Salt-Induced Shape-Memory Effect in Gelatin-Based Hydrogels

Hydrophilic biopolymers display a strong tendency for self-organization into stable secondary, tertiary, and quaternary structures in aqueous environments. These structures are sensitive to changes in external conditions, such as temperature, pH or ions/salts, which may lead to molecular and/or macroscopic transitions. Here, we report on biopolymer-based stimuli-sensitive switchable matrices showing a shape-memory function as an output being alternatively switched by two different input signals, such as environmental changes in salt concentration or temperature. This was realized by implementing a shape-memory function in hydrogels based on the coil-to-helix transition of protein chains in gelatin-based networks. The hydrogels exhibited mechanical properties similar to that of soft tissue (storage modulus G' = 1-100 kPa) and high swelling capabilities (Q = 1000-3000 vol %). In these gelatin-based networks, the covalent netpoints defined the permanent shape while after deformation helicalization of the gelatin acted as reversible stimuli-sensitive switches providing additional crosslinks capable of fixing the deformed temporary shape. By using either chaotropic salts to suppress gelatin helicalization or kosmotropic salts to support conformational changes of gelatin toward a helical orientation, these additional crosslinks could be cleaved or formed. In bending experiments, the strain fixity (R_f) and strain recovery ratios (R_r) were determined. While R_f ranged from 65 to 95% and was depending on the network composition, R_r were independent of the hydrogel composition with values about 100%. In addition, R_f and R_r were independent of the type of chaotropic salt that was used in this study, showing equal R_f and R_r values for MgCl₂, NaSCN, and Mg(SCN)₂.