Biomedical Engineering Reference
In-Depth Information
2 w
K 0 ¼ J o
oJ 2 j J ¼ 1 ¼ 2
D :
ð 3 : 460 Þ
Equation ( 3.460 ) sufficiently restricts parameter D to D [ 0.
The shear modulus at the initial stress-free reference state yields
2 w
oc 2 j c ¼ 0 ; k 1 ¼ k 2 ¼ 1 ¼ 2C 10
l 0 ¼ o
ð 3 : 461 Þ
with the single derivatives given in ( 3.426 ) together with
2
o
oc I 1 j c ¼ 0 ; J ¼ k 1 ¼ k 2 ¼ k 3 ¼ 1 ¼ 0 ;
o
I 1 j c ¼ 0 ; J ¼ k 1 ¼ k 2 ¼ k 3 ¼ 1 ¼ 2
oc 2
ð 3 : 462 Þ
o 2
oc 2 e k 2 E j ; iso j c ¼ 0 ; J ¼ k 1 ¼ k 2 ¼ k 3 ¼ 1 ¼ 0 :
and
Equation ( 3.461 ) sufficiently restricts parameter C 10 to C 10 [ 0.
In the context of material stability, it is shown in Balzani et al. (2006), for the
material constants k 1 and k 2 to require k 1 0 and k 2 [ 0.
3.5 Biomechanical Hypothesis
3.5.1 Introduction
Evaluation of the effectiveness of technical body support systems with respect to
preventive issues considered from a biomechanical point of view must be based on
an investigation of the effects on the respective body tissues. In this context
however, a qualified ''biomechanical hypothesis'' regarding mechanical tissue
stress thresholds for deeper tissues region is to be established.
For instance, primary goal when developing bedding systems in the area of
medical care must include pressure sore prevention. The term pressure sore is
defined by Chapman (1986) as follows: ''a pressure sore is a limited area of cell
damage caused directly by pressure effects or, is caused by frictional effects and
accompanying mechanical shear forces applied to the tissue''. A generally
accepted hypothesis assumes that by applying pressure on the tissue and the hereon
related pressure distribution in deeper tissue regions causes compression of the
(micro-) vasculature and thus reducing the blood and oxygen transport, respec-
tively, resulting in anaerobic cell metabolism accompanied by acidosis with
increased cell permeability and local edema formation. Prolonged anaerobic
metabolism finally leads to cell death (necrosis) and thus to sore development.
Already in 1722, the French surgeon De La Motte documented that pressure as
extrinsic factor plays an important role in the occurrence of tissue necrosis, but the
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