Effective endothelialization of polyurethane surfaces. Response to shear stress and platelet adhesion. Biomer and Mitrathane are thromboresistant polyurethane ureas that are suitable for coating artificial valves. Degeneration of the surface coatings, however, does occur over time, and in experimental animal studies adherent thrombi have been observed as a complication. In this study, morphometric techniques applied to scanning electron microscopy were used to assess whether vascular endothelial cells harvested from jugular veins of fetal lambs could be grown to confluence on these polyurethane surfaces, whether the monolayer would remain intact under conditions of shear stress (104 dynes/cm2), and whether this would result in decreased platelet adherence of sheep platelets relative to nonendothelialized surfaces. The results have shown that both Biomer and Mitrathane could be endothelialized: 88.8 +/- 5.1% of the Biomer surface consisted of intact confluent endothelial cells, as did 95.45 +/- 1.7% of the Mitrathane surface. After 6 hr of shear stress, a significant reduction in this feature was observed (p = 0.02), but both materials still maintained a high percentage of confluent endothelial cells (78.65% for Biomer and 85.58% for Mitrathane). After 48 hr of shear stress, however, the percent confluence was similar to control values, which suggested new cell growth. Endothelialized Biomer compared with Mitrathane had fewer adherent single platelets, or small or large aggregates (p = 0.001, p = 0.01, and p = 0.05, respectively); this feature was not affected by shear stress. Whereas endothelialization of the surface clearly decreased platelet adherence on Mitrathane (p = 0.01), only a trend was seen with Biomer. These studies therefore show that endothelialization of these polyurethane surfaces is feasible. 