The
advantage of the biphasic indentation technique for
cartilage is that a single measurement can give you the
aggregate modulus, the Poisson's ratio and the permeability
of cartilage. While there are other methods to measure
all of those things, this is the only technique that
can give a good value for all of them without assuming what
the others are. For instance, older methods of
determining the aggregate modulus often made an assumption
on what the Poisson's ratio was. Before this technique was
perfected, a common assumption was that the Poisson's ratio
for the solid phase of cartilage was near 0.5. We now
know that it is usually closer to 0 or 0.1.

One
of the exciting things that we have recently discovered is
that tissue engineering of some materials can modify their
viscoelastic properties so that they display a response that
fits the biphasic theory of cartilage, where they did not
before. Shown are two creep indentation curves of an
SIS (porcine small intestinal submucosa) material. The
indentation curve of the basic non-modified SIS material
does not match the theoretical indentation curves of
cartilage very well (Figure 1). But, after seeding the
material with canine chondrocytes and implanting it in the
back of a mouse for 24 weeks, one of our samples showed a
close fit to the theoretical curve for cartilage (Figure
2). This means that one can demonstrate cartilage-like
viscoelasticity by showing a close fit of the indentation
curve of the material to the biphasic theory.
When such tissue-engineered cartilage demonstrates
biphasic viscoelasticity, one can then determine the value
of its aggregate modulus, Poisson's ratio, shear modulus and
permeability from a single indentation test.