Platelets are one of the smallest cells in the human body, having discoid shapes with 2–4 μm diameters, but they play a large role in preventing blood loss when damage has occurred in a vessel [1
]. Platelets initiate hemostasis by using their glycoprotein receptors to form attachments to the damaged tissue, which arrests them from circulating in the blood () [3
]. Once attached, platelet release a variety of agonists and soluble adhesive proteins from within their granules to activate and recruit more platelets to the wound site () [5
]. Platelets can also act as biomechanical elements for the growing clot structure by using their glycoprotein receptors to form bridges between other platelets and the surrounding protein meshwork that forms the hemostatic plug (). They further reinforce the integrity of the plug by using their cytoskeletal filaments to undergo shape change [6
], forming protrusions that enable more physical connections with other platelets within the clot, while also using their actin-myosin interactions to pull the clot into a more compact structure that stabilizes it against the vessel wall [8
Figure 1 Platelet Adhesion and Aggregation: (a) Platelets adhere to the vessel wall when exposed to matrix proteins. (b) Adhered platelets undergo shape change and release soluble adhesive proteins from their α-granules. (c) A hemostatic plug is formed (more ...)
Glycoprotein receptors in platelets bind to ligands sites found within the extracellular matrix (ECM) of the vessel wall and soluble adhesive proteins that platelets release [9
]. ECM proteins of the vessel wall consist mainly of collagen and laminin, but soluble adhesive proteins like von Willebrand Factor (vWF), fibrinogen, and fibronectin can also deposit onto the wound site to enhance platelet adhesion () [10
]. The initial arrest of a platelet from the blood flow involves the glycoprotein receptor GPIb-IX-V, but subsequent engagement of GPIV to collagen can activate integrins α2
, which further assist in the adhesion process. P-selectin receptors on the surface of activated endothelial cells can mediate platelet adhesion through interactions with P-selectin glycoprotein ligand 1 (PSGL1) on a platelet’s membrane after degranulation [14
]. Moreover, platelet GPIbα receptors can also interact with P-selectin to aid in homing platelets to the site of injury [16
]. Receptor-ligand interactions are nano-scale and a single platelet can have a multitude of different receptor-ligand interactions during clot formation (). Understanding these small and complex interactions requires approaches that can specifically control the ligand presentation on a surface and have sufficient measurement sensitivity to interrogate their biophysical properties.
Physical forces also play a critical role in hemostasis by regulating the mechanobiology of platelets. When a platelet adheres to a wound site, adhesive forces keep the platelet attached and prevent it from being dislodged by the blood stream. Receptors GPIb-IX-V and αIIb
are known to have a large role in platelet mechanobiology because they regulate the initial tethering to the vessel wall and the activation of platelet shape change and force generation [17
]. Upon activation, G-actin monomers in platelets polymerize into F-actin filaments, allowing platelets to undergo shape change. Platelet activation also leads to phosphorylation of non-muscle myosin, which can in turn, engage with actin and form contractile filaments. The contractile forces produced by platelets are in the range of piconewtons for a single actin-myosin complex to nanonewtons for single platelets, but are vastly important in stabilizing a clot by compacting its structure [18
] and in strengthening platelet adhesions through integrin-related mechanotransduction [19
]. Another type of force that is important in hemostasis is shear forces applied to platelets due to flow of blood. Shear forces can cause platelets to detach, but are also known to have a major role in the steps from platelet adhesion to aggregation.
A multitude of engineered devices have been developed to look at adhesive, contractile, and shear forces and the role of agonist and receptor-ligand bindings on the clot formation process [5
]. Among the technological advances for studying platelets, micro- and nano-scale tools have been used recently to understand platelet biology and thrombus formation dynamics [24
]. The advantage of these tools is that platelets and their adhesion receptors are micro- and nano-scale is size, so devices that are in the same size range as platelets can be used as programmable materials, in which the physical and adhesive interactions between platelets and their surroundings can be controlled and measured. In this review, we will highlight the tools used to examine clot formation with an emphasis on the tools used to study the role of hemodynamic shear and platelet forces.