With this advance, the researchers claim that they have taken a step toward the design of functional textiles - clothes that gather, store, or transmit digital information. With further development, the technology could lead to clothes that act as antennas for your smart phone or tablet, monitor your fitness level or athletes' performance, bandages that tell doctors how well the tissue beneath it is healing or a flexible fabric cap that senses activity in the brain.
"A revolution is happening in the textile industry," said John Volakis, director of the ElectroScience Laboratory at Ohio State. "We believe that functional textiles are an enabling technology for communications and sensing, and one day even medical applications like imaging and health monitoring."
Recently, Volakis and research scientist Asimina Kiourti refined their fabrication method to create prototype wearables at a fraction of the cost and in half the time as they could previously.
In Volakis' lab, the functional textiles, also called ‘e-textiles’, are created in part on a typical tabletop sewing machine. The sewing machine embroiders the e-textile into fabric automatically based on a pattern loaded via a computer file.
According to the researchers, the shape of the embroidery determines the frequency of operation of the antenna or circuit. The shape of one broadband antenna, for instance, consists of more than half a dozen interlocking geometric shapes, each a little bigger than a fingernail that form an intricate circle a few inches across. Each piece of the circle transmits energy at a different frequency, so that they cover a broad spectrum of energies when working together.
"Shape determines function," Kiourti said. "And you never really know what shape you will need from one application to the next. So we wanted to have a technology that could embroider any shape for any application."
The researchers' initial goal was to increase the precision of the embroidery as much as possible, which necessitated working with fine silver wire. But that created a problem, in that fine wires couldn't provide as much surface conductivity as thick wires. So they had to find a way to work the fine thread into embroidery densities and shapes that would boost the surface conductivity and, thus, the antenna/sensor performance.
Previously, they used silver-coated polymer thread with a 0.5mm diameter, each thread made up of 600 finer filaments twisted together. The new threads have a 0.1mm diameter, made with seven filaments made from copper enamelled with pure silver.
Kiourti estimated that embroidering a single broadband antenna like the one mentioned above has a material cost of around 30 cents per antenna, 24 times less expensive than when Volakis and Kiourti created similar antennas in 2014.
In part, the cost savings comes from using less thread per embroidery. The researchers previously had to stack the thicker thread in two layers, one on top of the other, to make the antenna carry a strong enough electrical signal. But by refining the technique that she and Volakis developed, Kiourti was able to create the new, high-precision antennas in one embroidered layer of the finer thread.
One prototype antenna looks like a spiral and can be embroidered into clothing to improve cell phone signal reception. Another prototype, a stretchable antenna with an integrated RFID chip embedded in rubber, takes the applications for the technology beyond clothing into tyre monitoring.
Another circuit includes non-conductive scarlet and gray thread embroidered among the silver wires "to demonstrate that e-textiles can be both decorative and functional," Kiourti said.
Tests showed that an embroidered spiral antenna measuring approximately six inches across transmitted signals at frequencies of 1 to 5GHz with near-perfect efficiency.
