Biomedical Engineering Reference
In-Depth Information
4.3.2 FLUID SHEAR
While the application of fluid shear forces is more typically associated with vascular tissue engineer-
ing, it has been hypothesized that individual chondrocytes might sense shear forces as fluid flows
in and out of the solid matrix during compression. Cone viscometers have been used extensively to
understand the effects of shear on chondrocyte monolayers. More recent work has focused on using
high-shear fluid devices as stimulatory bioreactors or solely as seeding apparatuses.
One of the simplest “bioreactors” is the spinner flask, which uses an impeller to mix oxygen
and nutrients throughout the media. Cell-seeded scaffolds are fixed firmly inside the flask away from
the impeller. The samples benefit from increased nutrient and waste transfer, as well as experience
controllable levels of shear. The container shape and mixing rate can both affect the shear patterns
throughout the culture environment, leading some groups to investigate close alternatives to the
spinner flask, such as the wavy-walled bioreactor [
523
,
524
]. In all of these fluid shear bioreactors,
cells can either be seeded onto scaffolds before they are inserted into the flask or inoculated directly
into the media, gradually attaching to scaffolds already in the flask [
525
]. Much work has gone
into optimizing cell seeding of scaffolds using this technique, which will be discussed later in this
chapter. Another variation of the mechanically-stirred environment is the orbital shaker/rotating
plate, which can slowly mix media in a culture without much turbulence [
526
].
Cell-seeded scaffolds cultured in spinner flasks have shown both positive and negative results,
depending on the level of shear seen by the cells. In one study, cartilage constructs from fluid-
sheared cultures (at 50 rpm) were more regular in shape and contained up to 70% more cells,
60% more sulfated glycosaminoglycan, and 125% more total collagen [
527
]. The increase in matrix
constituents is likely due to the larger number of cells in the constructs compared to controls. While
the improved matrix composition is a plus, there are also undesirable side effects associated with
high shear systems. Cell damage has been observed at 150-300 rpm in microcarrier cultures [
528
],
and although there is no apparent physical cell damage at 50 rpm, a fibrous capsule does form on
the construct surface [
527
]. While fibrous encapsulation does occur in most systems because of
increased nutrient availability at the construct surface, its presence could also indicate a protective
response to shear forces. The local shear force experienced by the cells is produced by eddies created
by the turbulent flow of the impeller. Cell flattening, proliferation, and formation of an outer capsule
is caused by the pressure and velocity fluctuations associated with turbulent mixing [
527
]. Other
experiments have also seen increases in total collagen content for constructs cultured in spinner
flasks [
529
]. However, a large percentage is likely type I collagen since that is what composes the
capsule surrounding the construct. The mixing rate has a limited effect on the amount of proteins
secreted by cells but does affect what types of proteins are made and whether they are incorporated
into the construct. Cell-seeded scaffolds exposed to any intensity of mixing (80-160 rpm) synthesized
more collagen and GAG than controls but actually retained lower fractions of GAG within the
scaffold [
523
]. This loss of GAG from the construct is caused by the continual convective flow in
the spinner flask.
