Platelets play an important role in blood coagulation, particularly in the formation of primary thrombi. It is thought that the aggregation of platelets, which initiates primary thrombi formation, is mediated by a macromolecule called von Willebrand Factor (vWF). vWF is a long chain macromolecule that exists in the blood flow as a soluble form and in the vessel wall as an insoluble form. Figure 1 schematically shows normal (a) and pathological (b) thrombus formation processes as illustrated by Ikeda (1998) In both cases, platelets adhere to the injured vessel wall and then form a thrombus in cooperation with the fibrin network, red cells, and other blood constituents. vWF is thought to play a more important role in pathological thrombosis formation than in the normal hemostatic process, particularly due to its ability to react to hemodynamic stress.