Biomedical Engineering Reference
In-Depth Information
ZnO film on top of the wave path, this offers much greater flexibility in the fabrication of
highly integrated microfluidics. The SAW devices can be built on the isolated ZnO islands,
whereas the other components, such as the microchannels, chamber, and sensors, can be
directly fabricated on the Si substrate.
SAW Heating/Droplet Ejector
For SAW devices, a higher RF power generates faster streaming, hence a higher mixing
and pumping efficiency. However, high RF power will also induce localized heating. The
surface temperature of the ZnO SAW device increases with higher voltage amplitudes and
the duration of the RF signal and decreases with the distance from the IDT (Fu et al., 2010).
The maximum temperature can reach values of about 140°C for a signal voltage of 60 V
(see Figure 8.25). It should be pointed out that the temperature was measured without any
water on the device surface, and it would be expected that the temperature readings would
be lower when water is present (at least lower than the boiling point). Significant acoustic
heating is detrimental for many of the biosubstances that might be investigated and can
also induce severe detection errors due to temperature-induced resonant frequency shift.
For ZnO SAW-based liquid transportation and mixing, heating effects can be suppressed
by using a pulsed RF signal to maintain the temperature below 40°C. A pulsed RF signal
can also used to direct the droplet motion, as this offers more precise control of the distance
moved and droplet positioning. Although acoustic heating has many negative effects for
biosensing, controlled acoustic heating can be utilized as a remote heater for biomedical
and life science applications, such as in polymeric chain reactions (PCRs) to amplify the
DNA concentration for detection and in others to accelerate bioreaction processes.
When the RF power applied to the IDT of a ZnO SAW device is sufficiently high, tiny
liquid droplets will be ejected from the surface (see Figure 8.26a), and this has been fre-
quently reported for LiNbO
3
SAW devices (Chono et al., 2004). Ejection of small particles
and liquids has many applications ranging from inkjet printing, fuel and oil ejection, and
biotechnology. The authors have recently demonstrated that thin film ZnO-based SAW
devices can eject the droplets, which are so tiny that an atomization process occurs as
160
15 V
27 V
40 V
48 V
60 V
120
80
40
0
20
40
60
80
Duration (s)
FIGURE 8.25
Temperature change as a function of time with RF signal voltage as a parameter. Temperature rises rapidly
during the initial 20 s, and then increases slowly thereafter. (From Fu et al.,
Sens. Actuators B
, 143, 606-619, 2010.
With permission.)
