Biomedical Engineering Reference
In-Depth Information
then become myofibroblasts and thus have the character of smooth muscle cells in
being able to impose contraction forces on the edges of the wound bed in order to
close the wound.
1,4
Wound contracture is the primary cause for the wound bed to
be reduced to between 5-10% of its size within 6 weeks of the injury. Both types
of wound healing involve scar formation (since healing by second intention
involved greater loss of and damage to tissues, scar formation is generally greater
in this type of wound healing. A scar itself is fibrous tissue that replaces normal
skin that was damaged.
Scar tissue contains collagen fibers that are much more densely packed than
those in normal skin.
1
This causes scar tissue to be much stiffer than normal skin
tissue.
1
3.5
Modeling skin using finite element methods
In order to understand how skin responds to various forces and stresses placed
upon it, we can simulate skin using numerical techniques. One such technique
simulates skin by creating a fine network of elements which when put together
behave overall like the skin. This is the idea behind finite element methods. One
can visualize such techniques by thinking of the three-dimensional structure of
skin as being composed of small rectangular elements. These elements are then
combined mathematically to yield the response of the system being modeled at
each discrete element. For example, by using such techniques we are able to
ascertain the stresses within a material under the application of certain boundary
conditions (forces, displacements, pressures).
We created one such model of the skin in analyzing how micromechanical force
therapies, such as the vacuum assisted closure device, (VAC, Kinetic Concepts,
San Antonio, Texas) may work.
5
The skin was modeled as linear, elastic, homoge-
neous and isotropic. The mechanical properties of the skin were captured by the
Young's modulus (measures stiffness) and the Poisson's ratio (measures
compressibility).
5
Because wounded skin may have very different mechanical
properties from normal skin, and because normal skin itself may have different
properties from one person to another (based on age, sex, individual variation, and
so on), each of the above mentioned properties were varied. Poisson's ratio was
varied between 0.36 and 0.5, while the Young's modulus was varied from 50-100
kPa.
The VAC device applies suction to a wound bed through a sponge packed within
a sealed dressing in the wound bed. With the application of the suction the
wounded tissue is exposed to sub-atmospheric pressures and is pushed by the
external pressure underneath the wound (alternately one can also consider the
wound bed to be 'pulled' into the sponge by the sub-atmospheric pressure). The
sponge itself collapses into a thin layer at the application of the negative pressure.
The wound is thus blocked from rising upwards by the sponge struts and thus the
wound rises into the empty space between the sponge struts (roughly hexagonal in
