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Page 16 of 28 Park et al. Soft Sci 2024;4:28 https://dx.doi.org/10.20517/ss.2024.22

[141]
by thermal iCVD do not form crosslinked structures, whereas those produced by photo iCVD do . By
using both thermal and photo iCVD simultaneously, the degree of crosslinking can be adjusted, which
affects the formation of the plateau region in frequency sweeps and the rubbery region in temperature
sweeps [Figure 7D].

Adjusting the UV light intensity or exposure time can effectively control the crosslinking density of
adhesives prepared by photocuring. Kim et al. demonstrated that increasing the UV exposure time, while
maintaining a constant light intensity, enhances the gel content. This increase in gel content promotes the
formation of the crosslinked network, allowing for precise control over stress relaxation properties [142,143] .
Employing this approach, they fabricated patterned adhesives with varying crosslinking densities by
manipulating the UV dosages. They cured the low-cured area via primary UV curing and formed the high-
cured area through secondary curing using the photomask. Compared to the non-patterned sample, the
patterned sample showed a significant improvement in the recovery performance. The patterned sample
also exhibited a relatively high modulus compared to the non-patterned sample, while demonstrating
similar adhesion strength. Subsequently, by changing the gray scale and size of the patterned mask, they
[144]
effectively tuned the density of crosslinked network . Smaller pattern sizes enhanced adhesion forces and
recovery properties, demonstrating the potential of UV-patterned adhesives as flexible adhesives
[Figure 7E]. These studies highlight that the crosslinked network can be controlled not only by changing
materials but also through manufacturing processes, offering significant insights into the versatility of
adhesive technologies.

Recently, Back et al. successfully developed a highly resilient adhesive with low crosslinking density but
excellent strain recovery characteristics by leveraging polymerization-induced microphase separation
[145]
(PIMS) that incorporates both hard and soft domains to form a nano-scaled bicontinuous phase . When a
macro chain transfer agent (CTA) and monomers are blended and cured, the linear monomer blocks grown
from the CTA ends are kinetically trapped by the crosslinked polymer domains, resulting in phase
separation that ultimately forms a nanoscale bicontinuous phase [Figure 7F]. In the bicontinuous phase, the
soft domains, formed solely by entanglement without crosslinking, deform immediately under folding
strain, allowing for a wide range of deformation, while the hard domains, possessing a crosslinked network,
exhibit superior recovery characteristics that enable them to return to their original state once external
deformation is removed. This research presents a significant strategy that overcomes the limitations
previously faced in controlling the behavior of traditional flexible adhesives, where it was challenging to
independently control flowability and recovery properties solely based on gel contents, i.e., the content of
crosslinked polymers.

Sustainability
For several decades, environmental concerns have driven increased attention to the sustainability of
materials across various fields [146-152] . This societal interest has significantly influenced the research and
development of adhesives, with studies focusing on monomer perspectives, manufacturing methods, and
post-use considerations.

To produce acrylate monomers in an environmentally friendly manner, AA reacts with hydroxy-containing
materials derived from natural sources [91,153-163] . AA, a crucial precursor for acrylate monomers, has already
been commercialized using an eco-friendly method [164-167] . Commonly used hydroxy-containing sources
include terpenoid and lignin-based materials [Figure 8A]. Depending on the type of hydroxy-containing
material used, either high T monomers or low T monomers are produced. High T monomers such as
g
g
g
menthyl acrylate and isobornyl acrylate have been extensively studied [168-171] , while innovations in low T
g