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Park et al. Soft Sci 2024;4:28 https://dx.doi.org/10.20517/ss.2024.22 Page 5 of 28

Stretchable displays, unlike previously mentioned device forms, undergo unexpected three-dimensional
deformations. Current research on textile displays and medical sensors aims to advance the technological
maturity of free-form displays [56-61] . Despite significant progress in driving cell technologies [58-67] , such as
serendipity circuits, island-type driving layers, and micro light-emitting diode (μLED) emission layers,
research on adhesives for stretchable displays remains insufficient, and the required properties are not well-
defined. However, insights from related fields, such as drug delivery systems or skin adhesives for medical
sensors, could help identify necessary properties for display adhesives [57,68-75] .

In addition to stretchability, adhesives for the drive part must be flowable enough to fill and flatten
substantial unevenness caused by μLEDs and serendipity circuits. For applications such as medical sensors
or textile displays, properties such as biocompatibility, easy detachability, water resistance, and chemical
resistance are also crucial. Since stretchable devices often use elastic materials such as polydimethylsiloxane
(PDMS) or polyurethane as substrates [65,76-82] , conventional acrylic adhesives may not provide sufficient
adhesion. This has led to suggestions for using silicone adhesives or adding adhesion promoters to acrylic
adhesives. Hydrogel adhesives are also being researched as candidates due to their excellent biocompatibility
and stretchability [83-86] . While the required characteristics of adhesives for stretchable displays can be
anticipated, specific specifications remain uncertain. Thus, in-depth research is needed to clearly define
these properties.

Table 1 summarizes the essential properties of adhesives for flexible devices with different form factors.
Rigid and foldable adhesives have well-defined specifications based on the requirements of commercially
available display devices. In contrast, the parameters for rollable and stretchable adhesives are less
established, as they are still in the research and development stage and are derived from recent publications.
These specifications are approximate and may vary depending on the device's size and purpose. In the next
chapters, we explain in detail how Table 1 was derived. We will first define the key parameters for adhesives
used in flexible devices with different form factors and explore strategies to achieve these parameters.
Additionally, we will review the solutions researchers have developed to address the trade-offs among these
key parameters.

KEY PARAMETERS FOR ADHESIVES IN FLEXIBLE DEVICES
Low modulus across a wide temperature range
Adhesives for flexible devices relieve stress across device modules by forming multiple neutral planes, which
decouple strain in adjacent layers. The formation of these neutral planes and the resulting strain decoupling
reduce the amount of stress and strain acting on each layer, thus preventing defects such as cracks or
delamination. For these neutral planes to be effective, the adhesive’s complex shear modulus must be
4
significantly low. An adhesive with a shear modulus of under 5 × 10 Pa at room temperature (RT) can
alleviate overall module stress [15,25,35,39,87-91] [Figure 3A]. It is crucial to maintain the adhesive’s modulus close
to its RT value across a wide range of operating temperatures to ensure the temperature reliability of flexible
devices. Near and below the glass transition temperature (T ), a rapid increase in complex shear modulus
g
can disrupt the neutral plane design strategy. At higher temperatures, the polymer might become more
flowable and lose elasticity, degrading its recovery properties. To prevent a rapid increase in modulus at low
temperatures, low T monomers are introduced to lower the adhesive’s T close to the lower boundary of the
g
g
industrial temperature range. This results in increased flowability near the upper boundary of the industrial
temperature range, thus necessitating control through an appropriate introduction of chemical/physical
crosslinking to manage cohesion. This strategy has been chosen to maintain low modulus across a wide
temperature range. This approach helps maintain designed strain decoupling and neutral plane splitting
within a broad temperature range, based on the properties set at RT. Therefore, to preserve the mechanical
stability of flexible devices, the adhesive must maintain an extremely low modulus across a broad
temperature range [24,42] .