Self-scrolling ability of differentially acetylated chitosan film.

Chitosan film cast on a glass slide was exposed to acetic anhydride vapor, resulting in an acetylation gradient in the film, with preferential acetylation of the exposed surface. The difference in degree of acetylation between the two surfaces of the peeled film was confirmed by attenuated total reflection infrared spectroscopy. Upon immersion of the film in water, differential swelling occurred because the more highly acetylated surface absorbed less water, and the resulting bending moment caused self-scrolling with the more highly acetylated surface inside. Simultaneous peeling and scrolling of films with a thickness of micrometer order, using dilute aqueous hydrofluoric acid, afforded tightly rolled chitosan microtubes. This simple self-scrolling mechanism is potentially applicable for micro-scale design with various naturally occurring polymers.