Biomedical Engineering Reference
In-Depth Information
Sample preparation
a.
The specimens are often small in size and may need to be embedded in resins before they are pro-
cessed for nanoindentation. However, the mechanical properties of the tissues will be affected, to
different extents, depending on the resin used for embedding.
b.
The high surface smoothness requirement of nanoindentation can be achieved in two ways: micro-
tome sectioning and mechanical polishing. Microtome sectioning produces a specimen surface
of lower surface roughness with less subsurface damage in comparison to mechanical polish-
ing. Both of these features are advantageous when attempting to delineate the differences in the
nanoscale structures of the mineralized tissues.
Hydration
c.
The specimens have to be hydrated with an appropriate fluid throughout storage, processing, and
experimentation to preserve the mechanical properties.
d.
The presence of fluid during an experiment may induce errors in the data recorded by the nanoin-
denter; therefore, these data have to be corrected for the effects of the fluid with appropriate
techniques.
Indenter tips
e.
There are four major types of nanoindenter tips, namely, Berkovich, cube corner, cono-spherical,
and flat-end tips.
f.
Berkovich tips are most commonly used and they have the same projected area to depth ratio as
Vickers indenter tips.
g.
Cube corner tips are sharper than the Berkovich tips and they are often used for small structures
and also for characterizing the fracture toughness of materials.
h.
Cono-spherical tips have the unique ability to capture the smooth transition from elastic to plastic
contact since the strain field they produce is not self-similar. Depending on the tip radius, these
tips can be used for either hard or soft materials.
i.
Flat-end tips have a defined contact area that is suitable for soft materials.
Load function
j.
The strain rate associated with conventional triangular or trapezoidal load functions decreases
with increased loading. If a constant strain rate is required, the load function has to be in exponen-
tial form.
k.
A trapezoidal load function with a long hold period at the maximum load and fast unloading can
minimize the viscoelastic effects in the measured contact stiffness.
l.
To fully remove the viscoelastic effect, the data need to be analyzed by a suitable method.
Microstructural influence
m.
The oral mineralized tissues are heterogeneous and so large scatter may exist in the measured data.
n.
AFM-based indentation techniques could be used for investigating the mechanical properties of
small structures in the mineralized tissues.
o.
Nanoindentation can be coupled with ultrasonic measurements, or FIB milling to investigate the
anisotropic properties of the mineralized tissues.
