Stretchable conductors for wearable electronics

By Gary Elinoff, contributing writer

Wearable electronics, whether in sensors attached to the human body or embedded within clothing, will have to be able to stretch and extend along with us. This will require electrical conduits that will be able to flawlessly conduct power and signals. A new material has been developed at the University of Tokyo to meet this need. In the form of a pasty ink, it can be imprinted on textiles and rubbers and is still functional when stretched up to five times its original length.

The raw materials for the ink are micron-sized flakes of silver, fluorine rubber, fluorine surfactant, and an organic solvent. With resistivity defined as the resistance over a conductor with a uniform cross-section, conductivity, its reciprocal, is measured in siemens. A trace of the new material, after being first printed and then heated onto a stretchable base, was demonstrated to exhibit a conductivity of 4,972 siemens per centimeter (S/cm). When stretched to first twice and then to five times its original length, the conductivity declines only to 1,070 S/cm and 935 S/cm, respectively.

Nanoparticles are the key

On examination, the scientists discovered that the silver flakes had reformed themselves into nanoparticles about one-thousandth of their original size. The investigators also discovered that altering the composition of the other components could vary the size and distribution of the nanoparticles.

The chemistry of why nanoparticles of silver can be made to produce the effects described is complex and is being actively investigated. They also have the interesting effect of being antimicrobial, possibly by piecing the biofilms that bedevil conventional antibiotics, as described in Microbepost. This effect, obviously, means that silver nanoparticles can be medically significant and are not to be taken lightly.

The fact that the silver flakes transition into silver nanoparticles means that much less of the metal is needed to conduct electrical signals. But silver is expensive in any amount, and the Tokyo team is actively exploring alternatives to the metal.

Stretchable connectors at work

To demonstrate practicality, the scientists installed pressure and temperature sensors onto textile garments. The new stretchable conductors were employed within the transducers. In the experiment, the sensors worked as designed even when they were stretched to 250% of their original size.

Stretchable connectors at work. Source: Someya Group, University of Tokyo.

This high rate of performance means that the stretchable connectors are suited for the new wave of “smart clothing” that will be of great interest in the coming years for athletes and for others. They will also be important factors in the development of robotic appendages, which will need to bend and stretch to a greater extreme than any human knee or wrist.