  Collagen networks provide the main structural component of most tissues and
represent an important ingredient for bio-mimetic materials for bio-medical
applications. Here we study the mechanical properties of stiff collagen
networks derived from three different echinoderms and show that they exhibit
non-linear stiffening followed by brittle fracture. The disordered nature of
the network leads to strong sample-to-sample fluctuations in elasticity and
fracture strength. We perform numerical simulations of a three dimensional
model for the deformation of a cross-linked elastic fibril network which is
able to reproduce the macroscopic features of the experimental results and
provide insights into the internal mechanics of stiff collagen networks. Our
numerical model provides an avenue for the design of collagen membranes with
tunable mechanical properties.
