Figure 3.

(a) Wetting theory. (b) Diffusion theory. (c) Fracture theory. Figures modified from [35] with permission. (d) Mechanical theory. (e) The principle of double network hydrogel enhances adhesion. The blue network represents polyacrylamide, the orange network represents alginate, and the red dots represent calcium ions. (f) Principle of topological binding. The chitosan chains are dissolved in a solution at pH = 5. The chitosan chains diffuse into the gel at pH = 7, using hydrogen bonds to form a new network. Figures modified from [36] with permission.