Power/Performance Bits: Nov. 5

Conductive yarn
Researchers at Drexel University created an electrically conductive coating for yarn that withstands wearing, washing, and industrial textile manufacturing.

Rather than using metallic fibers, the coating is made up of different sized flakes of the two-dimensional material MXene, which was applied to standard cellulose-based yarns. Titanium carbide MXene can be produced in flakes of varying sizes; to create the best performing conductive yarn, the individual fibers were infiltrated with small flakes before the entire yarn strand was coated with larger flakes via dip-coating, a standard dyeing method. The team used cotton, bamboo, and linen yarns, which were then used in an industrial knitting machine.

“Some of the biggest challenges in our field are developing innovative functional yarns at scale that are robust enough to be integrated into the textile manufacturing process and withstand washing,” said Genevieve Dion, an associate professor in Westphal College of Media Arts & Design at Drexel. “We believe that demonstrating the manufacturability of any new conductive yarn during experimental stages is crucial. High electrical conductivity and electrochemical performance are important, but so are conductive yarns that can be produced by a simple and scalable process with suitable mechanical properties for textile integration. All must be taken into consideration for the successful development of the next-generation devices that can be worn like everyday garments.”

MXene-coated yarns were tested by using them to knit textiles in three common patterns — single jersey, half gauge and interlock — to determine the optimal configuration for knitting with the conductive yarn. (Source: Drexel University)

Each type of yarn was knit into three different fabric swatches using three different stitch patterns – single jersey, half gauge and interlock – to ensure that they are durable enough to hold up in any textile from a tightly knit sweater to a loose-knit scarf. The yarn was used to create touch-sensitive textiles that didn’t see diminished performance even after twisting, bending, and dozens of washing cycles.

“Researchers have explored adding graphene and carbon nanotube coatings to yarn, our group has also looked at a number of carbon coatings in the past,” said Yury Gogotsi, Distinguished University and Bach professor in Drexel’s College of Engineering. “But achieving the level of conductivity that we demonstrate with MXenes has not been possible until now. It is approaching the conductivity of silver nanowire-coated yarns, but the use of silver in the textile industry is severely limited due to its dissolution and harmful effect on the environment. Moreover, MXenes could be used to add electrical energy storage capability, sensing, electromagnetic interference shielding and many other useful properties to textiles.”

The team will continue work on the conductive yarn, including tuning the coating process and collaborations with a textile manufacturer. “With this MXene yarn, so many applications are possible,” Gogotsi said. “You can think about making car seats with it so the car knows the size and weight of the passenger to optimize safety settings; textile pressure sensors could be in sports apparel to monitor performance, or woven into carpets to help connected houses discern how many people are home – your imagination is the limit.”

Transparent conductive films
Scientists at Aalto University and the University of Vienna combined graphene and carbon nanotubes to create an improved transparent conductive film. Transparent conductive films (TCFs) are used in touch screens, organic LEDs, and solar cells. Efforts are underway to replace the metal-oxide films currently used with new materials.

By combining carbon nanotubes and graphene, the conductivity of the film was improved beyond what is possible when using each of the component structures separately. The researchers used a process called thermophoresis to deposit nanotubes on prefabricated graphene electrodes. The hybrid films’ conductivities were roughly twice as high as predicted.

For some time, the team has been at work developing techniques to place densely-packed and clean random nanotube networks on graphene. “This is another application of the technologies we have developed over the past decades. Put simply, this work is about how the two materials are put together without solvents,” said Esko Kauppinen, a professor at Aalto.

In tests, the strong electrical interactions of graphene enhanced the flow of electrons between the nanotubes by encouraging charge-tunneling. The team used a scanning transmission electron microscope to look at the material on the scale of individual atoms, and saw that the van der Waals interaction between the graphene and nanotubes was strong enough to collapse the circular nanotube bundles into flat ribbons.

“This is really an ingenious approach. The charge transport in nanomaterials is very sensitive to any external factors. What you really want is to avoid unnecessary processing steps if your goal is to make the ideal conductive film,” said Kimmo Mustonen, lead scientist of the Vienna group. “It actually is quite remarkable. We of course knew that the interaction is quite strong. For instance, think of graphite; it is just a large number of graphene layers bound together by the same mechanism. Yet we did not expect that it has such a strong impact on conductivity.”

Stretchable sensors
Researchers at the University of Illinois at Urbana-Champaign applied kirigami techniques to make wearable graphene sensors more flexible and tolerant of strain.

Kirigami is similar to origami, but involves cutting as well as folding. “To achieve the best sensing results, you don’t want your movement to generate additional signal outputs,” said SungWoo Nam, associate professor of Mechanical Science and Engineering at Illinois. “We use kirigami cuts to provide stretchability beyond a material’s normal deformability. This particular design is very effective at decoupling the motion artifacts from the desired signals.”

Key to developing the design was creating an online software platform to simulate and analyze how kirigami structures behave and deform as different materials. Graphene was chosen because it is already capable of withstanding significant deformation. The layer of graphene was encapsulated between two polyimide layers before cuts were made.

“Because graphene is sensitive to external changes, yet also a flexible semimetal conductor, people are very interested in creating sensors from it,” Nam said. “This sensitivity is well suited for detecting what is around you, such as glucose or ion levels in sweat.”

The active sensing element was placed on an “island” between two “bridges” made from kirigami graphene. While the graphene did not lose any electrical signal despite the bending and tilting, it still took the load from the stretching and straining, enabling the active sensing element to remain connected to the surface.

Along with increasing stretchability, the kirigami architecture made the graphene sensor strain-insensitive and free from motion artifacts, meaning that even as it was deformed, there was no change in electrical state. The team said they found that the graphene electrodes exhibited strain-insensitivity up to 240 percent uniaxial strain, or 720 degrees of twisting. “What’s interesting about kirigami is that when you stretch it, you create an out of plane tilting,” Nam said. “That is how the structure can take such large deformations.”

The team is working on improvements for a second version and think the design could extend to other atomically thin materials, such as TMDs.