Thromboresistence polymers

It seemed reasonable to anticipate that the synergism of these two features (high heparin content and stability of the resultant materials) would result in long-term thromboresistant polymers. The in vivo tests revealed, however, their extremely low thromboresistance as compared to the ionically bound heparin-containing polymers, in particular. The effect is assumed to be caused by a lack of sufficient mobility of the polymer-bound heparin molecules, which prevents the performance of the intrinsic anticoagulant properties of heparin. 

Chlorosulfonated styrene resins and carboxyaminoacid polymers were also found to possess thromboresistant properties by Josefonwicz and coworkers [483]. Studies included investigation of the effect of spacer length between amine and carboxylic groups as well as modification of styrene/isoprene/styrene blocks with chlorosulfonyl isocyanate giving sulfamate and carboxylic functionality [484],... 

Methods for preparing heparin-containing polymeric materials by means of ionic and covalent immobilization of heparin on various polymers are surveyed. The data on the biological activity of heparin are discussed as well as the probable mechanisms of thromboresistance enhancement endowed to polymeric materials by this anticoagulant. Some approaches toward an increased efficiency of anticoagulant properties of immobilized heparin are analyzed, and the position of heparin-containing polymers among other biomedical polymers is discussed. 

Of these the last one has been most widely used, since heparin-modified polymeric materials exhibit the highest and by today unsurpassed effects of thromboresistance enhancement. Many of these materials have not only proved to be potent in trials on animals, but have already found clinical application. These achievements have stimulated continuous interest in heparin-containing polymers (HCP) which is best manifested by listing the investigations performed in the field in recent years and still under way. They involve the new procedures for the synthesis of HCP providing minimal loss of activity of bound heparin, the studies of interactions of HCP with blood and its individual components, as well as on the mechanism of enhanced thromboresistance of HCP, and the search for new tasks for HCP. 

The year of 1961, when Vincent Gott11 observed the inhibition of thrombus formation by immobilized heparin for the first time, was marked as the second birth date of heparin, since it was for the first time isolated from liver tissue. Its anticoagulant action was detected in 1892. Although more than 20 years have passed since Gott s publication, there is still much confusion concerning the views on the mechanism of enhanced thromboresistance of heparin-modified polymers, which greatly hinders the introduction of HCP into clinical practice. 

GBCH-polymers were the first synthetic materials that displayed relatively high thromboresistance. For instance, poly(methyl methacrylate) grafts, having been coated with GBCH and implanted in dog s vena cava, were patent for 14 days, while uncoated PMMA grafts were totally covered with thrombin within the first 2 hours44). 

The heparin content of the materials involving anion-exchange resins goes up to 800 ng/cm2, whereas the maximal heparin content of graphite-based polymers is 0.78 ng/cm2. All of the synthesized polymers, although exhibiting sufficiently high thromboresistance in in vitro tests (Table 3), drastically varied in their stability in various model media (Table 4). 

Polystyrene itself is not used for endoprosthetic purposes and its application is accounted for only because of easy substitutions in benzene rings. The method was subsequently modified for heparinization of silicone and natural rubber, polyethylene, polypropylene, polyethylene terephthalate), and other polymers. Styrene was first grafted onto the polymers by y-radiation and then the above-described reaction was performed in the second step. All the polymers synthesized in this way contained sufficiently large amounts of immobilized heparin (2.8—15.7 ng/cm2) and displayed good thromboresistance when tested in vitro — recalcified blood was not clotted for several hours. 

The fact that thromboresistance of HCP is dependent on the method of immobilization of heparin, together with a rather low activity of covalently immobilized heparin, makes the idea of long-term enhancement of thromboresistance of polymers on their heparinization doubtful 54,70,71J. Naturally, the answer can be given only after a detailed analysis of the interaction of HCP with blood and its components and clarification of the mechanism of the effect of the immobilized heparin on the blood clotting system and relying on the results of in vivo tests of these materials are necessary. 

Table 12. Thromboresistant properties of some polymers containing covalently...

What are the experimental data underlying the first hypothesis It is common knowledge today that the covalently bound heparin (if bound to the polymer rigidly and inexorably) is usually less active than ionically attached heparin which is capable of elution. In a series of works a group of Japanese investigators, who are today probably the only supporters to this concept, has disclosed the correlation between thromboresistance of the polymer and the amount of heparin released into the bloodstream 68 71). The minimal elution rate of heparin providing sufficient thromboresistance was found to be 4 x 10s g/cm2 min. The decrease of the rate of elution resulted in a drastic decrease of thromboresistance. 

To clarify the mechanism of enhanced thromboresistance of such polymers, the state of the blood clotting system before and after the contact with the polymer was examined I36) (Table 19). Obviously, the changes of the blood clotting system parameters are of the same type as those accompanying the interaction of blood with HCP (see Table 16) but essentially exceeding the latter. Consequently, the heparin-protease-... 

Wilson, at Bishop College, and Eberhart and Elkowitz at University of Texas (27) have irradiated a silicone substrate in the presence of chloromethylstyrene monomer to produce a reactive graft polymer that can be quarternized with pyridine and reacted with sodium heparin to produce a thromboresistant heparinized product that has a higher blood compatibility than the untreated silicone. The same group has used essentially the same methods to create a heparin grafted polyethylene surface. 

Members of the biomaterials community continue to search for experimental animal models that assess the thromboresistance of polymers. We developed a new animal model involving rapid, simple retrograde cannu-lation of the goat s carotid arteries. The method promises to assess potential biomaterials, evaluate drugs that may decrease thrombus growth, and measure real-time thrombus growth and dissolution. A critical aspect of this new experimental model is that the continuously monitored net platelet retention data can be modeled mathematically. 

To circumvent many of these undesired side effects associated with systemic heparin administration, many investigators have endeavored to immobilize heparin to blood-contacting polymers to form thromboresistant surfaces. Considering that heparin binds to the endothelium following systemic injection (I), this approach appears attractive. 

The enhancement of albumin binding, and its potential effectiveness in improving thromboresistance, is further demonstrated in the results of simultaneous fibrinogen-albumin exposure, shown in Figure 12. Reduction of fibrinogen adsorption is proportional to add-on of albumin, presumed to be bound to octadecyl residues. Varying yield may depend on the manufacturer s polymer process variables, the derivatization process variables, and the protein solution exposure technique. 

Chitosan-heparin coated polymers display excellent thrombo-resistance properties. The lifetime of the thromboresistance can be extended by covalently binding the heparin to chitosan with the aid of sodium cyanoborohydride. This surface treatment is useful for biomedical applications requiring blood compatibility for periods as long as four days. 

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