Biomedical Engineering Reference
In-Depth Information
biosensor incorporates both acoustic wave mass-sensing and impedance
spectroscopy on the same device. Bovine aortic endothelial cells were
seeded on the gold electrodes and used to detect the presence of dif er-
ent toxicants: aldicarb, nicotine and ammonia. h e presence of toxicants
will kill the BAECs and cause changes in both the resonance frequency
and impedance measurements. Linear correlation between resonant fre-
quency shit s and concentration of toxicants were obtained using this
sensor. h e usage of dual, simultaneous detection methods (impedance
and mass-sensing) allows cross-validation and reduces false-positives
during measurements.
8.6
Surface Acoustic Wave Mass Sensors
As mentioned earlier, another form of acoustic wave mass sensors uti-
lize surface waves. h is type of sensor has its transducers only on top of
the piezoelectric layer, making it easier to fabricate. SAW sensors also
typically operate at higher resonance frequencies (200 MHz - 1GHz)
compared to QCMs, making them more sensitive to mass changes as
sensitivity is dependent on resonant frequency. h e SAW's transducers
are ot en in the form of interdigital transducers (IDTs) and the spac-
ing between the alternating transducers determines the resonance fre-
quency of the device. When alternating voltage is applied the input IDTs
induce surface waves in the piezoelectric material. h e surface waves
are detected by the output IDT and induce an alternating current.
Depending on the cut of the piezoelectric material, the surface acous-
tic waves can have either compressional (Figure 8.7A) or shear (Figure
8.7B) components.
When used as a biosensor, the surface between these two sets of IDTs is
covered with a biological layer sensitive to the analyte to be detected. h e
absorption of the analyte on the sensitive layer will produce a time delay
in the acoustic wave propagation. h e main disadvantage of the Rayleigh
wave based devices when used as biosensors is the degradation of perfor-
mance due to liquid damping. In liquid the quality factor
Q
drops (usually
more than 90% reduction) and negatively af ects the device sensitivity.
Since most of the biological applications are performed in liquid
only very few Rayleigh wave acoustic wave devices could be integrated
in microl uidic channels, without signii cant degradation of the sensor
performance.
