Long term thromboresistance of heparinized surfaces

In vitro haemocompatibility and stability of two types of heparin-immobilized silicon surfaces

Heparin was covalently immobilized onto a silicon surface by two different methods, carbodiimide-based immobilization and photo-immobilization. In the former method, a (3-aminopropyl) trimethoxysilane (APTMS) self-assembled monolayer (SAM) or multilayer was first coated onto the silicon surface as the bridging layer, and heparin was then attached to the surface in the presence of water-soluble carbodiimide. In the latter method, an octadecyltrichlorosilane (OTS) SAM was coated on the silicon surface as the bridging layer, and heparin was modified by attaching photosensitive aryl azide groups. Upon UV illumination, the modified heparin was then covalently immobilized onto the surface. The hydrophilicity of the silicon surface changed after each coating step, and heparin aggregates on APTMS SAM and OTS SAM were observed by atomic force microscopy (AFM). In vitro haemocompatibility assays demonstrated that the deposition of APTMS SAM, APTMS multilayer and OTS SAM enhanced the silicon's haemocompatibility, which was further enhanced by the heparin immobilization. There is no evident distinction regarding the haemocompatibility between the heparin-immobilized surfaces by both methods. However, heparin on silicon with APTMS SAM and multilayer as the bridging layers is very unstable when tested in vitro with a saline solution at 37 degrees C, due to the instability of APTMS SAM and multilayer on silicon. Meanwhile, photo-immobilized heparin on silicon with OTS SAM as the bridging layer showed superb stability.

Antithrombogenic heparin-bound polyurethanes

Many kinds of heparin-bound polyurethanes have been developed. Polyurethanes are a family of elastomers displaying better blood-compatibility than other polymeric materials. It is useful to modify this material by heparinization. Several approaches to heparinization have been devised: 1) a general method of heparinization, applicable to all polymeric materials, 2) a heparinization method specific to polyurethanes, and 3) the design of heparinizable polyurethane derivatives. These three approaches are first explained in detail. Then, the antithrombogenic mechanism of the heparinized polymers is discussed. Finally, the interactions of the heparinized polymers with blood coagulation factors, plasma proteins, and platelets are discussed.

Surface characterization of heparin-complexing poly(amido amine) chains grafted on polyurethane and glass surfaces

Poly(amido-amine) chains grafted onto polyurethanes and glass form stable complexes with heparin yielding potential nonthrombogenic surfaces. The characterization of the surfaces, and the product of each chemical reaction including final heparinized surfaces, has been studied by contact angle data and scanning electron microscopy (SEM). Air in water, octane in water, and drop-on-plate contact angle data were used to estimate surface (gamma sv) and interfacial (gamma sw) free energies. Solid-water work of adhesion (Wa) and its dispersive (Wda) and polar (Ipsw) components were calculated for all studied surfaces. It has been found that the viscosity of polyurethane solution used for film casting influences wetting properties of these films. It has also been found that a direct correlation exists between the Ipsw/Wda values and the degree of coverage of the surfaces by cellular deposits after their exposure to platelet-rich plasma. Final heparinized polyurethane and glass materials are hydrophilic, their Ipsw/Wda ratio is high, and little or no cellular deposit is observed on their surfaces.

Inhibition of surface induced coagulation by preadsorption of albumin-heparin conjugates

Surface coatings of the albumin-heparin conjugates were developed to improve the blood compatibility of polymeric materials. Glass, PVC, Biomer and cellulose acetate were coated with albumin-heparin conjugate and its adsorption and desorption behavior on glass in particular was studied using 3H and 51Cr radiolabeled conjugates. Precoated materials showed a significant prolongation of the Lee-White clotting time as compared with noncoated ones. It was demonstrated that the prolonged clotting time for pretreated glass was due to surface bound conjugate. Prolonged recalcification times of plasma exposed to glass, Biomer, and PVC were obtained using albumin-heparin conjugate precoated surfaces. Albumin-heparin conjugates with high affinity for antithrombin III gave more prolonged clotting times as low affinity conjugates when used as coatings for glass. This indicates that the behaviour of heparin in preadsorbed conjugates resembles that of heparin in solution.

Interaction of antithrombin III with preadsorbed albumin-heparin conjugates

The adsorption of antithrombin III (AT III) onto polystyrene surfaces preadsorbed with albumin or albumin-heparin conjugates was studied using a two step enzyme immuno assay. When AT III-buffer solutions were used, the highest adsorption values were measured on high affinity albumin-heparin conjugate pretreated surfaces. Less AT III adsorption was found on nonfractionated albumin-heparin conjugate preadsorbed surfaces. AT III adsorption could also be detected on low affinity conjugate and albumin coated surfaces. When AT III was adsorbed from plasma or plasma dilutions with buffer, only AT III on surfaces preadsorbed with high affinity or nonfractionated albumin-heparin conjugate was found. These results demonstrate that the heparin moiety of the conjugate is directed to the solution phase whereas the albumin moiety contacts the polystyrene surface.

On the mechanism of enhanced thromboresistance of polymeric materials in the presence of heparin

Polymeric materials with covalently immobilized heparin were shown to display enhanced thromboresistance in vitro and in vivo experiments. This property of heparin-containing polymers is due to the specific effect of immobilized heparin for every step of interaction of a polymer with blood. The presence of heparin substantially changes the character of adsorbed proteins on a polymer surface and the number of adhered platelets. Thromboresistance enhancement is largely carried out by the interaction of immobilized heparin with plasma proteins which is accompanied by the decrease in total blood coagulant activity, by the decrease in fibrinogen, prothrombin and thrombin concentrations, and by the supression of fibrinstabilizing factor activity. The free heparin content in blood is not changed. It was found that immobilized heparin forms complexes with fibrinogen, thrombin and plasmin that produce lytic action on unstabilized fibrin.

Covalently bound conjugates of albumin and heparin: synthesis, fractionation and characterization

Covalently bound conjugates of human serum albumin and heparin were prepared as compounds which could improve the blood-compatibility of polymer surfaces either by preadsorption or by covalent coupling of the conjugates onto blood contacting surfaces. The conjugates (10-16 weight % of heparin) were obtained by a condensation reaction between albumin and heparin using 1-ethyl-3-(dimethylaminopropyl)-carbodiimide. Unreacted albumin and heparin were removed by diethyl-aminoethyl (DEAE)-cellulose and Cibacron Blue Sepharose chromatography respectively. The activity of the heparin component incorporated in the albumin-heparin conjugates (Ac) was compared with that of the heparin used for the synthesis of the conjugates (Anat) by thrombin time, inhibition of Factor Xa and the activated partial thromboplastin time (APTT) assays. The Ac/Anat ratio for the above assays was as follows: Thrombin time 1.25, Factor Xa inhibition 0.5. and APTT 0.5. Gel filtration chromatography showed broad-molecular weight distributions. The conjugates were fractionated using immobilized antithrombin III (ATIII). High ATIII and low ATIII affinity conjugate fractions showed the same behavior as ATIII fractionated heparin with respect to thrombin times and Factor Xa inhibition.

Development of semicrystalline poly(vinyl alcohol) hydrogels for biomedical applications

Swollen crosslinked poly(vinyl alcohol) (PVA) networks were developed and tested as potential biomaterials. They were prepared by electron-beam irradiation of aqueous PVA solutions at various temperatures and doses of irradiation. These materials were characterized by low mechanical properties and especially by low elongation at break and ultimate tensile strength. Reinforcement was achieved by a two-stage dehydration-annealing process, introducing crystallites in the polymeric network. Improved mechanical properties were achieved due to the presence of this crystalline PVA phase. Due to their structure, these hydrogels are proposed to be used as biomembranes for selective transport of macromolecules and as biomaterials for synthetic articular cartilage applications. Surface heparinization was accomplished by reacting heparin to the hydroxyl groups of the PVA surface, through covalent acetal bridges.