Lignin-Based Hydrogels: Synthesis and Applications
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polymers Review Lignin-Based Hydrogels: Synthesis and Applications Diana Rico-García 1, Leire Ruiz-Rubio 2,3,* , Leyre Pérez-Alvarez 2,3 , Saira L. Hernández-Olmos 1, Guillermo L. Guerrero-Ramírez 1 and José Luis Vilas-Vilela 2,3 1 Chemistry Department, University Center of Exact Sciences and Engineering, University of Guadalajara, 44430 Guadalajara, Mexico; [email protected] (D.R.-G.); [email protected] (S.L.H.-O.); [email protected] (G.L.G.-R.) 2 Macromolecular Chemistry Group (LQM), Physical Chemistry Department, Faculty of Science and Technology, University of the Basque Country, 48940 Leioa, Spain; [email protected] (L.P.-A.); [email protected] (J.L.V.-V.) 3 BCMaterials, Basque Center for Materials, Applications and Nanostructures, UPV/EHU Science Park, 48940 Leioa, Spain * Correspondence: [email protected]; Tel.: +34-946017972 Received: 19 November 2019; Accepted: 19 December 2019; Published: 3 January 2020 Abstract: 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. Keywords: lignin; hydrogels; renewable sources 1. Introduction As a result of the gradual depletion of fossil resources and the constant increase in the world’s population, the interest in finding an alternative, clean, and globally available resource has arisen. Biomass is the second most abundant material and, due to its composition, it has the ability to replace many petroleum products. The plant biomass consists of 40–45 wt% of cellulose, 25–35 wt% of hemicellulose, 15–30 wt% lignin, and up to 10 wt% of inorganic components [1]. Among the three main components, cellulose and hemicellulose are the ones with the highest added value owing to their broad applications in industry, while lignin is considered inexpensive, so currently, 98% is burned to generate energy in production plants and the remaining 2% is used in the manufacture of livestock feed, vanillin, fillers in cements, in the manufacture of carbon fibers, as well as complement in agglomerates and adhesives [2–7] Lignin plays a fundamental role in plants, in that it helps the cell wall and internal fibers to transport water and nutrients, protects them from microbial attack, and provides mechanical support to the plant. Previous studies have determined the amount of lignin in different plants, being the coconut fiber with 45% the plant that contains the highest percentage of lignin [8], follow by rice husk Polymers 2020, 12, 81; doi:10.3390/polym12010081 www.mdpi.com/journal/polymers Polymers 2020, 12, x FOR PEER REVIEW 2 of 23 Polymers 2020, 12, x FOR PEER REVIEW 2 of 23 Polymers 2020, 12, 81 2 of 23 coconut fiber with 45% the plant that contains the highest percentage of lignin [8], follow by rice husk with 35% [9] and wood containing 10%–35% lignin [10]. However, even though coconut fiber is the withone that 35% contains [9] and wood the highest containing amount 10%–35% of lignin, lignin its [ 10consumption]. However, and even production though coconut is much fiber lower is the than one thatone that contains contains the highest the highest amount amount of lignin, of lignin, its consumption its consumption and production and production is much is much lower thanlower wood. than wood. Wood is the main raw material in the paper industry and consequently, an important by- Woodproduct is theof this main industry raw material is lignin. in the paper industry and consequently, an important by-product of thisproduct industry of this is industry lignin. is lignin. Two types of wood are normally used for paper production, hardwood and softwood. HardwoodTwo types is usually of wood obtained are normally from usedpine, forfir, paperspruce production, and larch trees hardwood and softwood and softwood. is obtained Hardwood from isHardwood usually obtained is usually from obtained pine, fir,from spruce pine, and fir, larchspruce trees and and larch softwood trees and is softwood obtained fromis obtained eucalyptus, from eucalyptus, poplar and birch trees. Although both types of wood are used to produce different types poplarof paper, and the birch most trees. used Although is softwood. both This types has of an wood advantage are used for to obtaining produce lignin, different since types this of type paper, of theof paper, most usedthe most is softwood. used is softwood. This has This an advantagehas an advantage for obtaining for obtaining lignin, sincelignin, this since type this of type wood of wood contains a higher percentage of lignin and, consequently, makes lignin an extremely abundant containsand inexpensive a higher product. percentage of lignin and, consequently, makes lignin an extremely abundant and inexpensiveand inexpensive product. product. 1.1. Lignin 1.1. Lignin Chemically, lignin is a highly branched, amorphous biomacromolecule that normally presents Chemically, ligninlignin isis aa highlyhighly branched,branched, amorphousamorphous biomacromoleculebiomacromolecule that normallynormally presents molecular masses between 1000 and 20,000 g/mol. At present, it does not exist a defined lignin molecular masses between 1000 and 20,000 gg/mol./mol. At At present, present, it it does not exist a defineddefined lignin structure, since the determination of the lignin structure is extremely complex. The structure of lignin structure, sincesince the determination ofof the lignin structurestructure is extremely complex.complex. The structure of lignin is constituted by different structures and repeated units, which depend on the species, location, is constitutedconstituted by by di differentfferent structures structures and and repeated repeat units,ed units, which which depend depend on the on species, the species, location, location, growth growth duration of the plant, and the extraction process used to separate it from cellulose and durationgrowth duration of the plant, of the and plant, the extraction and the processextraction used process to separate used itto from separate cellulose it from and hemicellulose.cellulose and hemicellulose. However, it is known that the structure of the lignin, regardless the plant and the However,hemicellulose. it is knownHowever, that it the is structureknown that of thethe lignin,structure regardless of the lignin, the plant regardless and the extractionthe plant process,and the extraction process, is composed mainly of three different phenylpropane monomers called sinapyl isextraction composed process, mainly is ofcomposed three diff mainlyerent phenylpropane of three different monomers phenylpropane called sinapylmonomers alcohol, called coniferyl sinapyl alcohol, coniferyl alcohol, and ρ-coumaryl alcohol [11] which give rise to units of p’-hydroxyphenyl alcohol, andconiferylρ-coumaryl alcohol, alcohol and ρ [-coumaryl11] which givealcohol rise [11] to units which of p’-hydroxyphenylgive rise to units of (H), p’-hydroxyphenyl guaiacyl (G) and (H), guaiacyl (G) and syringyl (S). Figure 1 shows the three monolignols that are considered the syringyl(H), guaiacyl (S). Figure (G) and1 shows syringyl the three(S). Figure monolignols 1 shows that the are three considered monolignols the building that are blocks considered of lignin. the building blocks of lignin. Figure 1. The three monolignols considered as the building blocks of lignin. These subunits contain many different functional groups, such as hydroxyls (20% to 30%), These subunits contain many different functional groups, such as hydroxyls (20% to 30%), carbonyls (20%) and methoxyls (90%–95%), which are active sites for further chemical modification of carbonyls (20%) and methoxyls (90%–95%), which are active sites for further chemical modification lignin [12]. Figure2 shows the structure of lignin from softwood. of lignin [12]. Figure 2 shows the structure of lignin from softwood. Figure 2. Structure of softwood lignin. Reprinted with permission from Zakzeski et al. [5]. Copyright Figure 2. Structure of softwood lignin. Reprinted with permission from Zakzeski et al. [5]. Copyright (2010)Figure American 2. Structure Chemical of softwood Society. lignin. Reprinted with permission from Zakzeski et al. [5]. Copyright (2010) American Chemical Society. Polymers 2020, 12, 81 3 of 23 The heterogeneous nature of wood means that there is not currently available method for the quantitative isolation of lignin without the risk of structurally modifying it during the