Thrombus Silastic

None of the smooth surfaces prepared by the depositing of plasma polymer on the smooth surface of Silastic tubing showed detectable thrombus formation by gamma camera imaging. Therefore, they were evaluated by measurement of relative rates of platelet consumption. Table 35.7 shows the results obtained using the plasma polymers described previously [5]. 

When Silastic is on the front portion of the rod and blood contacts it first, little thrombus forms on the Silastic, while a distinct thrombus forms on the back polypropylene section (Figure 9a). Thus the thrombus did not propagate upstream, against the flow of blood. When the order of materials was reversed, a frank thrombus developed on the polypropylene, as expected, but the thrombus did not propagate downstream onto the surface of the Silastic. (There is a slight zone of overlap of a few millimeters.) Thus, blood sensitization was not significant. 

The embolization process on the fibronectin-coated PVC surface was quite different than the embolization process on fibronectin-coated Silastic. On fibronectin-coated PVC, 97% of the deposited thrombus [both platelets and fibrin(ogen)] were shed after 120 min of blood contact, while on fibronectin-coated Silastic only 75% of the deposited platelets and none of the fibrin(ogen) was released. This behavior is consistent with the observations shown in Figures 17 and 18 (fibronectin-coated PVC at 30 min and... 

Figure 18. Fibronectin-coated Silastic after 30 min of blood contact. Large adherent thrombus (center) is beginning to embolize. Surrounding surface coverage appears to be mostly fibrin with a few adherent platelets.

The influence of the surface on the thrombotic response was most apparent in the case of a2-macroglobulin. On PVC, a2-macroglobulin promoted greater thrombus formation whereas on Silastic it prevented extensive thrombus buildup. This behavior indicates that different molecular interactions are taking place between the surface and the protein and between the protein and blood. 

To augment the poor mechanical properties of Silastic, Akutsu (11) fabricated a sac-type artificial heart from calendered Silastic sheets reinforced with Dacron mesh. Because it was easily workable, they were able to construct a heart pump in one piece, but unfortunately not without seams. So even though the flex life of the material was extended, thrombus formation (at the seams) remained a problem. 

Liotta (12) constructed his artificial heart from a combination of different materials - Lucite, teflon, polyester urethane, and silk. It is difficult to evaluate the thrombogenic potential of this combination of materials because the longest survival was 13 hours in dogs. The principle cause of death in these animals was low cardiac output secondary to inadequate venous return. The following year in a different series of experiments Liotta (13) tried a different combination of materials. Here, although the experiments were acute in scope, thrombus formation at the blood-plastic interface was a major problem. The left ventricular assist device (LVAD) was a tube-type with the housing and valves constructed of Estane. The internal elastic tube was made of either natural rubber. Silastic, or natural rubber covered externally with Silastic. 

Note the thrombus at the junction of the natural tissue and Silastic. 

Thrombus at the junction of a rough (Dacron graft) and smooth (Silastic) surface. Although a smooth neointima is formed on the graft (below the suture line), thrombus is evident at the graft Silastic junction. 

Figure 7. A passive pump bladder recovered virtually free of thrombus. A mistake while coating the graft-Silastic transition region resulted in the small thrombus seen on the graft.

Figure 9. Recovered Silastic LVAD of identical design as that shown in Figure 8. Thrombus-free bladder is the result of a fabrication change.

The early Baylor heart assist (left ventricular bypass) pumps used Dacron velour as the blood contact material backed with Silastic (Figure 2). These pumps were employed therapeutically in several patients (5), however, transformation of the thrombus into connective tissue on the impervious surface did not occur. In longer term experiments with calves, propagation of thrombus on the surfaces continued, eventually compromising flexion of the diaphragm, and therefore, of pump function. 

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