A blood clot needs to have the right degree of stiffness and plasticity to stem the flow of blood and yet be digestable by lytic enzymes so as not to form a thrombus, causing heart attacks, strokes, or pulmonary emboli, but the origin of these mechanical properties is unknown. Clots are made up of a three-dimensional network of fibrin fibers stabilized through ligation with a transglutaminase, factor XIIIa. We developed methods to measure the elastic moduli of individual fibrin fibers in fibrin clots with or without ligation, using optical tweezers for trapping beads attached to the fibers that functioned as handles to flex or stretch a fiber. Here, we report direct measurements of the microscopic mechanical properties of such a polymer. Fibers were much stiffer for stretching than for flexion, as expected from their diameter and length. Elastic moduli for individual fibers in plasma clots were 1.7 ؎ 1.3 and 14.5 ؎ 3.5 MPa for unligated and ligated fibers, respectively. Similar values were obtained by other independent methods, including analysis of measurements of fluctuations in bead force as a result of Brownian motion. These results provide a basis for understanding the origin of clot elasticity.fibrinogen ͉ optical trap ͉ viscoelasticity ͉ microrheology ͉ cardiovascular B lood clots play an essential role by stopping bleeding, but they can also cause heart attacks and strokes. Clots are formed when the enzyme thrombin cleaves fibrinogen to generate fibrin monomers, which polymerize to produce a threedimensional network of fibers (1-8). Fibrin is stabilized by ligation, ¶ the formation of intermolecular covalent bonds at specific sites with a transglutaminase, factor XIIIa, rendering the whole clot stiffer and resistant to fibrinolytic dissolution (9, 10). The viscoelastic properties of clots and their major constituent fibrin are normally finely tuned to optimize how they stop bleeding while also minimizing their effect in cardiovascular disease, because bleeding occurs if clot stiffness is too low; a decreased rate of fibrinolysis and increased thrombosis and thromboembolism are generally associated with stiff and friable clots, although such relationships are complex (10 -14). Although much is known of fibrin assembly mechanisms (1)(2)(3)(4)(5)(6)(7)(8)(15)(16)(17)(18), the origin of clot viscoelasticity remains to be established.The elasticity of a fibrin clot, like that of rubber-like polymers, is characterized by very large deformability with essentially complete recovery (19). However, the elasticity of the fibrin clot cannot be rubber-like, because it is not a random-coil network made up of thin, highly flexible strands; instead, it is a network made up of thick branching fibers. As an example of how unrealistic such rubber-like models are, it can be calculated from clot stiffness that there would be an average of only one fibrin molecule per branch point for a rubber-like model (20), yet electron micrographs show that the clots used for these experiments commonly have Ϸ1 million fibrin molecules bet...
