NANOINDENTATION OF SOFT TISSUES AND OTHER BIOLOGICAL MATERIALS - Nanoindentation with Biological Applications - page 279

observed between mechanical properties measured through indentation
experiments and cartilage disease or degeneration have led to the
more recent development of commercial hand-held indenters for
diagnostic use during arthroscopy surgeries. 16,17 Using either quasi-
static 17 or dynamic 16 compression, these hand-held indenters are used to
probe joint tissue during surgery and identify regions of cartilage
degeneration. These developments demonstrate the value placed on
mechanical characterization of cartilage tissue as a marker for disease
or degeneration, and the important role of indentation in the field of
cartilage biomechanics.
The use of nanoindentation to characterize mechanical properties in
cartilage is a natural extension of the popular indentation testing
technique. While macroscale and microscale indentation work well for
large joints from large animal models, such as dog or cow knee joints,
they cannot be used to map mechanical properties of articular cartilage
from small animal models (such as murine models) or from small joints
(such as the finger joints) because the cartilage is too thin and mapping
requires finer spatial resolution. It is therefore not surprising that
researchers have turned to nanoindentation to map mechanical properties
under conditions that require finer spatial or displacement resolutions.
However, it is important to note that nanoindentation studies of
cartilage have not simply replicated the traditional load-relaxation tests
at a smaller length scale. Load-relaxation tests require displacement-
control for implementation, and typically take many minutes or
hours to reach equilibrium. In addition, biphasic poroelastic analysis
sometimes requires the use of a porous indenter tip to allow fluid flow.
Due to instrumentation and experimental limitations with commercial
nanoindentation systems (discussed in Section 4), it can be difficult to
exactly replicate the traditional load-relaxation test protocol and biphasic
analysis. Instead, most nanoindentation experiments in cartilage have
either been based on quasi-static nanoindentation testing of polymers
with nominally elastic analysis or have involved development of entirely
new experimental protocols. The next section will review the types of
experiments that have been performed on cartilage, and the types of
research questions that are being addressed. Advantages and limitations