Self-Sustaining Cellulose Nanofiber Hydrogel Produced by Hydrothermal Gelation without Additives

We prepared a self-sustaining hydrogel from 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibers (TOCNs) via hydrothermal treatment at 160 °C. The self-sustaining hydrogels could be obtained at less than 1 wt % TOCNs without any additives. Brownish hydrogels obtained after the hydrothermal treatment could be rendered transparent by immersing them in distilled water at 5 °C. The compressive modulus of the hydrogel increased with increasing heating time. X-ray diffraction studies reveal that the crystal structure of the internal layers of the TOCNs remained intact after the hydrothermal treatment and depigmentation. The hydrothermal treatment caused the hydrolysis of molecules, especially the glucuronate units, from the external layer of TOCN. The elimination of the glucuronate units decreased the net negative surface charge of the TOCNs, resulting in their aggregation into a three-dimensional network structure owing to the predominance of attractive forces. Such additive-free hydrogels that can be shaped into diverse forms are promising for medical applications.