Page 272 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

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Encyclopedia of Physical Science and Technology EN016J-783 August 1, 2001 10:58

Tissue Engineering 823

The design of more sophisticated cultured tissues us-
ing more than one cell type can be enhanced by spatially
controlling the seeding process. For this purpose, various
methods for patterning the deposition of extracellular ma-
trix or other cell attachment factors onto surfaces have
been developed. Photolithography involves spin-coating
a surface (typically silicon or glass) with an ∼1-µm thick
layer of photoresist material, exposing the coated material
to ultraviolet light through a mask containing the pattern
of interest, and treating the surface with a developer so-
lution which dissolves the exposed regions of photoresist
only (Fig. 1). This process leaves photoresist in previ-
ously unexposed areas of the substrate. The exposed areas
of substrate can be chemically modified for attaching pro-
teins, etc., or can be treated with hydrofluoric acid to etch
the material. The etching time controls the depth of the
channels created. Subsequently, the leftover photoresist is
removed using an appropriate solvent, which leaves a sur-
face patterned with different molecules and/or grooves.
A disadvantage of this method is that it uses chemicals
toxic to cells and generally harsh conditions which could
denature proteins are used. FIGURE 2 Patterning using soft lithography. The silicon mas-
The etched surfaces produced by photolitography can ter is used as a mold to create a flexible replica made of
be used to micromold various shapes in a polymer called poly(dimethylsulfoxane) (PDMS). The replica can be used as a
poly(dimethylsiloxane) (PDMS). The PDMS cast faith- stamp to deposit protein on a substrate, as a stencil to cover up
selected regions of the substrate during protein coating, or as a
fully reproduces the shape of the silicon or glass mold to
series of flow channels to deliver a protein-coating solution onto
the micron scale and can be used in various “soft lithog- the substrate.

raphy” techniques, including microstamping, microfluidic
patterning, and stencil patterning (Fig. 2). An infinite num-
ber of identical PDMS casts can be generated from a
single master mold, which makes the technique very in-
expensive. Soft lithography methods can be used on virtu-
ally any type of surface, including curved surfaces, owing
to the flexibility of PDMS. Another patterning method
which works well at larger size scales is microprinting us-
ing laserjet technology, which can also be used to create
three-dimensional structures (Fig. 3).
In using these approaches, it is important that the base
material be resistant to physisorption, or the selectiv-
ity of the adhesive groups may be significantly reduced
in vivo. A successful approach to prevent adhesion to the
base material is via covalent attachment of anti-adhesive
factors on the remaining functional groups.
Micropatterning is especially desirable to maximize
heterotypic cell–cell interactions between a parenchymal
cell such as hepatocyte and supporting or “feeder” cells
FIGURE 1 Patterning using photolithography. A silicon (or glass) such as fibroblasts. Keeping in mind that cells cultured
wafer coated with photoresist is exposed to ultraviolet light in areas on surfaces do not usually layer onto each other (except
determined by a mask overlay. A developer chemical selectively
removes the photoresist and the exposed areas of silicon can be for malignant cancer cell lines), random seeding using a
either etched for use in soft lithography techniques (see Fig. 2) or low ratio of parenchymal cells to feeder cells will achieve
coated with proteins. this goal, but at the expense of using a lot of the available