Abstract
Slow, large deformations of human brain tissue--accompanying cranial
vault deformation induced by positional plagiocephaly, occurring
during hydrocephalus, and in the convolutional development--has surprisingly
received scarce mechanical investigation. Since the effects of these
deformations may be important, we performed a systematic series of
in vitro experiments on human brain tissue, revealing the following
features. (i) Under uniaxial (quasi-static), cyclic loading, brain
tissue exhibits a peculiar nonlinear mechanical behaviour, exhibiting
hysteresis, Mullins effect and residual strain, qualitatively similar
to that observed in filled elastomers. As a consequence, the loading
and unloading uniaxial curves have been found to follow the Ogden
nonlinear elastic theory of rubber (and its variants to include Mullins
effect and permanent strain). (ii) Loaded up to failure, the "shape"
of the stress/strain curve qualitatively changes, evidencing softening
related to local failure. (iii) Uniaxial (quasi-static) strain experiments
under controlled drainage conditions provide the first direct evidence
that the tissue obeys consolidation theory involving fluid migration,
with properties similar to fine soils, but having much smaller volumetric
compressibility. (iv) Our experimental findings also support the
existence of a viscous component of the solid phase deformation.
Brain tissue should, therefore, be modelled as a porous, fluid-saturated,
nonlinear solid with very small volumetric (drained) compressibility.
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