Interfacing stretchable circuits for deformable devices and displays

Electronic devices are changing. Flexible devices such as foldable mobile phones, rollable electronic watches with wraparound displays, and extendable displays that widen their screens have entered our lives. Can a display that folds like paper to put away in our pockets really become a reality? For such deformable devices, their parts also need to be flexible. But the core technology for an interface material that connects various parts is still not secured. To this, a research team at POSTECH has recently developed a deformable conductive film that connects flexible electronic devices.

A research team led by Professor Unyong Jeong and Ph.D. candidates Hyejin Hwang and Minsik Kong of Department of Materials Science and Engineering in collaboration with Professor Ho-Jin Song of Department of Electrical Engineering and Professor Soojin Park of Department of Chemistry at POSTECH have together developed a stretchable anisotropic conductive film (S-ACF) that can connect other electrodes physically and electrically regardless of the rigidity, flexibility or elasticity of the circuit line. The findings from this study were published in Science Advances, an authoritative international journal.

For stretchable devices such as stretchable displays, electronic skin, and implantable devices, it is essential to make a deformable circuit board. Circuit boards that can be formed into different shapes require high extensibility of many materials and components like wirings, displays, sensors, as well as rechargeable energy supply devices such as batteries. Methods for connecting high-resolution circuits so far include soldering, wire bonding, anisotropic conductive film, and flip-chip bonding, but there remains the issue of stably maintaining the physical and electrical properties even when their shape is altered.

To this, the research team produced a S-ACF by arranging metal particles at regular intervals in SEBS-g-MA, an extensible block copolymer, which maintains a strong interfacial adhesion while securing stable electrical connection even when its shape is changed via chemical bonding with the substrates.

In particular, maleic anhydride present in SEBS-g-MA enables chemical bonding between substrates, creating strong adhesion at low temperatures. The researchers verified that the electrical and physical connection was effectively formed when the S-ACF was placed at the contact interface between the two substrates with mild temperature (80°C) treatment for about 10 minutes.

In addition, S-ACF can be selectively patterned so that particles are arranged in a desired part, which increases the polymer contact surface in an area that does not require electrical connection to increase bonding strength, and is economical by reducing the use of metal particles. The film produced in this way adds stretchability to the conventional anisotropic conductive films and enables high-resolution circuits connection (50μm), low-temperature processing, and production scalability.

“This film enables connecting devices with more complex structures in the future,” explained Professor Unyong Jung who led the study. He added, “I hope that it will serve as a launchpad for integrating and manufacturing stretchable devices – which have been independently studied – into one substrate and integrated system.”

If S-ACFs are produced in the form of tapes in the future, wouldn't it be possible for anyone to connect stretchable high-resolution circuits with a little piece of tape?

The basic research on the arrangement of conductive microparticles began with the long-term support from the Samsung Future Technology Development Project from 2014 to 2018 which enabled the launch of the start-up and the subsequent technology transfer. Through additional research and development for commercialization, the project has been newly selected as a Materials & Components Technology Development Program by the Korea Evaluation Institute of Industrial Technology (KEIT), and is awaiting domestic production of high-precision anisotropic conductive films. Professor Jeong commented, “We anticipate this research to be a prime example of a bold investment in basic research leading to commercialization.”

This research was conducted with the support from the Nano and Material Technology Development Program funded by the National Research Foundation of Korea.