Biomedical Engineering Reference
In-Depth Information
d
n
4
t
3
n
g
|
1
Figure 1.14
Basis of detection based on combination of a nanowire and field-effect
transistor.
Although offering attractive possibilities in terms of potential device sensi-
tivity and miniaturization, it remains to be established if issues of convenient
biochemical attachment and overall incorporation into required circuitry can
be solved from a routine perspective. There is also the ever present specter of
how such structures would be expected to behave if employed in complex media
such as blood. The enhanced sensitivity referred to above may actually
represent an Achilles' heel unless the interfering surface effects can be
minimized or eliminated.
d
n
3
.
1.4.2 Acoustic Wave Physics and Devices
The inception of the use of acoustic wave devices for the detection of
biochemical interactions is generally regarded to have taken place in the 1980s.
Development of the technology has had a checkered history in that controversy
surrounded earlier research efforts. The first attempt with a surface acoustic
wave (SAW)
62
device for immunochemical detection was later shown not to
involve Rayleigh waves at all,
63
and around that time it was commonly believed
that the thickness shear mode (TSM) device could not be used in fluids because
of severe damping effects. Despite this notion, Konash and Bastiaans
64
were
able to use a bulk acoustic wave device as a liquid chromatographic detector by
exposing only one face of the seminar to fluid. Moreover, with respect to the
latter, the invalid concept that the TSM operating in liquids is exclusively a
'mass' sensor persists to modern times.
Acoustic wave sensors are very much associated with piezoelectric physics.
However, a number of structures are available which do not employ piezoe-
lectric components in a direct sense. Accordingly, the latter are included for
completeness in this comprehensive review of the technology.
1.4.2.1 Essentials of the Piezoelectric Phenomenon
An excellent definition of piezoelectricity has been provided by Cady,
65
'piezoelectricity is the electric polarization produced by mechanical strain in
crystals belonging to certain classes, the polarization being proportional to the
strain and changing sign with it'. We now recognize that the reverse is true also,
that is, a specific crystal can be mechanically strained when subjected to an
electric polarization. The latter phenomenon is often referred to as the converse
