232 Dean et al.: Translation of vascular tissue engineering to pediatric CHD ologic scaffolds, such as decellularized human and porcine vessels and human umbilical vein. These materials have shown a high degree of matching of compliance between graft and natural vessels with little risk of immunogenic response [30,31]. However, these meth- ods have not yet been evaluated in clini- cal trials, and their applicability in TEVG constructions requires further study. Cells for Seeding Several different cell types have been considered for use in TEVGs, in- cluding endothelial cells, smooth mus- cle cells, embryonic stem cells, induced pluripotent stem cells, and BM-MNCs. The ideal seeded cell should be easily harvested, require a short incubation time, and rapidly give rise to neotissue f igure 1. Cell seeding of TEVg . a) Static formation. While the use of endothelial seeding. A cell suspension is pipetted onto the cells and smooth muscles cells is entic- outer surface and inner lumen of the scaffold. b) ing due to their matching of natural ves- Vacuum seeding. A cell suspension is forced sels, they require long in vitro incubation through a scaffold by internal pressure, thus times and thus increased risk of contam- lodging the cells within the pores of the scaffold. ination [32,33]. Similarly, embryonic Rt(r2ae0pm1s0a.)i(nA[i3nd7ga]paatsnepddirUwatdietheilsspmecroamlnleis(cs2tie0od1n1if)nro[s3mp8e]V)c.iilmlaelonna scteelmls chealvlse abnedenincdouncseiddeprleudridpuoetetnot sthteemir potential to differentiate into the en- functionality as a temporary vascular conduit dothelial and smooth muscle cells of a ma- until neotissue formation takes place. Cur- ture vessel. However, these cells are not rent scaffold material can be broken up into always available, carry significant risk of the subgroups of synthetic and biological. teratoma formation, and require more inves- Traditionally, synthetic materials have been tigation before clinical use can be consid- used, and scaffolds have been constructed ered [34,35]. Conversely, BM-MNCs are using woven polymers of polyglycolic acid readily available via bone marrow aspiration (PGA), polylactic acid (PLA), and poly- and require a relatively short, 2-hour scaf- caprolactone (PCL). By combining and vary- fold incubation time prior to implantation. ing the concentrations of these polymers, Our group has pioneered a novel process of scaffolds can be generated to meet the com- separating BM-MNCs from the aspirate pliance specifications of the vascular envi- through the use of a specially designed filter ronment in which they are to be introduced to separate cells of a particular size [36]. [26-28]. However, the degree to which the While the process of harvesting bone mar- compliance and biomechanical properties of row mononuclear cells is not without some woven polymer scaffolds can be fine-tuned risk to sterility and cell viability, its relative is significantly less than that found in newer speed and safety makes it the current fron- construction techniques such as electrospin- trunner in the construction of TEVGs. ning, in which an electrical field is used to Seeding Techniques mold individual polymer fibers into a tubular structure upon a rotating metal rod [29]. Traditionally, scaffolds have been stati- Other groups have focused on the use of bi- cally seeded through a process in which cells