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Porous structures in the scanning electron microscopy (SEM) images of (a) lignin-based nanocapsules (reproduced from Ref. [20] with permission from the American Chemical Society), (b) lignin-based fiber mat (reproduced from Ref. [21] with permission from the Royal Society of Chemistry), and (c) lignin-based hydrogel (reproduced from Ref. [22] with permission from the Royal Society of Chemistry).
