Design and fabrication of sub-mm-sized modules containing encapsulated cells for modular tissue engineering.

We have proposed modular tissue engineering as a strategy to construct vascularized tissues containing multiple cell types. To create a modular construct, instead of seeding a preformed scaffold, cells were encapsulated within sub-mm modules, and the outer surface of these modules was covered with a layer of endothelial cells. Modules were then added to a larger structure (here by filling a tube) to form the modular construct. Through a systematic process of materials selection, collagen, human umbilical vein endothelial cells (HUVECs), and HepG2 cells, a human hepatoma cell line, were identified as suitable components for module formation, at least for initial studies. A method, which involved cutting and shaping the modules within a tubular mold, was developed to fabricate sub-mm, cylindrical, collagen modules that contained viable, functioning HepG2 cells and that could be seeded with a surface layer of HUVECs. Module dimensions were reproducible and easily altered in a controlled fashion if desired. The module fabrication process developed here not only generated modules suitable for the assembly of a prototype modular construct, but also could potentially be used more generally for other applications for which the goal is to form submm-diameter cylinders from soft hydrogels.