MXene-coated yarns as platform technology for e-textiles

(Nanowerk Spotlight) MXenes – the large family of two-dimensional (2D) transition metal carbides and nitrides – show outstanding physical properties and have found applications ranging from energy storage to medicine and optoelectronics (read more in our MXene primer).
MXenes' inherently good conductivity and excellent volumetric capacitance makes them a very attractive material for fabricating textile-based, wearable electronics (e-textiles) that can be worn like everyday garments. This requires the fabrication of conductive yarns that can be manufactured by simple and scalable processing and possess mechanical properties that makes them robust enough to be suitable for the wear and tear experienced by everyday textiles.
So far, though, there have been no comprehensive studies investigating the scalability, manufacturability and washability of MXene-based conductive yarns.
Enter Professor Yuri Gogotsi's Nanomaterials Group at Drexel University – the team that discovered MXenes in 2011. In recent work, reported in Advanced Functional Materials ("Knittable and Washable Multifunctional MXene-Coated Cellulose Yarns"), the group established an approach that combines the versatile chemistry and promising electrical and electrochemical properties of MXenes with commercial cellulose-based yarns.
This is the first study demonstrating highly conductive MXene-based yarns that can be washed and knitted just like conventional yarns – offering a potential platform technology for e-textile-based devices with tunable performance.
knitted MXene-coated cellulose-based yarns
Seamlessly knitted MXene-coated cellulose-based yarns. Concept illustration of a garment integrated with energy storage and harvesting device with a capacitive pressure sensor. Insets show actual device prototypes comprising of a) knitted energy storing fabric with alternating MXene-coated cotton yarn (black) and a nonconductive commercial viscose yarn (green). b) Knitted energy harvesting fabric with alternating MXene-coated linen yarn (black) and a commercial Teflon yarn (brown) can be placed strategically to harvest energy from body movements. c) Capacitive pressure sensor device knitted with MXene-coated bamboo yarn, where the device can sense different applied pressures ranging from low to high. (Reprinted with permission from Wiley-VCH Verlag)
"Our motivation was to design and develop conductive yarns, which can be seamlessly integrated into everyday garments," Gogotsi tells Nanowerk. "The lack of conductive yarns that offer washability and manufacturability using existing textile processing technologies – in the same way as traditional commercial yarns – has prevented consumers from experiencing the added functionality of conductive yarns with functionalities such as energy storage and sensing."
"Instead of demonstrating only the electrochemical performance of our MXene-coated yarns as supercapacitors, we took the extra step and worked on crucial properties like washability, advanced manufacturability, and scalability," he points out. "These concepts are vital for the long way from lab to fab, i.e. scaling up laboratory research to industrial production processes."
MXenes have been incorporated into yarns by a variety of methods, including dip-coating, drop-casting, and biscrolling, and processed into fibers via wet-spinning and electrospinning. The dip-coating process the Drexel team used in the present study is the most facile, scalable, and environmentally friendly (no organic solvent required) method.
"Unlike the dip coating processes reported in the literature, we developed a two-step coating process to maximize the loading of MXene both on the fiber and the yarn level," explains Simge Uzun, the paper's first author. "We first infiltrated the internal yarn structure with small Ti3C2 MXene flakes to coat the individual fibers before coating the external yarn with large Ti3C2 MXene flakes. We were able to achieve almost 80 wt.% MXene loading on the yarns without any loss from MXene flaking off during handling and washing."
Using low-cost, commercial cellulose-based yarns, the team found that yarns with 78 wt.% MXene loading exhibited the highest specific length capacitance among the cellulose-based yarn-shaped supercapacitors reported to date.
Different stitch patterns commonly used in knitted fabrics
Different stitch patterns commonly used in knitted fabrics. a) Single jersey. b) Half gauge. c) Interlock. d) Unsuccessful attempt to knit MXene-coated cotton yarn (black) in single-jersey pattern. e) MXene-coated cotton yarn knitted with half-gauge pattern resulted in a porous fabric. f) MXene-coated cotton yarn knitted with interlock pattern resulted in a dense fabric. (Reprinted with permission from Wiley-VCH Verlag) (click on image to enlarge)
With an eye towards scalable manufacturing processes, the researchers also demonstrated that fabrics using MXene-coated yarns can be produced on industrial equipment. MXene composite yarns produced with other methods (such as for instance electrospinning) are not currently strong enough to be knitted or woven on industrial machines.
Moreover, knitting cotton yarns coated with active material has been a challenge for a long time. The main reason is that cotton yarns consist of shorter fibers (1-4 cm) in comparison to other cellulose-based yarns (5-8 cm for bamboo and 60-120 cm for linen), thus the cotton fibers are more likely to pull apart from each other while under tension during knitting.
Understanding the properties and the limitations of conductive yarns enabled the team to adjust the stitch patterns and the corresponding knitting parameters in order to knit these yarns.
To demonstrate the practical applicability of their highly conductive and electroactive yarns, the researchers prepared a fully knitted textile-based capacitive pressure sensor that offers high sensitivity (gauge factor of ∼6.02); wide sensing range of up to ∼20 % compression; and excellent cycling stability (2000 cycles at ∼14 % compression strain).
"To the best of our knowledge, this is the first study demonstrating the washability and manufacturability of functional yarns with high conductivity," notes Uzun. "We hope that our work will encourage scientists and engineers to take the extra step to produce conductive yarns that meet industry standards in terms of processability."
Watch a video directed by Simge Uzun that describes the team's work designing and producing functional fibers/yarns for wearable smart textile applications (the video is Second Place Winner (SciVid 2018-H) in the 2018 Science in Video Competition for the MRS Fall meeting).
MXene-coated yarns could be utilized for various types of smart textile applications where conductivity is essential. While this work specifically focuses on MXene-coated cellulose-based yarns, demonstrating their energy storage and pressure sensing applications, these yarns offer electrical and electrochemical properties that can meet the requirements of other applications – triboelectric generators; other types of sensors (e.g., strain, moisture, and temperature); antennas; and electromagnetic interference (EMI) shielding.
According to Uzun, the coating process can be easily tailored based on the specific requirements of the target application. For instance, a maximum amount of MXene coating is desirable for supercapacitors since the specific capacitance is directly proportional to the active material loading. On the other hand, uniform, continuous and thin MXene coating on the yarn surface is ideal for electromagnetic interference (EMI) shielding applications.
Going forward, the team now will expand the dip-coating process to synthetic fibers and yarns, taking advantage of their mechanical properties (e.g., higher tensile strength and modulus) compared to natural fibers.
Since the electrical properties of these yarns can be tuned according to the requirements of specific applications, MXene-coated cotton yarns can be utilized to rapidly prototype various knitted textile-based devices including supercapacitors and antennas.
By Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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