Date of Award

12-2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical and Chemical Engineering and Sciences

First Advisor

Chris Bashur

Second Advisor

Vipuil Kishore

Third Advisor

Mehmet Kaya

Fourth Advisor

Yi Liao

Abstract

The primary goal of this project was to produce and develop a tissue-engineered small-diameter vascular graft which can replace the arteries and function well for long-term grafting. Implanting an electrospun tubular conduit into the peritoneal cavity to recruit autologous cells prior to grafting the conduit into the artery is a method to achieve this goal. Incorporating an epsin-mimetic peptide on the conduit prior to grafting is another proposed method to reach the goal. This will be accomplished through four complementary studies: (1) determining the effect of a pouch in peritoneal pre-implantation and reducing the potential peritoneal adhesion formation;(2) determining the impact of collagen to poly(ε-polycaprolactone)(PCL)ratio on mesh hemocompatibility;(3)assessing the impact of the percentage of collagen incorporated and peritoneal pre-implantation on graft patency with long-term grafting; and (4) delivering of epsin-mimetic UPI peptide from electrospun conduits. In the first study, we designed a novel pouch from poly (ethylene glycol) diacrylate to lower the potential for peritoneal adhesion formation (Chapter 2). The hydrogel was designed with macropores to also let peritoneal fluid penetrate inside. Mechanical assays and finite element modeling were performed to ascertain the structure integrity of the pouch inside the cavity. Significant reduction in peritoneal adhesion with the new pouch is a crucial step for clinical transition, which we managed to show. In the second study, blended scaffolds made with natural and synthetic biomaterials have been investigated for the possibility of thrombosis formation that could lead to graft failure (Chapter 3). Even though we previously showed benefits of 10% collagen incorporation with PCL within tissue-engineered vascular grafts (TEVGs), collagen can activate platelets and is known to be prothrombotic. Interestingly, we did not find thrombosis cases. In this study, 10% and 25% collagen incorporated electrospun meshes had lower platelet attachment compared to 50% collagen and at comparable levels as pure PCL. These conditions had less activated platelets than 50% collagen, which exhibited large platelet aggregation, but also less than pure PCL. The biomaterial mechanism for this result with low amounts of incorporated collagen was also investigated. Images of picrosirius red staining confirmed collagen presence on the surfaces of the fibers, and we further characterized the biomaterial to understand reasons why this collagen available for interaction with cells may not lead to platelet activation. These results can hopefully help to optimize the PCL/collagen conduits for aortal grafting. In the third study, conduits of different ratios of PCL and collagen with and without peritoneal pre-implantation were grafted into the abdominal artery of rats to determine graft remodeling, patency, and endothelialization in the arterial microenvironment for extended periods (i.e., ten months) (Chapter 4). Previously, we have shown that pre-implantation resulted in reduced overall lipid oxidation and expression of an M1 macrophage marker after a shorter grafting time (e.g., six weeks). This study was performed for ten months because other studies have suggested that graft responses in rodent models changes after the standard 6 months. Importantly, patency of grafts with preimplantation was 100% while the non-implanted ones had 60% patency. With pre-implantation, conduit remodeling was seen and luminal endothelialization occurred. Also, we demonstrated after 10 month of aortal grafting CD45 positive cells are appeared while sections were negative for macrophages, so further analysis is required. Finally, in the last study, preliminary results suggest that local UPI peptide delivery from electrospun scaffolds could promote cell proliferation leading to early graft endothelialization (Chapter 5). Early development of a functional endothelium is crucial for graft survival and lowering the intimal thickness which is a clinical transition barrier. The impact of exogenous peptide delivery and coated scaffolds was investigated in vitro with endothelial cells (ECs). Cells on electrospun meshes did not respond to UPI peptides delivered via media as well as the incorporation of the peptides within the scaffolds, which improved the ECs and their cell-cell contacts. These coated scaffolds were also investigated for hemocompatibility, and the fibrin coating was shown to not promote platelet adhesion. Overall, these results demonstrated beneficial impacts of peritoneal pre-conditioning and optimized collagen incorporation, and potentially also delivery of epsin-mimeticpeptides, which can lead to long-term survival of the aortal grafting and hopefully address challenges for TEVGs. Future work will involve investigating the compounds and proteins present on the pouch and enclosed conduits for understanding the pre-implantation benefits impact and utilizing those compounds to modify the grafts to provide the same benefit while avoiding the peritoneal pre-implantation surgery (Chapter 6).

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