ALL-POLYSACCHARIDE SCHIFF-BASE HYDROGELS FOR PH-RESPONSIVE SOFT ACTUATORS

Soft actuators are stimulus-responsive materials that change shape in response to external stimuli such as pH, temperature, and light.¹  Herein, pH-responsive hydrogels based on chitosan and oxidized dextran (Figure 1a) and crosslinked by reversible imine bonds (Figure 1b) were prepared. The influence of the polysaccharides concentration, molar ratio between the amine and aldehyde ([NH₂]/[CHO]), freeze-thaw process (FT), and pH (5.5 or 7.2) on the mechanical and swelling properties (Figure 1c) were investigated aiming to develop a pH-responsive soft actuator (Figure 1d). The hydrogels exhibited swelling ratio (SR) values at pH = 5.5 and 7.2, as well as Young Modulus (E) in the range from 90 to 830, 13 to 93, and 0.6 to 6 kPa, respectively, by increasing polysaccharides concentration from 10 to 20 mg mL^-1 and decreasing [NH₂]/[CHO] from 15 to 2.5. After the freezing-thawing process, the hydrogels are tougher, with an increase of 5-10 folds in E values, and the swelling ratio is remarkably less affected by the pH. Therefore, the combination of an active swelling-shrinking hydrogel layer with a passive frozen-thawed hydrogel layer is an interesting strategy for constructing a pH-responsive soft actuator based on polysaccharides and functional within the pH range from 5.5 to 7.2.²