“Laser Tweezers” to Study Blood Clots

Clotting of the blood is an exceedingly complex process that involves numerous precursors and regulatory mechanisms. Interestingly enough, the body also posseses a separate process–the fibrinolytic system–that makes sure that blood clots get dissolved, if needed. Likewise, this system is also very complex. Now researchers from the University of Pennsylvania School of Medicine have introduced a new method to study composition of blood clots (in the picture, fibrin fibers are blue, platelet aggregates are purple, and red blood cells are red):

For the first time ever, using “laser tweezers,” the mechanical properties of an individual fiber in a blood clot have been determined by researchers at the University of Pennsylvania School of Medicine. Their work, led by John W. Weisel, PhD, Professor of Cell and Developmental Biology at Penn, and published in this week’s early online edition of the Proceedings of the National Academy of Sciences, provides a basis for understanding how the elasticity of the whole clot arises.
Clots are a three-dimensional network of fibrin fibers, stabilized by another protein called factor XIIIa. A blood clot needs to have the right degree of stiffness and plasticity to stem the flow of blood when tissue is damaged, yet be digestible enough by enzymes in the blood so that it does not block blood-flow and cause heart attacks and strokes.
Weisel and colleagues developed a novel way to measure the elasticity of individual fibrin fibers in clots-with and without the factor XIIIa stabilization. They used “laser tweezers”-essentially a laser-beam focused on a microscopic bead ‘handle’ attached to the fibers-to pull in different directions on the fiber.
The investigators found that the fibers, which are long and very thin, bend much more easily than they stretch, suggesting that clots deform in flowing blood or under other stresses primarily by the bending of their fibers.
Weisel likens the structure of a clot composed of fibrin fibers to a microscopic version of a bridge and its many struts. “Knowing the mechanical properties of each strut, an engineer can extrapolate the properties of the entire bridge,” he explains. “To measure the stiffness of a fiber, we used light to apply a tiny force to it and observed it bend in a light microscope, just as an engineer would measure the stiffness of a beam on a macroscopic scale. The mechanical properties of blood clots have been measured for many years, so now we can develop models to relate individual fiber and whole clot properties to understand mechanisms that can yield clots that have vastly different properties.”
He states that these findings have relevance for many areas: materials science, polymer chemistry, biophysics, protein biochemistry, and hematology. “We present the first determination of the microscopic mechanical properties of any polymer of this sort,” says Weisel…

The press release…