MICROFLUIDIC FORMATION OF CELL-LADEN HYDROGEL MODULES FOR TISSUE ENGINEERING - Micro and Nanotechnologies in Engineering Stem Cells and Tissues - page 196

8.5.2 Random Assembly of Microtissue Units
Random assemblies of microtissue units result in vascularized tissues (Fig. 8.5c and
d). 55 For this application, collagen gel rods seeded with endothelial cells (HUVEC
cells) and encapsulating hepatocyte cells (HepG2 cells) are used. After preparing
rod-shaped collagen gels encapsulating hepatocyte cells, endothelial cells are spread
on the gel and cultured until a confluent layer of endothelial cells grows on the
surface of the gel. The rod-shaped HUVEC-HepG2 microtissues are assembled into a
larger tube with perfusion of medium or whole blood. The perfusion induces
remodeling of cells and produces a perfusable tissue with a microporous body
because the spaces between microtissues become microchannels. The simple method
using a random assembly creates functional tissue equivalents and potentially
engineer organ grafts.
8.5.3 Controlled Assembly of Microtissue Units
In contrast to random assembly, a controlled assembly method can make arbitrarily
shaped tissues. In this method, the surface tension force at the water-oil interface is
used to aggregate hydrogel blocks (Fig. 8.5e-g). 56 First, PEG-methacrylate
(PEGmA) gel blocks containing cells are produced by photolithography using
UV light. After gelling, the PEGmA gel blocks soaked in sol solution are transferred
into mineral oil, where the gel blocks are assembled by mechanical agitation.
Subsequently, exposure to UV light gelates the gel blocks into assembly. This
method also generates 3D cell structures containing multiple types of cells. In
particular, lock-and-key assemblies composed of cross-shaped gel blocks and plural
rod-shaped gel blocks form coculture tissues without any additional steps. Using this
method, the tissues formed from microtissue units are fabricated reproducibly
without complicated assembly and handling procedures.
In the approach using microfluidic devices, collagen blocks containing cells
fabricated by a micromolding method are assembled into microchannels or micro-
chambers (Fig. 8.5h-i). 57 When assembling collagen blocks in microchambers, 3D
cell structures are generated, but hydrodynamics in the narrow microchannel forces
the collagen blocks to produce ordered structures. The advantage of this method lies
in its ability to form tissue structures containing multiple types of cells. The 3D cell-
lined structure comprising hierarchical coculture tissues will allow studying 3D cell-
cell interactions and signaling.
In another approach, convex PEG gel blocks, microtrains, are produced by flow
lithography and are fluidically assembled along a concave rail (Fig. 8.5i and k). 58
Using the rails as guides, complex 2D structures are built by fluidically assembling
microtrains with zero error and incorporating all microtrains as components in the
structures (Fig. 8.5l). Furthermore, heterogeneous fluidic assembly of microtrains
containing different types of living cells is achieved by the same guiding mechanism
(Fig. 8.5m). The guided and fluidic assembly method require the convex hydrogel
blocks and the concave rails but has strong potential, owing to its flexibility, to
produce heterogeneous and complex cell-laden structures as living tissues.