Biomedical Engineering Reference
In-Depth Information
sophisticated methods involve hardware modifications such as the use of fluid cells and irrigation
systems
[27,28]
.
The presence of a fluid medium that covers the specimen during nanoindentation, on one hand,
maintains the hydration of the biological tissues; however, conversely it may induce undesirable sur-
face tension forces during the recording of data
[14,29]
. The nanoindenter records the total force on
the tip comprising the surface tension force at the fluid meniscus, in addition to the force from the
contact with the sample surface. Furthermore, depending on the protocol used to define the zero refer-
ence for the displacement data, the latter may be mistaken to be the air-fluid interface, instead of the
fluid-specimen interface. The problem of incorrectly identifying the contact point, as shown schemat-
ically in
Figure 16.3
, may be overcome by fitting the initial portion of the load-displacement curve
with the Hertzian contact relationship whilst subtracting the meniscus pulling force from the fluid
[30]
, i.e.,
3 2
/
P
A h
(
h
)
Kh
(16.5)
a
The fluid will also apply an additional traction force on the indenter tip at the onset of unload-
ing at which the mechanical properties are measured (cf. Eqs. (16.1)-(16.3)), but the effect of such a
force can be removed by the same method for correcting viscoelastic effects that will be described in
Section 16.3.4.
16.3.3
Indenter Tips
Most of the tips used in nanoindentation are made of diamond because the high stiffness ensures that
the deformation measured by the nanoindenter is mostly contributed by the specimen. The tip geom-
etry is the most important consideration when choosing a suitable tip for indenting biological tissues.
Four major kinds of tips are available, namely, Berkovich, cube corner, cono-spherical, and flat-end
tips (
Figure 16.3
).
The Berkovich tip is the most commonly used indenter tip installed in commercial nanoindenters.
It has a relatively large face angle of 65.3° which makes it less likely to be damaged under repeated
use. An important feature of the Berkovich tip is that the face angle gives the same projected area to
depth ratio as the Vickers indenters commonly used in micro-/macro-indentation. This allows the con-
ventional Vickers hardness scale to be applicable in analysis of the nanoindentation data.
Artificial displacement,
h
a
Displacement (nm)
K
FIGURE 16.3
Illustration of the load-displacement curve obtained from
a fluid-covered specimen surface (
h
a
artificial drift,
K
constant arising from fluid effects).
