Could self-powered ‘smart clothing’ be a step closer?

From denim jackets that connect with your smartphone, to yoga pants that help you optimise your form, smart clothing is becoming big business. While smaller, more specialist companies initially led the way in incorporating electronic devices into wearable items, the past few years has seen major clothing brands like Levi, Ralph Lauren, and Under Armour step up and take notice. There are now so many high-tech garments on the market that tech outlets are able to publish ‘best of’ lists.

But what those lists don’t mention is that most of these products are limited by their power source; typically a battery that needs to be removed prior to washing the garment. Researchers are investigating numerous alternatives for powering such devices, with energy harvesting technologies leading the way. In a new paper published in Nano Energy [DOI: 10.1016/j.nanoen.2021.106835], researchers from the Chinese Academy of Sciences, Lanzhou, report on their contribution to this effort – triboelectric nanogenerators (TENGs) made from fluorinated silk and nylon fabrics.

TENGs that generate a voltage via a coupling of the triboelectric and electrostatic effect have been in use for close to a decade, but those based on fabrics (F-TENGs) tend to have lower conversion efficiencies and low wear resistance, which limits their use in smart clothing. Washable F-TENGs, largely made by melt-spinning fluorinated materials such as PVDF and PTFE onto fabric substrates, have low air permeability. This makes them uncomfortable to wear. The Lanzhou team instead adapted a liquid-phase fluorination technique to dip-coat silk, cotton, polyester, flax and nylon with urethane perfluorooctyl silane (NHCOO-PFOTS).

This process had a negligible impact on the morphology and permeability of the fabric – fluorinated silk (F-silk) remained smooth and soft. However, it greatly increased the contact angle for a range of liquids, including water, milk, and orange juice, suggesting that it imparts some self-cleaning behaviour on the fabric.

Electrostatic tests showed that a pairing of F-silk and nylon offered the best voltage output, so an F-TENG was assembled using those materials, both of which were affixed to a commercially-available conductive fabric. A maximum instantaneous power density of 2.08 W⋅m−2 was obtained at ~10 M? load, and this output remained stable for 45,000 contact-separation cycles. This, the authors say, is sufficient to “harvest energy from arm swing movements and drive digital watches.”

In addition, after 70 hours of washing, the water contact angle of the F-silk was largely unchanged, and the output voltage of F-silk/nylon pairs was 96.77% of the original value (449.94 V compared to 465 V). Abrasion tests also showed the F-silk to be remarkably durable to high friction loads. Finally, the team sewed an F-silk/nylon pair into a t-shirt and carried out a range of tests, including powering a sensor that reacts to the presence of sodium chloride in water. This, they say could be basis of a “drowning warning system.” Whatever the final application, these results are promising, and may offer one route to self-powered, washable and comfortable smart garments.

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Min Feng, Yang Wu, Yange Feng, Yang Dong, Yubo Liu, Jialiang Peng, Nannan Wang, Shiwei Xu, Daoai Wang. “Highly wearable, machine-washable, and self-cleaning fabric-based triboelectric nanogenerator for wireless drowning sensors,” Nano Energy 93 (2022) 106835. DOI: 10.1016/j.nanoen.2021.106835