Masters Theses

Date of Award

12-1997

Degree Type

Thesis

Degree Name

Master of Science

Major

Engineering Science

Major Professor

Judy L. Cezeaux

Committee Members

Roberto S. Benson, John H. Forrester, Jill E. Sackman

Abstract

Small diameter (≤ 6 mm ID) expanded polytetrafluoroethylene (ePTFE) vascular grafts, used as vessel replacements in patients without usable saphenous veins, have a cumulative patency rate of 38% after 5 years (Veith et al., 1986). Thrombus formation and anastomotic hyperplasia, an increase in luminal smooth muscle cells at the graft-host boundary which leads to progressive narrowing and eventual occlusion of the vessel, are primarily responsible for synthetic graft failure. In an effort to curtail these processes and, hence, increase patency rates of synthetic grafts, investigators have seeded grafts with endothelial cells prior to implantation. This technique has been moderately successful for the endothelialization of synthetic grafts in animal models, but not in humans thus far (Zilla et al., 1987; Wigod and Kitzman, 1993; Herring et al., 1994).

The carbon-fluorine bonds within ePTFE have high bond energy (486 kJ/mol) and low polarizability (Yamada et al., 1996), resulting in a hydrophobic surface. Studies have shown that endothelial cells (EC) exhibit poor adhesion to hydrophobic materials (Dekker et al., 1992); rather, EC adhere well on moderately water-wettable polymers (Van Wachem et al., 1985). Several methods of surface modification have been developed aimed at making polymers more hydrophilic. The purpose of this study is to evaluate the effect of vacuum UV (VUV) modification of ePTFE on endothelial cell adhesion and proliferation. VUV modification of ePTFE has shown increased initial adhesion of fibrin glue (Szymankiewicz et al., 1997), providing some reason to believe VUV modification will help EC adhesion.

Pieces of ePTFE graft material were exposed to 10 W, 20 W, or 40 W VUV radiation for 10, 20, or 40 minutes using an ultraviolet excimer lamp. After modification, the graft pieces were autoclaved and cut into pledgets, half of which were coated with fibronectin (20 µg/ml). Static cell adhesion was measured by seeding tritiated thymidine-(3H-thy-) labeled human umbilical vein endothelial cells (HUVEC) onto the modified and unmodified pledgets for 60 minutes at 37°C. Following a wash, pledget radioactivity was measured using β-scintillation counting. For the cell proliferation experiments, pledgets were seeded with unlabeled HUVEC which were allowed to adhere to the graft material for 18 hours at 37°C. The cells were then exposed to ³H-thy (1 µCi/ml) for 52 hours. Again, incorporation of "H-thy was measured using ẞ-scintillation counting.

VUV modification had no significant effect on EC adhesion or proliferation on ePTFE vascular graft material for the power levels and exposure times examined. In the presence of fibronectin, cell proliferation on ePTFE exposed to 20 and 40 W for 40 and 10 minutes, respectively, appeared to be improved as compared to proliferation on unmodified ePTFE, but these results were not statistically significant. Analysis of variance showed significant improvement in adhesion and proliferation in the presence of fibronectin. Statistical significance was assumed to be at P<0.05. Scanning electron microscopy demonstrated that cyanoacrylate used to secure the graft pledgets for the adhesion and proliferation experiments may have invalidated the results by altering the graft surface characteristics.

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