The Advantage of the Biphasic Indentation Technique

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.

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