Yokohama, Japan -- As tiny as bubbles may seem, in engineers' hands they can spark big innovations.
Yokohama National University scientists have developed a promising bubble printing method that enables high-precision patterning of liquid metal wiring for flexible electronics. This technique offers new options for creating bendable, stretchable, and highly conductive circuits, ideal for devices such as wearable sensors and medical implants.
Their study was published in Nanomaterials on Oct. 17.
Wiring technology is part of our daily lives. This technology creates pathways that connect electronic components, carrying signals and power throughout a device. Traditional wiring — made of physical wires and circuit boards — powers most electronics, from phones to computers. With growing demand for wearable electronic devices, however, traditional wiring is revealing inadequacies.
"Conventional wiring technologies rely on rigid conductive materials, which are unsuitable for flexible electronics that need to bend and stretch," said Shoji Maruo, a professor at the Faculty of Engineering of Yokohama National University and corresponding author of the study.
Alternatives to such rigid materials, like liquid metals, show promise, but using them comes with certain challenges.
"Liquid metals provide both flexibility and high conductivity, yet they present issues in wiring size, patterning freedom, and electrical resistance of its oxide layer," said Masaru Mukai, an assistant professor at the Faculty of Engineering and the study's first author.
The research team addressed these limitations by adapting a bubble printing method — traditionally used for solid particles — to pattern liquid metal colloidal particles of eutectic gallium-indium alloy (EGaIn). Bubble printing is an advanced technique for creating precise wiring patterns directly onto surfaces, especially on non-traditional or flexible substrates, using particles that are moved by the flow generated by bubbles.
The team employed a femtosecond laser beam to heat the EGaIn particles, generating microbubbles that guide them into exact lines on a flexible-glass surface.
"The key is to improve conductivity by replacing the resistive gallium oxide layer with conductive silver via galvanic replacement," Maruo said.
The resulting wiring lines were not only incredibly thin and conductive, but also highly flexible.
"Our liquid metal wiring, with a minimum line width of 3.4 μm, demonstrated a high conductivity of 1.5 × 105 S/m and maintained stable conductivity even when bent, highlighting its potential for flexible electronic applications," Mukai said.
By achieving reliable, ultra-thin liquid metal wiring, this method opens up possibilities for creating soft electronics in wearable technology and healthcare applications, where both flexibility and precise functionality are essential.
The team aims to further enhance the flexibility and elasticity of their liquid metal wiring by incorporating even more adaptable substrates.
"Our ultimate goal is to integrate this method with electronic components, such as organic devices, enabling practical, flexible devices for everyday use," Maruo said. "We see potential applications in areas like wearable sensors, medical devices, and other technologies that require flexible, durable wiring."
Tatsuya Kobayashi, Mitsuki Sato, Juri Asada, Kazuhide Ueno and Taichi Furukawa at Yokohama National University contributed to this research. JST CREST JPMJCR1905 helped support this research.
