## 3 Clinical Study in Venous Model  
In 2001, [PERSON] et al. reported the successful transplantation of a TEVG which replaced a 2 cm segment of a four-year-old girl's pulmonary artery [58]. The initial conduit was seeded with cells originally from an explanted peripheral vein. These cells were expanded for eight weeks in culture, then subsequently seeded onto a tubular biodegradable scaffold made of a 50:50 copolymer of \(\varepsilon\)-polycaprolactone-polylactic acid and reinforced with woven poly (glycolic acid) (PGA) fibers. The graft was fabricated in conjunction with the Japanese textile company GUNZE LTD. The PLCL sponge layer was selected because of its biodegradability and pore size which allow cell infiltration and neotissue formation. The material properties of the PLCL allow for flexibility and ease of handling, however its suture retention strength was limited. A woven layer of PLA or PGA was selected to reinforce the graft as it could be easily incorporated into the manufacturing process of the PLCL sponge layer (Figure 2a-d). The patient had no postoperative complications and displayed a patent graft upon follow-up. After the successful procedure and two additional proof of concept large animal studies, we began a pilot clinical trial investigating the use of biodegradable TEVGs in congenital heart surgery. Between September 2001 and December 2004, 25 Japanese patients underwent extracardiac total cavopulmonary connection (TCPC) operations with a TEVG [59]. Patient demographics, diagnosis, graft type, and size at the time of implantation are presented in Table 1. The median patient age at the time of operation and mean follow-up time was 5.5 and 11.1 years, respectively [60; 61]. There were no incidences of graft-rupture, aneurysal formation, or ectopic calcification. While eight patients died within the follow-up period, none of the mortalities were due to graft-related complications. In fact, the autopsy of one patient who died 13 years after TEVG implantation revealed a structure similar to that of a native vein or pulmonary artery (Figure 3) [62]. However, seven patients presented with asymptomatic graft stenosis and underwent successful balloon angioplasty, including one that required a repeat catheterization and stent placement. Additionally, one patient's TEVG had a thrombus formation one year after implantation, but this was successfully resolved with anticoagulation drug therapy. When contacted, all surviving patients reported no functional limitations (Figure 4) [61]. _Appl. Sci._**2019**, \(9\), 1274  
Figure 2: (**a**) Proposed mechanism of neovesel formation after implantation of a cell-seeded biodegradable scaffold. Early pulse of monocyte chemoattractant protein-1 (MCP-1) and related cytokines from seeded bone marrow-derived mononuclear cells (BM-MNCs) enhances early monocyte recruitment to the scaffold. Infiltrating monocytes release multiple angiogenic cytokines and growth factors (e.g., vascular endothelial growth factor (VEGF)), which recruit smooth muscle cells. (**b**) Appearance of tissue engineered vascular graft (TEVG). TEVG has a thickness of 1 mm, and mainly has a length of 13 cm; they are trimmed and implanted according to the anatomy of the patient. (**c**) SEM image of Woven poly (glycolic acid) (PGA). (**d**) SEM image of luminal PCLA; the bar shows 100 \(\upmu\)m adopted with permission from [PERSON] et al. In our next steps, we focused on elucidating the cellular and molecular mechanism resulting in neotissue formation and graft stenosis. Surprisingly, in a murine model, we found that that the cells seeded onto the graft all but disappeared within 24 hrs and were replaced by an abundant macrophage response [63; 64]. To characterize the effect of the macrophages, we ran experiments that knocked out macrophage function, only to find that the grafts failed to remodel into neotissue. On the other hand, in unseeded grafts, too much macrophage infiltration was found to lead to stenosis [63; 64].