Preparation of high-strength α-chitin nanofiber-based hydrogels under mild conditions

Previously, we reported the preparation of high-crystalline hydrogels from alpha-chitin nanofibers by a simple NaOH treatment at low temperatures. In the present study, mild conditions were applied to both alpha-chitin powder and nanofibers to make hydrogels using calcium chloride dehydrate-saturated methanol (Ca solvent). Interestingly, nanofibrillation of powder occurred during the treatment. Compared with the powder, here, both wet chitin nanofiber (WChNF) and methanol-treated chitin nanofiber (MChNF) sheets could easily form into hydrogels at room temperature without the reflux conditions. In this system, water was an important factor in limiting the destruction of chitin crystalline structures caused by calcium ions. This was evident in the WChNF-based hydrogel, which exhibited higher crystallinity and retained its original continuous nanofiber network structure. Finally, we examined the tensile properties of samples prepared from NaOH solution and Ca solvent in the wet state. Although the hydrogel made from Ca solvent was not formed via interdigitation, the WChNF-based one exhibited considerably high tensile strength at around 1.3 MPa, which was close to the NaOH/ethanol-treated sample (tensile strength: 1.8 MPa).