Page 159 - Read Online

Page 20 of 31 Lee et al. Soft Sci 2024;4:38 https://dx.doi.org/10.20517/ss.2024.36

Conversely, elastomer substrates with inherent stretchability offer substantial advantages over the previously
discussed structured plastic substrates from a free-form display standpoint [134,135] . These benefits include
enabling high-resolution applications and maintaining stable deformation in environments with dynamic
and multiple deformations. To fully leverage these benefits, it is desirable for the light-emitting parts formed
on the substrate to also possess stretchability. Proposed methods include forming wrinkle structures on the
elastomer surface to impart structural stretchability to the light-emitting parts [136,137] and developing new
materials to construct light-emitting parts with inherent stretchability [138,139] . In summary, the ideal
combination of light-emitting components and strain-engineered substrates is chosen based on the specific
demands of free-form displays, with Table 1 presenting the properties of each stretchable substrate and key
considerations for their application in free-form displays.

Display applications on structured plastic substrates
As previously mentioned, structured plastic substrates have been widely studied in early stretchable display
research due to their structural simplicity and high manufacturing efficiency. This approach allowed
companies to maintain existing display processes while avoiding complex additional steps, leading to
extensive adoption and the presentation of prototype products at numerous exhibitions. For instance, Jeong
et al. employed a plastic composite substrate with precisely controlled short wrinkle wavelengths to mitigate
pixel distortion caused by traditional wrinkle structures used for stretchability in flexible substrates and to
reduce spatial loss in displays . In this study, a thin PI-based plastic composite substrate was utilized to
[140]
fabricate a thin-film LED (TFLED). Compared to conventional macroscopic wrinkle structures, the short-
wavelength wrinkle structure with a 20 µm wavelength significantly reduced visible image distortion,
demonstrating excellent visual quality and potential for application in high-resolution free-form displays
and wearable devices [Figure 16A]. Kim et al. developed a wearable QD-LED for stretchable optoelectronic
applications by incorporating a wrinkle structure on a polyethylene naphthalate (PEN) substrate, which
[141]
offers superior thermal stability compared to PET . After forming the device on the PEN substrate, it was
transferred to a pre-stretched elastomer substrate, creating a wrinkle structure that allowed the device to
operate stably under stretch conditions of up to 70% [Figure 16B]. Furthermore, the elastomer beneath the
plastic layer enabled easy attachment to the skin, demonstrating potential for use in various wearable
electronic applications. Additionally, Yin et al. developed an energy self-powered stretchable display system
by transferring a stretchable OLED and a stretchable polymer solar cell onto a plastic substrate, then
[142]
forming a periodic buckling structure . Using a stencil-based deposition method, they created alternating
adhesive and non-adhesive regions on the substrate, attaching devices to a pre-stretched substrate and
releasing it to achieve the buckled display system. This configuration achieved a maximum stretchability of
100% and consistently maintained performance even after more than 20,000 cycles of stretching and
recovery [Figure 16C].

Kirigami-structured flexible plastic substrates are widely used in stretchable display applications,
particularly for controlling in-plane deformations such as Poisson’s ratio. Jang et al. introduced research
that combines an auxetic meta-structure with a Kirigami pattern to improve image distortion in stretchable
[143]
displays . The auxetic metamaterial, which has a negative Poisson’s ratio, expands laterally when stretched
axially, allowing for stable display operation even under 24.5% stretch conditions. The designed Poisson’s
ratio of -1 helps maintain the original image after stretching, and this design allows for uniform
deformation when attached to curved surfaces [Figure 17A], making it suitable for skin-attached
phototherapy devices or displays that can be affixed to non-spherical surfaces. Deng et al. implemented a
display device using a PEN substrate with a rotating square Kirigami pattern, designed for applications in
curved tensile environments . The integration of the driving flexible printed circuit board (FPCB) and
[144]
LED elements on the PEN substrate provided up to 57% bidirectional stretchability, and the device
demonstrated stable operation even on complex 3D curved surfaces such as cylindrical, spherical, and