Tokyo: University of Tokyo researchers have developed a new ink that can be printed on textiles in a single step to form highly conductive and stretchable connections.

This functional ink will enable electronic apparel such as sportswear and underwear incorporating sensing devices for measuring a range of biological indicators such as heart rate and muscle contraction.

Using this ink, the 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.

This sensor can measure muscle activity by detecting muscle electrical potentials over an area of 4x4 square cms with nine electrodes placed two centimetres apart in a 3x3 grid.

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

‘The biggest challenge was obtaining high conductivity and stretchability with a simple one-step printing process.’

Current printed electronics, such as transistors, light emitting diodes and solar panels can be printed on plastic or paper substrates but these substrates tend to be rigid or hard.

However, it has proved difficult to make an ink that is both highly conductive and elastic without a complicated multi-step printing process.

The new ink is comprised of silver flakes, organic solvent, fluorine rubber and fluorine surfactant and exhibits high conductivity even when it is stretched to more than three times its original length.

‘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,’ Someya said.