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Arab Hassani. Soft Sci 2023;3:31 https://dx.doi.org/10.20517/ss.2023.23 Page 7 of 33

Mechanoreceptor Resistive, capacitive and Multi (Temperature, LCR meter and parameter analyser; personal PVA or PDMS Power supply Health monitoring, humanoid
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(Touch) photocurrent sensor array strain, humidity, and light computer; and 3D mapping robotics, prosthetics, and human-
magnetic fields) machine interfaces
ADC: Analogue-to-digital conversion; MCU: micro-controller unit; PCA: principal component analysis; PDMS: polydimethylsiloxane; PEN: poly(ethylene naphthalate); PET: polyethylene terephthalate; PI: polyimide;
PVA: poly(vinyl alcohol); PVDF: poly(vinylidene fluoride); P(VDF-TrFE): poly(vinylidene fluoride-trifluoroethylene).

Soft eye image-sensing arrays
The human eye comprises a lens that collects light and a hemispherical retina. Dense integration of 100-120 million photoreceptors and rod and cone cells in
the retina helps convert incident light to action potentials that are transmitted to the brain through millions of nerve fibres [Figure 2A] [27,130] .

Gu et al. developed a biomimetic artificial electrochemical eye (EC-EYE) consisting of a hemispherical retina with a high-density perovskite nanowire array
[27]
acting as the photosensor [Figure 2B] . An ionic liquid electrolyte mimicking vitreous humour served as the front-side common contact with the nanowires.
Freestanding hemispherical porous aluminium oxide membranes (PAM) were used to grow a high-density array of perovskite nanowires inside nanochannels.
Nanochannels were selectively opened in the PAM layer by using a focused ion beam. A magnetic field was used to align nickel (Ni) microneedles with three
exposed nanowires on the PAM layer. Each Ni microneedle formed a pixel with a lateral size of 1 μm and pitch of 200 μm. Thin tubes filled with liquid metal
were used to connect to each Ni microneedle through a PDMS socket. The PDMS socket was fabricated using a 3D-printed hedgehog-shaped mould to pattern
a 10 × 10 hole array on the socket. Thin soft tubes filled with eutectic gallium indium liquid metal were used to form liquid-metal wires mimicking human
nerve fibres behind the retina. Then, 100 such tubes were inserted into the PDMS socket holes, and the socket was attached to the PAM/nanowire surface to
form the photodetector array. The liquid metal tubes were connected directly to a computer-controlled 100 × 1 multiplexer on a printed circuit board (PCB)
[Figure 2C]. Several optical patterns (e.g., character “A”) were projected onto the EC-EYE, and the photocurrent of each pixel was measured using a current
meter. A personal computer (PC) was used for both processing the current data and controlling the multiplexer. The photocurrent values were converted to a
grayscale number between zero and 255 to reconstruct the detected objects. Figure 2D shows the character “A” imaged by the EC-EYE and its projection onto
a flat plane. The EC-EYE achieved a maximum photocurrent sensitivity of 303.2 mA/W, and the high-density nanowire arrays provided high image resolution.
The EC-EYE was demonstrated as a vision system for humanoid robots that resembled the human eye in appearance and characteristics.

Unlike the EC-EYE, which reconstrued the entire eye, Choi et al. focused on developing a soft high-density image sensor and electrode array (i.e., a closed-
loop system) integrated with the human retina, as shown in Figure 3A . The device consists of a high-density, vertically stacked, and ultra-thin soft
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molybdenum disulfide (MoS )-graphene phototransistor (CurvIS) array and ultra-thin neural-interfacing electrodes (UNE) [Figure 3B]. Each stacked electrode
2
and phototransistor is connected through a flexible PCB (FPCB) coated with soft and thick silicone rubber (i.e., soft FPCB). An external light signal is focused
by the lens onto the CurvIS array [Figure 3B]. The output photocurrent generated by each phototransistor in response to the external light is amplified by a
trans-impedance amplifier, passed through an inverter, and fed into a micro-controller unit (MCU). Figure 3C shows all these components integrated into the
soft FPCB. The MCU measures and processes the amplified signal and generates electrical pulses. These electrical pulses are then applied to the relevant UNE