Biomedical Engineering Reference
In-Depth Information
further penetration upon a creep hold at maximum load may further
expand the contact volume. Finally, interpretation of nanoindentation
results in bone may differ from indents made to a constant maximum
applied load, rather than a constant maximum contact depth, due to the
inherent heterogeneity of the bone material.
Table 7-2. Typical values for the radius of contact circle:
a
, contact area:
A
c
, mean
pressure:
p
m
, and an estimate of the test volume contributing to elastic modulus
measurement:
V
e
; and [mean (SD), number
n
=] are presented for plane strain modulus,
E
', for a Berkovich tip (5 mN maximum applied load), an
R
= 5
μ
m spherical tip
(15 mN), and an
R
= 21
m spherical tip (100 mN) in both wet and PMMA embedded
equine cortical bone. Data from Bushby
et al.
(2004).
43
μ
Berkovich
(5 mN)
R
= 5
m
(15 mN)
μ
R
= 21
m
(100 mN)
μ
Wet
a
(
2.54
3.05
5.85
μ
m)
m
2
)
20.3
29.2
107.5
A
c
(
μ
p
m
(MPa)
248
513
931
m
3
)
V
e
(
μ
1150
2000
14100
E'
(GPa), n
11.2 (2.8), 308
9.9 (1.9), 117
11.7 (1.7), 326
Embedded
a
(
μ
m)
1.61
1.81
4.54
m
2
)
8.14
10.3
64.8
A
c
(
μ
p
m
(MPa)
612
1460
1540
m
3
)
296
419
6630
V
e
(
μ
E'
(GPa), n
19.5 (2.7), 349
18.1 (2.4), 548
19.4 (2.1), 283
In general, the measured modulus of bone has been shown to
decrease with increased max load ranging from 17.3 GPa measured at
100 N to 12.3 GPa at 1 N.
73
As the contact area and plastic deformation
increase at higher loads, it is likely that more pores and regions high in
organic volume fraction are incorporated into a test volume to create a
“softening” influence.
73
The consideration of the maximum depth of
penetration with regards to the 3-D microstructure is clearly of great
importance. Indentation to various depths can be used to probe bone's
hierarchical structure. For example, shallow indents (<1000 nm) have
been commonly applied to investigate bone's microstructural features -
such as individual lamella.
44,60,64
These shallow indents, often performed
with an AFM-based indentation system, are more sensitive to
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