Biomedical Engineering Reference
In-Depth Information
52 CHAPTER 3.
IN VITRO
TISSUE ENGINEERING
dependent on cell size. If binding sites are distributed evenly across a surface at an average density,
then cells should spread, proliferate, and migrate on the material with ease. If high densities of
binding sites are placed in small areas, then a rounded morphology will result instead. Both systems
can be used to study different aspects of cell-biomaterial interactions.
Micropatterning is not limited to stamping peptides and proteins. It can also alter the to-
pography of a surface, which in turn, can affect cell attachment, proliferation, and gene/protein
expression [
451
]. By controlling how strongly a cell is bound to the surface, micropatterning affects
the migration and proliferation of attached cells. In general, rough surfaces at the sub-micron scale
allow for weak cell attachment but inhibit extensive spreading whereas smooth surfaces promote
strong attachment and spreading as well as proliferation and migration. Aligned topographies have
also been shown to affect cell morphology and differentiation [
452
].
3.4.3 CATABOLIC ANDOTHER STRUCTUREMODIFYING FACTORS
While it is counterintuitive to apply catabolic factors to fabricate a piece of tissue, the enzyme
chondroitinase-ABC (C-ABC) has been applied to deplete GAG content to subsequently improve
biomechanical properties. C-ABC increased tensile properties of self-assembled articular cartilage
without compromising compressive properties as GAG levels return post-treatment [
453
]. Multiple
C-ABC treatments further increased tensile properties, reaching values of 3.4 and 1.4 MPa for the
tensile modulus and ultimate tensile strength, respectively [
454
]. C-ABC represents an exciting
method for engineering functional articular cartilage by departing from conventional anabolic ap-
proaches. Another structure modifying agent is lysyl oxidase, which acts to crosslink collagen [
455
].
Attempts to affect collagen crosslinking (and thus mechanical properties) have targeted this enzyme
with an inhibitor, beta- aminopropionitrile [
456
-
458
].
3.5 MECHANICAL STIMULATION
Bioreactors are a critical component for growing a mechanically functional tissue. For articular
cartilage, compressive, tensile, and frictional properties are of the utmost importance, as are the tissue's
general wear characteristics, so an
in vitro
tissue engineering approach should focus on improving
these properties before implantation. Direct compression and, especially, hydrostatic pressure have
been shown to help stimulate the secretion of proteins that are necessary for compressive strength.
Low- and high-shear bioreactors also have shown promise in growing functional constructs, perhaps
due to the increased nutrient transfer during stimulation. While mentioned here, these bioreactors
are discussed in more detail in the next chapter.
3.6 CHAPTER CONCEPTS
While the
in vivo
environment has been thought to contain all the necessary factors (though
not necessarily the cells) in effecting cartilage repair,
in vitro
tissue engineering is gaining
popularity due to the well-controlled environment it offers.
