Hardwood Kraft Lignin-Based Hydrogels: Production and Performance

Crosslinked lignin starch copolymer as a sustainable and thermally stable drilling fluid controller

Fluid loss is a well-known challenge of drilling operations. In this work, a novel sustainable starch-lignin-based polymer was synthesized for possible use in drilling fluid applications. The X-ray photoelectron spectroscopy (XPS) analysis confirmed that kraft lignin was crosslinked with starch via ether covalent bonds. The X-ray diffraction (XRD) analysis confirmed the loss of crystallinity in starch and emerging of new amorphous structures in crosslinked starch-lignin (CSL) polymers after crosslinking with lignin. The incorporation of lignin and new covalent ether bonds improved the thermal stability of starch. The CSL had a rougher surface morphology, higher hydrophilicity, and significantly higher water absorption than starch. CSL-2, with its higher lignin content, demonstrated higher hydrophilicity, better water absorption capacity, and thermal stability than CSL-1. The rheology analysis of the CSL-2 polymer suggested that crosslinking starch with lignin would increase G' more than G" and reduce tan δ of the polymer solution, resulting in more elastic properties and more stability against the angular frequency. Due to its improved swelling, thermal, and rheological properties as compared to native starch, the produced sustainable lignin-starch copolymer could be used as a new viscosity and rheology modifier, such as a fluid loss controller for oil extraction from wells.

A comprehensive review of sustainable hydrogels from lignin for advanced wastewater solutions

Throughout the recent years, water bodies have been significantly contaminated via various industrial and pollution wastes posing threats to the living. To tackle the situation, Lignin-Based Hydrogels have appeared as a material with great potential for wastewater treatment. Biomass-derived polymers for wastewater treatment present a sustainable replacement to plastics based on petroleum owing to its biocompatibility, affordability, eco-friendliness and biodegradability. After cellulose the lignin is the second highest polyaromatic bio-polymer in plants. And serves as the Earth's primary renewable source for aromatic materials. Structurally, it is a cross-linked polymer enriched with hydrophilic functional groups like hydroxyls, methoxyls, carbonyls that makes it a promising precursor towards hydrogel development. This review focuses on hydrogels based on lignin, highlighting their synthesis, traits and potential functions in water treatment. It also examines various methods for extracting lignin from different raw materials. Challenges and limitations associated with real-world applications of LBHs are addressed and along with prospects for future research. Ultimately, this review provides valuable insights into developing sustainable hydrogels based on lignin for efficient elimination of aquatic contaminants.

Bridge-rich and loop-less hydrogel networks through suppressed micellization of multiblock polyelectrolytes

Most triblock copolymer-based physical hydrogels form three-dimensional networks through micellar packing, and formation of polymer loops represents a topological defect that diminishes hydrogel elasticity. This effect can be mitigated by maximizing the fraction of elastically effective bridges in the hydrogel network. Herein, we report hydrogels constructed by complexing oppositely charged multiblock copolymers designed with a sequence pattern that maximizes the entropic and enthalpic penalty of micellization. These copolymers self-assemble into branched and bridge-rich network units (netmers), instead of forming sparsely interlinked micelles. We find that the storage modulus of the netmer-based hydrogel is 11.5 times higher than that of the micelle-based hydrogel. Complementary coarse grained molecular dynamics simulations reveal that in the netmer-based hydrogels, the numbers of charge-complexed nodes and mechanically reinforcing bridges increase substantially relative to micelle-based hydrogels.

Facile fabrication and antibacterial properties of chitosan/acrylamide/gold nanocomposite hydrogel incorporated with Chaetomium globosium extracts from Gingko biloba leaves

Microorganisms are a unique part of our ecosystem because they affect the survival of living organisms. Although pathogenic microorganisms could be detrimental to the plants, animals, and humans, beneficial microbes have provided significant improvement in the growth and development of living organisms. In this study, the fungus Chaetomium globosium was isolated from the medicinal tree Gingko biloba, and then incorporated into a polymerization system to fabricate chitosan/acrylamide/gold (CS/Am/Au) nanocomposite hydrogels. The as-prepared hydrogel displayed increased mechanical strength due to the reinforcement of Au (gold) nanocomposites within the hydrogel matrix. Also, the equilibrium pH responsive swelling rates of the hydrogels gradually increased as the pH increases due to partial acid and basic hydrolysis occurring in the hydrogel as well as formation of hydrogen bond. In addition, the hydrogel demonstrated promising antibacterial activities against selected gram-positive (Staphylococcus epidermidis and Staphylococcus aureus) and gram-negative (Pseudomonas aeruginosa) bacterial strains with an average MIC90 of 0.125 mg/mL at a dosage of 1.0 mg/L. The obtained results are quite promising towards resolving several health challenges and advancing the pharmaceutical industries.

Construction and mechanism study of lignin-based polyurethane with high strength and high self-healing properties

Lignin is a natural polymer with abundant functional groups with great application prospects in lignin-based polyurethane elastomers with self-healing abilities. In this study, a lignin self-healing polyurethane (PUDA-L) was specially designed using lignin as the raw material of polyurethane, combining lignin with Diels-Alder (DA) bond and hydrogen bonds. The experimental results showed that PUDA-L was prepared with good thermal stability, fatigue resistance, shape memory effect, excellent mechanical strength, and self-healing ability by partially replacing the crosslinking agents with bio-based lignin and hydroxylated modified lignin to increase the hydroxyl content. Polyurethane has a tensile strength of up to 29 MPa and an elongation at break of up to 500 %. The excellent self-healing ability of PUDA-L originates from the internal DA bonds and cross-linked hydrogen bonds. After the dumbbell sample was fused and heated at 130 °C for 4 h, the elastomer could be completely healed, the tensile strength was restored to 29 MPa, and the self-healing efficiency was up to 100 %. The developed PUDA-L elastomer has promising applications in sensors and smart skins.

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.

Preparation and Adsorption Properties of Lignin/Cellulose Hydrogel

With the development of global industry, industrial wastewater pollution has caused serious environmental problems, and the demand for green and sustainable adsorbents is increasingly strong in the society. In this article, lignin/cellulose hydrogel materials were prepared using sodium lignosulfonate and cellulose as raw materials and 0.1% acetic acid solution as a solvent. The results showed that the optimal adsorption conditions for Congo red were as follows: an adsorption time of 4 h, a pH value of 6, and an adsorption temperature of 45 °C. The adsorption process was in line with the Langmuir isothermal model and a quasi-second-order kinetic model, which belonged to single molecular layer adsorption, and the maximum adsorption capacity was 294.0 mg/g. The optimal adsorption conditions for Malachite green were as follows: an adsorption time of 4 h, a pH value of 4, and an adsorption temperature of 60 °C. The adsorption process was consistent with the Freundlich isothermal model and a pseudo-second-order kinetic model, which belonged to the chemisorption-dominated multimolecular layer adsorption with the maximum adsorption capacity of 129.8 mg/g.

Development of shelf life-extending packaging for vitamin C syrup based on high-density polyethylene and extracted lignin argan shells

Biobased packaging is an essential parameter in the pharmaceutical industry. In the present work, bio-composites consisting of high-density polyethylene (HDPE) as a matrix and lignin recovered from argan nut shells as filler were developed to investigate their potential use as packaging materials for vitamin C drugs. The lignin was extracted via alkali and klason processes, and the effects of the extraction method as well as the lignin content on the thermal, morphological, mechanical, and rheological properties of the produced composites, as well as their application for vitamin C packaging, were investigated. Among all the prepared packaging materials, the one with desirable results in pH, color stability, hardness, and mechanical characteristics was based on alkali lignin. It achieved its highest Young's modulus enhancement, 10.12 %, at 10 % alkali lignin loading, while the highest yield strain enhancement (4.65 %) was obtained with 2 % loading. When compared to neat HDPE and HDPE/klason lignin packaging materials, vitamin C solutions packed with this composite showed a lower oxidation rate, attributed to the extremely low pH variation and high color stability of the material, which decreased the rate of vitamin C degradation. According to these findings, HDPE/alkali lignin composite is a promising vitamin C syrup packaging material.

Recent advances in pharmaceutical and biotechnological applications of lignin-based materials

Lignin, a versatile and abundant biomass-derived polymer, possesses a wide array of properties that makes it a promising material for biotechnological applications. Lignin holds immense potential in the biotechnology and pharmaceutical field due to its biocompatibility, high carbon content, low toxicity, ability to be converted into composites, thermal stability, antioxidant, UV-protectant, and antibiotic activity. Notably, lignin is an environmental friendly alternative to synthetic plastic and fossil-based materials because of its inherent biodegradability, safety, and sustainability potential. The most important findings related to the use of lignin and lignin-based materials are reported in this review, providing an overview of the methods and techniques used for their manufacturing and modification. Additionally, it emphasizes on recent research and the current state of applications of lignin-based materials in the biomedical and pharmaceutical fields and also highlights the challenges and opportunities that need to be overcome to fully realize the potential of lignin biopolymer. An in-depth discussion of recent developments in lignin-based material applications, including drug delivery, tissue engineering, wound dressing, pharmaceutical excipients, biosensors, medical devices, and several other biotechnological applications, is provided in this review article.

Lignin nanoparticle reinforced multifunctional polyvinyl alcohol/polyurethane composite hydrogel with excellent mechanical, UV-blocking, rheological and thermal properties

In this study, we innovatively synthesized a multifunctional PVA/PU-LNP composite hydrogel with integrated distinguished UV-blocking, mechanical strength, dynamic viscoelasticity and thermal properties by introducing lignin nanoparticle (LNP) into polyvinyl alcohol (PVA) and polyurethane (PU) mixed matrix through freeze-thaw cycle. The rigid porous network structure was established by hydrogen bond interactions among the well-distributed LNP and PVA/PU molecular chains, which endowed excellent mechanical strength, viscoelasticity, thermal stability and flexibility with PVA/PU-LNP composite hydrogel. The elongation at break and tensile strength of PVA/PU-LNP composite hydrogel were markedly improved from 227.3 % and 247.1 KPa to 460.1 % and 950.4 KPa with the LNP loading of 2 % based on PVA weight, respectively. Meanwhile, PVA/PU-2%LNP hydrogel exhibited prominent compressive resistance and pleasing shape recovery capability. Moreover, the blending of LNP at a low dosage (0.5 %) based on PVA weight effectively shielded 99.34 % of UV light and penetrated 42.27 % of visible light, indicating that PVA/PU-LNP composite hydrogel demonstrated outstanding anti-UV performance. In addition, the incorporation of LNP caused a remarkable decline in the pore size of PVA/PU-LNP composite hydrogel (4.39 ± 0.46 μm to 1.54 ± 0.22 μm), which slightly reduced water uptake capacity of composite hydrogel. Therefore, this work provided a new approach to constructing a multifunctional composite hydrogel.

Designing Lignin-Based Biomaterials as Carriers of Bioactive Molecules

There is a need to develop circular and sustainable economies by utilizing sustainable, green, and renewable resources in high-tech industrial fields especially in the pharmaceutical industry. In the last decade, many derivatives of food and agricultural waste have gained considerable attention due to their abundance, renewability, biocompatibility, environmental amiability, and remarkable biological features. Particularly, lignin, which has been used as a low-grade burning fuel in the past, recently attracted a lot of attention for biomedical applications because of its antioxidant, anti-UV, and antimicrobial properties. Moreover, lignin has abundant phenolic, aliphatic hydroxyl groups, and other chemically reactive sites, making it a desirable biomaterial for drug delivery applications. In this review, we provide an overview of designing different forms of lignin-based biomaterials, including hydrogels, cryogels, electrospun scaffolds, and three-dimensional (3D) printed structures and how they have been used for bioactive compound delivery. We highlight various design criteria and parameters that influence the properties of each type of lignin-based biomaterial and corelate them to various drug delivery applications. In addition, we provide a critical analysis, including the advantages and challenges encountered by each biomaterial fabrication strategy. Finally, we highlight the prospects and future directions associated with the application of lignin-based biomaterials in the pharmaceutical field. We expect that this review will cover the most recent and important developments in this field and serve as a steppingstone for the next generation of pharmaceutical research.

Pharmaceutical applications of lignin-derived chemicals and lignin-based materials: linking lignin source and processing with clinical indication

Lignocellulosic biomass is one of the most abundant bioresources on Earth. Over recent decades, various valorisation techniques have been developed to produce value-added products from the cellulosic and hemicellulosic fractions of this biomass. Lignin is the third major component accounting for 10-30% (w/w). However, it currently remains a largely unused fraction due to its recalcitrance and complex structure. The increase in the global demand for lignocellulosic biomass, for energy and chemical production, is increasing the amount of waste lignin available. Approaches to date for valorizing this renewable but heterogeneous chemical resource have mainly focused on production of materials and fine chemicals. Greater value could be gained by developing higher value pharmaceutical applications which would help to improve integrated biorefinery economics. In this review, different lignin extraction methods, such as organosolv and ionic liquid, and the properties and potential of the extracted chemical building blocks are first summarized with respect to pharmaceutical use. The review then discusses the many recent advances made regarding the medical or therapeutic potential of lignin-derived materials such as antimicrobial, antiviral, and antitumor compounds and in controlled drug delivery. The aim is to draw out the link between the source and the processing of the biomass and potential clinical applications. We then highlight four key areas for future research if therapeutic applications of lignin-derived products are to become commercially viable. These relate to the availability and processing of lignocellulosic biomass, technologies for the purification of specific compounds, enhancements in process yield, and progression to human clinical trials.

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.

Biological activity of multicomponent bio-hydrogels loaded with tragacanth gum

Producing hydrogels capable of mimicking the biomechanics of soft tissue remains a challenge. We explore the potential of plant-based hydrogels as polysaccharide tragacanth gum and antioxidant lignin nanoparticles in bioactive multicomponent hydrogels for tissue engineering. These natural components are combined with TEMPO-oxidized cellulose nanofibrils, a material with known shear thinning behavior. Hydrogels presented tragacanth gum (TG) concentration-dependent rheological properties suitable for extrusion 3D printing. TG enhanced the swelling capacity up to 645 % and the degradation rate up to 1.3 %/day for hydrogels containing 75 % of TG. Young's moduli of the hydrogels varied from 5.0 to 11.6 kPa and were comparable to soft tissues like skin and muscle. In vitro cell viability assays revealed that the scaffolds were non-toxic and promoted proliferation of hepatocellular carcinoma HepG2 cells. Therefore, the plant-based hydrogels designed in this work have a significant potential for tissue engineering.

Delivery of DNA into Human Cells by Functionalized Lignin Nanoparticles

Lignin is an aromatic plant cell wall polymer that is generated in large quantities as a low-value by-product by the pulp and paper industry and by biorefineries that produce renewable fuels and chemicals from plant biomass. Lignin structure varies among plant species and as a function of the method used for its extraction from plant biomass. We first explored the impact of this variation on the physico-chemical properties of lignin nanoparticles (LNPs) produced via a solvent exchange procedure and then examined whether LNPs produced from industrial sources of lignin could be used as delivery vehicles for DNA. Spherical LNPs were formed from birch and wheat BioLignin™ and from poplar thioglycolic acid lignin after dissolving the lignin in tetrahydrofuran (THF) and dialyzing it against water. Dynamic light scattering indicated that the diameter of these LNPs was dependent on the initial concentration of the lignin, while electrophoretic light scattering indicated that the LNPs had a negative zeta potential, which became less negative as the diameter increased. The dynamics of LNP formation as a function of the initial lignin concentration varied as a function of the source of the lignin, as did the absolute value of the zeta potential. After coating the LNPs with cationic poly-l-lysine, an electrophoretic mobility shift assay indicated that DNA could adsorb to LNPs. Upon transfection of human A549 lung carcinoma basal epithelial cells with functionalized LNPs carrying plasmid DNA encoding the enhanced green fluorescent protein (eGFP), green foci were observed under the microscope, and the presence of eGFP in the transfected cells was confirmed by ELISA. The low cytotoxicity of these LNPs and the ability to tailor diameter and zeta potential make these LNPs of interest for future gene therapy applications.

Methacrylated alkali lignin grafted P(Nipam-Co-AAc) copolymeric hydrogels: Tuning the mechanical and stimuli-responsive properties

The current study reports the preparation of lignin grafted temperature and pH responsive hydrogels through copolymerization of N-isopropylacrylamide, acrylic acid and varying amount of lignin methacrylate (LMA = 50, 100, 150 and 200 mg) as crosslinker adopting radical polymerization technique. Functional group and structural characterizations were carried out to confirm hydrogels synthesis and their network structure. The variation in pore size on addition of lignin revealed the tuning of pores as well as swelling capacity of the hydrogels by suitable amount of LMA. All LMA grafted hydrogels showed temperature responsive behavior and pH dependent sensitivity in swelling, with reduced equilibrium swelling capacity values compared to sample without lignin. In alkali medium at room temperature, the maximum swelling capacity with 48% higher retention was noticed, while a significant reduction in swelling was observed at 40 °C in all media. The addition of lignin still preserved the tensile strength up to 100 kPa and compressive load bearing ability up to 30 kPa in freeze dried state with adequate interfacial stress transfer. An increase in lignin concentration showed enhanced storage modulus (~two-fold increase), adequate loss modulus values and improved cell viability, which paves the way for possible biomedical applications.

Recent advances in lignin-based porous materials for pollutants removal from wastewater

Water pollution is one of the most serious threats facing mankind today and has obtained widespread attention. Significant advances have been made in the past decades to apply porous materials in wastewater treatment, due to their large specific surface areas (S_BET) for interaction with the aimed ions or molecules. However, the majority of porous materials are prepared from fossil-based resources and still possess some drawbacks, such as high cost and non-degradability, which inevitably cause secondary pollution to the environment from their production to disposal. Lignin is the most abundant and the only scalable renewable aromatic resource on earth. Due to its unique physicochemical properties including high carbon content, plentiful functional groups and environmental friendliness, the lignin-based porous materials (LPMs) have shown promising prospects in efficient removal of soluble pollutants from wastewater. In this review, we firstly described the structural and chemical basis of LPMs, following presented the recent progress in the decontamination of heavy metal ions, organic dyes, antibiotics, anions and radionuclides from aqueous systems. Additionally, the outlook was provided to promote more practical implementation of LPMs in the near future.

Supertough Lignin Hydrogels with Multienergy Dissipative Structures and Ultrahigh Antioxidative Activities

Hydrogels derived from lignin are typically weak and contain only a small amount of lignin, which limits their broad application prospects. In the present work, a novel lignin/poly(N,N-dimethylacrylamide) (PDMA) hydrogel with a high lignin content, superb toughness, and ultrahigh antioxidative performance is constructed by employing a facile dissolve-dry-swell solvent exchange method. Through this process, lignin and PDMA are self-assembled into a multienergy dissipative structure containing rigid lignin-rich domains. Precisely, the PDMA chains both interpenetrated inside and adhered on the surface of these domains through hydrophobic associations. This structure enables the lignin hydrogels to dissipate energy efficiently during the fracture process. At an optimized ultrahigh lignin content of 58% (dry weight basis), the prepared lignin hydrogel exhibited remarkable mechanical properties, such as a high elastic modulus (2.5 MPa), tensile strength (2.5 MPa), and super tensile strain (11.3), and an extremely high fracture energy above 16 000 J m^-2. In addition, the tough lignin hydrogel exhibited a commendable antioxidant property and nontoxicity. All these advantageous properties provide the lignin/PDMA hydrogels with the potential for use in biomedical materials applications.

Insights into the Potential of Hardwood Kraft Lignin to Be a Green Platform Material for Emergence of the Biorefinery

Lignin is an abundant, renewable, and relatively cheap biobased feedstock that has potential in energy, chemicals, and materials. Kraft lignin, more specifically, has been used for more than 100 years as a self-sustaining energy feedstock for industry after which it has finally reached more widespread commercial appeal. Unfortunately, hardwood kraft lignin (HWKL) has been neglected over these years when compared to softwood kraft lignin (SWKL). Therefore, the present work summarizes and critically reviews the research and development (R&D) dealing specifically with HWKL. It will also cover methods for HWKL extraction from black liquor, as well as its structure, properties, fractionation, and modification. Finally, it will reveal several interesting opportunities for HWKL that include dispersants, adsorbents, antioxidants, aromatic compounds (chemicals), and additives in briquettes, pellets, hydrogels, carbon fibers and polymer blends and composites. HWKL shows great potential for all these applications, however more R&D is needed to make its utilization economically feasible and reach the levels in the commercial lignin market commensurate with SWKL. The motivation for this critical review is to galvanize further studies, especially increased understandings in the field of HWKL, and hence amplify much greater utilization.

Lignin-Based Hydrogels: Synthesis and Applications

Polymers obtained from biomass are an interesting alternative to petro-based polymers due to their low cost of production, biocompatibility, and biodegradability. This is the case of lignin, which is the second most abundant biopolymer in plants. As a consequence, the exploitation of lignin for the production of new materials with improved properties is currently considered as one of the main challenging issues, especially for the paper industry. Regarding its chemical structure, lignin is a crosslinked polymer that contains many functional hydrophilic and active groups, such as hydroxyls, carbonyls and methoxyls, which provides a great potential to be employed in the synthesis of biodegradable hydrogels, materials that are recognized for their interesting applicability in biomedicine, soil and water treatment, and agriculture, among others. This work describes the main methods for the preparation of lignin-based hydrogels reported in the last years, based on the chemical and/or physical interaction with polymers widely used in hydrogels formulations. Furthermore, herein are also reviewed the current applications of lignin hydrogels as stimuli-responsive materials, flexible supercapacitors, and wearable electronics for biomedical and water remediation applications.

Impact of Counter Ions of Cationic Monomers on the Production and Characteristics of Chitosan-Based Hydrogel

Chitosan-based hydrogel has received considerable interests because of its appealing properties and applications in many areas. The primary objective of this work was to produce novel cationic chitosan-based hydrogels via polymerizing chitosan with two cationic monomers of the same structure but with different counter ions [2-(methacryloyloxy)ethyl]trimethylammonium methyl sulfate (METMS) and [2-(methacryloyloxy)ethyl]trimethylammonium chloride (METAC). Polymerization of chitosan with the cationic monomers performed under the conditions of 50 °C, 5 h, 7 pH, and 2/1 mol/mol monomer/chitosan led to chitosan-METMS and -METAC with the cationic charge densities of 3.22 and 2.88 mequiv/g, respectively. Elemental analysis, gel permeation chromatography, Fourier transform infrared, X-ray diffraction, and differential scanning calorimetry analyses were used to confirm the impact of counter ions of cationic monomers (i.e., polarizability of monomers) on their polymerization performance and the characteristics of induced chitosan-based hydrogels. Also, the results of this work postulated that the counter ions associated with the monomers could dramatically impact the water uptake and swelling properties of the generated chitosan-based hydrogels as well as their performance in adsorbing an anionic dye from a simulated solution.

Lignin-based hydrogels: A review of preparation, properties, and application

Lignin as the second most abundant and the only polyaromatics-contained bio-polymer in plant has been most studied for various applications. In the past decade, the utilization of lignin for value-added materials has been extensively sought after since lignin valorization represents one of the main challenging issues of the paper industry and lignocellulosic biorefinery. Among these researches, making lignin into hydrogels has great potential for upgrading lignin into functional materials. In this review, lignin hydrogel is wrapped up with preparation strategies, properties and applications. The major cross-linking strategies to synthesize lignin-based hydrogels were reviewed first, including monomers copolymerization, crosslinking of monomers with reactive polymer precursors and polymer-polymer crosslinking. Two most important properties of mechanical and porous structures of lignin hydrogel were then discussed. More importantly, we extensively reviewed current applications of lignin hydrogel, including absorption, controlled release, smart materials for stimuli sensitive, biosensors and electrodes. These applications have paved avenues for lignin valorization. Overall, this paper covers recent advancements regarding lignin-based hydrogel and represents a timely review of this promising material.