Figure 17. Design principle for tough adhesion of hydrogels – integrate tough dissipative hydrogels and strong interfacial linkages.

a. definition of interfacial toughness and the 90-degree peeling test to measure the interfacial toughness. F is the peeling force, $F_{plateau}$ is the plateau peeling force, and W is the width of the sample. The interfacial toughness can be calculated as ${\text{Γ}}^{\text{inter}}=F_{platenu}/W$ based on the values of $F_{plateau}$ and W measured in the 90-degree peeling test. b. weak interface can give the adhesive failure mode. c. brittle hydrogel matrix can give the cohesive failure mode. d. integration of tough dissipative hydrogels and strong interfacial linkages gives tough adhesion of hydrogels⁴³. The contributions of strong interfacial linkages and mechanical dissipation in the process zone to the total interfacial toughness are ${\text{Γ}}_0^{\text{inter}}$ and ${\text{Γ}}_D^{\text{inter}}$, respectively. The total interfacial toughness of the tough adhesion is ${\text{Γ}}^{\text{inter}}={\text{Γ}}_0^{\text{inter}}+{\text{Γ}}_D^{\text{inter}}$ d is adopted from Ref⁴³.