# 3D Printing Applied to Tissue Engineered Vascular grafts
###### Abstract
The cells colonize the scaffold, produce their own extracellular matrix, and foster vessel regeneration, as well as graft patency [9]. 3. The addition of SMCs, fibroblasts, or stem cells within the vessel wall may be beneficial. [PERSON] et al., [10] provided evidence that the cell content of the implanted TEVGs correlates with the patency rate. Even though the seeded cells disappear from the grafts within a few days, it was demonstrated that they stimulate the host response in a paracrine manner, via the release of growth factors and cytokines, and promote remodeling and regeneration [11]. As a consequence, initially seeded cells are replaced by a neotissue. The evidence of the remodeling process of the TEVG post-implantation has fostered the idea that biodegradable biomaterials may be appropriate if the vessel integrity is maintained for several weeks, while extracellular matrix (ECM) produced by the host cells replaces the scaffold material as it degrades, allowing the development of an autologous graft [12]. 4. Finally, the anchorage of specific biologically active molecules to the scaffolds stimulates the in-situ remodeling and the regenerative process and therefore limits hyperplasia. Accordingly, the tissue-engineered strategies for TEVG corresponding to biomaterial-centered and/or cell-based approaches have led to the development of a broad variety of combinations of cells and scaffolds. Recent investigations have shed light on promising TEVG composed of synthetic vascular grafts seeded with either endothelial cells [13], adipocyte-derived vascular cells [14] or bone marrow mononuclear derived cells [15]. Other TEVGs development provided evidence of the feasibility of using bioresorbable scaffold [12] or decellularized biological matrices, such as the bioengineered human acellular vessel (HAV) [16]. In this approach, human vascular smooth muscle cells (hVSMCs) were seeded into polyglycolic acid polymer scaffolds and subjected to pulsatile distension for 8 weeks before the removal of the cells with detergents, preserving the tubular structure composed of extracellular matrix proteins. Furthermore, scaffold-free grafts composed of fibroblast and SMC sheets rolled on a mandrel to produce a tubular structure (named Tissue Engineering by Self-Assembly (TESA) [17]) have been successfully developed. To date, several biological active vascular prostheses have been tested in clinical trials for vessel reconstruction in pediatric patients with congenital cardiac diseases [18], peripheral arterial disease [14] and hemodialysis access [16; 17]. Nevertheless, at the moment, no biological active vascular grafts have received market approval from the Food and drug Administration. The urgent need for small diameter TEVGs has challenged the scientific community and resulted in the development of numerous multi-disciplinary approaches. Regarding the complexity of the multilayered vessel structure, 3D printing has opened new avenues for the engineering of structures composed of cells, growth factors, and scaffolds assembled in an organized way. In the present review, we specifically address the use of 3D printing for the fabrication of TEVGs and describe various combinations of deposition techniques, biomaterials, and cells.