Biomedical Engineering Reference
In-Depth Information
Fig. 5.8 Scanning electron microscope photographs of the papillary layer of the human dermis
a healthy, b-c partially damaged, and d severely damaged. Magnification: a, b, c: x700, d: x200
dermis is mainly composed of extracellular matrix components, collagen and
elastin embedded in ground substance. It is assumed that the dermis is responsible
for most of the skin's mechanical properties. These properties, however, vary with
body region, age, sex, and physical condition. The subcutis comprises loose
connective tissue and fat and connects the dermis to underlying tissue, such as
muscle or organs. The subcutis contains blood vessels, lymphatics, nerve, and fat
cells (adipocytes).
Fat cells provide energy, thermoregulation, insulation and mechanical cush-
ioning. The functional purpose of adipose tissue is reviewed by (Klein et al. 2007)
in a report on subcutaneous fat. The mechanical properties of porcine subcuta-
neous (white) adipose tissue have recently been investigated under low and high
strain. Distinct time-dependent changes in viscosity at specific strain have been
demonstrated in adipose tissue, (Geerligs et al. 2008) and (Geerligs et al. 2010).
Adipose tissue samples in these studies were from the porcine middle layer, which
is reported to be comparable to the deep subcutaneous layer in the abdominal
region of humans.
In addition, to provide qualitative microscopic insight into pathologic changes
of the papillary layer of the human dermis adjacent to a pressure sore at the
sacrum, tissue samples taken post mortem from a 87 year old subject are shown in
Fig.
5.8
. The sampling locations in Fig.
5.8
provide (a) healthy tissue as well as
tissue taken from regions at the pressure sore boundary (b), (c) and (d) within a
stage-II pressure sore, (Arao et al. 1998).
5.1.3 Biological and Mechanical Properties
From a mechanobiological point of view, more or less large cell conglomerates of
human soft tissue, especially connective and muscle tissue, consist of a filling
which exhibits viscoelastic properties. The filling is enclosed by a membrane of
entangled protein fibres, permitting water exchange (enclosed-filling-problem),
(Otto 1985) and (Benderoth 1984).
The degree of filling is of central importance for the deformation behaviour due
to an outer load. Only the living organism guaranties a stationary filling state
