Abstract
Tissue softening is commonly reported during mechanical testing of
biological tissues in vitro. The loss of stiffness may be due to
viscoelasticity-induced softening (the time-history of load-caused
softening) and strain-induced stress softening (the maximum previous
load-caused softening). However, the knowledge about tissue softening
behaviour is presently poor. The aims of this study were to distinguish
whether the loss of the stiffness during preconditioning was due
to strain softening or viscoelasticity and to test the tissue softening
in circumferential and longitudinal direction in the guinea pig oesophagus.
Eight repeated pressure controlled ramp distensions and eight uniaxial
tensile-release ramp stretches in three series were done on eight
guinea pig oesophagi. The stress-strain curves were used to display
the time-dependency (viscoelasticity) and the maximum previous load-caused
softening (strain softening) in circumferential and longitudinal
directions. For both the longitudinal and the circumferential softening,
the peak stress and stiffness produced during the first loading were
bigger than those produced in the remaining loadings. The stress
loss due to strain softening was about three times more than that
due to viscoelasticity in the longitudinal direction. The strain
increased more than two times between the strain softening and viscoelastic
softening in the circumferential direction. With a stress level of
20�kPa, the stiffness in the circumferential direction lost more
than that in the longitudinal direction (P<0.05), indicating the
anisotropic softening properties in the oesophagus. In conclusion,
the stiffness loss during preconditioning is mainly attributed to
strain softening, appears irreversible and is anisotropic.
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