Bioinspired Devices - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 224

8.4 Devices Mimicking Biological Organs/Functionalities

Several human organs have been mimicked in man-made devices that try to match
the sensitivity of the natural counterparts. One example is the nose. Owing to the
extensive research in nanosensors able to detect minute concentrations of several
gases, one would expect that an array of specialized/appropriately functionalized
sensors can easily replace the nose as gas (odor) sensor. However, a true biomimetic
nanotechnology olfactory sensor (NOSE) does not consist of an array of sensors
in which each sensor detects a single analyte but of an array of sensors that
respond to several gases, such that identification of a particular gas reduces to
identifying the distinct signature of that gas on the entire array of cross-reactive
sensors. So, whether it is not difficult to fabricate bioinspired sensors, it is also
important to understand the sensing mechanism of the biological organ in order to
produce biomimetic NOSEs. For detection and identification of gases in biomimetic
NOSEs, one should first collect the simultaneous responses from all sensors in
the array and then to apply pattern recognition algorithms such as artificial neural
network techniques or principal component analysis to determine the combination
of sensor values for which one can obtain the maximum variance of data points
in mutually orthogonal directions. The gases are identified by their signatures
in the planes/volumes spanned by these orthogonal directions, as illustrated in
Fig. 8.12 . The nature of the sensors is not relevant. The detection can be made
by arrays of micrometer-scale silicon cantilevers ( Baller et al. 2000 ) functionalized
with different polymers that swell when exposed to analytes, case in which the
fingerprint of different odors is extracted from sets of deflection data collected
from all cantilevers at different times, or one can detect gases via electrical signals
recorded by an array of percolating SnO 2 nanowires ( Sysoev et al. 2007 ). In the last
case, the discriminating power of the sensor increases by inducing a temperature
array across the nanowire layer deposited on top of an array of Pt strips, the detection
limit for CO, for example, reaching 150-200 ppb in a response time of less than
30 s. Again, this low detection limit can only be achieved by recording conductivity
patterns across the nanowire array and employing multivariate pattern recognition
methods for signal processing in an optimized coordinate system with a number of
dimension equal to the number of gases in a mixture minus one. In a similar manner,
a functional electronic nose can be implemented in a single crystal SnO 2 wedge-
like nanobelt, with a variable diameter, on which an array of Pt nanoelectrodes is
patterned ( Sysoev et al. 2010 ). In addition to different dimensions of the nanobelt
sensing element seen by the cross-sensing detectors in the array, the nanobelt could

gas 1

P2

Fig. 8.12 Signatures of
different gases in the
planes/volumes spanned by
orthogonal directions defined
by pattern recognition
algorithms

gas 3

gas 2

P1