  Collagen forms the structural scaffold of connective tissues in all mammals.
Tissues are remarkably resistant against mechanical deformations because
collagen molecules hierarchically self-assemble in fibrous networks that
stiffen with increasing strain. Nevertheless, collagen networks do fracture
when tissues are overloaded or subject to pathological conditions such as
aneurysms. Prior studies of the role of collagen in tissue fracture have mainly
focused on tendons, which contain highly aligned bundles of collagen. By
contrast, little is known about fracture of the orientationally more disordered
collagen networks present in many other tissues such as skin and cartilage.
Here, we combine shear rheology of reconstituted collagen networks with
computer simulations to investigate the primary determinants of fracture in
disordered collagen networks. We show that the fracture strain is controlled by
the coordination number of the network junctions, with less connected networks
fracturing at larger strains. The hierarchical structure of collagen fine-tunes
the fracture strain by providing structural plasticity at the network and fiber
level. Our findings imply that structural disorder provides a protective
mechanism against network fracture that can optimize the strength of biological
tissues.
