The middle layer or tunica media responsible for integrity and mechanical strength of the blood vessel is composed of smooth muscle cells (SMCs) in circle of rows and elastic fibers. This tight monolayer also functions for antithrombosis, anti-infection or -inflammation, and regulation of cells in other layers by detecting physicochemical and biological changes in the blood. The tunica intima which is the inner layer is made up of endothelial cells (ECs) providing a pathway for frictionless flow of blood. However, the thickness of the layers vary depending on their physiological role. Types of the blood vessels and their structuresĪrteries and veins are composed of three distinct layers: (1) tunica intima, (2) - media, and (3) - adventitia (Fig. Then, the deoxygenated blood is collected by venules and further transported to veins which returns to the heart. The actual interactions between local cells and blood such as nutrient supply and waste removal take place in the capillaries. Arteries further branches into arterioles and then divide into capillaries, the smallest blood vessels. The native circulation system starts with the outflow of oxygen-rich blood through the aorta which branches into arteries to other organs and tissues. Prior to developing tissue engineered blood vessels, understanding of the structure and functions of the native blood vessels is the fundamental step. This review introduces the current development in tissue engineered vascular scaffolds created using extrusion-based 3D cell-printing technique. Through this technique, cell-encapsulated microtubular structures can be fabricated for VTE applications (Fig. Especially, the introduction of 3D cell-printing allowed the deposition of cells accurately and uniformly in the desired region of the scaffold. Recently, advancements in tissue engineering have enabled mimicry of tissues and organs in a more precise manner. Other requirements are that the vascular graft should reduce the risk of thrombogenicity while enhancing the regenerative potentials. Size mismatch between the host and the transplanted vessels can cause uncontrolled turbulence or resistance. Besides the mechanical properties, the diameter of the tubular scaffold should match to that of the host blood vessel. Low mechanical strength may lead to rupture of the graft due to the constant blood pressure, while high stiffness can cause compliance mismatch between the graft and the native blood vessel leading to intimal hyperplasia or atherosclerosis. One of the most important factors to imitate is the mechanical properties which should match with the host blood vessel. The purpose of VTE is to develop tissue engineered tubular scaffolds mimicking the in vivo environment of the native blood vessels. By doing so, vascular tissue engineering (VTE) has emerged to bridge the gap. Therefore, new approaches were needed to compensate the limitations of autologous and synthetic vascular grafts. However, these FDA approved nondegradable synthetics may not be the best option due to the risk of thrombosis, intimal hyperplasia, and graft failure in small diameter environments. Since then, various synthetic vascular grafts were commercialized using Dacron, expanded poly(tetrafluoroethylene) (ePTFE), and polyurethane (PU) (Table 1). Although autografts have various benefits including appropriate mechanical properties, the major drawback is the limited availability. Up till now, autografts are known as the gold standard of vascular replacement. The first use of autologous artery was reported in 1896. Vascular autografts are widely used in treatment of vascular diseases and in surgeries such as tissue reconstruction and replantation. The need for an ideal small diameter vascular graft
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