Biomaterials for MEMS

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.

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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.

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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.

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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.

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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.

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Figure 5.6. N-isopropylacrylamide (NiPAAm).

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