Strengthening and Self-Reinforcement of Polyacrylamide/Chitosan Hydrogel through Salting-Out Treatment and Mechanical Training

Applications of hydrogels─such as in biomimetic artificial muscles─demand a high capacity for large deformation, exceptional strength, and unique self-reinforcement properties. However, designing hydrogels that combine all these attributes remains a challenge. In this work, we strengthen the polyacrylamide/chitosan composite hydrogel by employing a salting-out treatment. Subsequent mechanical training through cyclic loading induces self-reinforcement: when the external deformation exceeds the training deformation, i.e., the historical maximal stretch ratio, the hydrogel exhibits a higher stress compared to the case of monotonic loading. The mechanisms of strengthening and self-reinforcement were elucidated through a combination of mechanical experiments and molecular dynamics simulations. The strengthening of the hydrogel is attributed to the aggregation of chitosan chains, driven by both the shielding of electrostatic repulsion and hydrophobic association. Additionally, cyclic loading promotes the integration of residual dissociated chitosan chains into these aggregated chains, forming larger and stronger cluster domains that further reinforce the hydrogel. Moreover, hydrogels treated with a lower concentration of sodium chloride solution exhibited more pronounced self-reinforcement, a phenomenon effectively explained by the proposed physical picture. This work offers deeper insights into the mechanisms of hydrogel self-reinforcement and provides valuable guidance for designing hydrogels with superior self-reinforcement, thereby presenting a feasible method for developing soft biomimetic devices.