Bioreactors - Articular Cartilage Tissue Engineering - page 62

62 CHAPTER 4. BIOREACTORS
to increase nutrient and waste transfer to increase cell metabolism during culture. The third type of
shear, direct fluid perfusion, is a stimulus that has less connection to the physiological conditions in
normal joint motion but, instead, was developed primarily to facilitate nutrient transfer through the
bulk of a three-dimensional scaffold. The last category, utilizing low shear “microgravity” bioreactors,
applies minimal loading to constructs floating in a fluid environment that has flow characteristics
that greatly enhance mass transfer to and from the cells.
4.3.1 CONTACT SHEAR
Shear loading is one of several physiological condition that provides mechanical stimulation in
normal joint function. While solid-on-solid shearing is minimal because of fluid pressurization in
cartilage [ 7 ], small amounts of contact shear still exist. The rubbing of two solid materials can have
elements of compression and tension that affect the response of cells within the tissue. Bioreactors
that replicate this form of mechanical stimulation often are attempting to induce cells to synthesize
cartilaginous matrix molecules, as well as trying to create a surface that has frictional properties more
akin to the native tissue. As yet, only a few instances of “contact shear” bioreactors have been reported
in the literature, likely due to the non-uniform stimulation that is applied through the depth of a
sheared construct. Future studies might begin to incorporate contact shear stimulus as a later-stage
stimulus once significant matrix has already been deposited in the construct.
Shear bioreactors can be designed to apply translational or rotational strains. In translational
shear devices, the construct typically remains fixed to the bottom surface while the top surface
moves along one axis [ 518 , 519 ]. Rotational shear devices apply a small amount of compressive
strain and then rotate around the z-axis to produce strains in the construct [ 520 - 522 ]. This is the
same motion as used for torsion tests or rheometry although for purposes of stimulation rather than
characterization. Alternatives to these traditional approaches do exist, one of which tries to replicate
the physiological mechanical environment with a loading shaft that rolls across the tops of fixed
constructs, applying a low level of frictional shear (0.5 N normal force) in a cyclic manner [ 389 ].
As with direct compression and hydrostatic pressure, dynamically applied shear strains showed
more promising results than static conditions. Dynamic shear of 2% at 1 Hz produced constructs
with 40% more collagen, 25% more proteoglycan, and 6-fold higher equilibrium modulus [ 519 ].
Interestingly, loading duration was minimal - only 6 minutes of shear every other day produced these
increases after four weeks. Another research group showed that dynamic shear of 1-3% at 0.01-1 Hz
could increase protein synthesis 50% and proteoglycan synthesis 25% [ 522 ]. The addition of IGF-I
to cultures undergoing shear was found to have a synergistic effect on protein and proteoglycan
synthesis that was independent of any improvement of convective diffusion [ 521 ]. Applying an
interface motion i.e., frictional shear) to cell-seeded scaffolds has been shown to increase cartilage
oligomeric matrix protein expression [ 518 ]. Future research will investigate the role contact shear
has on matrix organization through the depth of engineered constructs.