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Page 2 of 28 Park et al. Soft Sci 2024;4:28 https://dx.doi.org/10.20517/ss.2024.22

foldable smartphones and laptops [Figure 1A]. Additionally, the growing interest in optimizing space and
enhancing interior design has driven the creation of rollable TVs, with research focused on reducing the
curvature radius and improving the reliability of these displays. In healthcare, the need for personalized
solutions has increased interest in realistic displays, such as feelable or implantable ones, used in medical
sensors. Consequently, extensive research is being conducted on flexible, large-form-factor displays to meet
[3-7]
diverse application needs [Figure 1B].
Adhesives, particularly pressure-sensitive adhesives (PSAs), are widely used in various fields ranging from
tapes and labels to electronics, automotive, and sealing because they can easily attach to numerous solid
surfaces with light pressure . Among them, acrylic PSAs lead the market due to their resistance to
[8,9]
ultraviolet (UV) light, humidity, and temperature, as well as their excellent optical properties and cost-
[10]
effectiveness, showing a steady growth in market share . Specifically, optically clear adhesives (OCAs), a
type of acrylic PSA, play a vital role in the diverse designs of modern displays. In traditional rigid displays, it
is crucial to use OCAs that ensure strong adhesion for each functional layer, maintain optical transparency,
preserve organic light-emitting diode (OLED) light brightness, and offer high reliability under diverse
environmental conditions. With the increasing demand for space-saving and ubiquitous communication,
the newly released foldable phone features additional integrated layers made of thinner, more flexible
materials compared to existing rigid phones, necessitating the use of more adhesives [Figure 2A, right]. As
these functional layers become thinner, they exhibit lower bending stiffness, which increases their
flexibility [11-15] [Figure 2B]. Consequently, the layers used in flexible displays are composed of thin and
flexible materials [16-23] . With the transition to flexible displays, the role of adhesives has become more
significant. These adhesives are crucial because they not only bond adjacent layers but also mitigate stress
caused by deformation [24-26] . They must accommodate dynamic movement while maintaining optical clarity
and color integrity. Depending on the application and curvature radius of the flexible display, the required
properties of the adhesive can vary. Therefore, the development of flexible adhesives must consider
mechanical properties, adhesion properties, optical properties, and environmental stability.

As devices undergo structural and functional advancements, high-tech flexible adhesives must further
enhance their already well-defined properties. Future adhesives need to retain their current properties while
being manufactured thinner or maintaining stable performance across a broader temperature range. With
the advancement of display technology, new functionalities such as UV blocking, stimuli responsiveness,
and shock resistance are being added [27-29] . Additionally, to enhance eco-friendliness from an environmental,
social, and governance (ESG) perspective, research on sustainable adhesives throughout all stages of
production, use, and processing is essential.

In this paper, we explore the development of adhesives for flexible displays as influenced by changes in
display form factors. We identify the key parameters required for these adhesives and, based on current
research, consider how they should evolve as display form factors, functionality, and usability become more
sophisticated. Additionally, we review the current status of eco-friendly adhesives and offer insights into
future directions for developing flexible adhesives that meet both sustainability and functionality
requirements.

THE DEVELOPMENT OF ADHESIVES FOR FLEXIBLE DEVICES
The advent of flexible display devices has necessitated the development of new technologies across various
fields to ensure their sophisticated and stable operation. OLED-based devices, in particular, have been
widely adopted and studied due to their inherent flexibility. To improve adoption in wearable sensors and
smart textiles, alternative display technologies such as mechanoluminescence [30-32] and thermolu-