Page 172 - Read Online

Page 20 of 33 Arab Hassani. Soft Sci 2023;3:31 https://dx.doi.org/10.20517/ss.2023.23

Soft odour identification sensor arrays
The human olfactory system is responsible for our sense of smell. The nose houses various receptor families
[144]
that detect volatile chemical stimuli which dissolve in the mucus lining . The action potentials generated
in the receptors travel along their axons, which terminate in the olfactory bulb. The olfactory information
[145]
then travels to the olfactory cortex for processing, as shown in Figure 12A . The electric nose (e-nose) that
could mimic this operation could include an array of chemical sensors, electronic circuitry (e.g., sampling,
filtering, and signal conditioning), and data analysis software [Figure 12A] .
[145]
Lorwongtragool et al. developed a wearable e-nose by integrating a 4 × 2 chemical sensor array with a
Zigbee wireless communication system [Figure 12B] . The sensor array comprised eight fully inkjet-
[125]
printed gas sensors on a PEN film with Ag ink IDEs. The resistance of each sensor was determined by
connecting it to a voltage divider. The output voltages of eight sensors were then applied to an eight-
channel analogue multiplexer connected to a USB data acquisition (DAQ) device. To fabricate the sensing
layers of sensors 1-4, layers of a multi-walled CNT (MWCNT) solution were inkjet-printed on electrodes,
followed by the printing of a polymer layer composed of one of the following polymers: polyvinyl chloride
(PVC), poly(styrene-co-maleic anhydride), cumene terminated poly(styrene-co-maleic anhydride)
(Cumene-PSMA), polysulphone (PSE), and polyvinylpyrrolidone (PVP). For sensors 5-8, each polymer was
blended with the MWCNT solution and printed on electrodes. The sensor array was exposed to four
individual volatile organic compounds (VOCs) present in the volatiles released from the axillary skin. The
sensor response percentages for the four VOCs are shown in Figure 12C. The e-nose was used to monitor
the body odour of a participant before exercise, after exercise, and after relaxing for a long period post-
exercise. The data were collected for 5 min in each step, and the entire data-collection process was repeated
thrice. Principal component analysis (PCA) based on unsupervised learning was used to observe the odour
progression in each minute and classify the odour of the armpit when performing different activities. This
wearable e-nose could be used to detect unusual odours corresponding to various health statuses.

Zheng et al. developed a skin odour detection sensor array (i.e., e-nose system) consisting of 3 × 2 sensors
based on CNTs and polymer suspension solutions as the sensing materials . This flexible sensor array data
[126]
was read using a voltage divider circuit and interfaced with an MCU for data acquisition. A Bluetooth chip
and a button cell were used for wireless transmission and as the power supply, respectively [Figure 13A]. To
prepare the sensing material solutions, one of the three polymers [hydroxypropyl methylcellulose (HPMC),
poly(methyl vinyl ether-co-maleic anhydride) (PMVEMA), and polyvinyl pyrrolidone (PVP)] was mixed
with carboxylated or hydroxylated CNTs by ball milling. Ag IDEs were screen-printed on a PET substrate.
Then, 5 μL of the sensing solution was drop-casted on the respective electrodes and dried. To avoid direct
contact with sweat, a hydrophobic and breathable PVDF membrane was laminated onto the sensor array. A
schematic of the wearable e-nose system integrated into an armband is shown in Figure 13B. The sensor was
placed under the armpit, while the related circuitry was placed on the bicep. The responses of each sensor to
5-25 ppm of hexanoic acid, dodecane, and decanal (i.e., elements with an odour similar to sweat) were
measured to calculate their sensitivities [Figure 13C]. The e-nose system was tested on eight participants
while they walked for 63 min on a treadmill. The experiment was repeated on four consecutive days. The
responses of the sensors were then used for feature extraction, and a K-means clustering algorithm was
applied to identify human odour. This e-nose system that can detect human skin odour can be used for
individual identification or to determine individual physiological status.

Soft sound sensor arrays
Sound energy is conveyed by the pinna and auditory canal to the eardrum [146,147] . Vibrations of the eardrum
are amplified and transmitted by the middle ear bones, namely, the malleus, incus, and stapes, to the inner
ear fluid. The cochlea is a fluid-filled duct in the inner ear that is responsible for converting sound