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Several approaches have been explored to
colonize cells within the porous scaffold
outside the body. In a traditional cell culture
practice, cells are populated in a batch culture
on flat, stationary plates with a certain amount
of medium containing necessary factors. In these
batch culture systems, the primary driving force
for nutrient distribution within the porous
structure is diffusion which is dictated by
Fick’s first law. Cells consume nutrients and
relying on limited amount of nutrients always
leads to starvation and non- uniform
distribution. A way to improve the nutrient
distribution is by constant mixing or applying
flow systems for continuous replenishment where
nutrient distribution is facilitated by
convection. In addition to improving the
nutrient distribution, fluid flow can also
introduce shear force on the cells. Many parts
of the body are exposed to stresses either due
to the weight they carry (such as bone), the
function they perform (such as bladder and
cartilage) or due to the flow of fluid (lung,
blood vessels). Thus, it is important to grow
the cells by exposing them to the same
conditions that they are exposed to within the
body. |
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Different types of bioreactors have been
designed to regenerate tissues with the
intention of improving the nutrient distribution
while applying mechanical stimuli. However, the
fundamental concepts in developing these
reactors are not well defined. For example, many
tissues (skin, bladder, and cartilage to name a
few) have a high aspect ratio i.e., large
surface area relative to the thickness of the
matrix and contain multiple cell types. However,
flow within bioreactors containing large porous
structures with high aspect ratios has not been
studied. In these scale-up systems, one has to
understand the fluid distribution and the effect
of shape of the reactor. Non-uniform flow
patterns within the reactor could lead to i)
poor distribution of nutrients and ii)
non-uniform shear stress distribution. These
factors affect cellular colonization,
proliferation, and function. Further, tissue
regeneration is a dynamic process where the
porous characteristics change due to
proliferation of cells, de novo deposition of
matrix components, and degradation of the porous
architecture. These changes affect the transport
characteristics which ultimately determine the
quality of the regenerated tissue. Thus to
develop improved quality tissues, one has to
understand the influence of these factors. |
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The objective of the project is to understand
the fundamental governing characteristics in
various reactor configurations that lead to
regeneration of high quality tissues. We use a
set of integrated studies to understand the
influence of various parameters on tissue
regeneration. First, we use computational fluid
dynamic (CFD) tools such as Comsol and CFX to
understand the effect of reactor configurations
on fluid distribution through the porous
structure. In the simulation various factors are
considered for evaluation. Some of these factors
include i) reactor shapes (rectangular,
circular), ii) flow rate, iii) inlet-outlet
location, iv) inlet-outlet size which regulate
velocities, v) permeability of the scaffold, vi)
nutrient consumption (particularly oxygen and
glucose) characteristics (oxygen profiles in one
of reactor configuration is shown in the figure
above), vii) different types of cells (smooth
muscle cells, chondrocytes, hepatocytes,
fibroblasts, cord blood stem cells), and viii)
mechanical properties of the porous structures.
For example, we assess the effect of changing
porous architecture due to cell growth and
deposited matrix elements on fluid distribution,
shear stresses and pressure drop. We also
evaluate minimum flow requirements to satisfy
nutrient requirements. Secondly, we perform
experiments to validate the simulation results.
We evaluate the effects of changing porous
characteristics during tissue regeneration
attributed to de novo synthesis of matrix
elements and cell colonization by synthesizing
matrixes with different pore characteristics. We
evaluate changes in cellular behavior under
fluid flow (figure above). We also measure
diffusivities in porous structures, and account
for changes in dimensions of the scaffolds using
mechanical properties. |
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