Antithrombotic Mechanisms of Urokinase lmmobilized Polyurethane

Influence of Washing and Sterilization on Properties of Polyurethane Coated Fabrics Used in Surgery and for Wrapping Sterile Items

The objective of this work was to determine the influence of washing and sterilization under real hospital conditions on properties of microbial barrier offered by polyurethane coated fabrics used in surgery and for wrapping sterile items. Emphasis was put on the change of surface polyurethane coating by using FTIR analysis. The permeability and durability of the microbial barrier were determined after 0, 10, and 20 washing and sterilization procedures according to previously developed methods. Bacterial endospores of the apathogenic species of the genus Bacillus Geobacillus stearothermophilus and Bacillus atrophaeus were used. Mechanical damage to medical textiles in the washing and sterilization process was determined according to standard HRN EN ISO 13914-1:2008 and associated with changes in physical and mechanical properties. Chemical changes of PU coatings were determined using FTIR analysis. The results showed an exceptionally efficient microbial barrier and its durability in all samples after 0, 10 and 20 washing and sterilization procedures and for a period of one, two and three months.

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.

The use of antithrombotic therapies in reducing synthetic small-diameter vascular graft thrombosis

BACKGROUND

Thrombosis of synthetic small-diameter bypass grafts remains a major problem. The aim of this article is to review the antithrombotic strategies that have been used in an attempt to reduce graft thrombogenicity.

METHODS

A PubMed/MEDLINE search was performed using the search terms "vascular graft thrombosis," "small-diameter graft thrombosis," "synthetic graft thrombosis" combined with "antithrombotic," "antiplatelet," "anticoagulant," "Dacron," "PTFE," and "polyurethane."

RESULTS

The majority of studies on antithrombotic therapies have used either in vitro models or in vivo animal experiments. Many of the therapies used in these settings do show antithrombotic efficacy against synthetic graft materials. There is however, a distinct lack of human in vivo studies to further delineate the performance and limitations of therapies displaying good antithrombotic characteristics.

CONCLUSION

Very few antithrombotic therapies have translated into clinical use. More human in vivo studies are required to assess the efficacy and safety of such therapies.

Mechanical properties of small-diameter polyurethane vascular grafts reinforced by weft-knitted tubular fabric

Polyester filament yarns of different Deniers were knitted into tubular fabrics with different densities and thicknesses on a specially designed weft-knitting machine. The developed tubular fabric was used to reinforce polyurethane vascular graft and thus a kind of composite vascular graft was fabricated with a small inner diameter of 4 mm. Tensile properties of the reinforced composite vascular grafts were compared with the control tubular fabric and the pure PU vascular grafts. Elasticity and strength of the reinforced vascular grafts were improved compared with the weft-knitted tubular fabrics. Strength of the reinforced composite vascular grafts was almost 5-10 times of the strength of the pure PU vascular grafts. As the PU content increased in the reinforced composite vascular grafts, the wall thickness of the vascular graft and its strength increased, but the initial modulus of the reinforced composite vascular grafts remained similar to that of the weft-knitted tubular fabric, and the PU content showed little influence on the initial modulus of the reinforced composite vascular grafts. Microporous structure can also be fabricated in the wall of the reinforced composite vascular grafts.

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.

Urokinase-immobilization suppresses inflammatory responses to polyurethane tubes implanted in rabbit muscles

Urokinase and plasmin appear to have antiinflammatory activity in some injury models, and urokinase immobilization has been clinically used to prevent thrombus formation in various implants, including intravenous indwelling catheters and subcutaneous drainage tubes. In the present study, polyurethane tubes were embedded in rabbit muscle for 3 months and the effect of urokinase immobilization on inflammatory responses to the implanted tubes was studied at 1 week, 1 month, and 3 months. Mononuclear leukocyte accumulation occurred around implanted polyurethane tubes and peaked after 1 month, but was reduced significantly by urokinase immobilization. The treatment also lessened as well as delayed eosinophil accumulation, but did not affect fibrosis caused by implanted tubes. These results indicate suppressive effects of urokinase immobilization on polyurethane-elicited inflammatory responses and suggest that an approach to develop persistently active urokinase immobilization is rational for successful long-term device implantation.

Expression of fibrinolysis activators and their inhibitor in neointima of polyester vascular grafts

The aim of the study was to evaluate dynamic changes in the expression of fibrinolytic system components in neointima forming in polyester vascular grafts. The study was carried out on 18 mongrel dogs divided into three groups, that underwent replacement of abdominal aorta with a polyester double velour prosthesis. Grafts were removed at 1, 4 and 12 months. The specimens were fixed according to AMeX method. Immunohistochemical labeling for von Willebrand factor (vWf), tissue plasminogen activator (t-PA), urokinase (u-PA), its receptor (u-PAR), plasminogen activator inhibitor type 1 (PAI-1) and D-dimer (DD) was performed. Increasing intensity of vWf expression on the graft luminal surface was found in successive periods of the study. A light positive t-PA and u-PA staining was shown in neointima at 1 month and its intensity was significantly increased at 4 and 12 months. Expression of u-PAR appeared at 4 months. A light positive PAI-1 and DD staining was demonstrated in neointima in all periods of the study. The results demonstrated increasing expression of fibrinolysis activators in neointima of polyester vascular grafts. Intensive expression of plasminogen activators, when compared to their inhibitor may reduce thrombotic properties of graft neointima particularly in the late period after prosthesis implantation.

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.

In vitro blood compatibility of polymeric biomaterials through covalent immobilization of an amidine derivative

We present a surface coating with anticoagulant characteristics showing significantly reduced coagulation activation. The synthesis of a monomeric conjugate containing a benzamidine moiety was carried out and its inhibitory activity against human thrombin, the key enzyme of the blood coagulation cascade, was determined using a chromogenic assay. Based on that, low-thrombogenic interfaces were prepared by covalent attachment of this low-molecular weight thrombin inhibitor on poly(octadecene-alt-maleic anhydride) copolymer thin films and characterized using ellipsometry, XPS and dynamic contact angle measurements. The in vitro hemocompatibility tests using freshly drawn human whole blood showed, in agreement with the SEM images, that a PO-MA film modified with a benzamidine moiety using a PEG spacer decreased the activation of coagulation, platelets and the complement system. The decreased protein adsorption, in addition to the specific inhibition of thrombin, effectively enhanced the short-term hemocompatibility characteristics.

Effect of blood access on the platelets of azotemic patients initiating dialysis therapy

The effect of blood access on platelets and clotting factors was investigated in 46 azotemic patients. Arteriovenous fistula was used in 10 patients (AVF group), and polyurethane double-lumen catheters were inserted through the subclavian vein in 6 patients (PUS group) or through the femoral vein in 15 patients (PUF group). Indwelling urokinase-immobilized single-lumen catheters and double-lumen catheters were placed in the femoral vein of 5 patients (UKS group) and 10 patients (UKD group), respectively. Blood cell counts, beta-thromboglobulin (beta-TG), platelet factor 4 (PF4), prothrombin time, and activated partial thromboplastin time were measured before insertion while catheters were indwelling and after catheters were pulled out. Although the platelet count decreased to 83% of the initial value during indwelling in the PUF group and 89% in the PUS group, it did not decrease in the AVF, UKS, and UKD groups. There were no differences between the PUF and PUS groups nor between the UKS and UKD groups. Plasma beta-TG increased in the PUF and UKD groups with indwelling catheters but did not change with the AVF. From these results, we conclude that the AVF did not activate platelets, the urokinase-immobilized catheter activated platelets, and the polyurethane catheter activated and decreased platelets. This might be due to the different surface properties of each blood access. Thus, the urokinase-immobilized catheter seems to be more favorable than the polyurethane catheter for emergency blood access.