Biomedical Engineering Reference
In-Depth Information
and formation of an outer capsule, are caused by the
pressure and velocity fluctuations associated with tur-
bulent mixing.
Table 7.2-5
represents the high-shear
bioreactor summary
[18]
.
Using impellers is popular in basic cell culturing be-
cause it increases the rate of mass transfer to the cells,
but impellers create many problems. Non-uniform mass
transfer rates, nutrient and pH gradients, and shear gra-
dients impart a non-uniform mechanical stimulus over
the construct, resulting in the formation of tissue that is
inferior to that produced by other culturing techniques.
The shear force at the surface of the impeller is reported
to be 10 times higher than anywhere else within the
bioreactor. Because of this, cells located closer to the
impeller will exhibit more of an injury response because
of the high levels of shear. Cells farther away will not
produce a fibrous capsule as long as the rotation rate is
low enough, but they will receive fewer nutrients and
experience a higher pH because of the lessened mixing.
Three-neck flask
Incubator
Needle
Scaffold
Cell suspension
Magnetic bar
Stirrer
Cell attachment
Fig. 7.2-21 Schematic view of a spinner flask designed to
assist initial cell attachment. The cell-attached scaffolds are
transferred onto 24-well plates for subsequent cell proliferation
and differentiation assays.
seeding (typically 24 h), low efficiency at low cell con-
centrations, and undesirable side effects associated with
mechanically stirred bioreactors of high shear rates. The
degree of shear stress depends on stirring speed and the
morphology of the scaffold. Cell damage has been ob-
served at 150-300 rpm in microcarrier cultures, and
although there is no apparent physical cell damage at
50 rpm, a fibrous capsule does form on the construct
surface. The local shear force experienced by the cells is
produced by eddies created by the turbulent flow of the
impeller. The smallest turbulent eddies are on the order
of several hundreds of micron with velocities of ap-
proximately 0.5 cm/sec. Cell flattening and proliferation,
7.2.6.3.2 Perfusion system
Mechanically stirred bioreactors are used in scaffold-
seeding experiments but they might not be optimal for
producing tissues such as cartilage. The most successful
results in any bioreactor are based on the use of scaffolds
seeded dynamically and at high densities. However, 3-D
hydrogels that include cells during gel formation do not
need to be seeded in this manner and cells can be dis-
tributed evenly at high densities without the use of
a mechanically stirred bioreactor. For 3-D porous
Table 7.2-5 High-shear bioreactor summary
First author
Proteoglycan
Collagen
Additional notes
Gooch (construct, 2- to 4-week
bovine knee, 18 10
6
cells/cm
3
PGA, 6 weeks)
36% decrease in GAG
composition; 2- to 3-fold
increase in GAG synthesis
80% increase in collagen
composition
Constructs retained low levels of
GAG after synthesis
Vunjak-Novakovic (construct,
2- to 3-week bovine knee,
25 10
6
cells/cm
3
PGA, 8 weeks)
60% increase in GAG
composition
125% increase in collagen
composition
Fibrous capsule formed
2-fold increase in [
35
S]-sulfate
incorporation
Smith (monolayer, adult bovine wrist,
1.5 10
5
cells/cm
2
,
3 days)
Not evaluated
High levels of prostaglandin E
2
and metalloproteinase released
Freed (construct, 2- to 3-week bovine
knee, 29 10
6
cells/cm
3
PGA, 6
weeks)
No significant change
50% increase in collagen
composition
Primarily type I collagen
Dunkelman (construct, 4- to
8-month rabbit joints, P2, 25 10
6
cells/cm
3
PGA, 4 weeks)
25% dry weight GAG (15-30%
in native cartilage)
15% dry weight collagen
(50-73% in native cartilage)
Direct perfusion; more tissue
formed on periphery than middle
Refer to the original work for the references.
