Cellulose hydrogels prepared from micron-sized bamboo cellulose fibers.

We demonstrated for the first time that dimensionally stable hydrogels could be obtained from bamboo pulp fibers through dialysis against distilled water followed by a short time of ultrasonic treatment. Micron-sized short fibers rather than cellulose nanofibrils constituted the majority of fibers in the hydrogels. During the pulping process with HNO3 and KClO3, carboxylic groups could be introduced to cellulose due to the mild oxidation of hydroxyl groups. When presented in aqueous NaOH, the carboxylic groups could be converted into their sodium salt form. The subsequent dialysis treatment against water made the negatively charged COO(-) groups extensively exposed. The negatively charged cellulose fibers could induce considerable electrostatic repulsion between them, which was discovered to govern the formation of hydrogels. In addition, it was revealed that homogeneous hydrogels could be formed when the pH was at 7, 9 and 11. However, when salt was added, no dimensionally stable hydrogel was obtained.