Biomedical Engineering Reference
In-Depth Information
that are constructed from these movements. As with other forms of wave
propagation, the confinement of a wave within boundaries leads naturally to
the instigation of the phenomenon of resonance. This involves, to a first
approximation, the complete constructive interference of the standing waves
mentioned above. In this connection, with respect to acoustic wave devices, the
term 'piezoelectric acoustic resonator' is often employed.
d
n
4
t
3
n
g
|
1
1.4.2.2 Piezoelectric Bulk Acoustic Wave Biosensors
In this type of device, particle motion and the acoustic wave involve the complete
thickness of the structure. We introduce two sensors that display these char-
acteristics—the TSM and its sister device, the electromagnetic acoustic wave sensor
or EMPAS. The TSM, because of its low cost and convenience in operation, is
represented by far the most research activity and applications of any acoustic wave
system; accordingly, it is dealt with in more detail than other configurations.
The TSM is composed of an electroded (often gold) piezoelectric wafer
(usually AT-cut quartz) (Figure 1.15). An oscillating electrical potential is
applied via the electrodes in order to drive mechanical motion in the device.
A resonant acoustic shear wave is generated which travels through the
piezoelectric material with little energy dissipation. The wave is reflected at the
device-surroundings interface in order to maintain a standing wave
(Figure 1.15). In chemical and biosensor applications material is generally
added to the device surface, for example, species involved in a bimolecular
interaction. On a simple theoretical basis this scenario was the subject of very
early work by Sauerbrey
71
who showed that, when material is deposited on the
device surface, the resonant frequency is changed according to his famous
equation, which is expressed in eqn (1.6) for quartz as the piezoelectric material:
d
n
3
.
Df
¼
2f
0
Dm
ð
1
:
6
Þ
p
A
r
q
m
q
where Df is the frequency change, f
0
is the primary resonant frequency of the
sensor, A is the effective surface area associated with the piezoelectric process, r
q
Figure 1.15
Thickness-shear mode acoustic wave device and standing wave in device
with propagation of a damped acoustic shear wave into liquid.
