Direct Energy, 2018a

46 2.3 Piezoelectric Devices
tions. Quartz is piezoelectric, and it is durable, readily available, and
nontoxic. Engineers have developed piezoelectric devices designed for use
outside of the body and to be implanted inside the body. Some piezoelectric
devices are used as sensors. For example, piezoelectric sensors can monitor
knees or other joints [3]. Also, ultrasonic imaging is a common diagnostic
technique. Piezoelectric devices are used both to generate the ultrasonic
vibrations and to detect them[33]. Other biomedical piezoelectric devices
are used as a source of electrical power. Articial hearts, pacemakers, and
other devices require electricity, and they are often limited by battery technology
available to supply the energy [36]. Piezoelectric generators have
no moving parts to wear out, and they can avoid the problem of needing
to change the batteries. Physical activity can be classied as continuous,
such as breathing, or discontinuous, such as walking. Both types of physical
activity can be used as a source of mechanical energy for piezoelectric
devices [36]. The amount of power required for dierent biomedical devices
varies quite a bit. For example, an articial heart may require around 8 W
while a pacemaker may require only a few microwatts [36]. Piezoelectric
devices may be able to capture energy from typical physical activity and
convert it into electrical energy to power the device. A piezoelectric device
in an articial knee has produced 0.85 mW [36], and a device in a shoe has
generated 8.4 mW from walking [36].
Piezoelectric devices are used in other types of imaging systems besides
biomedical imaging systems. One of the earlier applications was in sonar
systems. Around the time of WWI, the military actively developed sonar
systems to detect boats and submarines. Today, sonar systems are used to
detect sh and to measure the depth of bodies of water [33]. Sonar imaging
is also used to analyze electrical circuits and to detect imperfections and
cracks in steel and in welds [33].
Piezoelectric devices are used in a variety of other applications too.
Piezoelectric sensors are used in some buttons and keyboards [36]. Piezoelectric
devices are used to make accelerometers [37, p. 353], and they are
used to measure pipe ow [33]. Speakers, microphones and buzzers can all
be made from piezoelectric devices, and they can operate at both audio
and ultrasonic frequencies. Piezoelectric devices that generate ultrasonic
signals can be used to emulsify dyes, paints, and food products like peanut
butter [33]. Also, they are used in some barbecue grill ignitions where
mechanical stress induces an electric spark [23, ch. 15].