Biomedical Engineering Reference
In-Depth Information
lateral side facing the medullary cavity, such as 3′, the measured piezo-
voltage between the pair of electrodes 3-3′ was input into a bioamplifier
(BMA-931, CWE Inc., United States)via an ultrahigh-input impedance (over
1012 Ω) head stage (Super Z, CWE Inc., United States). The amplified volt-
age signals were then recorded by a computer in the measurement sys-
tem. Mechanical loadings were applied using an Instron 1343 closed-loop
servo-hydraulic machine controlled by an 8800 Control Tower, and the
loading signals in the control tower were also input into the computer for
recording.
The testing sample and the head stage were enclosed in a double electro-
magnetic shield box with the outer shield connecting to earth and the inner
to the head stage common terminal. This arrangement kept the electric field
constant around the sample [4].
A trapezoidal loading profile was used, with a loading (or unloading) rate
of 600 N s -1 , load holding time of 6 s, and a holding load of 300 N, which pro-
duced maximum tensile (or compressive) stresses from 24 to 45 MPa in the
samples—well below the tensile strength of 50-100 MPa of cortical bovine
bone [36]. The trapezoidal loading waveform is shown in Figure  9.16(a).
Figure  9.16(b) shows a typical piezovoltage waveform between electrodes
3-3′ under three-point bending. The waveform has two pulses with different
signs, corresponding to the loading and unloading processes, respectively.
The peak of the first pulse corresponds to the loading end point and the
second pulse peak corresponds to the unloading end point. This shows that
once the loading or unloading process ended, the corresponding piezovolt-
age began to decrease toward zero. In other words, the polarized charges
or piezovoltages appeared on the bone surfaces in pulse form as the bone
deformation varied with loading [4].
9.5.2 Results
Figure 9.17 shows a group of typical results of a type A sample. Figure 9.17(a)-(e)
corresponds to the arrangement of electrodes on the bone samples. There
are  two plots of piezovoltage versus time in each figure. The curves I plot
represents the result from the three-point bending test and the curves II
represents the result from the four-point bending test.
Fu et al. [23] found that all five type A samples showed similar plots of
piezovoltage versus time under three-point bending. However, in the plots
from the four-point bending, the shapes were similar but the signs were
opposite. Tables 9.5 and 9.6 show all the voltage peak values of the sample
type A plots, under three- and four-point bending tests.
Figure  9.18 shows the results of piezovoltage versus time for the type B
samples under the loading profile shown in Figure 9.16. It can be seen from
Figure 9.18 that the signs of the piezovoltage are the same as those shown in
Figure 9.17. Table 9.7 lists all the peak values of piezovoltage for the type B
samples.
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