Investigation of mechanical properties of small caliber fibrous vascular grafts

Abstract

Cardiovascular diseases remain the most common cause of mortality worldwide, resulting in the deaths of 17.9 million people in 2019. Furthermore, previous cardiovascular diseases are a significant risk factor for COVID-19-related complications and deaths. According to the World Health Organization, the number of deaths would increase by 24.5% by 2030. The most frequent type of cardiovascular disease is coronary artery disease, which necessitates a surgical procedure called bypass grafting that involves arterial replacement. In bypass surgery, an autologous vein or synthetic graft is used to restore a diseased blood vessel that has become damaged or clogged. However, autologous grafts pose significant challenges due to scarcity and difficulties in graft harvesting operations. On the other hand, commercial synthetic ones are also problematic to be used as smaller diameter vascular grafts (< 6 mm) due to poor patency rates, thrombogenicity, and compliance mismatches, as well as neointimal hyperplasia in the peri-anastomotic regions. The compliance mismatch between the native vessel and the rigid synthetic graft at the anastomosis sites results in low blood flow rates and turbulent blood flow in small-diameter grafts. These mechanical issues lead to thrombosis and luminal narrowing due to intimal hyperplasia, which results in poor long-term patency, together with the thrombogenicity of the scaffold material and a lack of endothelialization. In order to address the demand for suitable scaffolds that can be utilized in bypass procedures by using new materials and production processes, researchers have concentrated on building an alternative tissue-engineered small-caliber vascular graft that can imitate the native artery in all ways. There is currently no small-caliber biodegradable vascular graft that has reached commercial success, despite the fact that significant breakthroughs have been made in the research that has intensified in recent years. The vascular graft is supposed to give structural support and promote cellular activity for the body to generate its vessels. The fundamental difficulty with vascular tissue engineering is still creating a perfect vascular graft that can replicate the structural, biological, and mechanical characteristics of the native blood vessels and be used in place of the disabled blood vessel. In this context, morphology and cellular analysis are typically given top priority, whereas mechanical aspects are only briefly discussed. To improve the clinical performance of vascular grafts, expose physiological stresses, and prevent graft failure brought on by intimal hyperplasia, thrombosis, aneurysm, blood leakage, and occlusion, it is essential to create grafts with good mechanical qualities comparable to native vessels. The mechanical characteristics of scaffolds, such as compliance, burst pressure, nonlinear elasticity, modulus, and suture retention strength, must match those of the native tissues because even a slight mechanical mismatch between the graft and the native vessel can cause graft failure. The mechanical properties of the vascular grafts are significantly influenced by the material and design. In this thesis, a detailed literature review was carried out to understand the native blood vessel structure and to provide a broad and comparative overview of recent studies on the mechanical properties of fibrous vascular grafts, with an emphasis on the effect of structural parameters on mechanical behavior in the experimental part. The purpose is to shed light on the design parameters needed to maintain the mechanical stability of vascular grafts that can be used as a temporary and biodegradable backbone, allowing an autologous vessel to take its place. An experimental study is carried out to produce fibrous vascular scaffolds made out of various biopolymers and their combinations with different fiber orientations and constructions and assess their physical, morphological, and mechanical properties. The first experimental part of the thesis is a preliminary study that includes the production of planar and tubular scaffolds made of neat PCL and PLA and their blends with the PCL/PLA blending ratios of 90/10, 80/20, 70/30, 60/40, and 50/50 by using an open system electrospinning unit. PCL is a flexible biopolymer with a long biodegradation time, whereas PLA is a strong polymer with high brittleness, higher biocompatibility, and a faster biodegradation time than PCL. The reason for utilizing these polymers together is to combine their mechanical and biological advantages and eliminate their inadequacies. The effect of the polymers and collector type on the fiber morphologies, diameters, and orientations, sample thickness, as well as the mechanical properties was assessed. It was observed from the results that all the samples were successfully produced, and they all have distinctive morphologies with smooth and continuous fibers. The tensile stress and elongation results revealed that polymer composition is highly effective on the tensile properties. Neat PCL samples had considerable elongation value with 390% whereas PLA showed good tensile strength with 2.73 MPa. When the blended samples were observed, it was seen that the blending affected the mechanical properties negatively based on the blending ratio that was used because of the immiscible characteristics of the polymers. The addition of PLA gradually improved the tensile properties, while using PCL in higher amounts caused better elongation values in blended samples, which shows the importance of the selection of a suitable blending ratio. According to the results of the planar samples, the PCLPLA90 and PCLPLA80 samples can be selected as they have moderate stress and strain values among the blended samples. Also, the use of tubular collectors enables the production of scaffolds with desired construction. On the second part, the monolayer tubular vascular prostheses were produced in a closed electrospinning system by using two rotational speeds to achieve scaffolds with randomly distributed or radially oriented fibers. In addition to the neat and blended samples made of PCL and PLA, two more polymers were added to the production stage, which are PLCL and PLGA. As PLCL is the copolymer of PCL and PLA and thought of as a better candidate to be used instead of physically blended scaffolds to eliminate the mechanical failure caused by blending, it was also used in the vascular graft fabrication process. On the other hand, PLGA has good biocompatibility and faster biocompatibility with good mechanical properties, which make it a good option to be used in vascular applications. When the physical and morphological results were investigated, it was seen that in a closed system, it is possible to produce vascular grafts with the desired thickness levels. Fiber orientation was also observed from SEM images in the radial direction within the tubular samples produced by using a high collector speed. The tensile test was performed on all the tubular samples in longitudinal and radial directions to see the effect of polymer composition, fiber orientation, and test direction on the tensile properties of the specimens. Results revealed that the neat PCL scaffolds showed more flexibility than the neat PLA samples, and the neat PLA samples show higher tensile strength than the neat PCL samples in general. Also in blended samples, tensile stress and elongation values were improved in some cases depending on the blending ratio, such as in PCLPLA90 and PCLPLA80 specimens. Also, neat PLCL samples had both higher elongation and strength values than all neat and blended scaffolds, with some exceptions. Generally, when the PLA ratio is increased, the tensile strength improves gradually, whereas the elongation values decrease. The maximum tensile strength belonged to PLCL100_O in the radial direction with 12.12 MPa, whereas it showed its highest elongation in the longitudinal test direction with 832%. In addition, PLGA100_R showed higher strength than the samples made of PCL and PLA, with very limited elongation. The strength values of PLGA samples were really promising, as it is a rigid polymer. On the other hand, radial fiber orientation greatly contributed to the tensile stress values in the radial direction and the elongation values in all directions compared to the samples with randomly oriented fibers. Higher stress values were obtained in the direction of the orientation whereas higher elongation values were achieved in the direction without fiber alignment. On the other hand, a custom-designed test device was specifically designed for vascular graft specimens to measure their burst strength and compliance. When the burst pressure values were assessed, the best results were obtained from the vascular grafts made of PLGA and then PCL/PLA blends with radial fiber orientations. The addition of PLA results in an increment in burst pressures up to a certain limit of PLA ratio. PLGA100_O showed the highest burst pressure at 2889 mmHg. According to the compliance measurements made using three different physiological blood pressure ranges, the scaffolds with higher flexibility possessed better compliance values. Thus, the samples with randomly distributed fibers had the highest compliance results when compared with the samples consisting of radially oriented fibers. PLCL100_R demonstrated the highest compliance with 4.924 mmHg %/100 mmHg at a 50–90 mmHg pressure range as the most flexible biopolymer among the others. Finally, considering the previously obtained biological analysis results, bilayer vascular grafts were fabricated by combining monolayer scaffolds with the best mechanical properties to obtain a prosthesis that could mimic the topography of the natural artery. The inner layer was constructed from randomly distributed fibers, whereas the radially oriented fibers were included in the outer layer. PCL100_R and PCLPLA80_R monolayers were selected as the inner layers while PLCL_O was used in the outer layers of the bilayered grafts due to their appropriate mechanical advantages. Results indicated that although the samples had a delamination problem in some cases, they had improved mechanical advantages in the tensile and bursting testing processes. On the other hand, the compliance results were still sufficient and comparable with the native blood vessels. All the results that have been achieved in this thesis shed light on the examination of the mechanical properties of vascular grafts and contain significant information for vascular prostheses to be produced in further research. The bilayered grafts that will be constructed in the future studies will be designed by considering the results of the mechanical assessments of the samples that have been optimized by using PCL, PLA, PLCL, and PLGA within the scope of this thesis and the biological examinations. In the following process, it is aimed to switch to in-vivo studies with the most appropriate bilayer scaffold designs to be obtained and to study the biological process in an interdisciplinary manner.