Hydrogel-Based Microfluidic Cell Culture 107
off the microheater.
Matrigel has been used on-chip to create cell-embedded regions within a
microfluidic chamber.
89
The cell-gel suspension was injected into the device using
a syringe. After the studies involving nanoparticle penetration into the gel were
completed, the Matrigel was dissolved using the metalloprotease dispase at 37
C
to recover the cells for subsequent flow cytometry analysis.
5.4.5 Transport Considerations
One of the key issues in cell encapsulation is ensuring adequate transport
of nutrients (oxygen, electrolytes) to the cells and metabolites away from the
cells. For example, stem cells are extremely sensitive to parameters in their
microenvironment such as the oxygen concentration. High oxygen conditions can
cause oxidative stress, which may alter protein expression and thus morphogene-
sis. For encapsulated islet cells, one persistent issue has been oxygen availability
at the interior of the capsules, with cell necrosis at the capsule core. A gradient in
oxygen is also seen in some tumors, and it has been found that some drugs are less
active in hypoxic environments.
13
The oxygen supply in PDMS devices may be insufficient without perfusion.
Leclerc et al. have found that either culture medium exchange was required to
maintain Hep G2 cell viability
90
or an additional oxygen supply chamber was
necessary.
91
An oxygen concentration gradient within a microfluidic cell culture system
could result in non-uniform results. Oxygen diffusion through permeable PDMS
structures is balanced by cellular uptake and convective transport by the flow
of culture medium. The diffusivity of oxygen in PDMS has been found to be
D = 4.1 × 10
5
cm
2
/s, while its solubility is 0.18 cm
3
(STP)/cm
3
atm.
92
However,
the permeability to oxygen can be significantly modified by protein adsorption
or plasma surface modification. Oxygen concentration in PDMS microfluidic
structures can be monitored using fluorescent dyes such as ruthenium tris(2,2’-
dipyridyl) dichloride hexahydrate (RTDP), which are quenched by oxygen in a
linearly dependent manner.
Recently, perfluorocarbons have been investigated as an oxygen carrier which
would increase oxygen solubility by 15–20 times compared to oxygen solubility
in water.
93
Perfluorocarbons are currently used as contrast markers in magnetic
resonance imaging (MRI). HepG2 cells in alginate capsules with added perfluoro-
carbons showed improved cellular metabolic activity and cell viability.
In order to better control the distribution of solutes within scaffold materials,
Choi et al. have fabricated microfluidic networks directly in the cell-seeded scaffold
material to permit delivery of nutrients into the bulk (Fig. 5.15). They have defined
a Krogh length, λ
c
D
s,g
c
0
/R,whereD
s,g
is the molecular diffusivity of the
solute in the scaffold, R is the consumption rate, and c
0
is the concentration of
the solute in the channel.
94
In order to minimize variations in concentrations of
metabolites or waste products, the microfluidic scaffold should have a spacing
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