How would you like it if your phone started charging whenever you dropped it into your pocket? That may eventually be possible, thanks to a carbon nanotube-based ink that can turn many common fabrics into conductors, or even components of a charge-storing supercapacitor. These won't be showing up in a clothing store near you anytime soon, however, as ensuring that the fabrics only direct the charge to appropriate devices and not, say, to a sensitive body part will be a separate engineering challenge.
Textile fibers are actually uniquely suited to transforming into electronics when combined with the seemingly ubiquitous carbon nanotube. Fibers made of cellulose, like cotton or polyester, are highly porous and can absorb large amounts of water and other polar solvents. When flexible single-walled carbon nanotubes are placed near polymers like these fibers, they have large van der Waals interactions with them, and can be treated with acid that helps them form hydrogen bonds with the fabric. This allows the flexible carbon nanotubes to wrap around the fibers in very high volumes, as the porous fabric gives the nanotubes a large surface area to work with.
In the new paper, the carbon nanotubes were mixed with a surfactant to form an ink (black, of course). The fabrics were dipped into the ink and dried in an oven for ten minutes to remove the water, resulting in a carbon nanotube-laden fabric. The bonds between the fabric and nanotubes are extremely strong, and neither further washing nor attempting to strip the fabric with tape causes the nanotubes to separate from the cloth.
Once treated, the fabric goes from simple cotton or polyester to a high-performing electrical, mechanical, and chemical material. Swatches of fabric were tested as both capacitor components and current collectors in simple circuits. Scientists found that the capacitance, or amount of charge stored per unit of electric potential, of the textiles was two or three times higher than that of carbon nanotubes on flat plastic substrates, probably because the porosity of the fabric allows the capacitor's ions to flow better.
In the experiments, scientists also found that the treated fabric's ability to conduct electricity increased when it was stretched up to 2.4 times its original length. The fabric's performance could also be improved with some chemical treatments. A nitric acid wash got rid of most of the surfactant from the ink, which reduced the resistance of the material by a factor of three. Treatment with manganese oxide increased the capacitance per unit area by a factor of 24.
Increasing the density of the fabrics also increased the capacitance, though with diminishing returns, since denser fabrics are less porous. The fabrics continued to perform well even after thousands of charge cycles, and their specific energy tops out at 20 Wh/kg, roughly on par with other supercapacitors in development—although the fabric substrate is significantly more cost-effective.
The researchers speculate that the relatively low cost of the carbon nanotube fabrics will facilitate the creation of wearable functional electronics, like solar cells and batteries, and may replace more unwieldy commercial electronics materials in other devices. They may also find use as a component of high-performance sportswear or as wearable health-monitoring equipment.
While the paper's authors outline an ambitious future for carbon nanotube fabrics, they are vague about how such a fiber would interact with the human body. It seems like a shirt that is also a capacitor could expose a person to a lot of electricity, but there's no mention of what sort of fabric structure would be necessary to prevent that interaction. It's probably safe to assume the first carbon nanotube fabrics would be integrated into a structure that can separate the enhanced fabric with other neutral layers, as with a lined pocket.
Despite the low cost of the textile substrate, single-walled carbon nanotubes are still somewhat difficult and expensive to obtain, so there's a long way to go until we can charge cell phones from our pockets, or until Jack Bauer can whip off his shirt to repair a dangling power line. Fantasies aside, the applications of carbon nanotube fabrics promise to be quite diverse if they're as easy to make as this paper implies.