Defense Date

2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Biomedical Engineering

First Advisor

Gary Bowlin

Abstract

Electrospinning is a polymer processing technique which allows for the production of nano to micro size fibers and scaffolds which can be composed of numerous synthetic biodegradable materials and natural biopolymers. Natively, elastin and collagen are the main components of vascular tissue. Arranged in a tri-layered structure, they create a specific mechanical environment that can withstand the rigors of circulation. The goal of this study was to develop a mechanically ‘biomimicking’ vascular graft composed of three distinct layers through the process of electrospinning. We hypothesize that the use of bioactive agents such as elastin, collagen, and silk to supplement poly(caprolactone) at specified ratios for each layer would provide a finely tuned vascular replacement. This was accomplished by establishing cross-linking parameters for the biopolymer materials and then assessing the mechanical properties of individual materials and eventually a whole tri-layered graft. Additionally, while mechanical testing can lead to a good graft, a replacement graft requires excellent cellular properties as well to promote cell infiltration, proliferation, and migration. Therefore, the conclusion of this study examines the integrin binding characteristics of the electrospun biopolymers. First, the results from the preliminary cross-linking study examined the dissipation of soluble elastin when uncross-linked v. cross-linked. It was determined through this initial study that synthetic scaffolds blended with soluble proteins such as elastin require a fixation in order to retain their protein mass within the scaffold. Retaining this mass, incrementally changed the material properties of the blended scaffolds. This initial study was then carried further to establish optimal cross-linking parameters using two different types of reagents: carbodiimide and genipin. It was found that lower cross-linking molarities produced excellent results based on assays performed to assess cross-linking percentages and rate of reaction. Some differences in mechanical properties were seen, but they did not constitute a choice of one cross-linker over the other. The next portion of this study aimed to design a tri-layered graft. This was performed with the aid of mathematical analysis to observe circumferential wall stresses based on simple tensile properties. A series of tri-layered grafts were electrospun using poly(caprolactone), elastin, and collagen. The medial layers of these grafts were changed while the intima and adventitia remained constant. Differences were demonstrated as the elastin content of the medial layer decreased, proving that each layer had an affect on the overall graft properties and that it was possible to tune graft mechanics. A larger tri-layered study looked to evaluate changes in the adventitial and medial layers while keeping the intimal layer constant using poly(caprolactone), elastin, collagen, and silk fibroin. In this study, differences were exhibited under compliance and burst strength testing, narrowing the scope of material choices. Results from a 4 week degradation study with the best tri-layered grafts revealed no evidence of degradation, but did generate some positive compliance results for two of the grafts. Finally, integrin binding and protein analysis portrayed results that were indicative of the existence of ligand binding sites for collagen scaffolds and the possibility of a small amount of ligand sites on silk. Elastin, however, displayed low to non-existent adhesion. These studies produced results that allowed us to continuously narrow the scope of materials as the experiment progressed towards an optimized tri-layered vascular graft.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

August 2011

Share

COinS