Biomedical Engineering Reference
In-Depth Information
FutureTrendsforZnOandAlNDevicesforLab-on-a-Chip
The elements required for operating detection as part of a lab-on-a-chip system include
(1) transportation of liquids, such as blood or biofluids containing DNA/proteins into an
area on which probe molecules have been predeposited; (2) mixing/reaction of the extracted
DNA or proteins with oligonucleotide or the antibody binders; and (3) detection of an asso-
ciated change in the physical, chemical, mechanical, or electrical signals. Thin film-based
acoustic wave devices can be used to fabricate lab-on-chip biodetection systems, which
combine the functions of microdroplet transportation, mixing, and biodetection.
ZnO- or AlN-based acoustic wave technologies can be integrated with other technologies,
such as the SPR method (Homola et al., 1999). SPR sensor technology has been commer-
cialized, and SPR biosensors have become a central tool for characterizing and qualifying
biomolecular interactions. A combination of SAW microfluidics and SPR sensing would
appear to be sensible for both microfluidic and detection functions. A potential problem is
that the surface temperature change induced by acoustic excitation may cause changes in
the refractive index, which is used for SPR sensor detection. A pulse mode SAW signals can
be used to minimize this effect. ZnO- and AlN-based acoustic wave microfluidic devices
can also be combined with liquid or gas chromatography that can be used to identify the
protein or molecules by mass spectroscopy (Sokolowski
et al., 2006). Integration of a SAW
with optical methods enables the simultaneous qualification of biological soft layers formed
on the sensor surface under different density, viscosity, thickness, and water content.
For digital microfluidics, there is a need to precisely and continuously generate liquid
droplets. ZnO and AlN acoustic wave technology can be used for the ejection of liquid
droplets, but it is rather difficult to precisely control the micro-droplet generation. A poten-
tial technology to overcome the drawbacks is to combine electrowetting-on-dielectrics
(EWOD) (Lee et al., 2007) with SAW microfluidics. In the past 10 years, EWOD technology
has been successfully developed to dispense and transport nanoliter to microliter biosam-
ples in droplet form at the exact volume required (Fair, 2007). However, one of the weak-
nesses is that EWOD technology does not provide efficient micromixing and requires the
integration of other technologies, for example, CMOS to realize bioreaction and biosens-
ing. A novel idea is to integrate the thin film-based SAW devices with the EWOD device
to form lab-on-a-chip equipped with well-developed functionalities of droplet generation,
transportation by EWOD, mixing, and biosensing using SAW technology (Li et al., 2009).
Acoustic wave devices can easily be integrated with standard CMOS technology. Dual
SAW or FBAR devices can be fabricated next to each other, so that the neighboring devices
can be used as a sensor-reference combination. One of the devices without predeposited
probe molecules can be used as a reference, whereas the other one with probe molecules
can be used to sense. Using such a combination, the errors due to temperature drift or
other interference on the sensing measurement can be minimized. Multisensor arrays can
be easily prepared on a chip, and a judicious selection of different immobilized biobind-
ers enables the simultaneous detection of multiple DNA or proteins, leading to accurate
diagnosis of a disease or detection of multiple diseases in parallel. The creation of these
cost-effective sensor arrays can increase the functionality in real time and provide parallel
reading functions.
Currently, one limitation of acoustic wave device applications is that they require expen-
sive electronic detection systems, such as network analyzers. A final product aimed at
the end user market must be small, portable, and packaged into a highly integrated cost-
effective system. The detection of a resonant frequency can be easily realized using stan-
