A blood clot is composed of ligated fibrin fibers that bend more than they stretch.


Fibrin fibers bend much more than they stretch, according to Jean-Philippe Collet, John Weisel (University of Pennsylvania, Philadelphia, PA), and colleagues. This flexibility lends the necessary elasticity to blood clots.Blood clots, which are composed of fibrin fibers, are both elastic and plastic—they mostly return to their original form after stretching but can also be irreversibly deformed. This viscoelasticity makes clots stiff enough to stem blood flow but pliable enough not to become obstructive.

Weisel's group investigated the mechanical properties of individual fibers that confer viscoelasticity to clots. They used laser tweezers to pull on beads attached to fibrin fibers within clots that were prepared from blood plasma. By measuring the force required to displace the bead a given distance, they calculated the fiber stiffness and found that individual fibrin fibers are 300 times more pliant for bending than they are for stretching. “From these measurements and from the clot structure,” says Weisel, “we can say that fibrin is not rubber-like,” which some scientists had previously hypothesized to account for clot elasticity.

Fibers could be stiffened nearly tenfold by the addition of factor XIIIa, an enzyme that stabilizes clots by creating covalent linkages between fiber molecules. Weisel's group now plans to model how the mechanical properties of individual fibers relate to the viscoelasticity of whole clots. Weisel notes, “I hope this research gets the attention of clinicians as well as researchers, because the mechanical properties of clots are important for understanding their function and pathology.”


Collet, J.P., et al.
Proc. Natl. Acad. Sci. USA.