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Page 4 of 33 Arab Hassani. Soft Sci 2023;3:31 https://dx.doi.org/10.20517/ss.2023.23

Electrostatic induction sensors can mimic the operation of electroreceptors. Sometimes, these sensors
include an electrode layer embedded in an insulating layer. If any naturally charged object approaches the
electrode, changes in electrical output are detected through electrostatic induction. If the electrode is
[52]
exposed to the object, both charge transfer due to physical contact and electrostatic induction occur .
Another example is a sensor consisting of a top layer composed of a pre-charged elastomeric electret, a layer
of conductive hydrogel acting as the ionic electrode, and a flexible insulating substrate. When a naturally
charged object approaches the top electret layer, the charge transfer induced in the ionic electrode generates
[53]
an electrical output .
Flexible capacitive, resistive, piezoelectric, and triboelectric pressure sensors can be used to detect low- and
medium-pressure stimuli, similar to the mechanoreceptors in the skin [54-56] . Resistive pressure sensors
comprise a soft/flexible pressure-sensitive conductive layer [e.g., carbon nanotubes (CNTs) and PDMS
composite materials], the resistivity of which changes with the application of a pressure stimulus. Capacitive
sensors consist of a soft insulating layer (e.g., PDMS) sandwiched between two electrodes, and the
capacitance value of the sensor changes when it is subjected to pressure. The piezoelectric effect observed in
materials such as poly(vinylidene fluoride) (PVDF) is used to convert applied pressure into voltage changes
in piezoelectric sensors . In triboelectric sensors, alternating contact and separation of two materials with
[57]
[58]
different electron affinities [e.g., polytetrafluoroethylene (PTFE) and aluminium (Al) layers ] during
pressure application induces surface charge generation (i.e., triboelectric effects) [59,60] .

Various types of sensors, such as biosensors, electrochemical, surface acoustic wave (SAW), resistive,
[61]
colourimetric, and optical sensors, have been developed to mimic olfactory receptors . For example, a
chemoreceptive ion gel was patterned on top of the poly(3,4-ethylenedioxythiophene) polystyrene sulfonate
(PEDPT:PSS) channel of an organic electrochemical transistor (OECT) to realise an electrochemical
olfactory sensor on a flexible substrate. The interaction of gas molecules with the ions in the ion gel
[62]
generates a potential that, in turn, gates the OECT channel .
Various types of chemical sensors and enzyme-modified triboelectric sensors have been developed to
[64]
[63]
mimic taste receptors. Chemical sensors are potentiometric or amperometric devices with sensing layers
(e.g., ion-selective and artificial lipid polymer membranes, metal oxides, and bioreceptors such as enzymes,
[65]
antibodies, nucleic acids, and cells ) that can detect a wide range of taste substances by measuring electrical
[66]
potential or current . One example of a flexible chemical sensor consists of laser-induced graphene
interdigitated electrodes (IDEs) patterned on a polyimide (PI) film and transferred onto a flexible Kapton
[67]
tape .

Acoustic sensors based on piezoelectric, triboelectric, capacitive, or piezoresistive mechanisms can mimic
auditory receptors [68,69] , and their operating principles are the same as those of mechanoreceptors. However,
the stimulus for acoustic sensors is sound. For example, a piezoelectric acoustic sensor on a thin flexible
polyethylene terephthalate (PET) substrate comprises a lead-zirconate-titanate (PZT) membrane with
multichannel IDEs for realising multi-resonant frequency band control that covers the entire voice
spectrum, similar to the human ear .
[70]

The significance of the use of these flexible sensors in bioreceptor-inspired sensor arrays lies in their
potential to offer large-area sensing, unprecedented sensitivity, selectivity, conformability, and energy
efficiency by mimicking the mechanisms used in natural biological systems [71-74] . This can greatly enhance
the fields of health monitoring, disease detection, environmental monitoring, and so on [75,76] .