Synthesis methods, characteristics, and applications of lignin-based hydrogels.
| Synthesis method | Crosslinking mechanism | Key advantages | Limitations | Applications | References |
|---|---|---|---|---|---|
| Free-radical polymerization | Covalent: lignin acts as a macro-initiator or macromonomer copolymerizing with vinyl monomers (e.g., acrylic acid, acrylamide). | High gel strength, highly tunable swelling ratio (> 500 g/g), excellent reproducibility. | Use of synthetic monomers, potential for toxic initiator residues, and complex purification. | Super-absorbents, wastewater treatment. | [16, 35, 37] |
| Chemical crosslinking | Covalent: reaction of lignin’s hydroxyl/carboxyl groups with crosslinkers, e.g., epichlorohydrin (EPI), glutaraldehyde, or PEG-diglycidyl ether. | Simple process, high structural stability, direct use of lignin. | Brittleness, potential toxicity of crosslinkers. | Controlled-release matrices (fertilizers, drugs), adsorbents. | [8, 47] |
| Enzymatic crosslinking | Covalent: oxidative enzymes (e.g., laccase, peroxidase) generate phenoxy radicals on lignin that undergo coupling. | Green and sustainable process, high biocompatibility, mild reaction conditions. | Slower kinetics, less control over network density, and high enzyme cost. | Biomedical applications (tissue engineering, drug delivery). | [19, 34, 44, 48] |
| Metal-ion coordination | Physical: non-covalent coordination between lignin’s functional groups (e.g., catechol, carboxyl) and multivalent cations (Fe3+, Al3+, Zn2+). | Injectable, self-healing, stimuli-responsive (pH, ions), reversible bonds. | Mechanically softer; stability can be environment-dependent. | Injectable drug depots, conductive composites, smart actuators. | [22, 49] |
| Hydrogen bonding/host-guest | Physical: non-covalent interactions with polymers [e.g., polyvinyl alcohol (PVA), chitosan] or designed host-guest pairs (e.g., β-cyclodextrin-adamantane). | Excellent biocompatibility, reversible and dynamic, shear-thinning. | Generally weak mechanical properties, sensitive to temperature/pH. | Wearable sensors, drug delivery carriers, and soft robotics. | [21, 50–52] |
AB: Conceptualization, Data curation, Investigation, Visualization, Writing—original draft. YA: Resources, Methodology, Software, Validation, Writing—review & editing. TKB: Conceptualization, Software, Supervision, Writing—original draft, Writing—review & editing. MTI: Conceptualization, Supervision, Project administration, Writing—review & editing. All authors read and approved the submitted version.
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