Article,
Method of testing very soft biological tissues in compression
Journal of Biomechanics, 38 (1): 153--158 (January 2005)
Abstract
Mechanical properties of very soft tissues, such as brain, liver,
kidney and prostate have recently joined the mainstream research
topics in biomechanics. This has happened in spite of the fact that
these tissues do not bear mechanical loads. The interest in the biomechanics
of very soft tissues has been motivated by the 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. Mechanical testing of very soft tissues provides a formidable
challenge for an experimenter. Very soft tissues are usually tested
in compression using an unconfined compression set-up, which requires
ascertaining that friction between sample faces and stress-strain
machine platens is close to zero. In this paper a more reliable method
of testing is proposed. In the proposed method top and bottom faces
of a cylindrical specimen with low aspect ratio are rigidly attached
to the platens of the stress-strain machine (e.g. using surgical
glue). This arrangement allows using a no-slip boundary condition
in the analysis of the results. Even though the state of deformation
in the sample cannot be treated as orthogonal the relationships between
total change of height (measured) and strain are obtained. Two important
results are derived: (i) deformed shape of a cylindrical sample subjected
to uniaxial compression 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 are highlighted.
