Knitted devices power up wearable electronics

The next step in mobile technology could see wearable textiles with built-in sensors to detect body movement, pressure detectors to monitor touch, and wireless communication devices to link to cellphones and computers. But all these functions require a power source, which – like the textile itself – needs to be flexible and comfortable to wear.

Researchers from Drexel University in the US and Deakin University in Australia led by Genevieve Dion and Yury Gogotsi believe they may have come up with the answer in the form of three-dimensional supercapacitors knitted from cotton or nylon yarn coated with a novel conductive material [Levitt et al., Materials Today (2020), https://doi.org/10.1016/j.mattod.2020.02.005].

“We [have] developed a simple method to produce tens of meters of highly conductive fibers and yarns capable of storing energy using MXenes, an emerging family of two-dimensional (2D) materials discovered at Drexel University in 2011,” explains PhD student and first author of the study, Ariana Levitt.

The team selected Ti₃C₂T_x, a 2D transition metal carbide, to coat the yarns because its hydrophilicity makes it solution processable, it is highly conductive, and, because its transition metal oxide-like surface undergoes redox reactions at high charge-discharge rates, enabling high capacitance in acidic electrolytes. A simple dip coating process deposits layers of Ti₃C₂T_x onto long lengths of 1- and 2-ply cotton and multi-filament nylon yarns without adversely affecting strength or flexibility.

“Once we achieved the desired loading of MXene onto the fibers/yarns, we knitted them into textiles using industrial knitting machines, the same machines used to produce the knitted textiles we wear every day,” says Levitt.

While fiber and yarn-based supercapacitors typically show good performance over short lengths, longer yarns have greater resistance. Knitting yarns into textiles gets around this problem by creating interconnected loops that give charge shorter routes to travel rather than having to follow the geometric length of the yarn. Knitting also creates denser fabrics with higher capacitance and better performance. The capacitance can be easily doubled or even tripled by simply knitting two or three yarns together.

Prototype supercapacitor devices fabricated from the knitted energy storage textiles show high capacitance (707 mF/cm²) in typical electrolytes and excellent stability over 10,000 cycles. Devices can be knitted in series to boost voltage (up to 1.5 V) or in parallel to increase current.

“Using the processes employed in this work, automated yarn coating, and industrial knitting technology, textile energy storage devices could be rapidly designed, programmed, prototyped, and ultimately, mass-produced,” says Levitt.

The researchers are aiming to integrate multiple knitted energy storage devices into a garment and demonstrate a prototype textile that can power LEDs and sensors. Long-term durability remains an issue, so the team is also exploring protective encapsulation methods.