  Strain-stiffening is a well-documented behavior in soft biological materials
such as liver and brain tissue. Measuring and characterizing this nonlinear
response, which is commonly considered as a mechanism for damage prevention, is
of great interest to engineers for design of better biomimetic materials, and
to physicians for diagnostic purposes. However, probing the elastic response of
soft or biological materials at large deformation in their natural habitat, is
an arduous task. Here, we present the Volume Controlled Cavity Expansion (VCCE)
technique as an in-vivo measurement method that offers the ability of
characterizing the stiffening response of materials in addition to identifying
their shear modulus. By employing minimal constitutive representations
involving only two constants (Mooney-Rivlin, Gent, and Ogden) we show that for
the conventional PDMS samples, this technique and an accompanying data analysis
method capture the shear modulus, as well as providing reliable measures of the
stiffening behavior of the samples.
