Biomedical Engineering Reference
In-Depth Information
Hauptmann, 2003; Grate et al., 2003). Acoustic wave sensors have the advantage that
they are versatile, both sensitive and reliable, being able to detect not only mass/density
changes but also viscosity, wave functions, elastic modulus, conductivity, and dielectric
properties. They have many applications in the monitoring of pressure, moisture, tem-
perature, force, acceleration, shock, viscosity, flow, pH levels, ionic contaminants, odor,
radiation, and electric fields (Shiokawa and Kondoh, 2004; Wohltjen et al., 1997). Recently,
there has been an increasing interest in acoustic wave-based biosensors to detect traces of
biomolecules through specific bioreactions with biomarkers. These include DNA, proteins
(enzymes, antibodies, and receptors), cells and tissues (microorganisms, animal and plant
cells, cancer cells, etc.), viruses, as well as the detection of chemical substances through
specific chemical absorption layers (Cote
et al., 2003; Kuznestsova and Coakley, 2007; Teles
and Fonseca, 2003). By detecting the traces of associated molecules, it is possible to diag-
nose diseases and genetic disorders, prevent potential bioattachment, and monitor the
spread of viruses and pandemics (Vellekoop, 1998; Shiokawa and Kondoh, 2004; Gizeli,
1997). Compared with other common biosensing technologies, such as surface plasmon
resonance (SPR), optical fibers, and sensors based on field effect transistors or cantilever-
based detectors, acoustic wave-based technologies have the combined advantages of sim-
ple operation, high sensitivity, small size, and low cost, with no need for bulky optical
detection systems (Lange et al., 2008).
The commonly reported acoustic wave-based biosensor is QCM (see Figure 8.1a) (Markx,
2003), which can be operated in a liquid environment using a thickness shear-mode.
Advantages of QCM include (1) simplicity in design and (2) a high Q factor. Problems asso-
ciated with QCM biosensors include low detection resolution due to the low-operating
frequency in the range of 5-20 MHz and a large base mass, a thick substrate (0.5-1 mm)
and large surface area (>1 cm
2
) which cannot be easily scaled down.
Because the SAW-based biosensors have their acoustic energy confined within a region
about one wavelength from the surface, the base mass of the active layer is roughly 1 order
of magnitude smaller than that of the QCM. Therefore, the sensitivity of the SAW devices
increases dramatically compared with that of the QCM (see Table 8.1). The longitudinal
or Rayleigh mode SAW device (Figure 8.1b) has a substantial surface-normal displace-
ment that rapidly dissipates the acoustic wave energy into the liquid, leading to excessive
damping, and hence poor sensitivity and noise. Waves in a SH-SAW device (Figure 8.1c)
propagate in an SH mode and, therefore, do not easily radiate acoustic energy into the
liquid (Barie and Rapp, 2001; Kovacs and Venema, 1992); hence, the device maintains a
high sensitivity in liquids. Consequently, SH-SAW devices are particularly well suitable
for biodetection, especially for “real-time” monitoring. In most cases, Love wave devices
(Figure 8.1d) operate in the SH wave mode with the acoustic energy trapped within a
thin waveguide layer (typically submicrometers). This enhances the detection sensitivity
by more than 2 orders of magnitude when compared with a SAW device owing to their
much reduced base-mass (Josse et al., 2001; Mchale, 2003). They are therefore frequently
employed to perform biosensing in liquid conditions (Lindner,
2008; Kovacs
et al., 1992;
Jacoby and Vellekoop, 1997).
In a manner similar to a SAW device, Lamb wave devices (Figure 8.1e) on a membrane
structure have been used for biosensing in liquid (Muralt et al., 2005). The wave velocity
generated in the flexural plate wave (FPW) or Lamb wave is much smaller than those in
liquids, which minimizes the dissipation of wave energy into the liquid. The detection
mechanism is based on the relative change in magnitude induced by the perturbation on
the membrane and not on the resonant frequency shift. Therefore, the sensitivity of these
devices increases as the membrane thickness becomes thinner (Nguyen and White, 2000;
