Biomedical Engineering Reference
In-Depth Information
epithelial layer with the mechanical strength needed for it to withstand various
stresses placed upon it.
The basement membrane separates the epidermis from the (relatively) much
thicker dermis (1- 2 mm in thickness and thus one order of magnitude thicker than
the epidermis). The dermis is responsible for the mechanical properties of skin.
1
The dermis is made up of fibers of connective tissue that run in all directions.
3
These fibers are made up of elastin and collagen. Collagen fibers provide strength
to the skin, while the elastin fibers provide recoil strength.
3
The tensile strength of
the dermis can range from 500 psi to 10 000 psi (3.4 to 68.9 MPa) (depending on
the type of species, orientation of the skin specimen tested and the location on the
body).
1
The collagen and elastin fibers are both embedded in an interfibrillar
matrix of proteoglycans.
3.3 Definition of mechanical properties
Stress is defined as the total force applied per unit area. In units, stress is exactly the
same as pressure (Newtons per meter). Or mathematically:
force
σ
=——
area
where
is the stress.
The strain is defined as the change in length of an object over the original length
written as:
σ
∆
l
ε
=
l
where
l
is the change in length and
l
is the original length. Strain
has no units since it is the ratio of length to length. Strain is usually presented as a
percentage.
The modulus of elasticity or Young's modulus,
E
, is defined as the ratio of stress
to strain. Or:
ε
is the strain,
∆
σ
E
=—
ε
The Poisson's ratio,
, is the ratio of negative strain in one direction to the strain in
an orthogonal direction and is given by:
-
ν
ε
x
=—
ε
y
Creep is the slow deformation of a material under the influence of stress over time
where the stress is constant over time, while stress relaxation refers to the change
in stress in a material which is subjected to a constant strain.
The stress-strain curve (
Fig. 3.2)
describes graphically how the material under
ν
