Thrombo-resistant

The ability of S -nitrosothiols to mimic many of the biological properties of NO itself may emanate from in vivo decomposition to generate NO. This decomposition is catalysed by Cu2+,n and may be important in the development of thrombo-resistant devices used in kidney dialysis or coronary by-pass surgery.196 It is also possible that direct transfer of NO from RSNO occurs in biological systems.197... 

However, in spite of the significant diversity in the quantitative evaluation of platelet adhesion, the platelets adhered onto heparinized surfaces are neither aggregated nor activated 4-78>. In other words, the intracellular contents of the adhered platelets, which may otherwise contribute to the blood clotting process, is not evolved into the bloodstream. This conclusion is verified by the results of Table 12, which indicate that the increased platelet adhesion does not result in a worse thrombo-resistance the blood clotting time at a HCP surface is an order of magnitude higher than that at the surface of the initial non-heparinized polymer. 

Heparin-like copolymers containing up to 100 units of sulfonated glucose or lactose have been prepared by polymerizing with acrylamide using arenediazonium salts with cyanate anions to form a thrombo-resistant heparinized surface. 

Furthermore, the effects on the anticoagulant activity caused by covalent coupling via specific functional groups on the heparin molecule were not investigated. When these aspects are considered, the improved thrombo-resistance of the heparinized materials is not necessarily due to the biochemical interactions of heparin with the appropriate blood factors. 

Material characteristics can be tailored to suit a desired application. For example, blood compatibility of various Blon elastomers was Investigated In the atria of goats by Dr. Williams group (15) In Toronto. Initial results Indicated that elastomers containing high levels of carbon black showed greater thrombo resistance than those with lower amounts or no carbon black. 

Figure 2.9 Critical surface tension (CST) values between 20 and 25 d per square centimeter are favorable for providing a thrombo-resistant surface. This figure illustrates a satisfactory CST of an umbilical vein graft flow surface after having been retrieved eight months following implantation. Textiles and prosthetics tend to have higher CSTs.

Tyrosine-derived polyearbonates have been tested for orthopedic implants, because of their high mechanical strength. Polyacrylates, instead, are used where a more flexible material is needed, for example for thrombo-resistant coating for blood-contacting devices. 

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. 

Other difficulties in in vitro studies arise from the finding that adsorption is changed by flow conditions and by the presence of blood formed elements. In addition, with blood, a stable protein adsorbate appears to be formed only after exposure of a day or more. Despite these difficulties, some adsorption characteristics appear to correlate with thrombogenicity. Preferential adsorption of albumin and low fibrinogen adsorption appear to denote thromboresist-ant surfaces and this is used by many as an indicator of thrombo-resistance. 

It has been well documented that most artificial materials, whether hydrophilic, hydrophobic, made electroconductive by the dispersion of conductive particles in it, smooth, rough or heparinized, were covered with protein immediately following implantation into the cardiovascular system (1,2). Regardless of this protein coating, some of the materials had higher thromboresistance than others. Many theories have been proposed to explain these thrombo-resistant properties, but a definitive answer to these questions would be premature at this time. We have also found that not every protein surface shows thromboresistance however, when the protein surface was treated with an aldehyde or heat. It resulted in a reproducibly improved thromboresistant surface. 

Aldehyde treatment was necessary to enchance the thrombo-resistance of the natural rubber mixed with albumin. Material made from natural rubber mixed with albumin did not show any enchanced thromboresistance without the aldehyde treatment. 

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