Figure 2: Scaffold Free Vascular Grafts. Aside from permanent or degradable polymeric scaffolds, vascular grafts are also fabricated using biological sources. These techniques include cell sheets, cell aggregates, and tissue printing strategies. Methods have been developed in which a sheet of cells is physically wrapped around a mandrel, followed by culture and eventual decellularization of the cell tube to leave behind ECM [90]. This ECM can then be seeded with autologous cells prior to implantation. Although successful in early clinical trials, this technique is both costly and time consuming. Additionally, cells may be seeded into a custom mold to promote cell fusion and assembly of cellular aggregates. While these tissues can be assembled rapidly and with controlled geometry and composition, difficulties often arise with low mechanical properties and tissue necrosis [99]. Finally, tissues can be printed using custom inkjet printers or bioink & biopaper combinations [159]. While such strategies offer rapid fabrication times and precise control of vessel geometry and composition, they are in the developmental stage, often requiring maturation prior to use and encountering difficulty matching required mechanical properties.