Biomedical Engineering Reference
In-Depth Information
PVDF has a glass transition temperature (
T
g
) of about
−
35
◦
C and is typically 50 - 60%
crystalline. To give the material its piezoelectric properties, it is mechanically stretched
to orient the molecular chains and then poled (placed under a strong electric field to
induce a net dipole moment) under tension. When poled, PVDF is a ferroelectric polymer,
exhibiting efficient piezoelectric and pyroelectric properties. These characteristics make
it useful in sensor and battery applications. Thin films of PVDF are used in some newer
thermal camera sensors. Unlike other popular piezoelectric materials, such as PZT, PVDF
has a negative d
33
value. Physically, this means that PVDF will compress instead of
expand or vice versa, when exposed to the same electric field. The piezoelectric properties
of PVDF are used to advantage to manufacture tactile sensor arrays.
PVDF is available in sheets that vary in thickness between 5 μm and 2 mm. It has good
mechanical properties and can be molded to an appropriate shape with little difficulty.
Metallization is used to apply a thin layer of metal on both sides of the PVDF sheet to
form electrodes and collect the charge accumulated. As mentioned earlier, deformation
caused by an external force results in an electrical charge that is a function of the applied
force. This charge results in a voltage
V=Q
/
C
, where
Q
is the charge developed, and
C
is the capacitance of the device.
Piezoelectric crystals act as transducers that turn force or mechanical stress into electri-
cal charge which, in turn, can be converted into a voltage. A schematic of a piezoelectric
sensor is shown in Figure 2.10.
Table 2.1 summarizes and compares different force sensing technologies. Of these
technologies, piezoelectricity using PVDF is elaborated in the next chapter. PVDF has
shown many outstanding features, such as high sensitivity, wide dynamic response, and
the ability to be integrated into MEMS devices, among many others.
Electrodes
Figure 2.10
Schematic for a piezoelectric sensor
