Mammalian platelets are anucleated cell fragments derived from megakaryocytes [1] that play a vital role in several physiologic and pathologic processes such as hemostasis and thrombosis [2], release of growth factors and modulation of inflammatory and immune responses [3]. The contribution to hemostasis, a process during which platelets have a key role in forming the plugs that seal injured vessels and arrest bleeding, is essential for the integrity of blood circulation [4, 5]. This complex function involves the ability to adhere to reactive subendothelial structures exposed at sites of injury and aggregate with one another in flowing blood [6]. To be efficient, this process must take place on both venular and arteriolar side of the circulation, the latter being where greatest is the physical challenge of overcoming the fluid dynamic forces that oppose adhesion and aggregation [7].

Platelets, with their ability of targeting vascular injury sites and releasing a variety of active substances during thrombogenesis, are a paradigm for a drug carrier in blood. Development of synthetic materials with the complex biochemical and physical attributes of natural cells may open new opportunities in medical therapy, but remains a difficult challenge. Reports have been published on synthetic blood cells [8, 9] and RGD-coated nanoparticles reproducing some of the functions exhibited by platelets during aggregation [10]. From the physical viewpoint, platelets are flexible and discoidal cell fragments, 2-4 μm in diameter and 0.5 μm in thickness. With respect to distinctive biological features, platelets express on their membrane surface an array of specific proteins that act as adhesive substrate receptors and are essential for localized hemostatic plug formation in areas of tissue damage. One of these receptors, the glycoprotein (GP) Ibα-a component of the GPIb-IX-V complex-is particularly relevant in that, by binding to the A1 domain of multimeric von Willebrand factor (VWF), endows platelets with the unique ability to tether to damaged vascular