Bioreactors - Articular Cartilage Tissue Engineering - page 61

exposed to intermittent hydrostatic pressure [ 509 ]. In addition to the biochemical and phenotypical
effects, hydrostatic pressure has been shown to influence mechanical properties critical to the function
of cartilage constructs. At the counterintuitive frequency of 0 Hz (i.e., static), 10 MPa, applied for
1 hour on days 10-14 of a 4 week culture, was shown to significantly increase aggregate modulus
values by 1.4-fold. This regimen also affected functional properties that seem to be difficult to
improve upon, namely tensile modulus and strength along with corresponding collagen content,
which increased over 2-fold [ 386 ]. Hydrostatic pressure has also been shown to act synergistically
with growth factors. A combination of 10 MPa static hydrostatic pressure, applied for 1 hour a day
for 5 days, and 30 ng/ml TGF- β 1 had an additive effect on the mechanical properties, increasing the
aggregate modulus by 164% and the Young's modulus by 231%, approaching 300 kPa and 2 MPa,
respectively. Additionally, the combined treatment had a synergistic effect on collagen content,
increasing it by 173% [ 390 ].
Since hydrostatic pressure appears to have a positive effect on the chondrocytic phenotype
of cells, recent efforts have used it to help stimulate chondrogenic differentiation in adult stem
cells [ 506 , 511 - 516 ]. Gene and protein expressions for synovium-derived mesenchymal stem cells
were enhanced by intermittent pressurization, showing upregulation of proteoglycan core protein,
type II collagen, and SOX-9 [ 514 ]. Adipose-derived stem cells expressed a chondrocytic phenotype
under hydrostatic pressure and accumulated a pericellular matrix more rapidly than non-loaded
controls [ 506 ]. A variety of hydrostatic pressure regimens had chondrogenic effects on bone marrow-
derived mesenchymal stem cells [ 511 - 513 , 515 ]. Furthermore, synergistic effects between growth
factors such as TGF- β 3 and hydrostatic pressure have been observed for adult stem cells [ 513 ]. The
loading regimen still plays an important role, though. Low magnitude stimulation has been found
to favor SOX-9 and aggrecan expression, whereas high magnitudes favor type II collagen expression
and synthesis [ 512 ]. These findings might help in determining a more complex loading regimen
that targets certain gene expressions at set times.
As with other mechanical stimuli, hydrostatic pressure might assist in organizing cartilage
matrix molecules into a more functional structure. Past work has shown that chondrocytes cultured
with exogenous chondroitin sulfate formed an abundant cell-associated matrix when exposed to cyclic
pressure. Control samples did not incorporate as much chondroitin sulfate and had a less organized
matrix when examined by transmission electron microscopy [ 517 ]. Self-assembling chondrocyte
cultures exhibited higher protein synthesis levels under intermittent pressurization and also showed
formation of lacunae surrounding the cells [ 312 ]. This structure is similar to that seen in native
cartilage and could be critical to the protection of cells in a mechanically-loaded tissue.

4.3 SHEAR BIOREACTORS

Four general categories of shear bioreactors have been investigated for tissue engineering studies.
The first is a solid-on-solid, contact shear that attempts to replicate the physiological situation where
cartilage rubs against either cartilage or meniscus. The second type, fluid shear, focuses on using fluid
flow as a source of shear for monolayer cell populations or cell-seeded constructs and is hypothesized