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Page 2 of 31 Lee et al. Soft Sci 2024;4:38 https://dx.doi.org/10.20517/ss.2024.36

INTRODUCTION
Displays play a crucial role in the industry, serving as a key component that enables efficient
[1,2]
communication between humans and machines by delivering visual information . Given that humans
heavily rely on vision to process information, displays are essential for conveying this information
[3,4]
accurately and efficiently . With the rapid advancement of communication technology, displays have
become indispensable in nearly every aspect of daily life, including smart homes, wearable devices,
[5-8]
automotive systems, and digital signage . As these diverse application areas continue to expand, research
and development efforts are increasingly focused on creating displays with broader functionalities and
innovative form factors. The development of flexible displays has been made possible by advancements in
material and process technologies, leading to the commercialization of foldable and rollable displays with
adjustable screen sizes, which have gained significant popularity among consumers due to their attractive
designs [9-11] . Additionally, the accumulated expertise in display materials and processes, along with
advancements in analyzing stress and strain during deformation, has further fueled research into displays
with cutting-edge multi-form factors [12-14] . Particularly, stretchable displays, which can freely change shape as
the device stretches, are regarded as the pinnacle of display form factor innovation and are expected to play
a vital role in the future evolution of display technology [15,16] .

In the effort to expand display form factors, extensive research and development have been conducted on
substrates, electrodes, light-emitting elements, and driving components, with substrate materials
consistently being a top priority [17-21] . For instance, flat panel displays have traditionally relied on glass
substrates that offer high transmittance along with excellent thermal durability [22,23] . On the other hand, the
emergence of flexible and foldable displays has shifted the focus toward researching thin and flexible glass
and plastic substrates, which are better suited to handle the mechanical demands of bending and
folding [24,25] . In this manner, different substrates are selectively utilized based on the display’s target form
factor. The selection process carefully considers not only the physical and chemical properties of the display
materials but also the temperature and pressure requirements during processing, as well as the functional
and aesthetic needs of the final application area. Therefore, to successfully realize display devices, it is
crucial to balance these factors and choose the most suitable substrate.

This review provides a comprehensive overview of strain-engineered substrate materials and their
applications in stretchable displays. It specifically categorizes and explains the differences between plastic
films with structural stretchability and elastomers with intrinsic stretchability [Figure 1]. For plastic films,
the review delves into the buckling and Kirigami structures that are commonly used to impart structural
stretchability, providing detailed explanations of the deformation behaviors that arise from various design
elements of these structures. For elastomers with intrinsic stretchability, it is emphasized that various
deformation dynamics can be programmed under the same tensile input through spatial modulus
patterning. This patterning can be achieved by aligning the elastomer network, adjusting the crosslink
density, or creating structures on the substrate surface. Additionally, this can be accomplished by
structurally designing heterogeneous materials, embedding them into the elastomer matrix, or forming
patterns on the matrix surface with heterogeneous materials. Finally, the review explores the utilization of
these strain-engineered stretchable substrates in display applications, adapted to different usage scenarios
and environmental conditions, by examining various examples.

STRUCTURALLY DESIGNED PLASTIC SUBSTRATES
To provide stretchability to flexible plastic films, strategies are commonly used that incorporate structures
capable of accommodating dimensional changes due to tensile deformation. To achieve this, various
structural designs have been introduced, including serpentine structures with zigzag patterns [26,27] , helical