Photoregulated Gradient Structure and Programmable Mechanical Performances of Tough Hydrogels with a Hydrogen-Bond Network.

Gradient materials exist widely in natural living organisms, affording fascinating biological and mechanical properties. However, the synthetic gradient hydrogels are usually mechanically weak or only have relatively simple gradient structures. Here, we report on tough nanocomposite hydrogels with designable gradient network structure and mechanical properties by a facile post-photoregulation strategy. Poly(1-vinylimidazole--methacrylic acid) hydrogels containing gold nanorods (AuNRs) are in a glassy state and show typical yielding and forced elastic deformation at room temperature. The gel slightly contracts its volume when the temperature is above the glass-transition temperature that results in a collapse of the chain segments and formation of denser intra- and interchain hydrogen bonds. Consequently, the mechanical properties of the gels are enhanced, when the temperature returns to room temperature. The mechanical performances of hydrogels can also be locally tuned by near-infrared light irradiation due to the photothermal effect of AuNRs. Hydrogels with arbitrary two-dimensional gradients can be facilely developed by site-specific photoirradiation. The treated and untreated regions with different stiffness and yielding stress possess construct behaviors in stretching or twisting deformations. A locally reinforced hydrogel with the kirigami structure becomes notch-insensitive and exhibits improved strength and stretchability because the treated regions ahead the cuts have better resistance to crack advancement. These tough hydrogels with programmable gradient structure and mechanics should find applications as structural elements, biological devices, etc.