AsianScientist (May 17, 2017) – Researchers from Japan have developed a printable elastic conductor that can retain high levels of conductivity even when stretched to as much as five times its original length. These results have been published in Nature Materials.
Wearable devices like health monitors require elastic conductive materials that can withstand high strains from repeated stretching. The new material, produced in paste-like ink form, can be printed in various patterns on textiles and rubber surfaces as stretchable wiring for wearable devices incorporating sensors, as well as give human skin-like functions to robot exteriors.
“We saw the growing demand for wearable devices and robots,” said University of Tokyo’s Professor Takao Someya, who supervised the current study. “We felt it was very important to create printable elastic conductors to help meet the need and realize the development of the products.”
To achieve a high degree of stretchability and conductivity, the researchers mixed four components to create their elastic conductor. They found that their conductive paste consisting of micrometer-sized silver flakes, fluorine rubber, fluorine surfactant—a substance that reduces surface tension in liquid—and organic solvent to dissolve the fluorine rubber markedly outperformed the elastic conductor they had previously developed in 2015.
Without stretching, printed traces of the new conductor recorded a high conductivity of 4,972 siemens per centimeter (S/cm). When stretched by 200 percent, or to three times its original length, the conductivity was 1,070 S/cm, which is nearly six times the value of the previous conductor (192 S/cm). Even when stretched by 400 percent, or to five times its original length, the new conductor retained a high conductivity of 935 S/cm, the highest level recorded for this amount of stretching.
Magnification by a scanning electron microscope and transmission electron microscope showed that the high performance of the conductor was due to the self-formation of silver nanoparticles—one-thousandth the size of the silver flakes and dispersed uniformly between the flakes in the fluorine rubber—after the conductive composite paste was printed and heated.
“We did not expect the formation of silver nanoparticles,” said Someya of their surprising discovery.
Furthermore, the scientists found that by adjusting variables like the molecular weight of the fluorine rubber, they could control the distribution and population of nanoparticles, while the presence of surfactant and heating accelerated their formation and influenced their size.
To demonstrate the feasibility of the conductors, the scientists fabricated stretchable pressure and temperature sensors wired with the printable elastic conductors on textiles. The sensors, which can be easily installed by laminating surfaces using heat and pressure, took precise measurements even when stretched by 250 percent. This is enough to accommodate high-stress flexible areas such as elbows and knees on conformable, form-fitting sportswear or joints on robotic arms often designed to surpass human capabilities and thus undergo higher strain.
The new material, which is durable and suitable for high-capacity printing methods like stencil or screen printing that can cover large surface areas, points to easy installation, and its properties of forming silver nanoparticles (which are a fraction of the cost of silver flakes) when printed provide an economical alternative for realizing a wide range of applications for wearables, robotics and deformable electronic devices.
The team is now exploring substitutes for silver flakes to further reduce costs, while also looking at other polymers like nonfluorine rubbers and various combinations of materials and processes to fabricate elastic conductors with similar high performance.
The article can be found at: Matsuhisa et al. (2017) Printable Elastic Conductors by in situ Formation of Silver Nanoparticles from Silver Flakes.
Source: University of Tokyo.
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