Bioinspired artificial platelets: past, present and future

Abstract

Platelets are anucleate blood cells produced from megakaryocytes predominantly in the bone marrow and released into blood circulation at a healthy count of 150,000-400,00 per μL and circulation lifespan of 7-9 days. Platelets are the first responders at the site of vascular injury and bleeding, and participate in clot formation via injury site-specific primary mechanisms of adhesion, activation and aggregation to form a platelet plug, as well as secondary mechanisms of augmenting coagulation via thrombin amplification and fibrin generation. Platelets also secrete various granule contents that enhance these mechanisms for clot growth and stability. The resultant clot seals the injury site to stanch bleeding, a process termed as hemostasis. Due to this critical role, a reduction in platelet count or dysregulation in platelet function is associated with bleeding risks and hemorrhagic complications. These scenarios are often treated by prophylactic or emergency transfusion of platelets. However, platelet transfusions face significant challenges due to limited donor availability, difficult portability and storage, high bacterial contamination risks, and very short shelf life (~5-7 days). These are currently being addressed by a robust volume of research involving reduced temperature storage and pathogen reduction processes on donor platelets to improve shelf-life and reduce contamination, as well as bioreactor-based approaches to generate donor-independent platelets from stem cells in vitro. In parallel, a complementary research field has emerged that involves the design of artificial platelets utilizing biosynthetic particle constructs that functionally emulate various hemostatic mechanisms of platelets. Here, we provide a comprehensive review of the history and the current state-of-the-art artificial platelet approaches, along with discussing the translational opportunities and challenges.

Keywords: Artificial Platelets; Bioinspired; Haemostasis; Platelets; Transfusion.

Figure 1.

Schematic showing the central hemostatic mechanisms driven by platelets involving adhesion to von Willebrand Factor (VWF) by platelet GPIb?, adhesion to collagen by platelet GPVI and GPIa/IIa (?2?1), aggregation via binding of fibrinogen to platelet integrin GPIIb-IIIa (?IIb?3), exposure of phosphatidylserine (PS) on activated platelet surface that renders the ‘thrombin burst’ for coagulation amplification and fibrin generation, and secretion of platelet granule contents to further enhance coagulation outputs; Scanning Electron Microscopy (SEM) images show characteristic adhesion and aggregation of activated platelets on a collagen surface, as well as formation of fibrin clot over activated platelets; The fibrin clot is ultimately degraded by the action of plasmin produced from plasminogen by the action of endothelial cell-derived tissue plasminogen activator (tPA).

Figure 2.

Platelet surrogate designs derived from (and dependent on) natural platelets, especially via extraction and utilization of platelet-derived membranes that partially retain the functions of hemostatically relevant glycoproteins and lipids.

Figure 3.

Artificial platelet designs that utilize particle surface coating with proteins and antibody fragments to emulate various hemostatic mechanisms of platelets; Examples include particle surface-decoration with fibrin-binding antibody fragments or nanobodies to emulate platelet-fibrin interactions for clot retraction, particle surface-decoration with fibrinogen to enhance platelet aggregation, and particle surface-decoration with recombinant glycoproteins that emulate VWF- and collagen-adhesive mechanisms of platelets.

Figure 4.

Artificial platelet designs that utilize particle surface coating with peptides to emulate various hemostatic mechanisms of platelets; Examples include particle surface-decoration with fibrinogen-relevant RGD and H-12 peptides to augment platelet aggregation, and VWF-binding as well as collagen-binding peptides (VBP and CBP) to emulate platelet adhesion mechanisms.

Figure 5.

Artificial platelet designs that utilize heteromultivalent surface decoration of particles to integrate the adhesive (VWF- and collagen-binding) and aggregatory (fibrinogen-to-GPIIb-IIIa binding) mechanisms of platelets in hemostasis; Examples include particle surface-decoration with VWFbinding, collagen-binding and fibrinogen-mimetic peptides (VBP, CBP and FMP).