The creation of an antithrombotic surface by apyrase immobilization

Unravelling Surface Modification Strategies for Preventing Medical Device-Induced Thrombosis

The use of biomaterials in implanted medical devices remains hampered by platelet adhesion and blood coagulation. Thrombus formation is a prevalent cause of failure of these blood-contacting devices. Although systemic anticoagulant can be used to support materials and devices with poor blood compatibility, its negative effects such as an increased chance of bleeding, make materials with superior hemocompatibility extremely attractive, especially for long-term applications. This review examines blood-surface interactions, the pathogenesis of clotting on blood-contacting medical devices, popular surface modification techniques, mechanisms of action of anticoagulant coatings, and discusses future directions in biomaterial research for preventing thrombosis. In addition, this paper comprehensively reviews several novel methods that either entirely prevent interaction between material surfaces and blood components or regulate the reaction of the coagulation cascade, thrombocytes, and leukocytes.

Ce-based solid-phase catalysts for phosphate hydrolysis as new tools for next-generation nanoarchitectonics

This review comprehensively covers synthetic catalysts for the hydrolysis of biorelevant phosphates and pyrophosphates, which bridge between nanoarchitectonics and biology to construct their interdisciplinary hybrids. In the early 1980s, remarkable catalytic activity of Ce4+ ion for phosphate hydrolysis was found. More recently, this finding has been extended to Ce-based solid catalysts (CeO2 and Ce-based metal-organic frameworks (MOFs)), which are directly compatible with nanoarchitectonics. Monoesters and triesters of phosphates, as well as pyrophosphates, were effectively cleaved by these catalysts. With the use of either CeO2 nanoparticles or elegantly designed Ce-based MOF, highly stable phosphodiester linkages were also hydrolyzed. On the surfaces of all these solid catalysts, Ce4+ and Ce3+ coexist and cooperate for the catalysis. The Ce4+ activates phosphate substrates as a strong acid, whereas the Ce3+ provides metal-bound hydroxide as an eminent nucleophile. Applications of these Ce-based catalysts to practical purposes are also discussed.

Control of Blood Coagulation by Hemocompatible Material Surfaces-A Review

Hemocompatibility of biomaterials in contact with the blood of patients is a prerequisite for the short- and long-term applications of medical devices such as cardiovascular stents, artificial heart valves, ventricular assist devices, catheters, blood linings and extracorporeal devices such as artificial kidneys (hemodialysis), extracorporeal membrane oxygenation (ECMO) and cardiopulmonary bypass. Although lower blood compatibility of materials and devices can be handled with systemic anticoagulation, its side effects, such as an increased bleeding risk, make materials that have a better hemocompatibility highly desirable, particularly in long-term applications. This review provides a short overview on the basic mechanisms of blood coagulation including plasmatic coagulation and blood platelets, as well as the activation of the complement system. Furthermore, a survey on concepts for tailoring the blood response of biomaterials to improve the hemocompatibility of medical devices is given which covers different approaches that either inhibit interaction of material surfaces with blood components completely or control the response of the coagulation system, blood platelets and leukocytes.

Combination strategies for antithrombotic biomaterials: an emerging trend towards hemocompatibility

Surface-induced thrombosis is a frequent, critical issue for blood-contacting medical devices that poses a serious threat to patient safety and device functionality. Antithrombotic material design strategies including the immobilization of anticoagulants, alterations in surface chemistries and morphology, and the release of antithrombotic compounds have made great strides in the field with the ultimate goal of circumventing the need for systemic anticoagulation, but have yet to achieve the same hemocompatibility as the native endothelium. Given that the endothelium achieves this state through the use of many mechanisms of action, there is a rising trend in combining these established design strategies for improved antithrombotic actions. Here, we describe this emerging paradigm, highlighting the apparent advantages of multiple antithrombotic mechanisms of action and discussing the demonstrated potential of this new direction.

Intrinsic Apyrase-Like Activity of Cerium-Based Metal-Organic Frameworks (MOFs): Dephosphorylation of Adenosine Tri- and Diphosphate

Apyrase is an important family of extracellular enzymes that catalyse the hydrolysis of high-energy phosphate bonds (HEPBs) in ATP and ADP, thereby modulating many physiological processes and driving life activities. Herein, we report an unexpected discovery that cerium-based metal-organic frameworks (Ce-MOFs) of UiO-66(Ce) have intrinsic apyrase-like activity for ATP/ADP-related physiological processes. The abundant Ce^III /Ce^IV couple sites of Ce-MOFs endow them with the ability to selectively catalyse the hydrolysis of HEPBs of ATP and ADP under physiological conditions. Compared to natural enzymes, they could resist extreme pH and temperature, and present a broad range of working conditions. Based on this finding, a significant inhibitory effect on ADP-induced platelet aggregation was observed upon exposing the platelet-rich plasma (PRP) to the biomimetic UiO-66(Ce) films, prefiguring their wide application potentials in medicine and biotechnology.

The blood compatibility challenge. Part 4: Surface modification for hemocompatible materials: Passive and active approaches to guide blood-material interactions

Biomedical devices in the blood flow disturb the fine-tuned balance of pro- and anti-coagulant factors in blood and vessel wall. Numerous technologies have been suggested to reduce coagulant and inflammatory responses of the body towards the device material, ranging from camouflage effects to permanent activity and further to a responsive interaction with the host systems. However, not all types of modification are suitable for all types of medical products. This review has a focus on application-oriented considerations of hemocompatible surface fittings. Thus, passive versus bioactive modifications are discussed along with the control of protein adsorption, stability of the immobilization, and the type of bioactive substance, biological or synthetic. Further considerations are related to the target system, whether enzymes or cells should be addressed in arterial or venous system, or whether the blood vessel wall is addressed. Recent developments like feedback controlled or self-renewing systems for drug release or addressing cellular regulation pathways of blood platelets and endothelial cells are paradigms for a generation of blood contacting devices, which are hemocompatible by cooperation with the host system. STATEMENT OF SIGNIFICANCE: This paper is part 4 of a series of 4 reviews discussing the problem of biomaterial associated thrombogenicity. The objective was to highlight features of broad agreement and provide commentary on those aspects of the problem that were subject to dispute. We hope that future investigators will update these reviews as new scholarship resolves the uncertainties of today.

Kinetic study of NTPDase immobilization and its effect of haemocompatibility on polyethylene terephthalate

Poor haemocompatibility of material surfaces is a serious limitation that can lead to failure of blood-contacting devices and implants. In this work, we have improved the haemocompatibility of polyethylene terephthalate (PET) surfaces by immobilizing apyrase/ecto-nucleoside triphosphate diphosphohydrolase (NTPDase) on to the carboxylated PET. NTPDase immobilized PET surfaces scavenge the ADP released by activated platelets, which prevents further platelet activation and aggregation. The surface properties of the modified PET were characterized by scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDAX), and contact angle measurement. The enzyme attachment and stability on the modified PET surfaces were evaluated. The kinetics of free enzyme and immobilized enzyme were studied and fitted using the Michaelis-Menten kinetic model. Both free and immobilized NTPDase followed Michaelis-Menten kinetics with similar Michaelis-Menten constants (K_m). This suggests that the activity of NTPDase was unchanged upon immobilization. Protein adsorption and %hemolysis was significantly reduced for carboxylated PET and NTPDase immobilized PET surfaces compared to unmodified PET. Lactate dehydrogenase assay showed that the number of adhered platelets reduced by more than an order of magnitude for the NTPDase immobilized PET surface compared to unmodified PET. These results clearly indicate that NTPDase immobilization significantly enhances the haemocompatibility of PET surfaces.

Dangerous liaisons: complement, coagulation, and kallikrein/kinin cross-talk act as a linchpin in the events leading to thromboinflammation

Innate immunity is fundamental to our defense against microorganisms. Physiologically, the intravascular innate immune system acts as a purging system that identifies and removes foreign substances leading to thromboinflammatory responses, tissue remodeling, and repair. It is also a key contributor to the adverse effects observed in many diseases and therapies involving biomaterials and therapeutic cells/organs. The intravascular innate immune system consists of the cascade systems of the blood (the complement, contact, coagulation, and fibrinolytic systems), the blood cells (polymorphonuclear cells, monocytes, platelets), and the endothelial cell lining of the vessels. Activation of the intravascular innate immune system in vivo leads to thromboinflammation that can be activated by several of the system's pathways and that initiates repair after tissue damage and leads to adverse reactions in several disorders and treatment modalities. In this review, we summarize the current knowledge in the field and discuss the obstacles that exist in order to study the cross-talk between the components of the intravascular innate immune system. These include the use of purified in vitro systems, animal models and various types of anticoagulants. In order to avoid some of these obstacles we have developed specialized human whole blood models that allow investigation of the cross-talk between the various cascade systems and the blood cells. We in particular stress that platelets are involved in these interactions and that the lectin pathway of the complement system is an emerging part of innate immunity that interacts with the contact/coagulation system. Understanding the resulting thromboinflammation will allow development of new therapeutic modalities.

Complement inhibition in biomaterial- and biosurface-induced thromboinflammation

Therapeutic medicine today includes a vast number of procedures involving the use of biomaterials, transplantation of therapeutic cells or cell clusters, as well as of solid organs. These treatment modalities are obviously of great benefit to the patient, but also present a great challenge to the innate immune system, since they involve direct exposure of non-biological materials, cells of non-hematological origin as well as endothelial cells, damaged by ischemia-perfusion in solid organs to proteins and cells in the blood. The result of such an exposure may be an inappropriate activation of the complement and contact/kallikrein systems, which produce mediators capable of triggering the platelets and PMNs and monocytes, which can ultimately result in thrombotic and inflammatory (i.e., a thrombo-inflammatory) response to the treatment modality. In this concept review, we give an overview of the mechanisms of recognition within the innate immunity system, with the aim to identify suitable points for intervention. Finally, we discuss emerging and promising techniques for surface modification of biomaterials and cells with specific inhibitors in order to diminish thromboinflammation and improve clinical outcome.

Blood compatible materials: state of the art

Devices that function in contact with blood are ubiquitous in clinical medicine and biotechnology. These devices include vascular grafts, coronary stents, heart valves, catheters, hemodialysers, heart-lung bypass systems and many others. Blood contact generally leads to thrombosis (among other adverse outcomes), and no material has yet been developed which remains thrombus-free indefinitely and in all situations: extracorporeally, in the venous circulation and in the arterial circulation. In this article knowledge on blood-material interactions and "thromboresistant" materials is reviewed. Current approaches to the development of thromboresistant materials are discussed including surface passivation; incorporation and/or release of anticoagulants, antiplatelet agents and thrombolytic agents; and mimicry of the vascular endothelium.

Biomaterials in cardiovascular research: applications and clinical implications

Cardiovascular biomaterials (CB) dominate the category of biomaterials based on the demand and investments in this field. This review article classifies the CB into three major classes, namely, metals, polymers, and biological materials and collates the information about the CB. Blood compatibility is one of the major criteria which limit the use of biomaterials for cardiovascular application. Several key players are associated with blood compatibility and they are discussed in this paper. To enhance the compatibility of the CB, several surface modification strategies were in use currently. Some recent applications of surface modification technology on the materials for cardiovascular devices were also discussed for better understanding. Finally, the current trend of the CB, endothelization of the cardiac implants and utilization of induced human pluripotent stem cells (ihPSCs), is also presented in this review. The field of CB is growing constantly and many new investigators and researchers are developing interest in this domain. This review will serve as a one stop arrangement to quickly grasp the basic research in the field of CB.

The formation of an anti-restenotic/anti-thrombotic surface by immobilization of nitric oxide synthase on a metallic carrier

Coronary stenosis due to atherosclerosis, the primary cause of coronary artery disease, is generally treated by balloon dilatation and stent implantation, which can result in damage to the endothelial lining of blood vessels. This leads to the restenosis of the lumen as a consequence of migration and proliferation of smooth muscle cells (SMCs). Nitric oxide (NO), which is produced and secreted by vascular endothelial cells (ECs), is a central anti-inflammatory and anti-atherogenic player in the vasculature. The goal of the present study was to develop an enzymatically active surface capable of converting the prodrug l-arginine, to the active drug, NO, thus providing a targeted drug delivery interface. NO synthase (NOS) was chemically immobilized on the surface of a stainless steel carrier with preservation of its activity. The ability of this functionalized NO-producing surface to prevent or delay processes involved in restenosis and thrombus formation was tested. This surface was found to significantly promote EC adhesion and proliferation while inhibiting that of SMCs. Furthermore, platelet adherence to this surface was markedly inhibited. Beyond the application considered here, this approach can be implemented for the local conversion of any systemically administered prodrug to the active drug, using catalysts attached to the surface of the implant.

Evaluation of the blood compatibility of materials, cells, and tissues: basic concepts, test models, and practical guidelines

Medicine today uses a wide range of biomaterials, most of which make contact with blood permanently or transiently upon implantation. Contact between blood and nonbiological materials or cells or tissue of nonhematologic origin initiates activation of the cascade systems (complement, contact activation/coagulation) of the blood, which induces platelet and leukocyte activation. Although substantial progress regarding biocompatibility has been made, many materials and medical treatment procedures are still associated with severe side effects. Therefore, there is a great need for adequate models and guidelines for evaluating the blood compatibility of biomaterials. Due to the substantial amount of cross talk between the different cascade systems and cell populations in the blood, it is advisable to use an intact system for evaluation. Here, we describe three such in vitro models for the evaluation of the biocompatibility of materials and therapeutic cells and tissues. The use of different anticoagulants and specific inhibitors in order to be able to dissect interactions between the different cascade systems and cells of the blood is discussed. In addition, we describe two clinically relevant medical treatment modalities, the integration of titanium implants and transplantation of islets of Langerhans to patients with type 1 diabetes, whose mechanisms of action we have addressed using these in vitro models.

Autoregulation of thromboinflammation on biomaterial surfaces by a multicomponent therapeutic coating

Activation of the thrombotic and complement systems is the main recognition and effector mechanisms in the multiple adverse biological responses triggered when biomaterials or therapeutic cells come into blood contact. We have created a surface which is auto-protective to human innate immunity by combining three fundamentally different strategies, all developed by us previously, which have been shown to induce substantial, but incomplete hemocompatibility when used separately. In summary, we have conjugated a factor H–binding peptide; and an ADP-degrading enzyme; using a PEG linker on both material and cellular surfaces. When exposed to human whole blood, factor H was specifically recruited to the modified surfaces and inhibited complement attack. In addition, activation of platelets and coagulation was efficiently attenuated, by degrading ADP. Thus, by inhibiting thromboinflammation using a multicomponent approach, we have created a hybrid surface with the potential to greatly reduce incompatibility reactions involving biomaterials and transplantation.

Corn trypsin inhibitor coating attenuates the prothrombotic properties of catheters in vitro and in vivo

Catheters initiate coagulation by activating factor (f) XII, which can lead to catheter thrombosis. Fondaparinux, which only targets activated fX (fXa), is associated with more catheter thrombosis than heparin, which targets fXa and thrombin. To render catheters less thrombogenic and fondaparinux more effective, we examined whether coating catheters with corn trypsin inhibitor (CTI), which blocks fXIIa, attenuates catheter-induced clotting and promotes fondaparinux activity. Compared with unmodified catheters, CTI-coated catheters demonstrated (a) decreased adsorption of fibrinogen and fXII, (b) greater inhibition of fXIIa generated by catheter-induced autoactivation, (c) attenuated fXIIa-mediated activation of fXI and (d) longer plasma clotting times in the absence or presence of fondaparinux. In an accelerated catheter thrombosis model in rabbits, (a) the time to catheter occlusion was longer with CTI-coated catheters than with unmodified catheters and (b) an intravenous dose of fondaparinux that had no effect on the time to occlusion of unmodified catheters extended the time to occlusion of CTI-coated catheters. These findings support the concept that the prothrombotic activity of catheters reflects their capacity to activate fXII and identify CTI immobilization as a novel approach for rendering catheters and other blood-contacting medical devices less thrombogenic.

Activation of coagulation and platelets by candidate membranes of implantable devices in a whole blood model without soluble anticoagulant

Implantable devices are challenged with thrombus formation at their biomaterial interface. Thus the importance of identifying compatible biomaterials that will help to improve the performance of these devices are becoming increasingly paramount. The aim of this study was to evaluate the activation of coagulation and platelets by candidate membranes considered for use in implantable devices on the basis of an adapted whole blood model without soluble anticoagulants. Evaluated materials were incubated with whole blood without soluble anticoagulant in wells coated with heparin. Prothrombin fragment 1+2 (PTF 1+2), thrombin-antithrombin complex (TAT), and β-thromboglobulin (BTG) were analyzed in plasma samples using enzyme immunoassays. The C5 inhibitor eculizumab was used to evaluate the role of complement. Incubation of two of the polyamide membranes PAR and PATF led to an increase in concentration of PTF 1+2 and TAT (p < 0.01 for PAR, ns for PATF). The BTG concentration was significantly increased for five materials [PAR, PATF, polycarbonate (PC), and two polyarylethersulphone membranes PAES-1 and PAES-2]. Complement inhibition had no effect on coagulation or platelet activation induced by PAR and PATF. In conclusion, PAR and PATF were not compatible with blood and should be avoided for use in implantable devices.

Protection of nonself surfaces from complement attack by factor H-binding peptides: implications for therapeutic medicine

Exposure of nonself surfaces such as those of biomaterials or transplanted cells and organs to host blood frequently triggers innate immune responses, thereby affecting both their functionality and tolerability. Activation of the alternative pathway of complement plays a decisive role in this unfavorable reaction. Whereas previous studies demonstrated that immobilization of physiological regulators of complement activation (RCA) can attenuate this foreign body-induced activation, simple and efficient approaches for coating artificial surfaces with intact RCA are still missing. The conjugation of small molecular entities that capture RCA with high affinity is an intriguing alternative, as this creates a surface with autoregulatory activity upon exposure to blood. We therefore screened two variable cysteine-constrained phage-displayed peptide libraries for factor H-binding peptides. We discovered three peptide classes that differed with respect to their main target binding areas. Peptides binding to the broad middle region of factor H (domains 5-18) were of particular interest, as they do not interfere with either regulatory or binding activities. One peptide in this group (5C6) was further characterized and showed high factor H-capturing activity while retaining its functional integrity. Most importantly, when 5C6 was coated to a model polystyrene surface and exposed to human lepirudin-anticoagulated plasma, the bound peptide captured factor H and substantially inhibited complement activation by the alternative pathway. Our study therefore provides a promising and novel approach to produce therapeutic materials with enhanced biocompatibility.

Hemocompatibility of silicon-based substrates for biomedical implant applications

Silicon membranes with highly uniform nanopore sizes fabricated using microelectromechanical systems (MEMS) technology allow for the development of miniaturized implants such as those needed for renal replacement therapies. However, the blood compatibility of silicon has thus far been an unresolved issue in the use of these substrates in implantable biomedical devices. We report the results of hemocompatibility studies using bare silicon, polysilicon, and modified silicon substrates. The surface modifications tested have been shown to reduce protein and/or platelet adhesion, thus potentially improving biocompatibility of silicon. Hemocompatibility was evaluated under four categories—coagulation (thrombin–antithrombin complex, TAT generation), complement activation (complement protein, C3a production), platelet activation (P-selectin, CD62P expression), and platelet adhesion. Our tests revealed that all silicon substrates display low coagulation and complement activation, comparable to that of Teflon and stainless steel, two materials commonly used in medical implants, and significantly lower than that of diethylaminoethyl (DEAE) cellulose, a polymer used in dialysis membranes. Unmodified silicon and polysilicon showed significant platelet attachment; however, the surface modifications on silicon reduced platelet adhesion and activation to levels comparable to that on Teflon. These results suggest that surface-modified silicon substrates are viable for the development of miniaturized renal replacement systems.