Based on silver nanoparticles, printable conductors retain conductivity even when stretched to the extreme

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 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

Figure 1. 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.