Research Team Develops Highly-Reliable Liquid Metal-Solid Metal Contacts for Stretchable Electronics

Stretchable Electronics
▲ Professor Yongtaek Hong of SNU Department of Electrical and Computer Engineering (Left), Researcher Eunho Oh

Korean researchers presented their new research that would advance the commercialization of soft electronic platform, a core technology of future wearable devices.

SNU College of Engineering (Dean Kook Heon Char) announced on 3rd that the research team led by Professor Yongtaek Hong of the Department of Electrical and Computer Engineering (Researcher Eunho Oh) has developed highly reliable liquid metal-solid metal contacts with corrugated carbon nanotube diffusion barrier for stretchable electronics.

With this development, the junction between semiconductor chip and stretchable electrode function stably not only in horizontal direction but also in vertical direction, which once was a tantalizing problem for the stretchable electronics field. This is a “green light” for the development of stretchable circuits and wearable devices.

Liquid metals like mercury and gallium are widely-used for stretchable circuits for they remain in liquid state at room temperature whilst possessing high electrical conductivity. However, most liquid metals faced limitation for their tendency to be easily transformed and permeated into their crystal structures when combined with other metals.

Hence, Hong’s team inserted a corrugated carbon nanotube diffusion barrier between the liquid metal and solid metal to prevent the spread of liquid metal while allowing electrons to pass through. They first printed silver films onto stretchable silicon rubber board at stretched state and then covered the top with 100 nanometer-thick layer of carbon nanotubes to return the board to its relaxed state. As a result, the silver and carbon nanotube thin film formed repeating wrinkles to allow the film to be stretchable like spring. Even when liquid metal is placed upon this “wrinkled” carbon nanotube, the silver thin film below prevents liquid metal from permeating.

In addition, the team presented a new method to attach surface-mount devices (SMD) onto stretchable circuits using corrugated diffusion barrier and liquid metal. With the conventional method, there was a technical limitation where the junction between soft stretchable circuit and hard SMD becomes damaged under stress due to vertical movement. The team responded to this issue by introducing “floating” where the junction of the SMD floats on the liquid metal to electrically bond with the soft board without stress.

Professor Hong explained, “Such developed circuits not only function stably when stretched, but also demonstrate outstanding performance without damage even after being repetitively pushed against the SMD. This technology can be applied to wearable displays and mobile devices which require user-interaction like pressing buttons.”

The research findings were published online on the Advanced Functional Materials on December 19th and the research was selected as the inside cover image of this 28th issue. This research was conducted with the support of the Institute of Information and Communications, Technology Planning and Evaluation.

▲ (From Left to Right) Stable Liquid Metal-Solid Metal Contact with Carbon Nanotube Diffusion Barrier / Display Circuit Integrating Liquid Metal Floating Which Uses the Carbon Nanotube Diffusion Barrier