Biomedical Engineering Reference
In-Depth Information
Table 3.1
Specific stiffness comparison between bone,
PTFE, and lead.
Density,
ρ
Elastic modulus, Specific stiffness,
(g cm
-3
)
E
/ρ
(GPa cm
3g-1
)
E
(GPa)
Bone
1.9
15
7.9
PTFE
2.0
0.5
0.25
Lead
11.3
13.5
1.2
PTFE and Lead data from Callister [3].
Given the strong interest in bone and bonelike materials, it is perhaps surprising
that little work had been done until recently considering the composite materials
aspects of bone. Pioneering work on the subject was performed by Katz in
the 1970s [5], but the subject then remained largely dormant until a recent
burst of activity. In the time elapsed since the work of Katz, there have been
significant and substantial advances in both experimental techniques, including
the miniaturization of mechanical testing technology to nanometer length scales,
and in computational power, thanks to rapid advances in the microprocessor
market. For these reasons, in this work, we take the work of Katz as a starting
point, but emphasize the recent developments and outstanding controversies. In
this chapter, we first review the components of the composite material bone,
including a critical assessment of what is known about the material properties of
each constituent phase. We then review basic aspects of composite mechanics, to
assess the degree to which bone can be considered in the framework of engineering
composite materials. This treatment includes an examination of the macroscopic
and microscopic aspects of bone as a composite, highlighting the areas in which
substantial further research is yet needed. Finally, following a critical examination
of current thinking about bone anisotropy, we conclude with recommendations for
future study.
3.2
Bone Phases
Bone is composed of three constituent phases: organic, mineral, and water. Adult
compact bone contains approximately, by volume, 30% organic, 50% mineral, and
20% water (Figure 3.1) [6, 7]. The dominant organic phase, type I collagen, forms a
scaffold that is mineralized by carbonated hydroxyapatite. Watery interstitial fluid
fills pore spaces at scales ranging from nanometers to millimeters. The properties
of the composite material bone depend on the properties and molecular structure
of each individual constituent phase, the arrangement of each phase in relation to
each other, and the organization of bone at the tissue level [8]. Bone is therefore a
hierarchical structural composite material.
