Development of sponge-like cellulose nanofiber hydrogels via freeze-concentrated structuring and acid-mediated interfibrillar assembly

Bio-based hydrogels are promising sustainable soft materials. However many polysaccharide- and protein-based hydrogels form brittle networks and show limited resistance to large compressive deformation. This study developed a freeze-concentrated gelation strategy for TEMPO-oxidized cellulose nanofiber (TOCNF) hydrogels using freeze–thaw processing and citric acid (CA)-mediated interfibrillar assembly. During freezing TOCNFs were confined and concentrated in the freeze-concentrated liquid (FCL). This reduced the interfibrillar distance and supported network formation during thawing. Controlled thawing in CA solution at 4 °C induced gradual protonation of surface carboxylate groups. As electrostatic repulsion decreased, TOCNFs associated into a continuous bulk network. Rapid thawing at 25 °C accelerated gelation but produced weaker structures, suggesting that slow acid diffusion and fibril rearrangement in the FCL are important for uniform network development. Mechanical behavior depended strongly on the carboxyl substitution degree. Higher substitution increased compression work and resistance to deformation. Under repeated compression at 80% strain the hydrogels showed sponge-like deformation governed by reversible water expulsion and reabsorption and maintained 85–95% height recovery without macroscopic fracture. SEM confirmed a honeycomb-like porous structure and a sheet-like interfibrillar network. FTIR and XRD further showed CA-induced protonation, increased crystallinity and fibril rearrangement within the freeze-concentrated structure. These results show that freeze-concentrated structuring with acid-mediated protonation can regulate TOCNFs assembly and produce sponge-like polymer hydrogels. This strategy offers a platform for bio-based soft materials including food structuring matrices, tissue-mimetic systems and functional porous polymer networks.