Figure 4: Vascular Network Formation. A primary challenge in tissue engineering involves assembling a complex vascular network throughout tissues to allow for required nutrient and waste transport. A variety of techniques can be used to create such assemblies, including the loading of angiogenic factors, microtissue building blocks, or MEMS pore fabrication. Angiogenic factors can be loaded into the scaffolds such that their gradual release promotes angiogensis within the scaffold [168]. Microtissue building blocks have been investigated as small subunits that can be assembled into a complex tissue [178,179]. These building blocks can then be loaded with cells and growth factors which promote angiongenesis. Finally, MEMS may be used to precisely fabricate a tissue scaffold with controlled channels and pores throughout to promote the assembly of a vascular network [173,175].