Biomedical Engineering Reference
In-Depth Information
menter may choose. Biological tissues also often show time-dependent effects
such as viscoelasticity (50). Predictions of in-vivo mechanical behavior can only
be made using a model that has been verified in vitro.
2.3.3. Effects of Fluid Mechanotransduction on Cells
Just as stresses transmitted by the ECM can affect cell behavior, so can
stresses transmitted by fluids flowing through an organ. Almost all physiological
surfaces that face flow are coated with a layer of endothelial cells, so these are
the only cells regularly affected by fluid mechanotransduction. The effect of
flow on endothelial cells is a well-studied topic. Investigators have reported on
hundreds of effects of flow mechanotransduction on endothelial cells (17,23,55)
(Figure 15), so that we know the shears and pressures that give rise to different
gene expressions, changes in intracellular makeup, physical deformation of the
cell, electrical behavior of cells (4), and others. Importantly for tissue engineer-
ing, fluid mechanotransduction guides the formation of blood vessels and vascu-
lature, and figures prominently in many vascular illnesses (23).
Figure 15
. Response of vessels to decreased flow: (
A
) at the start of experiment, the two
vessels are the same size; flow reduced in the left vessel; (
B
,
C
) two weeks later; note the
reduction in size of the left vessel in response to flow reduction (46).
2.3.4. Modeling of Blood Flow
The modeling of blood flow through a single vessel can be complicated;
while blood flow in almost all vessels is laminar, the highly non-Newtonian be-
havior of blood and distensibility of the vessel walls must be accounted for.
Blood's non-Newtonian behavior can be described by Casson's equation,
which gives an apparent viscosity:
2
N
¬
-
k
=+
k
2
-
®
,
[2]
-
-
1
Ud
/
