Toughness-enhancing metastructure in the recluse spider's looped ribbon silk

In nature's toughest materials, sacrificial bonds and hidden lengths play a key role in dissipating energy. Here, we show that the recluse spider (Loxosceles genus) spins its 50 nm-thin silk ribbons into sacrificial micro-loops, providing inspiration for the design of toughened uniaxial metamaterials. Previous attempts to incorporate sacrificial metastructure in cylindrical fibers have either failed to enhance toughness or required prohibitively complex manufacturing. In the recluse system, the loops of the ribbon-like strand are anchored by silk-to-silk bonds that do not compromise the fiber's ultimate strength upon release and require no additional adhesive. The silk's thin ribbon morphology facilitates the formation of these strong yet sacrificial bonds and reduces the risk of failure due to bending. Modeling and an experimental proof of concept are used to demonstrate that a looped ribbon metastructure can considerably enhance a material's toughness.