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Page 2 of 33 Arab Hassani. Soft Sci 2023;3:31 https://dx.doi.org/10.20517/ss.2023.23

responses of biological receptors (i.e., mechano, thermoreceptors, photo, chemo, electro, and magneto) to
internal or external stimuli constitute the foundation of remarkable innovations in wearable sensing
[2]
technology . Receptor organs, such as the eyes, ears, nose, mouth, and internal organs, contain various
[3]
types of receptor cells that promote parallel sampling and processing of sensory information .

Receptor types include neurons with free nerve endings embedded in tissues, neurons with encapsulated
endings in connective tissues, or specialised receptor cells with specific structures and sensitivity to specific
types of stimuli . Two examples of neurons with free endings are pain and temperature receptors in the
[4]
[5]
skin dermis (i.e., mechano and thermoreceptors, respectively) . Mechanoreceptors are divided into three
groups: Tactile receptors, proprioceptors, and baroreceptors . Tactile mechanoreceptors in the skin
[6-8]
include Merkel’s disks with free nerve endings, Meissner’s corpuscles, Ruffini endings, and Pacinian
corpuscles with encapsulated endings located near the skin surface, as well as deeper-level Krause end bulb
receptors with encapsulated endings in specialised regions . Proprioceptors can be found in the skin, joint
[6]
capsules, tendons, muscles, ligaments, and connective tissue, and they contribute to the position and
movement senses . Baroreceptors in the walls of blood vessels allow for the transfer of blood pressure
[8]
information to the autonomic nervous system . Thermoreceptors detect and respond to changes in
[7]
temperature. They can be found on the skin and inside the body . Specialised receptor cells called
[9]
photoreceptors in the retina respond to light stimuli . The nasal cavity houses olfactory receptor neurons
[10]
as chemoreceptors that respond to smells. The biomechanical activity of prey animals generates electric
[11]
fields that can be detected by electroreceptor cells on the skin of animals . Magnetoreceptor cells in
animals’ skin allow them to detect geomagnetic fields for different purposes related to navigation .
[12]
On the basis of the operating principles of bioreceptors, arrays of various types of sensors have been
developed and used to mimic receptor organs [13-17] . Figure 1 presents schematics of the photo, electro,
mechano, olfactory, taste, and auditory bioreceptors, which are responsible for the vision, touch, spatial
perception, smell, taste, and hearing senses, respectively [18-23] . Table 1 summarises the specific functions and
limitations of these six bioreceptors and the health conditions that could affect their performance [24-46] . In
addition, it lists suitable characteristics of the biomimetic soft sensor arrays that could replace these
bioreceptors, namely high sensitivity, resolution, selectivity, short response time, and high stability. In what
follows, we provide examples of soft sensors, explain their fundamental concepts, discuss the associated
materials, and summarise the functionalities needed to replicate the performance of these bioreceptors.

Nanowire-based photodetectors and phototransistors are examples of sensors that can respond to light
[47]
similarly to photoreceptors . Semiconductors (e.g., III-V semiconductors, metal oxides, perovskites, etc.)
are the most common materials used to fabricate nanowire-based photodetectors. Photoconductivity is a
known phenomenon in semiconductors, where incident light could lead to an increase in electrical
conductivity through the photogeneration of carriers . These nanowire-based photodetectors could be
[47]
fabricated as single nanowires or an ensemble (forest) of vertical nanowire arrays . Vertical nanowire array
[48]
photodetectors grown on flexible polymer substrates performed well in terms of restoring light responses
when replacing degenerated photoreceptors in the retina . Meanwhile, the light incident on a
[49]
phototransistor can modulate the channel charge carrier density by acting as an additional optically
controlled terminal apart from the three conventional transistor terminals. Metal oxide semiconductor
field-effect transistor (MOSFET)-type photodetectors (i.e., phototransistors) fabricated on flexible polymer
substrates offer high photosensitivity and responsivity, thereby mimicking the behaviour of mammalian
[50]
eyes . To detect colours similarly to cone cells in the eye, Zheng et al. used three types of nanoparticle
solutions, namely gold nanorods (Au NR), gold nanospheres (Au NS), and silver NS (Ag NS), to fill the
holes of a flexible polydimethylsiloxane (PDMS) substrate . One hole was filled with a carbon nanoparticle
[51]