Key mechanisms to mechanically improve polyacrylamide-based hydrogels

Hydrogels usually have weak mechanical properties (e.g., very low elastic modulus and strength of the polyacrylamide hydrogel). To improve the mechanical properties for broader applications, double-network hydrogels have been developed, such as the polyacrylamide-alginate hydrogels with the additional alginate network cross-linked by metal ions. In this paper, we reveal that the addition of the alginate alone (< 2 wt% of the hydrogel) cannot obviously change the material's microstructures leading to the improvement of its macroscopic mechanical properties. But the addition of a small amount of metal ions to help create the alginate network can significantly change the hydrogel's microstructures, which improve its mechanical properties by more than 100 times. It is further found by SEM observations (scanning electron microscope) that the modified porous microstructures of hydrogels significantly depend on the type of the metal ions. By adding divalent Ca2+ ions, the hydrogel's original pore walls become thicker, whereas the addition of trivalent Fe3+ ions results in a spider web-like microstructure where a fine low-scale network is built within the original pores. Therefore, our study here demonstrates different mechanisms (i.e., microstructural modifications) for improving hydrogel's mechanical properties, which would be helpful in further promotions of hydrogel applications.