A nonswellable gradient hydrogel with tunable mechanical properties

Susceptibility of traditional hydrogels to water leads to the deterioration of their mechanical properties and dimensional instability. Inspired by bone tissues, here, we report a nonswellable gradient hydrogel with adjustable mechanical properties via a simple acid-heat treatment of polyamide-based hydrogels. Both the hydrophilicity/hydrophobicity and crosslinking degree of the gel can be simultaneously regulated by taking advantage of the conversion of amide groups to imide groups in the hydrogel. A gradient imide structure is formed with a dense layer near the surface of the gel. The resultant hydrogel has no size change both in water and 0.9 wt% NaCl solution while it shows high strength with a compressive stress of 70 MPa at 70% strain at a higher imidization degree. At a lower imidization degree, the gel has excellent fatigue resistance and resilience and can return to its original state after 5 cycles of 90% strain.