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Arab Hassani. Soft Sci 2023;3:31 https://dx.doi.org/10.20517/ss.2023.23 Page 5 of 33

Soft bioinspired sensor arrays that are compatible with complex biological organs could help mimic natural
senses with the help of data acquisition [i.e., signal conditioning and analogue-to-digital conversion (ADC)
[77]
circuits], data processing, and visualisation techniques , as presented in Figure 1. Signal conditioning
involves signal amplification and noise filtering. Sensor signals are typically amplified by transistors. By
using transistor-based sensor arrays, such as OECT arrays, the intrinsic gain of transistors can be used to
obtain amplified signals at low applied voltages [78,79] . Chemical sensor arrays can help discriminate various
similar analytes, recognise specific patterns, and ease data processing for complex samples . Pressure
[80]
sensor arrays can track spatial pressure and map pressure changes with high resolution . These arrays have
[81]
been developed in various formats, such as second skin/patches, lenses, artificial organs, and clothes [82-84] .

The aforementioned sensor array systems are often integrated with actuating/electrical stimulation elements
to realise closed-loop systems; such systems are useful for developing smart systems that operate similarly to
biological organs in the body [59,85-88] . Biochemical [e.g., glucose and pH, thermal (temperature), mechanical
(e.g., strain and pressure)], and electrical [e.g., electroencephalogram (EEG) and electrocardiogram (ECG)]
signals detected using body sensors can change because of diseases. To regulate these changes, biochemical,
mechanical, and electrical actuators can be used for biochemical production, organ stimulation, and neural
stimulation, respectively [89,90] . For example, the pancreas is responsible for lowering blood sugar levels by
[91]
releasing insulin . In patients with type 1 diabetes, this regulation needs to be maintained using insulin
pumps . If a sensor can detect the blood sugar level, and based on this reason, if an actuator can release an
[92]
adequate amount of insulin, a closed-loop system can be formed (i.e., artificial pancreas). These actuators
are usually inspired by nature [93-98] . For instance, the octopus vulgaris arm with several suckers and an
interconnected nervous system has inspired the development of a soft adhesion actuator that consists of a
sucker with integrated strain sensors capable of picking up objects . Then, strain sensors can be used to
[99]
measure the mass of such objects and activate the actuator to ensure that it is firmly attached to the object.

The use of bioinspired and biomimetic strategies to develop energy storage and harvesting devices could
enhance and optimise the performances of these closed-loop systems [100-106] . The electricity generated by
[107]
bioinspired energy harvesting devices, such as triboelectric and piezoelectric nanogenerators and solar
[108]
[109]
cells , can be stored in storage units and used to power electronic devices . The two examples that are
[110]
inspired by nature are electrode design and materials of lithium-ion batteries and the solid-solid electrode/
[100]
electrolyte interfaces in solid-state batteries .
SOFT SENSOR ARRAYS TO REBUILD KEY SENSES
[111]
[112]
Several reviews have focused on bioinspired electronics. Valle and Jung et al. reviewed bioinspired
sensing systems based on the five traditional senses, namely the vision, touch, taste, smell, and auditory
senses. Xiao et al. focused on bioinspired ionic sensory systems, while Li et al. explored the prosthetic
[114]
[113]
interface applications of bionic sensor systems. Xue et al. summarised the processes and applications of
[115]
bioinspired sensor systems in healthcare . However, the present review focuses on bioreceptor-inspired
sensors and actuator arrays fabricated on a single soft, flexible substrate to rebuild one of the senses, such as
vision, spatial perception, touch, taste, smell, and hearing, to treat/monitor a specific condition or in
prosthetic applications. A summary of the key characteristics of the explored soft sensor and actuator/
electrode arrays is presented in Table 2 and discussed in the subsequent text [27,116-129] . This review focuses on
the bioreceptors that have inspired the development of sensor arrays, circuits required to build data
acquisition and processing units, power sources used in various systems, array fabrication processes, and
target healthcare applications.