Blood clots and thrombi consist primarily of a mesh of branched fibers made of the protein fibrin. We show how these networks give rise to the remarkable extensibility and elasticity of blood clots by determining structural and mechanical properties of the clot at the network, fiber, and molecular levels. The force required to stretch a clot initially rises almost linearly and is accompanied by a dramatic decrease in the clot volume. These macroscopic changes are accompanied by fiber alignment and bundling following forced protein unfolding. We develop constitutive models to integrate observations at spatial scales that span six orders of magnitude and indicate that fibrin clot extensibility and shrinkage are both manifestations of protein unfolding, which is not apparent in other matrix proteins such as collagen.

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