96 M. C. W. Chen and K. C. Cheung
microbeads. Although agarose has also been investigated as a cell encapsulation
material for transplantation,
30
it has not demonstrated immunoisolation.
31
5.2.4 Synthetic Hydrogels
Synthetic polymers can offer more controllable chemical composition and physical
properties compared to natural polymers.
5.2.5 Pluronic
The thermally reversible triblock copolymer poly(ethylene oxide)-poly(propylene
oxide)-poly(ethylene oxide) (PEO-PPO-PEO) are known as Poloxamers or Plu-
ronic. Historically, Pluronic has been added to mammalian cell culture medium
in low concentrations, from 0.05-0.1%. This additive lowers the surface tension of
the medium to improve transport of metabolites to cells, and also protects the cells
against shear-induced mechanical damage.
32
During experimental and industrial
manufacturing processes, gas bubbles may be introduced into the bulk liquid. Cell
damage occurs when the bubbles break at the liquid surface, and bubble rupture
is a major cause of cell lysis. Cell lysis can also occur due to the shear force in
draining liquid films if cells are trapped in a foam layer. The Pluronic protects cells
by preventing cell attachment to air bubbles.
33
The polymer self-assembles into micelles due to the hydrophilic PEO and
hydrophobic PPO blocks. At low temperatures the aqueous polymer solution is
a liquid. The polymer undergoes thermoreversible gelation to form a close-packed
arrangement of micelles. Pluronic F127 (molecular weight = 120 000 g/mol,
PEO
99
-PPO
65
-PPO
99
) has been approved by the Food and Drug Administration
(FDA) for controlled drug delivery applications. Pluronic F127 has also been
used as a tissue engineering scaffold material for chondrocytes, epithelial cells,
and fibroblasts.
34
Pluronic solutions in the range of 10–15% have an LCST at
physiological temperatures. Since non-modified Pluronic capsules require an
external membrane to prevent bead dissolution, Pluronic has also been modified
to add covalent chemical crosslinking in the polymer for better stability in cell
encapsulation.
35
Figure 5.6. N-isopropylacrylamide (NiPAAm).
36
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