The Poisson’s ratio of most natural materials typically range between 0 – 0.5. This means that the stretching of such materials usually induces compressive deformation perpendicular to the stretching direction. Electronic devices in stretchable electronics are generally fabricated on two-dimensional surfaces of the base material, allowing the devices to deform in the direction of the stretching deformation of the base material. Such deformation often distorts the initial shape of the stretchable devices.
The Poisson’s effect-induced shape distortion is a critical issue in stretchable displays as the distortion of the displayed images could significantly deteriorate customer experience. The display can be stretched for base materials with a Poisson’s ratio of -1 while maintaining the original shape even under uniaxial deformation. This unique stretching behavior is attributed to the expansion of the display in the same strain in a direction perpendicular to the stretching direction.
Mechanical metamaterials exhibit unique mechanical properties absent in nature and are ideal candidates for solving image distortion in stretchable displays by achieving a Poisson’s ratio of -1. The negative Poisson’s ratio can be tuned further by designing the cell structures. In particular, auxetic metamaterials are suitable for achieving meta-displays as they exhibit a negative Poisson’s ratio and tend to expand in the direction perpendicular to the applied force. Kirigami technology, compatible with conventional display fabrication processes, is commonly used to implement stretchability when fabricating auxetic metamaterials. However, contributing to this direction requires addressing critical obstacles to stretchable display implementation, including high deformation uniformity, large stretchability, and low image distortion.
Herein, Dr. Bongkyun Jang, Dr. Sejeong Won, Dr. Jaegu Kim, Dr. Juho Kim, Dr. Minsub Oh, Dr. Hak-Joo Lee, and Dr. Jae-Hyun Kim from the Korea Institute of Machinery and Materials (KIMM) implemented distortion-free and stretchable meta-display based on auxetic metamaterials. In their approach, the meta-display was constructed using micro-light-emitting diodes (LEDs) on a polyimide film. The micro-LEDs were transferred onto a kirigami-based auxetic meta circuit board designed based on finite element analysis. Finally, a scalable method for manufacturing the meta-display was proposed utilizing the roll-based transfer technology, an original KIMM technology. Their work is currently published in the journal Advanced Functional Materials.
The research team reported an auxetic meta-display with a Poisson’s ratio of -1 and stretchability of 24.5% without image distortion, even under uniaxial stretching. Compared with conventional displays based on organic LEDs and liquid crystals, micro-LED displays have better response time, contrast, and energy efficiency and do not need severe passivation. Additionally, micro-LED displays exhibit improved durability and long-term stability desirable for fabricating highly stretchable displays. Indeed, the biaxial and uniaxial tensile tests of the meta-displays confirmed that the distortion-free characteristic uniform extension emanated from the auxetic metamaterial.
The meta-display can be attached to surfaces with non-zero Gaussian curvatures, such as spherical or arbitrary 3D surfaces. This functionality outperformed the conventional displays currently available in terms of stretchability, image distortion, and durability. The applications of the meta-display for image distortion-free stretchable information displays and skin-attachable phototherapy devices were demonstrated. The biomedical applications were mainly attributed to conforming to the non-zero Gaussian curvatures.
In summary, the authors successfully realized a meta-display on a mechanical metamaterial. A combination of the micro-LEDs and kirigami-based auxetic metamaterials effectively addressed the critical obstacles comprising the implementation of stretchable displays. In a joint statement to Advances in Engineering, the authors stated that the newly developed design and fabrication technology could allow tunning metamaterials’ unique mechanical properties to broaden their applications’ scope.
Jang, B., Won, S., Kim, J., Kim, J., Oh, M., Lee, H., & Kim, J. (2022). Auxetic Meta‐Display: Stretchable Display without Image Distortion. Advanced Functional Materials, 32(22), 2113299.