A Facile Approach for Anisotropic Hydrogel with Light-Regulated Stiffness and Its Application to Achieve Mechanical Toughening.

Many load-bearing tissues in nature obtain high toughness by fabricating anisotropic structures with spatially regulated composition and modulus at the macroscale. This inspires a toughening strategy for hydrogels based on the controlling of modulus heterogeneity. Herein, a facile approach to realize light-regulated spatial modulus heterogeneity with large contrast in hydrogels is proposed. Ferric citric acid complex (Fe 3+ /CA-complex) is used as a light-responsive ionic cross-linker, which can first stiffen an alginate/polyacrylamide (Alg/PAAm) hydrogel by coordinating with the Alg to form another network, then realize light-triggered softening through photoreduction of ferric ions (Fe 3+ ). Based on this, a stripe-patterned hydrogel with alternating stiff and soft segments can be fabricated through photopatterning. The modulus contrast between the stiff and soft phases can be adjusted by control of several influence factors and the maximum modulus contrast can reach up to 87 times. As a result, the toughness of the stripe-patterned hydrogel is enhanced by 3.5 times comparing to that hydrogel without a pattern. This approach shows great potential in the synthesis of smart hydrogels with light-programmable mechanical performances, and may be widely applicable for the hydrogels with functional groups that can coordinate with metal ions.