Biomedical Engineering Reference
In-Depth Information
Fig. 5.4 Stained sections of a human skin, b white adipose tissue with adipose cells and
c skeletal muscle with muscle fibres (white), adopted from (University of Central Florida 2009)
and (University of Western Australia 2009)
and dentin. Muscle tissue can be subdivided in smooth and striated muscles, cf.
Fig.
5.2
b-d. Nervous tissue is composed of nerve cells and neuroglia (ectodermal
connective tissue) and is found in the central nervous system (brain and spinal
cord), as well as in the peripheral nervous system.
This differentiation circumscribes the anatomic and physiologic complexity of
human tissue. Figure
5.1
,
5.2
, and
5.3
a show histological slices of different tissues
and their structures (meso-, micro-, ultra-structures). Distinct material inhomo-
geneities from a continuum mechanical standpoint are reflected by different
scaling levels and accompanying material anisotropies. Figure
5.4
shows (micro-)
structural differences between skin, fat and skeletal musculature.
The mechanical behaviour of soft biological tissue depends on the fibrillar
components of the extracellular matrix, primarily elastin and collagen, as well as
nonfibrillar components in which the cells and fibres are embedded. The
mechanical properties of elastin and collagen determine the overall mechanical
tissue behaviour and will be discussed in more detail.
5.1.1.1 Collagen
According to Wikipedia (2009), collagen, derived from the Greek kolla gen
(producing glue) was used in ancient times as wood glue. Collagen is a structural
protein and is a basic constituent of the extracellular matrix of connective tissue in
animal and human bodies. 30 % of the total weight of all proteins is collagen,
making it the most widespread protein. Collagen constitutes a major part of bones,
teeth, cartilage, ligaments, tendons and skin. Its fibres have an extreme tensile
strength at almost zero strain and exhibit diameters in the range of 1-10 lm
(cf. Fig.
5.7
). Collagen thus represents the main load-carrying element.
Collagen is composed of helical peptide chains which untypically exhibit a left-
orientated helix structure (cf. Figs.
5.5
a, d and
5.7
b). Three of these helices,
