  Skin is the largest organ of many animals. Its protective function against
hostile environments and predatorial attack makes high mechanical strength a
vital characteristic. Here, we measured the mechanical properties of bass fish
skins and found that fish skins are highly ductile with a rupture strain of up
to 30-40% and a rupture strength of 10-15 MPa. The fish skins exhibit a
strain-stiffening behavior. Stretching can effectively eliminate the stress
concentrations near the pre-existing holes and edge notches, suggesting that
the skins are highly damage tolerant. Our measurement determined a
flaw-insensitivity length of several millimeters, which exceeds that of most
engineering materials. The strain-stiffening and damage tolerance of fish skins
are explained by an agent-based model of collagen network in which the
load-bearing collagen microfibers assembled from nanofibrils undergo
straightening and reorientation upon stretching. Our study inspires development
of artificial skins that are thin, flexible, but highly fracture-resistant and
widely applicable in soft robots.
