ENGINEERING ARTIFICIAL STEM CELL NICHES - Biologically-Responsive Hybrid Biomaterials - page 287

substrates and not in a near-physiological 3D setting. Since a stem cell in 3D is
exposed to an environment with a wide range of matrix porosities, stiffnesses,
proteolytic sensitivities, cell adhesion and cell signalling components, using
conventional experimental approaches, it would possibly take years to achieve a
comprehensive understanding of how the niche controls stem cell fate, just on
one stem cell type. Therefore, novel approaches aiming at high-throughput
screening of 3D microenvironmental conditions are urgently needed. In order
to achieve high-throughput micro-arraying of 3D matrices, 79 robotic liquid
dispensing and printing approaches could be combined with mild biomaterials
crosslinking chemistries. Combinatorial mixtures of liquid precursors of
hydrogel networks could for example be deposited in tiny volumes and high
density onto solid substrates. Murphy et al. fabricated 3D PEG hydrogel arrays to
screen for the individual and combinatorial effects of gel degradability, cell
adhesion ligand type, and cell adhesion ligand density on human MSC viability. 80
PEG network degradability and the presence of cell adhesion ligands were shown
to increase MSC viability in a dose-dependent manner. In future studies it will be
important to increase the throughput of such technologies, and to expand the
read-outs to relevant stem cell functions such as self-renewal or differentiation.
2.4. Dissecting cell-cell interactions in the niche in 3D
It is still poorly understood how cell membrane-bound signals emanating from
support cells in the niche control stem cell function. In order to study such cell-
cell signalling interactions in near-physiological 3D environments, novel model
systems based on 3D cell patterning technologies need to be devised. Bhatia
et al. have presented an interesting approach based on electropatterning of
mammalian cells within hydrogels. 81,82 Electropatterning allows localizing live
cells within hydrogels, such as photopolymerized PEG gels, using
dielectrophoretic forces. Parallel formation of large numbers of living
multicellular cell clusters of precise size and shape were formed in 3D. By
modulating the size of 3D clusters, this microscale tissue organization was for
example shown to influence bovine articular chondrocyte biosynthesis. This
platform should be interesting to apply to the study of the crosstalk of stem cells
with support cells in 3D. Another promising approach to investigate 3D cell-cell
interactions was demonstrated by Kamm and colleagues who combined gel
patterning with microfluidic technology to analyze angiogenesis in 3D co-
cultures. 83 Primary liver and vascular cells were cultured on each sidewall of a
collagen gel between two microfluidic channels. Morphogenesis of 3D
hepatocyte cultures was found to depend on diffusion and convection across the