A Japanese research group developed a new conductive ink that enables to form a fine circuit pattern on a cloth by one-time printing.
Even when the cloth is stretched to expand its length by three times, the conductivity of the circuit pattern is still high. If this technology is commercialized, it enables to use a stretchable cloth as a printed-circuit board (PCB).
In the future, it might become possible to print a myoelectric sensor on sportswear for training purposes as well as sensors that can collect biological information such as pulse and brain waves for medical and welfare purposes.
The group is led by Takao Someya, professor at the School of Engineering, the University of Tokyo.
As materials for forming electric circuits and electrodes on cloth, conductive thread and cloth have been proposed so far. Conductive thread can be used as circuits by forming linear patterns with a sewing machine. Conductive cloth is proposed to be used as electrodes by cutting and attaching it to clothes. However, with those materials and processes, it is difficult to form fine circuit patterns.
On the other hand, with a conductive ink, it is possible to form a circuit pattern with a line width of about 100μm at a time by conventional printing methods such as screen printing. When a cloth having a circuit pattern printed with the conductive ink was stretched by 3.15 times, its conductivity was 182S/cm.
The skin of human joints stretches by more than two times. Therefore, the group aimed to develop a material whose properties do not change even when stretched by three times. The conductivity of the material that Someya developed in 2009 was as low as 57S/cm, which lowered to 6S/cm when the material was stretched by 2.34 times.
To realize a conductive ink whose conductivity does not lower even when it is stretched, the research group used a fluorine-based surfactant in addition to silver (Ag) flakes (conductive material) and fluorine-based rubber (solvent).
"Silver is precipitated on the surface of the rubber, forming a 'conductive network' and making it easy to secure a path for electric conduction," Someya said.