Fig. 1.

Schematic illustration of the design principles for soft materials to be tough and strong. (A) Soft material contains a crack of size a and a process zone around the crack tip. (B) Toughening. High mechanical dissipation in the process zone, manifested as a hysteresis loop in the curve of nominal stress $s$ vs. stretch $\lambda$, toughens the material. (C) Flaw-insensitive strength. The fracture strength of a material increases with the decrease of flaw size until the flaw reaches a critical length scale $a_c$ below which the fracture strength is flaw-insensitive $s_c$. (D) High strength. High-functionality cross-links that accommodate hidden polymer chains can promote simultaneous stiffening and fracture of significant polymer chains, which gives soft material high fracture strength.