Biomedical Engineering Reference
In-Depth Information
500 mN and displacement range of 1 nm to 200 μm, this technique
bridges the gap between atomic force microscopy (AFM) and macroscale
mechanical testing, as illustrated in
Chapter 1
,
Fig. 1-2.
In typical
commercial instruments, displacement is monitored with a capacitance
gage, while force actuation is provided through electrostatic force
generation or magnetic coils. Load and displacement are monitored
continuously, and the elastic modulus and hardness of the substrate
material can be calculated from the resulting load-displacement curve
using well-established contact mechanics models.
6
Nanoindentation is
more readily adapted to the measurement of tissue-level properties than
AFM due to its higher load and displacement capabilities, normal sample
approach, and well-defined tip shapes. For this reason, nanoindentation
techniques have been used to measure local mechanical properties in
tissues ranging from bone and teeth to cartilage and arteries,
7
as well as
other biological materials such as wood
8
and spider silk.
9
Despite the recent rise in the use of nanoindentation to characterize
biological materials, there are many challenges associated with adapting
commercial nanoindentation instruments to the study of biological
materials. While all biological materials present some challenges, soft
tissues provide the greatest challenge to the field of nanoindentation
because their inherent properties are the most different from the
traditional indentation substrates for which commercial nanoindentation
systems were designed. Hence, soft tissue characterization will be the
primary focus of this chapter.
Nanoindentation was developed for application to silicon and other
engineering materials. These are materials that exhibit nominally elastic
behavior, are stiff (elastic moduli >> 10 GPa), can be polished to present
a flat surface, and can be tested in ambient air, without concern for
rigorous temperature or humidity control. Soft tissues, on the other hand,
generally exhibit time-dependent (viscoelastic or poroelastic) behavior,
are compliant (moduli on the order of kPa to MPa), have irregular
surfaces, exhibit significant amounts of creep, and have properties that
vary with degree of hydration and with temperature. Hence, numerous
modifications to traditional indentation techniques have been necessary
in order to apply commercial nanoindenters to the characterization of
these complex materials. These adaptations include developments in
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