Biomedical Engineering Reference
In-Depth Information
Because hydroxyapatite mineral is anisotropic, the properties do depend on
orientation to some degree. Velocity measurements, using scanning acoustic mi-
croscopy (SAM), performed on single-crystal mineralogical hydroxyapatite revealed
five independent elastic constants that describe a transversely isotropic material
[52]:
C
11
C
12
C
13
0
0
0
C
11
C
13
0
0
0
C
33
0
0
0
C
44
0
0
C
44
0
1
/
2
(
C
11
−
C
12
)
137
42
.
5 4
.
90 0 0
137
54
.
90 0 0
172
0 0 0
39
.
60 0
39
.
60
47
.
25
=
GPa
The density was reported as 3200 kgm
−
1
[52]. Using the Voigt-Reuss-Hill
approximation, anisotropic single-crystal elastic constants converted into isotropic
polycrystalline elastic Reuss and Voigt moduli [53] yield values of 116 and 119GPa,
respectively.
3.3.3
Water
Water is a principal constituent of bone, and thus the hydration state of the
sample plays a critical role in the measured mechanical properties. Dehydration
causes increased modulus of elasticity and tensile and bending strengths and
reduced values of fracture toughness [54-57]. Dehydration increases nanoinden-
tation modulus values by
15-25%, and up to a maximum of 50% [58-60].
While dehydration increases the magnitude of mechanical properties, the inherent
material relationships (e.g., anisotropy) are maintained.
Bone behaves in a time-dependent manner due, in part, to its organic phase [61,
62] and its water content with a corresponding poroelastic flow through the bone
material [63]. Viscoelasticity in bone has been shown to correlate with hydration
state [27, 61, 64, 65] and mineral content [62]. The time that it takes for fluid to
flow through the pore spaces in bone conveys poroelasticity. Bone tissue plasticity
and viscoelasticity are highly influenced by interactions between water and charged
sites on the collagen and other organic matrix constituents [26, 64, 66, 67].
Having examined the individual components of a composite mineralized tissue,
we now move on to examine composite mechanics models for prediction of the
elastic modulus based on proportions and properties of the constituent materials.
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