|dc.description.abstract||With the growing age of average population throughout the world, progress of biomaterials research is important to overcome current limitations. Vascular graft is one of the examples. Currently, synthetic vascular grafts made using expanded polytetrafluoroethylene (ePTFE) or Dacron are commercially available as treatments to cardiovascular diseases. These synthetic vascular grafts have good patency, which is a measure used to determine the success of the grafts, and are actively used in more than 400,000 life-saving procedures in the United States alone. However, the available synthetic grafts have limitations – their patency is poor when used in small-diameter application. Synthetic small diameter vascular graft (sSDVG) is defined by synthetic vascular grafts with internal diameter less than 6mm. sSDVG face limited success due to vascular wall thickening known as intimal hyperplasia (IH). IH develop at the distal anastomosis of the graft and are due to over proliferation and abnormal migration of vascular smooth muscle cells (VSMC). Mechanical compliance mismatch of the sSDVG is proposed as one of the key factors that contribute to the formation and development of IH. Compliance in vascular engineering is referring to the radial elasticity of the vascular graft or the blood vessel. Native blood vessels are highly elastic; compliance of human internal mammary artery with the diameter of 1-2 mm is reported to be 12% per 100mmHg. The compliance of sSDVG, however, is very low. The compliance of ePTFE graft, for example, is reported to be 1.2±0.3 % per 100mmHg. This discrepancy in the compliance of the native blood vessels and sSDVG is called compliance mismatch. Compliance mismatch is suggested to play an important role in IH because of mechanotransduction. Mechanotransduction refers to the cellular responses to mechanical stimuli. In the literature, many biological molecules inside VSMC, such as platelet-derived growth factor-BB (PDGF-BB), platelet-derived growth factor receptor, phosphorylated myosin light-chain kinase (pMLCK), have been found to be influenced by the mechanical stimulation. Compliance mismatch forces the cells around the anastomosis to be exposed to abnormal mechanical stimulation, which is translated to biological responses to increase proliferation and migration of VSMC.
Different biomaterials are being studied to develop compliant sSDVG. For example, polyurethane is being studied as a potential compliant vascular graft. Polyurethane vascular grafts were modified with gelatin and collagen to enhance endothelial cell adhesion and were modified using heparin to reduce thrombogenesis. Likewise, poly(vinyl alcohol) (PVA) vascular grafts were developed in 2008. PVA hydrogel is bio-inert, low-thrombogenic, and non-cytotoxic biomaterial with easily modified mechanical properties. The compliance of PVA vascular graft developed by Chaouat et al. had comparable compliance to the native blood vessels. However, the compliance, as well as other mechanical properties, of the PVA grafts were heavily influenced by the conditions in which the crosslinking of the hydrogel occurred. While the variation was observed, it was not studied systematically to identify the effects of the fabrication conditions on mechanical properties. In this thesis, the roles crosslinking density and interlayer adhesion play in compliance and burst pressure of PVA vascular grafts were studied. Fabrication parameters were categorized based on their effects on either crosslinking density or interlayer adhesion. PVA tubes with different fabrication conditions were made to yield tubes with lower crosslinking density, higher crosslinking density, lower interlayer adhesion, and higher interlayer adhesion. It was found that the higher crosslinking density resulted in higher burst pressure and lower compliance. Furthermore, it was found that higher interlayer adhesion resulted in higher burst pressure and lower compliance as well. Elastic modulus and suture-retention strength of the control, higher interlayer adhesion, and higher crosslinking density were compared as well. The result displayed that only the circumferential elastic modulus was affected by the interlayer adhesion and crosslinking density. Therefore, the study concluded that it is important to balance crosslinking density and interlayer adhesion to fabricate PVA grafts with desired compliance.
Consistency is important for research to ensure reliable result. In part due to the sensitivity to fabrication condition, the consistency of PVA vascular graft suffered from person-to-person variation in fabrication process. PVA vascular grafts are fabricated using dip-casting method. In this method, a cylindrical mold is dipped into PVA crosslinking solution to produce thin layer of PVA hydrogel on the mold. The dipping is repeated until the PVA graft reach desired wall thickness. Also, while PVA hydrogel crosslinked using chemical crosslinking method is observed to be non-degradable, the stability of PVA grafts were not studied. The batch-to-batch consistency of PVA tubes made using automated process was studied, as well as the long-term stability of PVA grafts. The automated fabrication method developed displayed similar capacity as those made using manual fabrication method. The grafts made using automated process displayed consistent variation in wall thickness, burst pressure, and compliance. The grafts made using automated process and manual process exhibited comparable burst pressure and compliance when accounting for the wall thickness. Lastly, physical dimensions were compared to study long-term stability of the PVA grafts. Wall thickness, graft length, and dry weights displayed less than 5% change after 180 days of incubation.
PVA hydrogel is also a good platform to study the effects of compliance mismatch on VSMC. Mechanical stimulation is essential for understanding the responses of VSMC on formation of IH. Studies have found that PDGF-BB and pMLCK are upregulated when VSMC are exposed to cyclic stretching. Furthermore, VSMC have been observed increase in proliferation and changes to migratory phenotype when exposed to cyclic strain. However, understanding the isolated effects of compliance mismatch has been difficult due to lack of continuous sample with two distinct regions of stiffness. In this thesis, a hybrid method using both physical and chemical crosslinking was developed to form continuous compliance mismatched samples using PVA hydrogel. The samples were characterized and found to be continuous in both parallel and perpendicular to the mismatch line. The samples were then used to perform in vitro experiments using human umbilical arterial smooth muscle cells (HUASMC) with exposure to cyclic stretching at 10% strain for 4 hours. Exposure to mechanical stimulation resulted in higher proliferation, change into migratory phenotype, compactification of PDGF-BB, and higher expression of pMLCK for the all the groups Among the stretched groups, compliance mismatch resulted in highest proliferation, pMLCK expression, and highest number of cells with concentrated PDGF-BB signal. However, no nuclear localization of yes-associated protein (YAP) was observed. The experiments were repeated with samples with higher compliance mismatch. Nuclear localization of PDGF-BB and YAP was observed when HUASMC cultured on higher compliance mismatched samples were exposed to cyclic stretch of 10% strain for 4 hours. Therefore, it can be concluded that not only does compliance mismatch affect cellular responses to cyclic stretching, but also the extent of compliance mismatch plays a role in the responses.||en