Multiple responsive hydrogel films based on dynamic Schiff base linkages

Stimuli-responsive hydrogels whose swelling degree changes in response to certain external stimulus are usually constructed by the introduction of a functional group. Here we show that they can also be constructed using dynamic bonds as crosslinks because the equilibrium of their formation and breakage can be shifted by external stimuli. As an example, hydrogel films were fabricated from partially oxidized dextran (PO-Dex) and chitosan (Chi) using the layer-by-layer assembly technique. The driving force for the film buildup is the in situ formation of Schiff base bonds between the aldehyde groups on PO-Dex and amino groups on Chi. The swelling of the film was studied using the Fabry–Perot fringes on the reflection spectra. Like ordinary hydrogels, the PO-Dex/Chi hydrogel films swell in water. Their swelling degree decreases with increasing oxidation degree of PO-Dex. The films swell to a larger degree when the pH is lowered. The pH-sensitivity was attributed to amino groups on chitosan and also the dynamic Schiff base linkages, because pH change shifts the equilibrium of the Schiff base reaction in the films, resulting in a change in the crosslink density and therefore a change in swelling degree. When the transient linkages were fixed by NaBH4 reduction, the pH-sensitivity of the films was significantly reduced. The films were found to be sensitive to other external stimuli, including temperature, L-lysine and pyridoxal. These stimuli-responsivities were also attributed to the dynamic Schiff base linkages in the film.