Article,
How to test very soft biological tissues in extension?
Journal of Biomechanics, 34 (5): 651--657 (May 2001)
Abstract
Mechanical properties of very soft tissues, such as brain, liver and
kidney, until recently have largely escaped the attention of researchers
because these tissues do not bear mechanical loads. However, developments
in Computer-Integrated and Robot-Aided Surgery -- in particular,
the emergence of automatic surgical tools and robots -- as well as
advances in Virtual Reality techniques, require closer examination
of the mechanical properties of very soft tissues and, ultimately,
the construction of corresponding, realistic mathematical models.
A body of knowledge about mechanical properties of very soft tissues,
assembled in recent years, has been almost exclusively based on the
results of compression, indentation and impact tests. There are no
results of tensile tests available. This state of affairs, in the
author's opinion, is caused by the lack of analytical solution relating
a measured quantity -- machine head displacement -- to strain in
simple extension experiments of cylindrical samples with low aspect
ratio. In the paper this important solution is presented. The theoretical
solution obtained is valid for isotropic, incompressible materials
for moderate deformations (<30%) when it can be assumed that planes
initially perpendicular to the direction of applied extension remain
plane. Two astonishing results are obtained: (i) deformed shape of
a cylindrical sample subjected to uniaxial extension is independent
on the form of constitutive law, (ii) vertical extension in the plane
of symmetry lambdaz is proportional to the total change of height
for strains as large as 30%. The importance and relevance of these
results to testing procedures in Biomechanics is highlighted.
