A researcher group, led by Takao Someya, Professor at the School of Engineering at the University of Tokyo, have developed a new conductive ink that enables the patterning of fine line circuits and lines on textiles using a simple one-time printing process.
The researchers said that what sets this conductive ink apart from others is that when the textile or cloth the ink has been printed on is stretched to three times it original length the conductivity of the printed circuit remains high.
It is known that conductive threads can be used to form electric circuits, and attached using a sewing machine. However, even with good materials and processes, it is difficult to form fine circuit patterns.
Figure: University of Tokyo - Printed circuit on cloth and a conductive ink
However, with the newly developed conductive ink, it is now possible to print circuits with a line width of about 100μm at a time using conventional printing methods such as screen printing. So when the printed cloth was stretched by 3.15 times, its conductivity was 182 S/cm.
For comparison human skin around joints stretches by more than two times. Therefore, the group set a research objective to develop a material whose properties do not change even when stretched by three times.
To produce a conductive ink whose conductivity does not fall even when it is stretched, the research group used a fluorine-based surfactant in addition to silver (Ag) flakes (a conductive material) and fluorine-based rubber (solvent).
If this technology was to be commercialised it could be a key enabler for stretchable electronics on cloth.
Figure: University of Tokyo - touch sensor using a circuit printed on a cloth
In the future this could make it possible to print a myoelectric sensor on sportswear for training purposes as well as sensors that monitor biological information such as pulse rates, and brain waves for medical and health purposes.
Printable elastic conductors with a high conductivity for electronic textile applications
Naoji Matsuhisa | Martin Kaltenbrunner | Tomoyuki Yokota | Hiroaki Jinno | Kazunori Kuribara | Tsuyoshi Sekitani | Takao Someya
Nature Communications 6 | Article number: 7461 | doi:10.1038/ncomms8461
Received 21 November 2014 | Accepted 11 May 2015 | Published 25 June 2015
Abstract
The development of advanced flexible large-area electronics such as flexible displays and sensors will thrive on engineered functional ink formulations for printed electronics where the spontaneous arrangement of molecules aids the printing processes. Here we report a printable elastic conductor with a high initial conductivity of 738 S cm−1 and a record high conductivity of 182 S cm−1 when stretched to 215% strain. The elastic conductor ink is comprised of Ag flakes, a fluorine rubber and a fluorine surfactant. The fluorine surfactant constitutes a key component which directs the formation of surface-localized conductive networks in the printed elastic conductor, leading to a high conductivity and stretchability. We demonstrate the feasibility of our inks by fabricating a stretchable organic transistor active matrix on a rubbery stretchability-gradient substrate with unimpaired functionality when stretched to 110%, and a wearable electromyogram sensor printed onto a textile garment.
