Biomedical Engineering Reference
In-Depth Information
each tissue type has one or multiple biological functions, i.e. any activity that con-
tributes to the correct functioning of the organism. Damage to each of these func-
tions requires an objective and quantitative method for evaluation. In both cases this
can be done either directly, through functional assessment, or indirectly, through
morphological assessment.
Damage to the mechanical function manifests itself through rupture or degrada-
tion of the mechanical constituents of the tissue. One way to quantitatively assess
this form of damage is to perform mechanical tests of tissues before and after the
induction of damage. For example, biaxial tensile testing on a patch of cardiovascu-
lar tissue can provide information on its stiffness in different directions. Excessive
tension will cause the gradual rupture of more and more of the collagen fibers which
will induce a measurable decrease in stiffness in the directions in which the collagen
fibers contribute.
Another, indirect way to assess mechanical function is to assess the morpholog-
ical integrity of the tissue. Imaging can provide insight into the composition of a
tissue and expose fractures in the different constituents. A microscopic image of a
patch of cardiovascular tissue can be stained to specifically show, for example, the
collagen fiber component (Schriefl et al.,
2012
). If imaged when brought to exces-
sive tension, image processing can reveal the percentage of collagen fibers that are
still intact, providing a quantitative measure of the damage to this constituent.
Damage to the biological function manifests itself through malfunction, function
switch or apoptosis of the involved cells. Quantification of the biological function
before and after damaging the tissue provides a measure of the induced damage.
Sometimes it is possible to directly measure this function. In other situations an
indirect approach is taken, by measuring the concentration of certain products, or
the expression of certain genes, as a biological function is often the result of a cell-
biological cascade of events.
For the specific case of arterial tissue, functionality refers to the vasoregulating
capability of the tissue, i.e., the potential of the smooth muscle cells to contract or
relax in order to regulate the blood pressure. This vasoregulating capability can be
quantified in an experimental setup, known as a 'myograph'. Schematically shown
in the top right image of Fig.
10.1
, the myograph consists of a water-jacketed or-
gan chamber in which an excised cylindrical section of an artery can be mounted
such that isometric tension can be recorded. The sample is immersed in a Krebs
buffer at 37 °C and continuously gassed with a mixture of 95 % oxygen and 5 %
carbon dioxide. After stabilization at the optimal preload level, Phenylephrine (PE)
at 10
−
6
M is added to the solution to induce contraction. PE is a contracting agent
that acts directly on the smooth muscle cells. Sodium nitroprusside (SNP) (10
−
6
M)
induces an endothelium-independent relaxation. Consequently, an adequate level of
SNP-induced relaxation will indicate intactness of the smooth muscle cells (Callera
et al.,
2000
). Absolute values of relaxation as well as the percentage of relaxation
relative to the amount of contraction are recorded and provide a quantitative mea-
sure of the damage to the smooth muscle cells when comparing these values to those
of an intact sample. More details on the experimental setup can be found in Famaey
et al. (
2010
). A similar custom-designed device to test active force generation in
response to electrical stimulation is reported in Böl et al. (
2012
).