Biomedical Engineering Reference
In-Depth Information
TABLE 9.7
Piezovoltage Peak Value (mV) for Type B
Sample under Three-and Four-Point Bending
Electrode 7-7
′
Electrode 8-8
′
Sample Type B
3P
4P
3P
4P
1
2.30
1.99
1.20
-0.20
2
1.86
2.03
0.65
0
3
3.07
3.04
1.38
0
4
1.79
1.90
1.76
0.30
5
3.68
3.40
4.01
0.18
three-point bending. This means that the signs of piezovoltage
depend on shear stress only. It should be mentioned that the fact that
signs of the piezovoltage at electrodes 7-7′ and 8-8′ were the same as
those at electrodes 1-1′ ~ 3-3′ under three-point bending also sup-
ports that conclusion, because electrodes 7-7′ and 8-8′ are located
around the neutral axes and only shear stresses act around them.
2. For both types of samples, there is only normal stress in any cross
section in the pure bending zone. Within the pure bending zone,
the piezovoltages at all the electrodes become lower, but they do not
approach zero except for electrodes 2-2′, 5-5′, and 8-8′, which are
located around the neutral axes. It can be concluded that the peak
values of the piezovoltages depend mainly on shear stress, although
normal stress still contributes to some extent.
3. As can be seen from Tables 9.5-9.7, the peak values at the same
electrodes in different samples are relatively irregular. These irreg-
ularities occur either between samples or between electrodes. This
indicates that the piezoelectricity of bone depends on the hierarchical
structure of bone, which might differ in different samples. The
important conclusion as to the macroscopic piezoelectric property of
bone, however, is that the signs of piezovoltages between two lateral
surfaces depend on shear stress only, not on normal stress.
Fu et al. [23] also noted from their analysis that irregularities in the
piezovoltage results suggested that the piezoelectric properties of bone are
relevant to its microstructure. Cortical bone has been regarded as a hierarchi-
cal composite material comprising mineral and organic phases. The mineral
phase is mainly composed of crystalline hydroxyapatite and the organic phase
consists mainly of collagen, which is the origin of the piezoelectricity [37,38].
Figure 9.19 shows a schematic illustration of collagen fibers [39,40].
A collagen molecule is about 300 nm in length and about 1.5 nm in diameter.
There is a 40 nm gap between the ends of collagen molecules in the longi-
tudinal direction, and 27 nm of a 300 nm collagen molecule length overlaps
