The nonlinear elastic properties of fibrin networks are crucial for normal blood clotting . Here, we show that the extraordinary strain-stiffening response of fibrin clots reflects the hierarchical architecture of the fibrin fibers , which are bundles of wormlike protofibrils . 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 clots of thick fibers can be modeled as networks of tight protofibril bundles. 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  enormous elastic resilience characteristic of blood clots.
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.
R
The
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
D
of fibrin networks
R
. Here,
, protofibrils self-assemble and bundle to form networks of semiflexible fibers. Here
R
clots reflects
networks is a direct reflection of
D
, which are bundles of wormlike protofibrils
D
,
R
clots
fibrin networks composed
R
At
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
A
nonlinearity and