Biomedical Engineering Reference
In-Depth Information
charge) to the change in applied pressure, is given by
z
1
z
P
∂σ
j
=
∂
1
A
∂
∂σ
j
1
A
∂μ
∂σ
j
z
2
z
∂σ
j
∂
d
3
j
=
=
−
P
,
∂
μ
∂σ
j
ln
−
∂
ln
z
∂σ
j
=
(8.4)
where
j
3 for the pressure imparted along the
z
-direction [11].
The first term of Equation 8.4 comes from the dipole moment of the nanofiber
web at constant thickness, which mainly depends on the number of effective
dipoles (
N
c
) present in the nanofiber web. The second term is known as the
elastic
compressibility constant
(
=
σ
3
j
) directly related to the dimensional changes. One of the
vital advantages of the nanofiber web-based pressure sensor or generator is that
the expected piezoelectric effect is much higher due to the high compressibility
associated with the large thickness change at the identical pressure compared to its
film form and to ceramic-based piezoelectric materials.
The appearance of the piezoelectric output signal from the pristine nanofiber
web-based sensor (Figure 8.16a) is another piece of evidence for the field-induced
preferential dipole orientation toward the thickness direction of the nanoweb
during electrospinning. However, after the as-electrospun nanofiber web is heat
treated at 130
◦
C for 15min, no AC type signal with amplitude (thin line) is
observed. This indicates that the piezoelectric coupling disappears completely. It
is believed that the preferentially oriented dipoles were completely randomized
(depoled) because of such heat treatment above the
T
c
. Therefore, in addi-
tion to the microscopic (based on the FTIR result) evidence, this is another
macroscopic confirmation of the preferential orientation of CF
2
dipoles during
electrospinning.
The next question then follows. Are the dipoles in the as-electrospun nanofiber
web really oriented perpendicular to the nanoweb surface (although we have some
idea as described by the FTIR results)? If so, is it also possible to verify this
macroscopically?
In order to get an idea about the direction of orientation of the dipoles, two
types of sensors were fabricated (illustrated in the insets of Figure 8.16b). One is
structure (a), when two layers of nanofiber web are stacked together with a (top-
bottom)-(top-bottom) series structure, and the other is structure (b), when another
two layers of nanofiber web are stacked together with a (bottom-top)-(top-bottom)
series structure. In both cases, the top and bottom electrodes are attached on each
outermost surface of these stacked layers like a single-layer nanofiber web-based
sensor described earlier. When compared with the output signal generated from
the one layer sensor shown in Figure 8.16a, the output signal was enhanced about
twofold (thick line) from structure (a), whereas the output signal from structure
(b) (thin line) was very much attenuated. Since the effect of the second term
of Equation 8.4 can be assumed to be the same for both cases, it is obvious
that the contribution from the first term is also significantly reduced due to
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