## 1 Introduction  
Cardiovascular diseases belong to the most frequent causes of death or life quality impairment. Currently, autologous vessel replacements such as the saphenous vein represent the gold standard grafts for small-diameter vessels (below 6 mm), and in many factors, they outperform synthetic alternatives. Autologous grafts, however, are of limited availability, and an invasive approach for their harvesting is required. In many cases, they are unsuitable for use due to unfavorable anatomy (gracility, branching, thrombosis) or previous harvest [1]. Allograft vessels from cadaveric donors provide an alternative source for bypass grafting. These grafts can be cold-stored or cryopreserved in tissue banks; nevertheless, they are not immediately available for emergent surgery, and also their long-term patency is limited [2]. Synthetic grafts act in many cases as suitable replacements; however, they show a reduced clinical efficacy, especially in small-calibre applications [3]. There is a great clinical demand for new biomaterials providing feasible solutions, including proper mechanical support, functional tissue regeneration, and a non-thrombogenic surface [4]. Not only tubular grafts but also planar patches are required with similar properties as cardiovascular prostheses [5]. These planar patches are indispensable in many vascular reconstruction procedures, i.e., endarterectomy [6]. Moreover, with pliable, non-thrombogenic, non-immunogenic patches seeded with autologous cells, we are just one step ahead of new heart valves' construction. Current heart valve bioprostheses, despite various types of advanced anti-calcification processing, are still subject to structural valve deterioration (SVD) in a long-term manner. The incidence of SVD is strongly dependent on age and the type of prosthesis, so it can vary widely between 3.3-20% at a 10-year follow up, even in the elderly population [7]. SVD strongly limits the clinical use of bioprosthetic valves in younger patients, especially in children. One possible solution to this problem is using xenogeneic grafts. However, animal tissues cannot be directly applied due to immune incapability, risk of infectious disease transfer, and other safety and regulatory issues. Therefore, a need for tissue processing and engineering is a crucial steptoward practical application. One possible way is graft decellularization. This process removes the immunogenic cellular elements and produces a non-immunogenic substitute suitable as an implantable graft or scaffold for further modifications. Also, the mechanical strength is preserved similarly to the native blood vessel, which is suitable for recolonization with the recipient's native cells in vivo [8; 9]. The decellularization is typically accomplished by treating the tissues with a combination of detergents, solvents, enzymes, osmotic gradients, and chelating agents [10; 11]. The process of decellularization affects the structural and functional features of the tissue, such as its swelling and stretching properties, suture retention, and the conductivity for cells [12]. Decellularized matrices for cardiovascular use need further modification. The first major thing is to improve the antithrombogenicity. Second is the support of an overall cell recolonization and stimulate re-endothelization in vivo. These steps could involve chemical modifications employing growth factors such as VEGF (Vascular endothelial growth factor), bFGF (Basic fibroblast growth factor) [13] to promote cell overgrowth, or use of heparin as an anti-thrombogenic agent, which also promotes cell grow [14]. On the other hand, growth factor use can be problematic for fulfilling the regulatory requirements for tissue grafts intended for human use and also potentially can induce unpredictable cell growth of host cells. The second approach to the modification of decellularized matrices is their recolonization with cells. Several studies have demonstrated the improved performance of these matrices in animal models.