Biomedical Engineering Reference
In-Depth Information
connective tissues, consisting of cells embedded in a matrix permeated by a system
of fibers.
2.3.1
Cartilage
During early fetal life the human skeleton is mostly cartilaginous. Subsequently, it
is largely replaced by bone but persists in, for example, synovial joints. It is essen-
tially a type of stiff, load-bearing connective tissue. Its distinctive properties are: a
low metabolic rate and a vascular supply confined to its surface or to large, perco-
lating tunnels, a capacity for continued and often rapid interstitial and appositional
growth, high resistance to tension, compression and shearing. Articular cartilage,
typical
hyaline cartilage
, has neither nerves nor blood vessels. Cartilage is covered
by a fibrous
perichondrium
except at the osseous junction; the synovial surfaces are
lubricated by the
synovial fluid
, carrying nutrients, secreted at the osseous junction,
and from the synovial membrane. The lubrication is absolutely fabulous: the
fric-
tion coefficient
of cartilage to cartilage can be as small as 0.002, and moreover, is
decreasing as loading increases!
Let us make here a parenthesis on viscosity.
Newtonian Fluids
Consider a thin layer of fluid between two parallel planes a distance dy apart as in
Fig.
2.11
.
One plate is fixed and a shearing force F is applied to the other. For steady con-
ditions, F will be balanced by an internal force in the fluid due to its
viscosity
.For
a laminar flow and a so-called Newtonian fluid, the stress is linearly proportional to
the velocity gradient dv/dy and the proportionality constant is the Newtonian vis-
cosity, independent of shear rate and only dependent on temperature and pressure.
Water is an example of a Newtonian fluid. More complex fluids with long molecules
or dispersions exhibit a complex viscous behavior. Three other generic classes of
viscous behavior of fluids will be distinguished (see Fig.
2.12
).
y
moving
veloc
ity
dv
F
dy
Fig. 2.11
Newtonian fluid
between parallel planes
x
stationary
