Ohio State University (OSU) researchers working to develop wearable electronics are now able to embroider circuits into fabric with 0.1 mm precision—the optimal size to integrate electronic components such as sensors and computer memory devices into clothing.

With further development, the technology could lead to shirts that act as antennas for a smartphone or tablet, workout clothes that monitor fitness level or a bandage that tells your doctor how well the tissue beneath it is healing.

John Volakis, director of OSU's ElectroScience Laboratory, and fellow researchers created the functional textiles, also called “e-textiles,” in part on a typical tabletop sewing machine. Like other modern sewing machines, it embroiders thread into fabric automatically based on a pattern loaded via a computer file. The researchers substituted the thread with fine silver metal wires that, once embroidered, feel the same as traditional thread to the touch.

OSU researchers are developing embroidered antennas and circuits with 0.1 mm precision. Image credit: OSU/Jo McCulty.“We started with a technology that is very well known—machine embroidery—and we asked, how can we functionalize embroidered shapes? How do we make them transmit signals at useful frequencies, like for cell phones or health sensors?” Volakis says. “Now, for the first time, we’ve achieved the accuracy of printed metal circuit boards, so our new goal is to take advantage of the precision to incorporate receivers and other electronic components.”

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.

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, the researchers had used silver-coated polymer thread with a 0.5-mm diameter, each thread made up of 600 even-finer filaments twisted together. The new threads have a 0.1-mm diameter, made with only seven filaments. Each filament is copper at the center, enameled with pure silver.

In previous experimentation, the researchers 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 their technique, they are now able to create the new, high-precision antennas in only one embroidered layer of the finer thread. So now the process takes only about 15 minutes for the aforementioned broadband antenna.

The researchers have also incorporated some techniques common to microelectronics manufacturing to add parts to embroidered antennas and circuits. 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.

Tests showed that an embroidered spiral antenna measuring approximately six inches across transmitted signals at frequencies of 1 to 5 GHz with near-perfect efficiency. The performance suggests that the spiral would be well suited to broadband internet and cellular communication.

Ohio State plans to license the technology for further development.