New strategy of platelet substitutes for enhancing platelet aggregation at high shear rates: cooperative effects of a mixed system of fibrinogen γ-chain dodecapeptide- or glycoprotein Ibα-conjugated latex beads under flow conditions

Platelet-inspired nanomedicine in hemostasis thrombosis and thromboinflammation

Platelets are anucleate cell-fragments derived predominantly from megakaryocytes in the bone marrow and released in the blood circulation, with a normal count of 150 000-40 000 per μl and a lifespan of approximately 10 days in humans. A primary role of platelets is to aid in vascular injury site-specific clot formation to stanch bleeding, termed hemostasis. Platelets render hemostasis by a complex concert of mechanisms involving platelet adhesion, activation and aggregation, coagulation amplification, and clot retraction. Additionally, platelet secretome can influence coagulation kinetics and clot morphology. Therefore, platelet defects and dysfunctions result in bleeding complications. Current treatment for such complications involve prophylactic or emergency transfusion of platelets. However, platelet transfusion logistics constantly suffer from limited donor availability, challenges in portability and storage, high bacterial contamination risks, and very short shelf life (~5 days). To address these issues, an exciting area of research is focusing on the development of microparticle- and nanoparticle-based platelet surrogate technologies that can mimic various hemostatic mechanisms of platelets. On the other hand, aberrant occurrence of the platelet mechanisms lead to the pathological manifestation of thrombosis and thromboinflammation. The treatments for this are focused on inhibiting the mechanisms or resolving the formed clots. Here, platelet-inspired technologies can provide unique platforms for disease-targeted drug delivery to achieve high therapeutic efficacy while avoiding systemic side-effects. This review will provide brief mechanistic insight into the role of platelets in hemostasis, thrombosis and thromboinflammation, and present the current state-of-art in the design of platelet-inspired nanomedicine for applications in these areas.

Bioinspired artificial platelets: past, present and future

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.

Recent Advances in Hemostasis at the Nanoscale

Rapid and effective hemostatic materials have received wide attention not only in the battlefield but also in hospitals and clinics. Traditional hemostasis relies on materials with little designability which has many limitations. Nanohemostasis has been proposed since the use of peptides in hemostasis. Nanomaterials exhibit excellent adhesion, versatility, and designability compared to traditional materials, laying a good foundation for future hemostatic materials. This review first summarizes current hemostatic methods and materials, and then introduces several cutting-edge designs and applications of nanohemostatic materials such as polypeptide assembly, electrospinning of cyanoacrylate, and nanochitosan. Particularly, their advantages and working mechanisms are introduced. Finally, the challenges and prospects of nanohemostasis are discussed.

Biomaterials and Advanced Technologies for Hemostatic Management of Bleeding

Bleeding complications arising from trauma, surgery, and as congenital, disease-associated, or drug-induced blood disorders can cause significant morbidities and mortalities in civilian and military populations. Therefore, stoppage of bleeding (hemostasis) is of paramount clinical significance in prophylactic, surgical, and emergency scenarios. For externally accessible injuries, a variety of natural and synthetic biomaterials have undergone robust research, leading to hemostatic technologies including glues, bandages, tamponades, tourniquets, dressings, and procoagulant powders. In contrast, treatment of internal noncompressible hemorrhage still heavily depends on transfusion of whole blood or blood's hemostatic components (platelets, fibrinogen, and coagulation factors). Transfusion of platelets poses significant challenges of limited availability, high cost, contamination risks, short shelf-life, low portability, performance variability, and immunological side effects, while use of fibrinogen or coagulation factors provides only partial mechanisms for hemostasis. With such considerations, significant interdisciplinary research endeavors have been focused on developing materials and technologies that can be manufactured conveniently, sterilized to minimize contamination and enhance shelf-life, and administered intravenously to mimic, leverage, and amplify physiological hemostatic mechanisms. Here, a comprehensive review regarding the various topical, intracavitary, and intravenous hemostatic technologies in terms of materials, mechanisms, and state-of-art is provided, and challenges and opportunities to help advancement of the field are discussed.

Nano- and micro-materials in the treatment of internal bleeding and uncontrolled hemorrhage

Internal bleeding is defined as the loss of blood that occurs inside of a body cavity. After a traumatic injury, hemorrhage accounts for over 35% of pre-hospital deaths and 40% of deaths within the first 24 hours. Coagulopathy, a disorder in which the blood is not able to properly form clots, typically develops after traumatic injury and results in a higher rate of mortality. The current methods to treat internal bleeding and coagulopathy are inadequate due to the requirement of extensive medical equipment that is typically not available at the site of injury. To discover a potential route for future research, several current and novel treatment methods have been reviewed and analyzed. The aim of investigating different potential treatment options is to expand available knowledge, while also call attention to the importance of research in the field of treatment for internal bleeding and hemorrhage due to trauma.

Bio-inspired nanomedicine strategies for artificial blood components

Blood is a fluid connective tissue where living cells are suspended in noncellular liquid matrix. The cellular components of blood render gas exchange (RBCs), immune surveillance (WBCs) and hemostatic responses (platelets), and the noncellular components (salts, proteins, etc.) provide nutrition to various tissues in the body. Dysfunction and deficiencies in these blood components can lead to significant tissue morbidity and mortality. Consequently, transfusion of whole blood or its components is a clinical mainstay in the management of trauma, surgery, myelosuppression, and congenital blood disorders. However, donor-derived blood products suffer from issues of shortage in supply, need for type matching, high risks of pathogenic contamination, limited portability and shelf-life, and a variety of side-effects. While robust research is being directed to resolve these issues, a parallel clinical interest has developed toward bioengineering of synthetic blood substitutes that can provide blood's functions while circumventing the above problems. Nanotechnology has provided exciting approaches to achieve this, using materials engineering strategies to create synthetic and semi-synthetic RBC substitutes for enabling oxygen transport, platelet substitutes for enabling hemostasis, and WBC substitutes for enabling cell-specific immune response. Some of these approaches have further extended the application of blood cell-inspired synthetic and semi-synthetic constructs for targeted drug delivery and nanomedicine. The current study provides a comprehensive review of the various nanotechnology approaches to design synthetic blood cells, along with a critical discussion of successes and challenges of the current state-of-art in this field. WIREs Nanomed Nanobiotechnol 2017, 9:e1464. doi: 10.1002/wnan.1464 For further resources related to this article, please visit the WIREs website.

Nanoparticle Effects on Human Platelets in Vitro: A Comparison between PAMAM and Triazine Dendrimers

Triazine and PAMAM dendrimers of similar size and number of cationic surface groups were compared for their ability to promote platelet aggregation. Triazine dendrimers (G3, G5 and G7) varied in molecular weight from 8 kDa-130 kDa and in surface groups 16-256. PAMAM dendrimers selected for comparison included G3 (7 kDa, 32 surface groups) and G6 (58 kDa, 256 surface groups). The treatment of human platelet-rich plasma (PRP) with low generation triazine dendrimers (0.01-1 µM) did not show any significant effect in human platelet aggregation in vitro; however, the treatment of PRP with larger generations promotes an effective aggregation. These results are in agreement with studies performed with PAMAM dendrimers, where large generations promote aggregation. Triazine dendrimers promote aggregation less aggressively than PAMAM dendrimers, a factor attributed to differences in cationic charge or the formation of supramolecular assemblies of dendrimers.

Adhesive Forces between A1 Domain of von Willebrand Factor and N-terminus Domain of Glycoprotein Ibα Measured by Atomic Force Microscopy

AIM

von Willebrand factor (VWF) plays an important role in the regulation of hemostasis and thrombosis formation, particularly under a high shear rate. However, the adhesive force due to the molecular interaction between VWF and glycoprotein Ibα (GPIbα) has not been fully explored. Thus, we employed atomic force microscopy to directly measure the adhesive force between VWF and GPIbα.

METHODS

We measured the adhesive force between VWF and GPIbα at the molecular level using an atomic force microscope (AFM). An AFM cantilever was coated with recombinant N-terminus VWF binding site of GPIbα, whereas a cover glass was coated with native VWF.

RESULTS

The adhesive force at the molecular level was measured using an AFM. In the presence of 1 μg/mL VWF, the adhesion force was nearly 200 pN. As per the Gaussian fit analysis, the adhesive force of a single bond could have been 54 or 107 pN.

CONCLUSION

Our consideration with the Gaussian fit analysis proposed that the adhesive force of a single bond could be 54 pN, which is very close to that obtained by optical tweezers (50 pN).

A factor VIII-derived peptide enables von Willebrand factor (VWF)-binding of artificial platelet nanoconstructs without interfering with VWF-adhesion of natural platelets

There is substantial clinical interest in synthetic platelet analogs for potential application in transfusion medicine. To this end, our research is focused on self-assembled peptide-lipid nanoconstructs that can undergo injury site-selective adhesion and subsequently promote site-directed active platelet aggregation, thus mimicking platelet's primary hemostatic actions. For injury site-selective adhesion, we have utilized a coagulation factor FVIII-derived VWF-binding peptide (VBP). FVIII binds to VWF's D'-D3 domain while natural platelet GPIbα binds to VWF's A1 domain. Therefore, we hypothesized that the VBP-decorated nanoconstructs will adhere to VWF without mutual competition with natural platelets. We further hypothesized that the adherent VBP-decorated constructs can enhance platelet aggregation when co-decorated with a fibrinogen-mimetic peptide (FMP). To test these hypotheses, we used glycocalicin to selectively block VWF's A1 domain and, using fluorescence microscopy, studied the binding of fluorescently labeled VBP-decorated nanoconstructs versus platelets to ristocetin-treated VWF. Subsequently, we co-decorated the nanoconstructs with VBP and FMP and incubated them with human platelets to study construct-mediated enhancement of platelet aggregation. Decoration with VBP resulted in substantial construct adhesion to ristocetin-treated VWF even if the A1-domain was blocked by glycocalicin. In comparison, such A1-blocking resulted in significant reduction of platelet adhesion. Without A1-blocking, the VBP-decorated constructs and natural platelets could adhere to VWF concomitantly. Furthermore, the constructs co-decorated with VBP and FMP enhanced active platelet aggregation. The results indicate significant promise in utilizing the FVIII-derived VBP in developing synthetic platelet analogs that do not interfere with VWF-binding of natural platelets but allow site-directed enhancement of platelet aggregation when combined with FMP.

A platelet-inspired paradigm for nanomedicine targeted to multiple diseases

Platelets are megakaryocyte-derived anucleated cells found in the blood. They are mainly responsible for rendering hemostasis or clotting to prevent bleeding complications. Decreased platelet numbers or deficiencies in platelet functions can lead to various acute or chronic bleeding conditions and hemorrhage. On the other hand, dysregulated hyperactivity of the clotting process can lead to thrombosis and vascular occlusion. There is significant evidence that beyond hemostasis and thrombosis, platelets play crucial mechanistic roles in other disease scenarios such as inflammation, immune response and cancer metastasis by mediating several cell-cell and cell-matrix interactions, as well as aiding the disease microenvironment via secretion of multiple soluble factors. Therefore, elucidating these mechanistic functions of platelets can provide unique avenues for developing platelet-inspired nanomedicine strategies targeted to these diseases. To this end, the current review provides detailed mechanistic insight into platelets' disease-relevant functions and discusses how these mechanisms can be utilized to engineer targeted nanomedicine systems.

Mimicking adhesive functionalities of blood platelets using ligand-decorated liposomes

Platelet transfusion is used for treating a variety of bleeding complications. Natural platelet-based transfusion products have very short storage life (3-7 days) and high risks of biological contamination and side effects. Consequently, there is significant clinical interest in synthetic platelet-mimetic constructs that can promote hemostasis, while allowing convenient large-scale production, easy portability, long storage life, and minimal biological risks. To this end, research efforts are being directed toward particles that can amplify aggregation of activated platelets or can mimic platelet's ability to undergo adhesion to various vascular matrix proteins. Here, we report on a synthetic construct design that combines the mimicry of platelet's shear-dependent adhesion to vWF and shear-independent adhesion to collagen under flow, on a single particle. For this, we have used 150-nm-diameter liposomes as model particles and have decorated their surface simultaneously with vWF-binding and collagen-binding recombinant protein fragments or synthetic peptide motifs. We demonstrate in vitro that these surface-modified liposomes are able to adhere onto vWF surfaces in a shear-dependent fashion and onto collagen surfaces in a shear-independent fashion under flow. Moreover, when the vWF-binding and the collagen-binding were integrated on a single liposomal platform, the resultant heteromultivalent liposomes showed significantly enhanced adhesion to a vWF/collagen mixed surface compared to liposomes bearing vWF-binding or collagen-binding ligands only, as long as the ligand motifs did not spatially interfere with each other. Altogether, our results establish the feasibility of efficiently mimicking platelet's dual adhesion mechanisms on synthetic particles.

Peptide-decorated liposomes promote arrest and aggregation of activated platelets under flow on vascular injury relevant protein surfaces in vitro

Platelet-mimetic synthetic hemostats are highly attractive in transfusion medicine. To this end, past research reports have described particles that either amplify platelet aggregation or mimic platelet adhesion. However, a construct design that effectively combines both functionalities has not been reported. Here we describe the design of a liposomal construct simultaneously surface-decorated with three peptides (a vWF-binding peptide (VBP), a collagen-binding peptide (CBP), and an active platelet clustering cyclic-RGD (cRGD) peptide), that can integrate platelet-mimetic dual hemostatic activities of adhesion and aggregation. We first demonstrate that surface-immobilized cRGD-liposomes are capable of aggregating activated platelets onto themselves. Subsequently, we demonstrate that hetero-multivalent liposomes bearing VBP, CBP, and cRGD, when introduced in flow with ≈ 20,000 activated platelets per microliter, are capable of adhering to vWF/collagen surfaces and promoting the recruitment/aggregation of platelets onto themselves. We envision that optimizing this construct can lead to a highly refined synthetic hemostat design for potential application in transfusion medicine.

Nanoparticle size and surface charge determine effects of PAMAM dendrimers on human platelets in vitro

Blood platelets are essential in maintaining hemostasis. Various materials can activate platelets and cause them to aggregate. Platelet aggregation in vitro is often used as a marker for materials' thrombogenic properties, and studying nanomaterial interaction with platelets is an important step toward understanding their hematocompatibility. Here we report evaluation of 12 formulations of PAMAM dendrimers varying in size and surface charge. Using a cell counter based method, light transmission aggregometry and scanning electron microscopy, we show that only large cationic dendrimers, but not anionic, neutral or small cationic dendrimers, induce aggregation of human platelets in plasma in vitro. The aggregation caused by large cationic dendrimers was proportional to the number of surface amines. The observed aggregation was not associated with membrane microparticle release, and was insensitive to a variety of chemical and biological inhibitors known to interfere with various pathways of platelet activation. Taken in context with previously reported studies, our data suggest that large cationic PAMAM dendrimers induce platelet aggregation through disruption of membrane integrity.

Biomechanics

This review covers research achievements in the field of biomechanics that have been obtained in the past few years and provides information to researchers in other fields of study.

Development of biodegradable nanosheets as nanoadhesive plaster

Sheet-shaped carriers having both obverse and reverse surfaces (thus, a large contact area for targeting a site and adhesive properties without any chemical cross-linker onto tissue surface) have several advantages as surgical dressings. These advantages include active targeting over spherically shaped carriers, which thus have an extremely small contact area for targeting sites. Here, we propose a novel methodology for preparation of a free-standing, ultra-thin, and biocompatible polymer nanosheet having heterosurfaces, fabricated through macromolecular assembly. In the context of biomedical applications, the targeted properties include injectable sheet-shaped drug carriers having precisely controlled size by exploiting micropatterned substrate, and giant polymer nanosheets composed of biocompatible polysaccharides. A huge aspect ratio, in excess of 106, is particularly applicable for novel surgical dressings. These biocompatible polymer nanosheets having heterosurfaces can thus be regarded as new biomaterials for minimally invasive treatment.