Researchers have used two-dimensional (2D) materials called MXenes to make stretchable, twistable conductive coatings on a wide range of materials, ranging from fabric to paper to plastic. The advance introduces a path to integrate sensors and energy storage devices into clothes, vehicles, and buildings.

Conductive coatings are a must for foldable displays, wearable sensors, and artificial skin. Researchers are exploring graphene, carbon nanotubes, metal nanowires, and organic materials for such coatings, but it has been hard to make coatings that retain conductivity while being bent, twisted, and stretched.

MXenes, a family of 2D materials, are an enticing material for conductive coatings. They are atoms-thick layers of transition metal carbides, carbonitrides, or nitrides with the chemical formula M_n+1X_nT_x, where M is a transitional metal, X is carbon or nitrogen, and T is a surface terminal group such as –F or -OH. The materials are highly porous, conductive and tunable surfaces that can be terminated with functional groups. Researchers are testing them in chemical sensors, desalination membranes, and energy storage devices.

Researchers have made titanium carbide (Ti₃C₂) MXene films with conductivity far higher than that of graphene. But making the films supple and stretchy has been a challenge. “Adhesion is a big problem,” says Jodie Lutkenhaus, a professor of chemical and mechanical engineering at Texas A&M University. “If you drop cast MXenes on a substrate, or dip a substrate into them, they will deposit but then flake off when the substrate is flexed.”

So Lutkenhaus and her colleagues made coatings one layer at a time. They deposited alternating layers of Ti₃C₂ and the polymer poly(diallyldimethylammonium chloride) (PDAC) either by spraying or dipping the substrate into the material. The polymer acts like a glue, binding the MXene layers to each other and to the substrate through electrostatic forces.

The researchers stacked up to 40 of the Ti₃C₂ -PDAC bilayers. They were able to make the coatings on various substrates including plastic films, silicone sheets, and nylon fibers, they report in a recent issue of Science Advances.

To test the coatings’ stability, the researchers repeatedly bent and stretched MXene-coated plastic and silicone films, through which the coatings retained their conductivity. They also connected a light-emitting diode to a battery through the MXene-coated plastic film. The LED stayed lit even when the film was folded in half. The silicone sheets retained their conductivity after being stretched by 40 percent.

Being able to coat different types of material surfaces with MXenes opens up many exciting possibilities, Lutkenhaus says. “You could have smart food packaging that senses spoilage, or fabrics with integrated energy storage to power gadgets.” With a new National Science Foundation (NSF) grant, the research team plans to explore other MXene compounds, and to make sensors and energy storage devices, she says.

The discoverer of MXenes, Yury Gogotsi, a professor of materials science and engineering at Drexel University, calls this “important, novel” research because it is the first report of “stretchable films with high conductivity that is retained after very large deformation.” These results were obtained with the most studied MXene, he says, but “other materials may have even better properties—we are just learning about their potential.”

Read the article in Science Advances.