Bioreactors - Articular Cartilage Tissue Engineering - page 67

pore structure and not on its face. This flow environment results in a construct that has a thick matrix
layer on one side and minimal matrix on the other. Additionally, matrix secretion in the bulk of the
scaffold is non-uniform since the energy associated with fluid flow either dissipates or concentrates in
regions according to the scaffold structure. Another problem is the induction of molecules associated
with injury response rather than matrix formation. Studies have shown that shear levels as low as
0.092 Pa (0.92 dyne/cm 2 ) can have adverse effect on cells [ 542 ]. While chondrocytes are considered
robust when exposed to mechanical stress, studies have shown that turbulent flow can produce a
negative effect even on chondrocytes [ 527 - 529 ]. The cells might not die, but their protein secretions
do become phenotypically altered, resulting in a deposited matrix that is mechanically inferior to
native cartilage. In this case, chondrocytes produce a thick, fibrous matrix composed mainly of
type I collagen that effectively isolates the cells from the turbulent flow [ 543 ]. High-shear direct
perfusion devices induce a fibrous response similar to the capsule formed in some spinner flask
cultures, although it is usually restricted to one side of the construct.
4.3.4 “LOW-SHEAR” BIOREACTORS
Flow-based bioreactors are attractive systems for tissue engineering because they improve mass
transfer rates, effectively increasing nutrient concentrations and decreasing waste levels in the culture
environment. While high-shear perfusion can successfully stimulate matrix production, the resulting
tissue is typically fibrous in nature rather than hyaline. Slower fluid flow rates are hypothesized to have
a general stimulatory effect on matrix synthesis while still allowing cells to express a chondrocytic
phenotype. This is the premise behind low-shear, rotating bioreactors.
Some of the most successful literature reports for cartilage bioreactors come from a modified
version of the clinostat, which was first described in 1872 by Julius von Sachs [ 544 ]. Its modern day
representation, the rotating wall bioreactor, provides a culture environment in which constructs are
continuously suspended in media. Sometimes described as a “microgravity” environment, this device
was developed by researchers to investigate the effect of free-fall on cell and tissue growth [ 545 ].
Within the past decade, rotating bioreactors have found success as a low-shear, high diffusion biore-
actor for many cell types. The ability of perfusion bioreactors to provide nutrient-rich environments
for cells in a stimulatory environment was carried over into the design of this more sophisticated
device. The original design is comprised of a media-filled, cylindrical vessel that rotates around
a central axis (also capable of rotation) at 15-30 rpm, which keeps constructs or cells floating in
suspension. Rotation speed has to be adjusted throughout the culture period to balance any gravity
effects on the samples. Gas exchange occurs through a gas permeable membrane that forms a hollow,
inner cylinder. Dynamic laminar flow in rotating bioreactors provides efficient oxygen supply and
allows newly synthesized macromolecules to be retained in the developing constructs [ 546 ]. Later
versions of the rotating bioreactor have modified the shape of the vessel and the mechanism for gas
and media perfusion. The culture environment present in rotating bioreactors make it attractive for
not only tissue engineering studies, but also more basic studies focusing on cartilage healing and
cell aggregation [ 547 , 548 ]. The major difference between rotating bioreactors and past perfusion