Researchers from the University of Windsor in Canada have recently demonstrated a technique for creating versatile light-emitting systems from knit textiles. As reported in Matter, the process introduces functionality to stretchable, ultrasheer fabrics without degrading softness and stretchability.

“Smart clothing” must appeal to consumers in both function and comfort if its capabilities are to be fully realized. “The current approach to producing light-emitting clothing involves sewing rigid elements like light-emitting diodes, wires, and optical fibers into textiles,” says lead researcher Tricia Breen Carmichael. “The problems with this approach involve the wearability: these devices reduce the intrinsic softness and stretchability of the clothing,” she says.

Thin film devices offer flexibility, but are designed for surfaces that are planar, rigid, or both. In contrast, Carmichael says, “textiles have a complex, thee-dimensional structure in which the yarns are interwoven and separated by void space that allows the fabric to move and stretch.” Researchers have previously woven light-emitting fibers into textiles, but that approach is limited to certain fabric structures and by the stretchability of optical fibers.

In this new research, the University of Windsor team took a textile-centered approach to integrating light-emitting capabilities with knit fabrics, soft and stretchy fabrics from which sweatshirts and lounge wear are often made. Their comfort stems from a void-filled looping structure. To preserve these properties, Yunyun Wu, a PhD student working with Carmichael and first author on the article, proposed utilizing the open structure of sheer knit fabric to create transparent electrodes.

The research team started with a stretchable, semitransparent knit fabric made of 87% nylon and 13% spandex, commercially available as pantyhose. The fabric was coated with a thin layer of gold by electroless nickel immersion gold (ENIG) metallization, a two-step method commonly used in the fabrication of printed circuit boards. The method involved first plating the fabric with a thin layer of nickel via an autocatalytic process, and then immersing the nickel-coated fabric in a solution that facilitated the replacement of nickel with gold.

Scanning electron microscopy (SEM) images of the fabric post-ENIG showed individual fibers coated with a uniform layer of gold, 100 nm thick. The fabric was highly conducting, with an unstretched surface resistivity of 3.6 Ω/sq. According to the researchers, this is slightly higher than that of a flat gold film of a similar thickness deposited by physical vapor deposition onto a glass substrate (0.9 Ω/sq). The coating had no significant effect on the material’s mechanical properties and the voids were unchanged.

Coating the fabric with gold reduced its 550 nm optical transmittance by 10%, to 37%. When the coated fabric was stretched, the loops expanded and transmission increased to a peak value near 60% in both primarily stretch directions. After peaking, transmission decreased slightly but remained well above the initial value up to 200% strain in both directions.

Stretching impacted the fabric’s conductivity as well. Under strain, the gold-coated nylon fibers straightened and the contact pressure between loops increased, lowering the resistance. The stretchy spandex fibers elongated under strain, causing cracks to form in their gold coating that increased the resistance. The combined effect of these responses was a peak resistance increase of about 1.5-times the initial value in both primary stretch directions up to 200% strain. However, the researchers note that this is still within the stability range required for wearable wiring and device interconnects.

As a proof-of-concept demonstration, the researchers fabricated an alternating current electroluminescent (ACEL) device using the gold-coated fabric as the top and bottom electrodes. When powered, an elastomeric emissive layer sandwiched between the electrodes produced a uniform blue light that easily shone through the electrodes. With a total thickness of 300 µm, the device retained the softness and stretchiness of its fabric components. The entire device was encapsulated in a thin protective layer of Ecoflex, a soft, transparent rubber, to prevent current leakage and remain operational. Intricate designs could be achieved by patterning the top electrode with wax before the gold coating was applied. The researchers also showed that the top electrode can be easily removed from the device and replaced, supporting interchangeable light-emitting patterns.

This research is a great example of how textile architecture can be harnessed to improve e-textiles, according to Jesse Jur, an expert in the science and engineering of smart textiles at North Carolina State University. The thermal and mechanical properties of textiles can be altered by varying their structure, yarn type, and composition, he notes, “with conformal coatings like this, you can now change them at the electrical level as well.”

Jur is optimistic about the future of textile patterning and the fabrication of multilayered structures that embrace the inherent properties of textiles, give these and other recent results. “It will be very exciting to see how this research evolves,” he says. Similarly, the Windsor researchers anticipate that explorations of other open structure materials—knitted, braided, woven, or tufted materials with a range of fiber compositions and densities—will open the door to many new opportunities for wearable devices.

Read the article in Matter.