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Page 18 of 28 Park et al. Soft Sci 2024;4:28 https://dx.doi.org/10.20517/ss.2024.22

Current adhesive production methods predominantly involve solvent-based thermal polymerization and
solvent-free photopolymerization. To make these processes more environmentally friendly, several
approaches have been suggested, including the use of water and other eco-friendly solvents [176-181] , as well as
substituting UV light with harmless visible light [182-188] . Noppalit et al. used cyclademol and tetrageraniol-
based (meth)acrylates to produce adhesives with a T of -30 °C, employing a miniemulsion polymerization
g
method that uses both toluene and water as solvents, reflecting an environmentally conscious
manufacturing approach . Kwon et al. successfully developed a highly efficient visible-light
[178]
photoinitiating system (PIS) by combining a previously developed visible-light-absorbing photocatalyst
combined with appropriate co-initiators [27,189] [Figure 8B]. Unlike existing visible-light PIS, this new system
can efficiently facilitate polymerization with a minimal amount of photocatalyst, enabling the production of
highly transparent OCAs suitable for display applications. Compared to existing UV systems, this method is
energy-efficient, ensures user safety, and allows for the creation of functional OCAs previously unattainable
with UV-light curing. The visible-light curing system can be easily scaled up using halogen lamps or LEDs,
making it a cost-effective alternative to expensive xenon lamps for UV curing and directly applicable to
industrial processes. Building upon this technology, they have developed UV-blocking [27,28] , UV-
debondable , and sustainable OCAs .
[190]
[91]
Research on the after-use of adhesives has gained attention due to the challenges of recycling or disposing of
them, mainly caused by their crosslinked networks and the tackiness of residues left on surfaces [190-197] .
Several research groups have been investigating methods, such as stimuli-responsive adhesive, to selectively
control adhesion forces for clean detachment from surfaces [190,196,197] . Stimuli-responsive adhesives are crucial
in the display industry as they enable error correction and facilitate the recycling of display components
after use. Recent studies by Kim et al. have focused on adhesion control for displays using benzophenone
moieties that respond selectively to UV light . This allows additional crosslinking responding to post-UV
[190]
irradiation, which increases cohesion while reducing adhesion force, thereby enhancing the recyclability of
displays or cover glass after use [Figure 8C]. Hwang et al. reported on an adhesive with switchable adhesion
[196]
properties activated by temperature changes . The lower critical solution temperature (LCST)
phenomenon of N-isopropylacrylamide is a key factor in modulating adhesive strength based on
temperature. They successfully demonstrated that the adhesion force is reversible, with high adhesion forces
at RT and 97% reduction in adhesion at 90 °C, by utilizing changes in inter- and intra-hydrogen bonding
formation [Figure 8D].

For several years, research on (bio)degradable adhesives has been actively advancing, with studies focusing
on copolymerizing acrylate monomers with degradable units and creating adhesives entirely from
degradable monomers [198-209] . Current research underscores the need to understand how degradable units
influence rheological properties. This insight is crucial for developing adhesives that maintain high
performance while minimizing environmental impact.

Various optical properties for flexible devices
The adhesive used above the light-emitting layer in display devices must be optically transparent to ensure
that light reaches the user without any loss. Additionally, its refractive index (n) should closely match that of
the cover glass to prevent light loss due to surface reflection or diffraction. Acrylic adhesives, with n = 1.49,
are well-suited for this purpose. Their refractive index is slightly lower than that of cover glass (n~1.50) and
slightly higher than that of TAC film (n~1.47) used in polarizers, minimizing light loss at interfaces.
Moreover, the adhesive must be colorless to avoid distorting displayed colors. This can be verified using
parameters such as the yellowness index and the Lab color index to ensure a true white color.