Printing Various Electrodes Freely with Liquid Metal

KAIST Research Team

[Asia Economy Reporter Kim Bong-su] KAIST announced on the 25th that a joint research team led by Steve Park from the Department of Materials Science and Engineering and Jae-woong Jeong from the Department of Electrical Engineering has developed a technology capable of printing stable forms of liquid metal at high resolution.

Liquid metal has been widely applied to flexible and stretchable electronic devices due to its high electrical conductivity and deformability similar to liquids. However, its instability in the liquid state and high surface tension have limited its use in electrodes requiring direct contact or wiring in electronic devices that demand high resolution.

To overcome this, research has been conducted to apply liquid metal in a stable form by pulverizing it into particles sized 6?10 μm (micrometers). However, this approach has the drawback of losing the originally high electrical conductivity due to oxidation occurring on the surface. To use these liquid metal particles in electronic devices, a process is required to remove the oxide layer on the surface through mechanical and chemical modifications to restore electrical conductivity.

To solve this problem, the research team developed a system that induces chemical modification by changing the composition of the suspension through evaporation promoted at the meniscus induced between the nozzle and the substrate during the printing process. First, the suspension used for printing was made increasingly acidic by evaporating a mixture of water and a weak acid (acetic acid) with a higher boiling point than water. Additionally, the team applied heat of about 60°C to the substrate to promote evaporation of the ink, acid activation, and chemical modification. Through this, they confirmed that the printed liquid metal particle wiring exhibited high electrical conductivity (1.5x10^6 S/m), comparable to metal, without any separate electrical activation process.

The research team enhanced mechanical and chemical stability by attaching electrolytes to the surface of the liquid metal particles, preventing clogging during the printing process and providing stretchability through bridging between the liquid metal particles. The printed liquid metal particle-based wiring showed little change in resistance even when stretched up to approximately 500%, indicating its potential use in various stretchable devices.

Since it can be rapidly deposited in various forms on different substrates through printing, it can be applied to various customized devices. In particular, the mechanical and chemical stability of the printed liquid metal particles demonstrated the possibility of using them as electrodes, which was previously impossible with conventional liquid metals.

Moreover, the electrolyte-attached liquid metal exhibits excellent biocompatibility, allowing it to be used as bioelectrodes that can come into direct contact with the skin. The research team deposited liquid metal on commercially available medical tape to fabricate an optimized EMG sensor (a sensor that detects subtle electrical signals caused by muscle movements) tailored to the user's body. Furthermore, they suggested the possibility of applying liquid metal electrodes deposited on biodegradable substrates as ECG sensors (electrocardiogram sensors) that do not generate medical waste after use.

The results of this study were published online on the 12th in the international academic journal Nature Communications.

Professor Steve Park said, "This is a meaningful result showing new application possibilities of liquid metal particle-based suspensions, which have recently attracted attention," adding, "It is expected to be utilized as a core technology in various flexible and stretchable electronic devices, including wearable and implantable monitoring electronics for healthcare."