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Lee et al. Soft Sci 2024;4:38 https://dx.doi.org/10.20517/ss.2024.36 Page 21 of 31

Table 1. Characteristics of strain-engineered stretchable substrates and their advantages and limitations in free-form display
applications
Plastic film Elastomers
Substrate type Network Crosslinking Structure
Buckling Kirigami
aligning control assembling
Stretchable Vertical compensation Void compensation Intrinsic stretchability
characteristic
Representative Pre-stretching Laser cutting Rubbing Thermal patterning Evaporation
manufacturing Mold transfer Etching Post-stretching Light patterning Direct printing
Advantages in free- Facilitates application in conventional display Suitable for high-resolution and multi-form applications
form display materials and process lines
Limitations in free-form Trade-off between stretchability and Requirement for developing new stretchable materials to
display resolution/reliability overcome process limitations

saddle-shaped surfaces [Figure 17B]. To address the issue of resolution degradation in stretchable displays
due to stretching, Lee et al. proposed a stretchable OLED device that maintains a high fill factor even after
stretching by exposing hidden active areas (HAA) during Kirigami deformation . They attached an
[145]
ultrathin OLED to a 3D Kirigami-structured substrate, allowing the HAA to become visible during biaxial
stretching, thereby preserving resolution and quality [Figure 17C]. This approach to compensating for
resolution loss effectively demonstrates the potential of the proposed method in maximizing the capabilities
of stretchable OLEDs.

Display applications on intrinsically stretchable elastomer substrates
Elastomers, which enable omnidirectional stretchability without compensatory structures, are considered
highly promising as substrates for free-form displays, with active research underway to enhance their
thermal stability and mechanical properties for effective display component integration [146,147] . Additionally,
as described in previous sections, efforts are also underway to expand the potential applications of free-form
displays through macroscopic and localized strain-engineering. Oh et al. developed a method to improve
image distortion in stretchable displays caused by the Poisson effect of elastomers by embedding various 1D
line patterns into an elastomer matrix . This approach effectively suppresses the contraction due to the
[148]
Poisson effect during stretching, resulting in a composite elastomer substrate with a near-zero Poisson’s
ratio [Figure 18A]. To minimize unwanted deformation of the substrate, the researchers meticulously
designed the modulus, dimensions, and shape of the line patterns and used transparent materials to ensure
excellent optical transparency of the composite substrate. To enable diverse deformation applications while
maintaining a zero Poisson’s ratio for the substrate, Choi et al. developed a meta-elastomer substrate with a
bidirectional zero Poisson’s ratio by embedding structurally designed honeycomb-shaped soft mechanical
[149]
metamaterials into an elastomer matrix . This substrate utilized soft materials for the frame, allowing for
independent tensile deformation along different axes [Figure 18B]. By optimizing the dimensions of the
structures and the modulus mismatch between the substrate and the matrix, they effectively controlled the
anisotropic Poisson’s ratio. The LED pixel arrays formed on this zero Poisson’s ratio meta-elastomer
substrate demonstrated linear and predictable deformation during stretching, indicating significant
potential for applications in stretchable displays that require various in-plane shape deformations. Lee et al.
proposed a stretchable display design where a structured plastic film with a negative Poisson’s ratio is
[150]
embedded within an elastomer matrix, allowing the display image to expand biaxially under tension
[Figure 18C]. The substrate integrates a rigid glass-fiber reinforced PDMS (GFRPDMS) region with a soft
elastomer matrix, optimizing the elastic modulus difference between the stiff and stretchable areas to
maintain stability under multi-directional stretching and achieve consistent strain control and performance
stability, even with repeated deformation. Using this substrate, they developed a stretchable display
incorporating a high-resolution micro-LED array and liquid metal electrodes, achieving up to 25% stretch
without image degradation.