Conductive ink on wrist-band measures muscle activity

An ink devised by Japanese researchers can be printed on textiles to form conductive and stretchable connections that measure body activity.

The functional ink can be used for sportswear and underwear fitted with sensing devices, according to researchers from the University of Tokyo, to measure heart rate or muscle contraction. The use of soft, stretchable material could pave the way for wearable devices that fit themselves to the human body.

Currently, printed electronics like transistors, light emitted diodes and collar panels can be printed on plastic or paper, but the substrates tend to be rigid or hard. Researchers designed an ink that is conductive and elastic, eliminating the multi-step printing process.

“Our team aims to develop comfortable wearable devices. This ink was developed as part of this endeavour,” said Takao Someya, the lead researcher.

The ink is made of silver flakes, organic solvent, fluorine rubber and fluorine surfactant, and preserves its conductivity even when it is stretched to more than three times its original length – the highest value reported for elastic conductors was two and a half times the length.

Using the ink the group of researchers created a wrist-band muscle-activity sensor by printing an elastic conductor on a sportswear material and combining it with an organic transistor amplifier circuit. The sensor can measure muscle activity by detecting muscle electrical potentials over an area of 4x4 square cm with nine electrodes placed 2 cm apart in a 3x3 grid.  

“The biggest challenge was obtaining high conductivity and elasticity with a simple one-step printing process,” Someya said. “We were able to achieve this by use of a surfactant that allowed the silver flakes to self-assemble at the surface of the printed pattern, ensuring high conductivity.”

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