin derivative during the stretching, the highest tensile strength, strain, and toughness of xylan-based hydrogels could reach 0.34 MPa, 2098.4 %, and 3.79 ± 0.95 MJ/m³, respectively. Furthermore, by incorporating MXene as the conductive fillers, the strength and toughness of hydrogels were further enhanced to 0.51 MPa and 5.95 ± 1.19 MJ/m³. Finally, the synthesized xylan-based hydrogels were able to serve as a reliable and sensitive strain sensor to monitor the movements of human beings. This study provides new insights to develop stretchable and tough conductive xylan-based hydrogel, especially utilizing the natural feature of bio-based resources.

Hemicellulose-based hydrogels for advanced applications

Hemicellulose-based hydrogels are three-dimensional networked hydrophilic polymer with high water retention, good biocompatibility, and mechanical properties, which have attracted much attention in the field of soft materials. Herein, recent advances and developments in hemicellulose-based hydrogels were reviewed. The preparation method, formation mechanism and properties of hemicellulose-based hydrogels were introduced from the aspects of chemical cross-linking and physical cross-linking. The differences of different initiation systems such as light, enzymes, microwave radiation, and glow discharge electrolytic plasma were summarized. The advanced applications and developments of hemicellulose-based hydrogels in the fields of controlled drug release, wound dressings, high-efficiency adsorption, and sensors were summarized. Finally, the challenges faced in the field of hemicellulose-based hydrogels were summarized and prospected.

Lignin-silver triggered multifunctional conductive hydrogels for skinlike sensor applications

Conductive hydrogels have attracted tremendous attention as a novel generation of wearable devices and body monitoring due to their great stretchability and high flexibility. Here, a multifunctional cellulose nanocrystal @sodium lignosulfonate-silver-poly(acrylamide) nanocomposite hydrogel was prepared by radical polymerization within only a few minutes. This polymerization rapidly occurred by lignosulfonate-silver (Ls-Ag) dynamic catalysis that efficiently activated ammonium persulfate (APS) to initiate the free-radical polymerization. In particular, the hydrogel exhibited excellent tensile strength (406 kPa), ultrahigh stretchability (1880 %), self-recovery, and fatigue resistance. Furthermore, due to the inclusion of Ls-Ag metal ion nanocomposite in the hydrogels, the composite hydrogel presented repeated adhesion to various objects, excellent conductivity (σ ∼ 9.5 mS cm^-1), remarkable UV resistance (100 % shielding of the UV spectral region), and high antibacterial activity (above 98 %), which enabled the hydrogel to be applied to epidermal sensors. In addition, the high-sensitivity (gauge factor of 2.46) sensor constructed of the hydrogel monitored the large and subtle movements of the human body and was used as a biological electrode to collect human electromyography and electrocardiographic signals. This work provided a novel strategy for the high-value utilization of lignin, which had potential application prospects in many fields such as wearable bioelectrodes.

Robust and adhesive lignin hybrid hydrogel as an ultrasensitive sensor

The fabrication of hydrogel for sensing purposes remains to be a challenge since the hydrogel needs to have both good mechanical strength and adhesiveness. This work reports a robust and adhesive hydrogel mainly constructed with AgNPs@lignin, polyacrylamide (PAM) and sodium alginate (SA). The silver nanoparticles (AgNPs) were in-situ generated via the reaction between lignin and silver ammonia ([Ag(NH₃)₂]⁺). The resultant lignin hybrid hydrogel exhibited a stress, strain and tearing energy up to 0.055 MPa, 1000% and 250 J·m^-2, respectively. Furthermore, the hydrogel adhered to different materials with an adhesion energy of higher than 230 J·m^-2. This hydrogel was demonstrated to be an ideal sensing material since it could detect both large-scale motions and tiny physiological signals including breathing and pulse. The hydrogel also exhibited good antibacterial performance and biocompatibility. This work provides a good example to design a lignin-based high-performance hydrogel material for sensing purposes.

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.

Degradable Hydrogel Adhesives with Enhanced Tissue Adhesion, Superior Self-Healing, Cytocompatibility, and Antibacterial Property

Degradable hydrogel adhesives with multifunctional advantages are promising to be candidates as hemostatic agents, surgical sutures, and wound dressings. In this study, hydrogel adhesives are constructed by catechol-conjugated gelatin from natural resource, iron ions (Fe³⁺ ), and a synthetic polymer. Specifically, the latter is prepared by the radical ring-opening copolymerization of a cyclic ketene acetal monomer 5,6-benzo-2-methylene-1,3-dioxepane and N-(2-ethyl p-toluenesulfonate) maleimide. By the incorporation of ester bonds in the backbone and the combination with quaternary ammonium salt pendants in the polymer, it exhibits excellent degradability and antibacterial property. Remarkably, doping the synthetic polymer into the 3,4-dihydroxyphenylacetic acid-modified gelatin network forms a semi-interpenetrating polymer network which can effectively improve the rigidity, tissue adhesion, and antibacterial property of fabricated hydrogel adhesives. Moreover, non-covalent bonds from coordination interaction between catechol and Fe³⁺ contribute to the fast self-healing of the developed hydrogel adhesives. These hydrogel adhesives with the multiple merits including the degradability, enhanced tissue adhesion, superior self-healing, good cytocompatibility, and antibacterial property show the great potential to be used as tissue adhesives in biomedical fields.