Studies of a novel human thrombomodulin immobilized substrate: surface characterization and anticoagulation activity evaluation

Strategies for surface coatings of implantable cardiac medical devices

Cardiac medical devices (CMDs) are required when the patient's cardiac capacity or activity is compromised. To guarantee its correct functionality, the building materials in the development of CMDs must focus on several fundamental properties such as strength, stiffness, rigidity, corrosion resistance, etc. The challenge is more significant because CMDs are generally built with at least one metallic and one polymeric part. However, not only the properties of the materials need to be taken into consideration. The biocompatibility of the materials represents one of the major causes of the success of CMDs in the short and long term. Otherwise, the material will lead to several problems of hemocompatibility (e.g., protein adsorption, platelet aggregation, thrombus formation, bacterial infection, and finally, the rejection of the CMDs). To enhance the hemocompatibility of selected materials, surface modification represents a suitable solution. The surface modification involves the attachment of chemical compounds or bioactive compounds to the surface of the material. These coatings interact with the blood and avoid hemocompatibility and infection issues. This work reviews two main topics: 1) the materials employed in developing CMDs and their key characteristics, and 2) the surface modifications reported in the literature, clinical trials, and those that have reached the market. With the aim of providing to the research community, considerations regarding the choice of materials for CMDs, together with the advantages and disadvantages of the surface modifications and the limitations of the studies performed.

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.

Polysiloxane Nanofilaments Infused with Silicone Oil Prevent Bacterial Adhesion and Suppress Thrombosis on Intranasal Splints

Like all biofluid-contacting medical devices, intranasal splints are highly prone to bacterial adhesion and clot formation. Despite their widespread use and the numerous complications associated with infected splints, limited success has been achieved in advancing their safety and surface biocompatibility, and, to date, no surface-coating strategy has been proposed to simultaneously enhance the antithrombogenicity and bacterial repellency of intranasal splints. Herein, we report an efficient, highly stable lubricant-infused coating for intranasal splints to render their surfaces antithrombogenic and repellent toward bacterial cells. Lubricant-infused intranasal splints were prepared by creating superhydrophobic polysiloxane nanofilament (PSnF) coatings using surface-initiated polymerization of n-propyltrichlorosilane (n-PTCS) and further infiltrating them with a silicone oil lubricant. Compared with commercially available intranasal splints, lubricant-infused, PSnF-coated splints significantly attenuated plasma and blood clot formation and prevented bacterial adhesion and biofilm formation for up to 7 days, the typical duration for which intranasal splints are kept. We further demonstrated that the performance of our engineered biointerface is independent of the underlying substrate and could be used to enhance the hemocompatibility and repellency properties of other medical implants such as medical-grade catheters.

Single and multi-functional coating strategies for enhancing the biocompatibility and tissue integration of blood-contacting medical implants

Device-associated clot formation and poor tissue integration are ongoing problems with permanent and temporary implantable medical devices. These complications lead to increased rates of mortality and morbidity and impose a burden on healthcare systems. In this review, we outline the current approaches for developing single and multi-functional surface coating techniques that aim to circumvent the limitations associated with existing blood-contacting medical devices. We focus on surface coatings that possess dual hemocompatibility and biofunctionality features and discuss their advantages and shortcomings to providing a biocompatible and biodynamic interface between the medical implant and blood. Lastly, we outline the newly developed surface modification techniques that use lubricant-infused coatings and discuss their unique potential and limitations in mitigating medical device-associated complications.

Preparation of chain-end clickable recombinant protein and its bio-orthogonal modification

Introducing unique functional group into protein is an attractive approach for site-selective protein modification applications. In this report, we systemically investigated four site-selective strategies to introduce azide functionality into recombinant thrombomodulin (TM456), via direct recombinant expression with unnatural amino acid, chemical, and enzymatic modification for its bio-orthogonal modification application. First, a straightforward recombinant method to express TM456 with azide functionality near C-terminus by replacing methionine with azidohomoanlanine from methionine auxotroph Escherichia coli cell was investigated. Next, a sortase-mediated ligation (SML) method to incorporate azide functionality into the C-terminus of recombinant TM456 was demonstrated. The third is to add azide functionality to the N-terminal amine of recombinant TM456via amidation chemistry, and the fourth is tyrosine selective three-component Mannich reaction to introduce azide functionality to recombinant TM456. Overall, SML of recombinant protein affords the highest overall yield for incorporating azide functionality into the C-terminus recombinant TM456 since the key protein expression step uses natural amino acids. Also, single site modification facilitates the highest TM456 activity.

Dual targeting of therapeutics to endothelial cells: collaborative enhancement of delivery and effect

Anchoring pharmacologic agents to the vascular lumen has the potential to modulate critical processes at the blood-tissue interface, avoiding many of the off-target effects of systemically circulating agents. We report a novel strategy for endothelial dual targeting of therapeutics, which both enhances drug delivery and enables targeted agents to partner enzymatically to generate enhanced biologic effect. Based on the recent discovery that paired antibodies directed to adjacent epitopes of platelet endothelial cell adhesion molecule (PECAM)-1 stimulate each other's binding, we fused single-chain fragments (scFv) of paired anti-mouse PECAM-1 antibodies to recombinant murine thrombomodulin (TM) and endothelial protein C receptor (EPCR), endothelial membrane proteins that partner in activation of protein C (PC). scFv/TM and scFv/EPCR bound to mouse endothelial PECAM-1 with high affinity (EC50 1.5 and 3.8 nM, respectively), and codelivery induced a 5-fold increase in PC activation not seen when TM and EPCR are anchored to distinct cell adhesion molecules. In a mouse model of acute lung injury, dual targeting reduces both the expression of lung inflammatory markers and trans-endothelial protein leak by as much as 40%, as compared to either agent alone. These findings provide proof of principle for endothelial dual targeting, an approach with numerous potential biomedical applications.

Recombinant thrombomodulin of different domains for pharmaceutical, biomedical, and cell transplantation applications

Thrombomodulin (TM) is a membrane glycoprotein mainly expressed by vascular endothelial cells and is involved in many physiological and pathological processes, such as coagulation, inflammation, cancer development, and embryogenesis. Human TM consists of 557 amino acids divided into five distinct domains: N-terminal lectin-like domain (designated as TMD1); six epidermal growth factor (EGF)-like domain (TMD2); Ser/Thr-rich domain (TMD3); transmembrane domain (TMD4); and cytoplasmic tail domain (TMD5). The different domains are responsible for different biological functions of TM. In the past decades, various domains of TM have been cloned and expressed for TM structural and functional study. Further, recombinant TMs of different domains show promising antithrombotic and anti-inflammatory activity in both rodents and primates and a recombinant soluble TM has been approved for therapeutic application. This review highlights recombinant TMs of diverse structures and their biological functions, as well as the complex interactions of TM with factors involved in the related biological processes. Particularly, recent advances in exploring recombinant TM of different domains for pharmaceutical, biomedical, and cell transplantation applications are summarized.

End-point immobilization of recombinant thrombomodulin via sortase-mediated ligation

We report an enzymatic end-point modification and immobilization of recombinant human thrombomodulin (TM), a cofactor for activation of anticoagulant protein C pathway via thrombin. First, a truncated TM mutant consisting of epidermal growth factor-like domains 4-6 (TM(456)) with a conserved pentapeptide LPETG motif at its C-terminal was expressed and purified in E. coli. Next, the truncated TM(456) derivative was site-specifically modified with N-terminal diglycine containing molecules such as biotin and the fluorescent probe dansyl via sortase A (SrtA) mediated ligation (SML). The successful ligations were confirmed by SDS-PAGE and fluorescence imaging. Finally, the truncated TM(456) was immobilized onto an N-terminal diglycine-functionalized glass slide surface via SML directly. Alternatively, the truncated TM(456) was biotinylated via SML and then immobilized onto a streptavidin-functionalized glass slide surface indirectly. The successful immobilizations were confirmed by fluorescence imaging. The bioactivity of the immobilized truncated TM(456) was further confirmed by protein C activation assay, in which enhanced activation of protein C by immobilized recombinant TM was observed. The sortase A-catalyzed surface ligation took place under mild conditions and occurs rapidly in a single step without prior chemical modification of the target protein. This site-specific covalent modification leads to molecules being arranged in a definitively ordered fashion and facilitating the preservation of the protein's biological activity.

Sequential co-immobilization of thrombomodulin and endothelial protein C receptor on polyurethane: activation of protein C

In an effort to control the surface-mediated activation of thrombin and clot formation, proteins and molecules which mimic the anticoagulant properties of the vascular endothelial lining were immobilized on material surfaces. When immobilized on biomaterial surfaces, thrombomodulin (TM), an endothelial glycoprotein that binds thrombin and activates protein C (PC), was shown to generate activated PC (APC) and delay clot formation. However, TM-mediated activation of PC on biomaterial surfaces was shown to be limited by the transport of PC to the surface, with maximum activation obtained at a surface density of ∼40 fmole TM cm(-2). This work investigates surface immobilized with TM and endothelial protein C receptor (EPCR), a natural cofactor to TM which increases the rate of activation of PC on the native endothelium. A sequential and ordered immobilization of TM and EPCR on polyurethane at an enzymatically relevant distance (<10 nm) resulted in higher amounts of APC compared with surfaces with immobilized TM or with TM and EPCR immobilized randomly and at TM surface densities (1400 fmole cm(-2)) which were previously shown to be transport limited. Ordered TM and EPCR samples also showed increased time to clot formation in experiments with platelet-poor plasma, as measured by thromboelastography. Surfaces immobilized with TM and its natural cofactor EPCR at an enzymatically relevant distance are able to overcome transport limitations, increasing anticoagulant activation and time to clot formation.

Novel thromboresistant materials

The development of a clinically durable small-diameter vascular graft as well as permanently implantable biosensors and artificial organ systems that interface with blood, including the artificial heart, kidney, liver, and lung, remain limited by surface-induced thrombotic responses. Recent breakthroughs in materials science, along with a growing understanding of the molecular events that underlay thrombosis, has led to the design and clinical evaluation of a variety of biologically active coatings that inhibit components of the coagulation pathway and platelet responses by surface immobilization or controlled release of bioactive agents. This report reviews recent progress in generating synthetic thromboresistant surfaces that inhibit (1) protein and cell adsorption, (2) thrombin and fibrin formation, and (3) platelet activation and aggregation.

Polymeric coatings that mimic the endothelium: combining nitric oxide release with surface-bound active thrombomodulin and heparin

Multi-functional bilayer polymeric coatings are prepared with both controlled nitric oxide (NO) release and surface-bound active thrombomodulin (TM) alone or in combination with immobilized heparin. The outer-layer is made of CarboSil, a commercially available copolymer of silicone rubber (SR) and polyurethane (PU). The CarboSil is either carboxylated or aminated via an allophanate reaction with a diisocyanate compound followed by a urea-forming reaction between the generated isocyanate group of the polymer and the amine group of an amino acid (glycine), an oligopeptide (triglycine) or a diamine. The carboxylated CarboSil can then be used to immobilize TM through the formation of an amide bond between the surface carboxylic acid groups and the lysine residues of TM. Aminated CarboSil can also be employed to initially couple heparin to the surface, and then the carboxylic acid groups on heparin can be further used to anchor TM. Both surface-bound TM and heparin's activity are evaluated by chromogenic assays and found to be at clinically significant levels. The underlying NO release layer is made with another commercial SR-PU copolymer (PurSil) mixed with a lipophilic NO donor (N-diazeniumdiolated dibutylhexanediamine (DBHD/N(2)O(2))). The NO release rate can be tuned by changing the thickness of top coatings, and the duration of NO release at physiologically relevant levels can be as long as 2 weeks. The combination of controlled NO release as well as immobilized active TM and heparin from/on the same polymeric surface mimics the highly thromboresistant endothelium layer. Hence, such multifunctional polymer coatings should provide more blood-compatible surfaces for biomedical devices.

An enzyme derivatized polydimethylsiloxane (PDMS) membrane for use in membrane introduction mass spectrometry (MIMS)

Membrane introduction mass spectrometry (MIMS) provides direct measurement of volatile and semivolatile analytes in condensed and gas-phase samples without sample preparation steps. Although MIMS has numerous advantages that include direct, on-line, real-time analysis with low detection limits, current applications of MIMS are predominantly limited to volatile and semivolatile analytes that permeate hydrophobic membranes (e.g., polydimethylsiloxane; PDMS). We report the first enzyme modified PDMS membrane for use with MIMS. This was achieved by immobilizing Candida rugosa lipase directly onto the surface of oxidized PDMS. These surface immobilized enzymes catalyze ester hydrolysis, releasing an alcohol product at the membrane interface that is readily detected. We have successfully used an enzyme modified membrane for the analysis and quantification of low-volatility and hydrophilic esters. We report the quantification of several carboxylic acid esters in dilute aqueous solutions, including a phthalate monoester carboxylate that is not readily detected by conventional MIMS. This new interface demonstrates the potential for extending the range and versatility of MIMS.

Covalently immobilized thrombomodulin inhibits coagulation and complement activation of artificial surfaces in vitro

Thrombomodulin (TM) serves as the endothelial cell receptor for thrombin and alters its characteristics from pro- to anticoagulant. Additionally, it promotes the formation of activated protein C. We evaluated the conservation of the overall outcome of these functions in recombinant TM linked to artificial surfaces by incubation with human whole blood in vitro. TM was covalently immobilized through poly(ethylene glycol) (PEG) spacers onto thin films of poly(octadecene alt maleic anhydride) covering planar glass substrates. TM binding to the polymer films was achieved after active ester formation at the carboxylic acid terminus of the PEG spacers and thoroughly characterized by HPLC-based amino acid analysis, immunofluorescence and ellipsometry. TM-coated samples were incubated for 3h with freshly drawn whole human blood anticoagulated with heparin (5IU/ml) using in-house developed incubation systems. The substantially reduced activation of blood coagulation (TAT) for TM-coated samples correlates well with the degree of contact activation (bradykinin and FXIIa formation) while no significant effects were observed for the platelet activation (PF4). Further, complement activation (C5a levels), was strongly diminished at the TM-containing surfaces. We conclude that the suggested method for preparation of TM immobilization may serve to prepare model substrates for studies on TM interactions but similarly provides a promising coating strategy for blood contacting medical devices.