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Stretchy, printable conductor could have applications in robotics and smart clothing

Image credit: Someya Group, The University of Tokyo

A printable conductor which retains high conductivity even when stretched to five times its original length has been developed by researchers at the University of Tokyo. The material could be printed on textiles and rubber surfaces to create wearable devices or give robots more human-like skin.

Wearable devices, such as those monitoring a person’s fitness, are becoming increasingly popular among health-conscious consumers, while robots expand into healthcare, retail and other sectors. Given these technological demands, it will become necessary to develop more flexible, robust electronics capable of withstanding stretching.

“We saw the growing demand for wearable devices and robots,” says Professor Takao Someya, who supervised the study. “We felt it was very important to create printable elastic conductors to help meet the need and realise the development of the products.”

The researchers, based at the University of Tokyo’s school of engineering, combined four components to create their new elastic conductor. They produced a conductor in a paste-like ink form, consisting of rubber, a surfactant (which reduces surface tension in liquids), a solvent and microscopic silver flakes.

This ink-like conductor could be printed using cheap, standard methods such as stencil or screen printing which cover large surface areas.

While relaxed, the printed conductor recorded a high conductivity of nearly 5,000 siemens per centimetre. When stretched to two, three, four and even five times its original length, the conductor retained a high conductivity of 900-1,000 siemens per centimetre. This is the highest level ever recorded for this amount of stretching.

Stretchable wiring

Someya Group, The University of Tokyo

Image credit: Someya Group, The University of Tokyo

Observing the material with a scanning electron microscope and transmission electron microscope, the researchers were surprised to find that the high conductivity was thanks to the formation of tiny silver nanoparticles, which dispersed uniformly in the rubber after printing and heating.

By adjusting the qualities of the rubber, quantity of surfactant and speed of heating, the researchers found that they could control the distribution and population of the silver nanoparticles.

To test whether their new conductor could work in practice, the Japanese team fabricated stretchable pressure and temperature sensors and wired them up with the elastic conductors printed on a fabric.

Even when stretched to several times their original length, the system was capable of taking precise measurements.

The researchers suggest that the durable material could be useful around the joints of robotic arms, where they could be printed on to skin-like material to provide sensory information. The conductor could also be used in sportswear, even proving durable and effective in high-stress, flexible areas such as the elbows and knees.

The research team reported its findings in the journal Nature Materials.

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