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“Lit up” fabric heralds new era in wearable electronics

By Dinsa Sachan August 28, 2018
Lit-up fabrics
Light-emitting fibers in which the diodes are placed 370 ± 110 mm apart in (a) an InGaN blue-color LED; (b) InGaN green-color LED; and (c) AlGaAsP red-color LED. Credit: Nature

Researchers have demonstrated a clever new way to integrate semiconductor diodes into clothes. The new method could be used to manufacture garments with integrated electronics that could, for example, measure a person’s heart rate or enable soldiers to communicate in a war zone.

But this was not an easy feat to achieve. Semiconductor diodes are the foundation of consumer electronics, so learning to embed light-emitting diodes and photodetector diodes into the fiber in the right manner was crucial to enable electronic functionality in them. The researchers had to first learn how to embed these two types of semiconductor devices into polymer fibers. The fibers were then woven into fabric. “As a result of this work, we’re going be able to consistently increase the functionality of fiber in the years ahead,” says Yoel Fink, a professor of materials science at the Massachusetts Institute of Technology and a co-author of the study that appeared in a recent issue of Nature. “The way that’s going to happen is by increasing the density of devices in the fiber, and the way that’s going to happen is by increasing the type and category of devices.” Fink is also the CEO of the non-profit enterprise, Advanced Functional Fabrics of America, which develops tech-enabled fabrics.

The research team used a process called thermal draw to produce their fibers. First, they developed a preform, which is a scaled-up structure of the fiber itself. A preform is a cylindrical object that is widely used to produce optical fibers. The preform was 2.5 cm wide, 1.25 cm thick, and 25 cm long. The fiber’s dimensions are 1/50th those of the preform. The semiconductor diodes were dropped into pockets drilled into the preform during the manufacturing process. The light-emitting diodes (LEDs) were made of indium gallium nitride (InGaN) and aluminum gallium indium phosphide (AlGaInP). The photodiodes were made of silica (Si) and gallium arsenide (GaAs). These diodes were in the form of microchips. The smallest device size that the researchers used was 170 × 170 × 50 µm.

The preform was heated in a furnace that had three temperature zones set at 150°C (top), 270°C (middle), and 110°C. (bottom). When the preform was heated, a thread-like fiber started to draw out of the preform which was then spooled. While the fiber was being drawn, the researchers fed tungsten or copper wires into hollow channels that were created during the manufacturing of the preform. These wires functioned to provide electric current to the LEDs in the fibers or as readout buses for the photodetecting devices integrated into the fibers in a similar fashion to the LEDs. The wires were not in physical contact with the diodes initially, but as the fiber was drawn, the wires established an electronic connection with the diodes. When power was supplied to fibers containing the LEDs, they lit up in shades of blue, green, and red (see Figure). This is the first time known that researchers have succeeded in integrating semiconducting devices in fibers using the thermal draw process.

A polycarbonate cladding serves as the outer layer of the fiber to protect the materials in the fiber’s core. The spacing of devices in the fibers could be varied. In some LED fibers, the devices were placed around 20 cm apart from each other. In others, that distance was as large as 200 cm.

In previous attempts, other researchers had co-drawn fibers with similar devices, but with metallic wires of low-melting temperatures, such as silicon, germanium, or amorphous chalcogenide material. “Unfortunately, to date, these materials prove to be hard to enable high-efficiency electronic devices in textile-ready fibers. For example, co-draw of silicon or germanium with metals to form interconnects in fibers has been challenging due to diffusion and mixing problems at the draw temperatures, while chalcogenide semiconductors display an overall inferior function relative to crystalline semiconducting materials,” says Michael Rein, a former graduate student at Fink’s laboratory and lead author of the article. Moreover, previous processes produced shorter fibers, but by using the new method, it was possible to draw out kilometers-long fiber with a single preform.

Enabling textile-grade fiber with electronic functions presents multiple problems that materials researchers have been grappling with for about 15 years. “We wanted to find materials that would be suitable for the thermal draw process, as well as combine different elements for electronic functionality, namely—insulators (polymer or glasses), conductors (metals such as copper/tungsten wires) and semiconductors (silicon, GaAs) in a fiber—and make them co-draw together successfully,” Rein says.

The cladding design also plays a significant role in the function of the fiber. In some fibers, the cross-section was designed like a lens. The authors described it as “a cylindrical lens extending along the whole length of the fiber.” This improves the transmission distance. The cladding cross-section makes light rays emitted from LEDs parallel and focuses the light on the photodetector diode.

The devices and the wires were shielded by the surrounding polymer cladding, protecting them from the environment. This enabled the fibers to endure several cycles of washing in a machine and even the rigors of the weaving process, proving their suitability for textiles. The experts at Inman Mills, a South Carolina-based technical fabric manufacturer, wove the fiber into fabric using a weaving loom.

The researchers demonstrated that the new technology could be used to monitor a person’s heart rate.  They placed an LED fiber at a distance of 5 mm from a photodetecting fiber. The LED fiber was supplied with current and emitted light. When a person placed a finger against the fiber, light from the LED fiber lit up the skin. The photo-detector fiber, which was connected to an oscilloscope, picked up the reflected light from the skin and the associated blood flow fluctuations. This system yielded results similar to those of a pulse sensor currently available in the market.

“This is pioneering research, but we shouldn’t get the idea that we’re going to make a shirt for $15 this way. Obviously, this is a higher-end application,” says John Badding, a professor of chemistry, physics, and materials science and engineering at The Pennsylvania State University. He was not involved in this study.

The research group will have to answer many questions before the technology can be commercialized. “There are always questions regarding health and privacy issues when you intend to place electronic components that monitor you and are placed in proximity to your skin for large periods of time,” says Juan P. Hinestroza, an associate professor of fiber science at Cornell University, who was also not a part of this research.

Hinestroza says the environmental impact of such technologies needs to be addressed, too. “What happens to these fibers post-consumer use?” he says. “Can they be disposed in a landfill as normal clothes? This is an especially important issue when it has been demonstrated that microfibers from synthetic fabrics, such as the one in this study, [have been] found in aquatic animals and are polluting water resources.”

Rein says that this breakthrough may have a wide set of applications. “Textiles are used in many [applications], not just garments,” he says. “For example, they are also used in furniture, curtains, and even military applications. Electronic functionality could be enabled in any of these textiles.”

Read the abstract in Nature.