Rubber-like electronics: Making electronic circuits with sound

Sound connects liquid metal droplets in a rubber matrix and makes rubber-like electronic circuits. These electronics are highly desirable for applications such as stretchable displays, health monitoring, medical implants, soft robotics, and augmented reality.

Article | Spring 2023

As portable devices have become popular, consumers have demanded more ease of use, comfort, and free-form factors. Electronic devices are required to be bendable and/or even stretchable. It is believed such a change would make them fit more easily in pockets—and they might also be wearable and attachable to human skin.

However, making electronics stretchable is difficult because the circuits inside also need to bend or stretch without losing their electronic functions. This has been a long-standing challenge because stretchable materials are generally not very good at conducting electricity, and their electronic pathways are easily broken while being stretched. One possible solution is liquid metal droplets, but they have problems as well. The droplets typically react with oxygen and form the native oxide skin, which is not conductive. Therefore, liquid metal droplets are not able to create electronic pathways inside a polymer. To address these problems, a research team under Professor Jiheong Kang in the Department of Materials Science and Engineering at KAIST invented a new way of using sound to connect liquid metal droplets inside a rubber matrix and create rubber-like electronic circuits. Liquid metal droplet circuits created by the team’s proposed method show high conductivity, high stretchability, and mechanical robustness. More importantly, electronic pathways in their circuits are strain-insensitive, meaning that electricity can be efficiently conducted under various deformations such as stretching, bending, and twisting.

 

The team printed liquid metal droplets dispersed on a polymer matrix on a rubber substrate to create stretchable electronic devices, while ultra-sound was applied. The ultra-sound led to the formation of mini-liquid metal droplets (10~100 nm), bridging the large liquid metal droplets (2~3 µm). This resulted in the formation of a conductive network—and that allowed for the construction of an electronic circuit. The research team has implemented a stretchable display and skin electronics using their liquid metal droplet circuits, in which numerous functional electronic components (sensors, transistors, diodes, LED, antenna, IC chips) are intimately integrated. The created material enables the rapid assembly of stretchable electronic devices with high stretchability and high functionalities. This technology provides a conductive platform on which to readily incorporate various electronic components with skin-like and even beyond-skin softness and deformability.

This research was published on November 11, 2022 in Science under the title “Universal assembly of liquid metal particles in polymers enables elastic printed circuit board” (Science 378, 637-641 (2022), Cover)