Biomedical Engineering Reference
In-Depth Information
7.3.5
Formation of Ordered and Disordered Structures
in Biominerals
Many living organisms rely on hard tissues, which are composed of biominerals
and protein matrices with exquisite microarchitectures, for support, protection,
and defense [
78
-
80
]. Natural composite materials such as teeth, bones, and shells
represent intriguing and diverse design paradigms for exploring the relationships
between structure and mechanical properties such as fracture toughness, stiffness,
and hardness[
81
]. Despite the variety of these complex hierarchical architectures,
biominerals are generally organized in a certain order to harden or stiffen tissues
in living organisms. Although it is quite clear that the composite character of these
materials plays an important role, some important questions need to be addressed.
For example, how can the self-assembled nanocomposites exhibit superior me-
chanical characteristics? What are the key structural factors leading to the superior
strength of hard tissues? Although the mechanical properties of hard tissues can be
affected by the factors, cf. the type of minerals, the degree of mineralization, and
size of mineral particles, there is still a striking variation in mechanical properties
even when the components of the composites are similar [
82
,
83
]. Combined with
nanoindentation, Jiang and Liu [
83
] applied the position-resolved small-angle X-
ray scattering and electron microscopy to examine the correlation between the
microstructure of the enamels of human teeth and the mechanical properties.
The human tooth enamels can be roughly regarded as a bundle of nano HAP
crystallite needles ( 94%) that sandwiched some proteins and water. It follows the
experiments [
83
] that the degree of ordering of the biominerals varies strikingly
within the dental sample, showing that both the hardness
H
and the elastic modulus
E
increase predominantly with the ordering of the biomineral crystallites [
83
](see
Fig.
7.11
).
The mechanism concerning the toughness enhancement of hard tissues
vs
the
improvement of the crystallite alignment are not entirely clear. Nevertheless, the
following so-called crack stopper model may provide some physical insight into this
matter. As indicated in Fig.
7.12
, normally, crystals are never perfect. Instead, many
defect lines or more seriously many grain boundaries may occur in crystals. The
letter can even form the mosaic structure, composed of micro-crystallite grains that
are misfit to a small degree with respect to each other (Fig.
7.12
a). At collision, the
crack may occur at the surface and easily propagate across the crystals along the
defect lines or the grain boundaries (Fig.
7.12
a). This results in the breakage of the
crystals. On the other hand, for a block of crystallite assembly in which lamella or
needle-like nano crystallites are packed in parallel to the surface (see Fig.
7.12
b),
a serious collision will also cause a crack at the surface of the block. Nevertheless,
the propagation of the crack along the defect lines, or the grain boundaries at the
surface, will be stopped at the interface between the parallel packed crystallites,
preventing the crack propagating across the block (see Fig.
7.12
b). In other words,
this parallel packing structure in a crystallite assembly will prevent the breakage
versus the penetration of the crack lines across the crystallite assemblies.
