An in situ inhibition strategy: Forming a physical barrier around ionic crosslinkers to toughen double-network hydrogels

Ionically crosslinked networks are extremely effective in reinforcing double-network (DN) hydrogels. But the simultaneous improvement of both homogeneity and ionic crosslinking density remains as a challenge for toughing DN hydrogels. Herein, an in situ inhibition method is proposed to solve this problem. In this method, inhibitor is generated in situ to form a physical barrier around the ionic crosslinker, thereby decreasing the ionic crosslinking rate and further toughening DN hydrogels in a controllable manner. The chemical structure, micromorphology and mechanical properties were investigated, and molecular dynamics simulations were performed to reveal the inhibition mechanism. The transmission electron microscopy (TEM) images confirm the formation of inhibitors. It is revealed that the introduction of inhibitor can decrease ionic crosslinking rate, leading to an improvement in homogeneity and mechanical properties. The fracture energy of alginate-Ca²⁺/polyacrylamide (PAAm) DN hydrogels is up to 19,638 ± 947 J/m², which is notably larger than the previously reported values (9000–16,000 J/m²). The inhibitor does not affect the chemical structures, thermal stability and micromorphology. This in situ inhibition method has significant potential to become a general method to toughen DN hydrogels containing ionically crosslinked networks, which will greatly promote the usage of hydrogels in diverse engineering applications.