  Bundles of polymer filaments are responsible for the rich and unique
mechanical behaviors of many biomaterials, including cells and extracellular
matrices. In fibrin biopolymers, whose nonlinear elastic properties are crucial
for normal blood clotting, protofibrils self-assemble and bundle to form
networks of semiflexible fibers. Here we show that the extraordinary
strain-stiffening response of fibrin networks is a direct reflection of the
hierarchical architecture of the fibrin fibers. We measure the rheology of
networks of unbundled protofibrils and find excellent agreement with an affine
model of extensible wormlike polymers. By direct comparison with these data, we
show that physiological fibrin networks composed of thick fibers can be modeled
as networks of tight protofibril bundles. We demonstrate that the tightness of
coupling between protofibrils in the fibers can be tuned by the degree of
enzymatic intermolecular crosslinking by the coagulation Factor XIII.
Furthermore, at high stress, the protofibrils contribute independently to the
network elasticity, which may reflect a decoupling of the tight bundle
structure. The hierarchical architecture of fibrin fibers can thus account for
the nonlinearity and enormous elastic resilience characteristic of blood clots.
