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Keum et al. Soft Sci 2024;4:34 https://dx.doi.org/10.20517/ss.2024.26 Page 23 of 32

both x-axis and y-axis directions, withstands over 180% strain, and remains durable through 10,000 cycles.
This novel approach mitigates the limitations caused by image quality degradation in stretchable displays,
which is often a consequence of the high Poisson’s ratio of elastomer substrates.

ADVANCED APPLICATIONS OF STRETCHABLE DISPLAY
The stretchable and compact multifunctional displays, capable of versatile deformation, are garnering
significant attention as a next-generation application of human-machine interfaces, surpassing its primary
role of information displays. Figure 13A shows the stretchable synaesthesia visual-acoustic encryption
[144]
display capabilities using a high-resolution mosaic image with a dot pattern of 300 µm . As shown in
Figure 13A, both visual and acoustic information were used as input signals for decoding during the display
operation. By utilizing variable sound signals to create multiplex QR codes, the applicability of the display
could extend to the next-generation authentication devices and security systems . As another example of
[144]
utilizing information displays, Yang et al. reported an asymmetrically enhanced coplanar-electrode (AECE)
EL device that operates based on impedance adjustment strategy . The reported AECE EL device
[145]
increases the brightness of one region at the sacrifice of another region, allowing the brighter region to serve
as a display and can be easily designed various shape pattern. The contrast between the bright and dark
regions of the display can be adjusted by varying the input voltage, frequency, relative area ratio of the
electrodes, thickness, and composition of the luminescent layer. The AECE was successfully implemented
on a soft substrate along with the luminescent layer to fabricate a stretchable ACEL display. Benefiting from
a similarity with the conventional sandwich structure, the AECE EL performed encryption/decryption
display applications as a patterned display. Stretchable displays offer versatile shape morphing capabilities,
making them promising candidates for next-generation displays by providing enhanced visual information
as 3D displays. Oh et al. developed a 3D display by integrating a low melting point alloy (LMPA)-graphene
nanoplatelet (GNP)-elastomer composite with a stretchable EL device . In this composite, LMPA imparts
[6]
shape memory properties, while GNP establishes conductive pathways. The 2D morphing display
transformed into complex 3D structures through electrothermal activation. They successfully demonstrated
applications such as 3D touch-sensing automotive displays, deformable wearable displays, and 3D art
displays and highlighted the potential of advanced next-generation visual-tactile interfaces. Figure 13B
shows the results of an ionic sensor application by incorporating an IL, ([EMI][TFSI]) and
electrochemiluminescent (ECL) luminophore [Ru(bpy) (PF ) ] into a porous poly(ethyl acrylate-ran-
6 2
3
styrene-ran-divinylbenzene) (PEA-r-PS-r-PDVB) foam . When pressure is applied to the porous ionic gel
[146]
foam, the bulk resistance decreases, leading to luminescence caused by the ECL annihilation reaction [146-149] .
The ECL ion-gel exhibited a proportional increase in the intensity with higher applied pressure. This result
demonstrates the successful integration of a deformable physical sensor device and a luminophore,
highlighting the application potential of skin displays in the wearable electronics. Moreover, Kim et al.
proposed an artificial stretchable sensory-neuromorphic system inspired by biological sensory nerve
systems, comprising a stretchable capacitive pressure sensor (artificial mechanoreceptor), resistive random-
access memory (artificial synapse), and QLED array (epidermal photon actuator) [Figure 13C] . In this
[150]
system, the role of the display array is to visualize the feedback through luminescence in response to
electrical feedback that is sensed as stimuli, processed, and inferred through memory. This demonstrates
that stretchable displays can be utilized in the emerging field of neuromorphic systems, which mimic the
structure and functions of the human brain. The deformable characteristics of the sensory-neuromorphic
system suggest the application direction towards the smart skin prosthetic systems. Furthermore, stretchable
displays are driving innovations in the field of soft robotics. As shown in Figure 13D, Zhang et al.
demonstrate successful integration of ZnS phosphor-based elastomer stretchable EL devices with a
pneumatic soft robot, fabricated using a self-extrusion multi-materials 3D printing process that enhances
the material’s flowability and printability to provide mechanical robustness of EL devices. This self-adaptive