photo of fabric with green light-emitting spots

Researchers from MIT and an associated research institute, Advanced Functional Fibers of America (AFFOA), have developed a method for producing functional fibers with embedded electronics. The fibers are said to be flexible enough to be woven into soft fabrics and made into wearable clothing. [Image: Courtesy of the researchers/MIT Press Office]

A research team led by Yoel Fink at the Massachusetts Institute of Technology (MIT), USA, has devised an ingenious—and apparently scalable—method for embedding tiny, electrically connected LEDs, photodetector diodes and other semiconductor devices into textile-grade fibers (Nature, doi: 10.1038/s41586-018-0390-x). The fibers reportedly are compatible with commercial-grade looms, and the fabrics using them can stand up to multiple runs through the laundry. The team even demonstrated bi-directional Li-Fi communications, across a 1-m distance, between two fabrics including the embedded light-emitting and photodetecting fibers.

In addition to fabrics that talk to one another, the team envisions a wide variety of other applications for textiles developed using the new method, from health monitoring to defense—with some applications expected to come on stream in as little as a year, according to a press release accompanying the work. Indeed, the researchers view the method as sufficiently scalable to open up the prospect of exponential expansion in textile functionalities—a sort of Moore’s law for smart fibers.

Getting the right draw

Drawing tower from bottom, with fiber emerging

A view of the fiber-drawing tower used in the MIT work. [Image: Courtesy of the researchers/MIT Press Office]

Fink’s lab and others have been working for many years on approaches to getting semiconductor devices into multifunction fibers for textiles—with some success. Most approaches to doing so have involved embedding semiconductor material and low-melting-point metals or conductors into a polymer fiber preform. The preform is then heated, extended and drawn into a long, thin filament, as with conventional optical fiber.

Thus, in earlier approaches to getting semiconductor functionality into textile fiber, the metal and the semiconductor material have themselves been drawn out together in a viscous state along with the fiber-forming polymer. The process works—but the quality of the resulting fiber-embedded semiconductor devices comes in far below what’s possible with industry-standard, wafer-based manufacturing of the same items.

In the new work, Fink, along with lead author Michael Rein and other researchers, took a different approach. Rather than have the embedded bulk semiconductor and low-melting-point metal conductor co-drawn and extended along with the fiber, the new method buries discrete semiconductor devices in the preform, and draws the fiber out around wires made of high-melting-point conducting metal (such as copper or tungsten).

In this scheme, both the semiconductor devices and the metal remain solid and unchanged during the fiber-drawing process. The method thus allows hundreds of high-quality diode devices, pre-manufactured using the best wafer-based processes, to be electrically connected by high-quality wires, within a thin fiber whose length can span hundreds of meters.

Trenches and tiny holes

As reported in the new study, the team’s approach begins with the creation of a fiber preform consisting of a series of polycarbonate slabs. In two of the slabs, which constitute the preform’s outer layers, the team excavates 1.25-mm-wide trenches running the length of the preform. In an inner layer, the researchers then drill a series tiny holes, roughly 100 microns in width, into which they embed sand-grain-sized diodes (LEDs or photodetectors) made using conventional semiconductor processes. The team then thermally consolidates the entire stack using a hydraulic press.

schematic of fiber-drawing process

(a) The process works by embedding a line of pre-made semiconductor devices into holes drilled in a layered polymer stack, which also includes two lengthwise channels. (b) The resulting preform is heated and drawn into fiber, with conducting wire continually fed into the channels as the fiber extends. During the process, necking and thinning of the preform pull the conducting wires closer to the embedded semiconductor devices until they make electrical contact (I and II). [Image: Reprinted with permission of Springer Nature, from M. Rein et al., Nature 560, 214 (2018); copyright 2018] [Enlarge image]

The consolidation step creates a single, integrated fiber preform roughly eight inches long, with two 1.25-mm-wide channels running the entire length of the preform and with a linear array of hundreds of evenly spaced semiconductors down the preform axis. Next, the team feeds the end of a spool of hair-thin tungsten or copper conducting wire into each of the two preform channels, and begins the process of thermally drawing the preform out into a long, narrow fiber.

As the draw proceeds, the embedded diodes are pulled out and evenly separated along the length of the fiber. The conducting wires, meanwhile, are unspooled and continually fed into the lengthening fiber. As the viscous preform necks and thins into the fiber, the non-melting conducting wires are pulled in closer to the line of embedded semiconductor devices until the wires touch the contact pads on the devices—at which point the draw stops.

The result: from a single eight-inch preform, the process can create draws of fiber as narrow as 350 microns wide and hundreds of meters in length—with hundreds of electrically connected, high-quality micro-diodes, at separations of less than 20 cm, running along the entire extent of the fiber.

Fabric A, say hello to Fabric B

To demonstrate potential applications in functional textiles, the team took the fiber to Inman Mills, a fabric manufacturer in South Carolina. Inman used a commercial loom to weave the fiber into a fabric, with threads of a nylon-cotton blend forming the warp and with the new semiconductor-enriched fibers, interspersed with conventional polyester fibers, forming the weft. The team found that the fibers held up to the commercial weaving—and that the resulting fabric remained functional even after ten cycles in a household washing machine.

fiber through needle's eye, with embedded green LED

The functional textile fiber, with embedded LEDs and electrical connections, can be fine enough to pass through a needle’s eye. [Image: Courtesy of the researchers/MIT Press Office]

Further, the researchers showed that the optical properties of the fiber could be enhanced by designing the external shape of the fiber preform so that it would have a “lensed” shape after drawing. Using such lensed fiber, the team was able to set up a 3-MHz bi-directional optical communications link, across a meter of free space, between fabrics containing the LED and photodetector fibers.

The researchers also created a cotton sock including adjacent LED and photodetector fibers, and demonstrated that it could function as a blood pulse monitor. The sock was able to pick up a pulse reading based on changes in light reflectance from the skin, due to heartbeat-driven expansion and constriction of capillaries near the skin surface.

“Moore’s-law-like” growth ahead?

According to a press release accompanying the new study, Fink expects that the first commercial products incorporating the team’s approach to creating functional fibers could hit the market as early as next year. Those applications will likely focus on specialized products geared to communications and safety. (Military applications seem a particularly likely early target; the work was funded in part by the U.S. Army.)

Beyond those initial applications, Fink and his team see a wide range of potential uses for a process that he calls “a scalable path for incorporating semiconductors into fibers.” Indeed, he suggests that as the technique gains traction, functional fibers and the fabrics incorporating them could advance at an accelerating pace, in a fiber analog Moore’s law for silicon chips.

“Moore’s law predicted that the basic capabilities of the chip are going to double every 18 motnhs or so,” Fink noted in a phone call with OPN. “What we're saying here is that, through the mechanism we’re describing in this paper, we can see how the basic capabilities [of functional fiber] can increase and grow in a Moore’s-law-like way.” (Fink stresses, though, that the specific time constant of the growth for high-tech textile fiber will likely differ from the 18-month prediction for computer chips.)

“What was done here is actually putting a very simple electronic device,” a diode, into a textile fiber, Fink said. “You could think about putting in other types of devices—not just diodes; transistors, memory and so forth. You could think about combining these things into systems and circuits, again in the fiber itself. By the time you’ve been through all of these different possibilities ... you suddenly realize, wow, you know, this could actually give you that kind of growth.”