Fiber flex

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...