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Keum et al. Soft Sci 2024;4:34 https://dx.doi.org/10.20517/ss.2024.26 Page 21 of 32

between the elastic pillars and bridges during stretching and reported that improving the layout of the
bridge interconnects or the elastic pillars could further enhance the mechanical stability. They also
explained that their unique structural approach is not limited to displays but can impart stretchability to
various electronics, including semiconductors, circuits, and electrodes.

SUBSTRATE FORM FACTORS
Further, one of the important components for obtaining the stretchability is the development of a
deformable substrate. Previously, various rigid LED devices on conventional wafer or glass substrates have
been developed, and achieved remarkable advances in pixel resolution, brightness, and device lifespan [129,130] .
Similar to the technological advancements in rigid-type displays, achieving advancements in various
technical performances of stretchable displays, particularly in deformable substrates and emissive layers, is
expected to facilitate their application as next-generation technologies including electronic skin-based
displays, soft robotics applications, and hybrid electronic devices with integrated displays. In this section, we
introduce various display form factors to provide stretchability by expanding from conventional rigid-based
substrates. This section includes textile-based displays based on fibers or fabrics [8,131-136] and unique meta-
structured displays [137-140] .

Textile-based stretchable displays
Fibers or textiles, with their inherent softness and highly deformable properties, are materials of interest in
the wearable electronics as the electronic textiles (e-textiles). In the e-textile displays, a key consideration is
[141]
the integration of electronic devices while maintaining the intrinsic advantages of the textile materials .
Textile-based form factors, composed of fabrics and fibers, inherently possess stretchability due to their
[142]
unique woven and knitted structures . When integrated with light-emitting layers, textile displays could
offer distinctive advantages such as breathability, wearability, and comfortability, making them highly
promising for next-generation wearable and stretchable display applications. Recently, there have been
numerous attempts to combine OLEDs, which are advantageous in terms of miniaturization and
integration, with textile form factors from both material and process technology perspectives. Ma et al.
fabricated a ZnS phosphor-based textile ACEL display using poly(vinylidene fluoride-co-
hexafluoropropylene) (PVDF-HFP) with high permittivity as a dielectric matrix on an elastic fabric and
[131]
incorporating barium titanate (BaTiO ) nano-particles into the composite . Stretchable AgNW bottom
3
electrodes, emissive layers, and PVDF-HFP dielectric films were sequentially printed using a screen printing
process on a thermoplastic polyurethane (TPU) film interface layer positioned on the fabric, and the AgNW
top electrode was formed by spray coating. They reported that the fabricated ACEL display exhibited
mechanically robust properties, with only a 5.62% decrease in luminous efficiency when stretched up to
80%, as well as stability under high temperature, humidity, and repeated washing. Additionally, it was
explained that it can be produced over a large area using a roll-to-roll (R2R) process [Figure 12A]. There are
also research examples of stretchable textile-based displays that adopt different engineering approaches,
utilizing 1D fiber-type structures instead of directly deposition of emissive layers and electrodes on the
[134]
fabrics. Song et al. developed a fiber-type OLED device with a patterned emission area . By arranging the
fiber OLED devices in a woven structure together with conductive fibers, a 2D textile display was realized
that could operate in a pixel matrix format [Figure 12B]. They deposited the constituent layers of a
phosphorescent green OLED, which has high brightness (~4,300 cd·m at 5 V) and high efficiency
-2
-1
(~46 cd·A , ~58 lm·W ), onto a rectangular indium-tin-oxide/polyethylene terephthalate fiber with
-1
patterned PU side barriers. This process was performed to create an interconnected luminescent fiber
composed of a 1D OLED pixel array. In addition, a robust and conductive encapsulation system, designed
to mitigate oxygen/water permeability and mechanical damages while allowing electrical current flow, was
achieved using a combination of Al base pad/molybdenum trioxide (MoO ) pre-barrier/PU top barrier/Al
3
contact pad. This system preserved the EL performance of the interconnected OLED fiber. The woven