Overview of Polymer Surface and Interface Science

While vascular prostheses are now in common use, they present several problems for long term stability (110). The first problem is the rapid adsorption of protein, triggering blood coagulation and leading to platelet adhesion. The second problem arises because of the difference in modulus and surface properties of vascular prostheses and normal blood vessels. As a result of the latter, the surfaces of the red blood cells may become abraded, resulting in hemolysis, platelet activation, and aggregation. Note that in this instance, it is not the polymer undergoing wear, but the red blood cell membranes. 

Initially, it was postulated that hydrophilic polymers or polymer surfaces would be suitable blood-compatible materials, i.e., they should not induce the coagulation process. A variant of this thinking was the use of a gradient modulus material, tough but flexible on the outside, but becoming soft and highly swollen on the inside. Other people examined the electrical properties of the surface of polymers, such as polarizability, net surface electrical charge, or overall surface potential. 

Another approach utilizes the grafting of the sulfate-containing polysaccharide heparin onto the blood-contacting surface. This material prevents coagulation and concomitant thrombosis. The strong anionic character of heparin enables it to bond ionically to cationic surfaces. The heparin is slowly released into the bloodstream. During this release process, the concentration of heparin near the surface is sufficiently high to prevent the formation of a thrombus for several days. Unfortunately, after this period of time, the release rate decreases. Thus, this approach is only suitable for use in short-term devices. 

A new approach involves growing of living cells on the blood-contacting surface. Bone marrow cells (112) and endothelial cell monolayers (113) both contribute to nonthrombogenicity. 

No material can come into being without creating a surface or interface An exanunation of the literature shows that modern polymer interface science dates only from about 1989, although clearly there are many older papers. In 

---