## 1 Introduction  
Congenital heart disease is the leading cause of neonatal death and affects almost 1% of all surviving births [1]. Almost a quarter of these patients need extensive reconstructive surgery [2]. In the adult population, cardiovascular diseases (CVD), such as aortic disease, coronary artery disease (CAD), and peripheral artery disease (PAD), are the leading cause of death worldwide [3].  
Current synthetic grafts are generally made from non-biodegradable materials such as polytetrafluoroethylene (e-PTFE(r) or polyethylene terephthalate (Dacron(r)) [4]. These grafts can be successful in large diameter surgeries, but in general, synthetic materials are not clinically suitable because they exhibit an increased risk of thrombosis, stenosis, calcification, and infection, while lacking in durability and growth potential [5]. Because synthetic materials are thrombotic in small-diameter blood vessels of 6 mm or less, the saphenous vein, internal thoracic artery, and radial arteries are often used in cases of coronary artery bypass surgery or peripheral artery bypass surgery for obstructive arteriosclerosis below the knee.  
Unfortunately, in affected patient populations, there is an insufficient amount of usable native tissue, either for reasons of anatomy or use in previous operations. In order to overcome the limitations associated with autologous and synthetic graft transplantation, the concept of tissue engineering was proposed [6]. Tissue engineering is a scientific field that may solve the problems that plague current grafts. Tissue engineering is defined as an interdisciplinary field which combines engineering and biomedical principles to create materials that integrate with a patient's native tissue to restore or improve physiological function.  
The classical paradigm of tissue engineering includes:  
1. Cells (i.e., progenitor cells, stem cells);
2. Scaffolds (i.e., synthetic, decellularized extracellular matrix);
3. Signals (growth factors, chemotactic factors) [6].  
The three components are interdependent and essential to each other when attempting to form organized neotissue. Since newly created neotissue is composed of autologous tissue, these constructs would theoretically be thrombo-resistant, less prone to infection, and have growth capacity. In 1986, the first tissue engineered blood vessels were reported by [PERSON] and [PERSON]. They consisted of fibroblasts and collagen gel embedded fibers integrated with bovine endothelial cells (EC), smooth muscle cells (SMC), and Dacron mesh [7]. Since then, hundreds of tissue engineered vascular grafts (TEVGs) have been developed and evaluated in animal models but very few have progressed to human clinical trials. In fact, there are only two that are currently undergoing clinical trials in the United States. In 2001, the first clinical trial to evaluate the use of TEVGs in the venous circulation was conducted in children with congenital heart disease by [PERSON] et al. [8]. In 2012, a separate clinical study conducted by [PERSON] et al. began to evaluate arteriovenous shunts for hemodialysis [9].  
This review focuses on the traditional role of scaffolds and cells in TEVGs, the status of recent venous and arteriovenous TEVG clinical studies, and discusses their limitations and future prospects.