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Park et al. Soft Sci 2024;4:28 https://dx.doi.org/10.20517/ss.2024.22 Page 7 of 28

Figure 3. (A) The ideal shear modulus of flexible adhesives in temperature sweep test. With low T adhesives, it facilitates the formation
g
of multiple neutral planes through the entire integrated structures; (B) DMA and rheometer are used to measure the viscoelastic
properties of flexible adhesives such as temperature sweep, frequency sweep, creep and recovery, S-S curve, and so on; (C) The
plastic-viscoelastic deformation occurs during the peel test. The stronger deformation occurs at peel front, the larger adhesion force is
detected. T : Transition temperature; DMA: dynamic mechanical analysis.
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Adhesion strength
Compared to adhesives used in conventional rigid devices, those for flexible devices are thinner and have
lower adhesion force, making it challenging to maintain sufficient bonding and ensure cohesive movement
of adjacent layers [34,100-103] . The adhesion force is primarily determined by the interfacial interactions and the
bulk deformation loss within the adhesive. During peel tests, energy dissipation occurs with the formation
of a plastic-viscoelastic zone at the peel front, where the debonding process takes place [Figure 3C]. The
formation of this zone is influenced by the adhesive thickness and peeling speed. As the thickness decreases,
a fully developed plastic-viscoelastic zone cannot form at the peel front, leading to a drop in adhesion
force . For adequate bonding with adjacent layers, the adhesion force of flexible adhesives should be at
[104]
least 1 N/cm, based on standards for foldable adhesives . To achieve this, flexible adhesives should be
[34]
formulated to allow for effective crosslinking, enhancing resistance to cohesive failure, and ensuring
sufficient wetting and bonding at the interface [105-109] .