Stretchable wearables powered by body motion

Nagoya University researchers have used a simple, cost-effective and scalable method to produce power generators that stretch with your body — a breakthrough that has been reported in the journal Nano Energy.

Wearable devices, like smartwatches and activity trackers, are typically powered by rechargeable batteries, which are relatively bulky and require a fair amount of time for charging. Moreover, the commonly used Li-ion batteries also face environmental concerns.

But what if the power source of wearable devices wasn’t a battery at all, but a stretchable material that follows the body’s motion and is small and lightweight for user comfort? Energy harvesting technologies that convert small amounts of ambient energy into electrical energy may provide an alternative to batteries for powering wearable devices.

Wearable energy harvesters must not only be flexible but also stretchable in order to follow the body’s three-dimensional body motion. Moreover, they must deliver sufficient power to drive a variety of electronic devices. However, the output power of harvesters at present is not enough to make stretchable triboelectric generation practical. Nagoya University researchers have now overcome the stretchability and low output power issue.

“We have realised a transparent and stretchable triboelectric nanogenerator (TENG) that can follow human motions by using a carbon nanotube thin film (CNT film) as an electrode for the TENG,” said Assistant Professor Masahiro Matsunaga, first author on the new study.

“The fabricated TENG has a simple structure: a CNT film is sandwiched inside a polydimethylsiloxane (PDMS) elastomer. It has a transparency of over 90%.” When the surface of the TENG is touched, the TENG converts mechanical energy into electricity by a process called ‘contact electrification’ and electrostatic induction.

The CNT film is made by a simple spray-coating method, which is cost-effective and scalable, with the research team able to fabricate a large TENG with area up to 12 x 12 cm. Moreover, using a plasma treatment during fabrication, the actual, realised power output density of the TENG is up to 8 W/m².

As a proof of concept, the team demonstrated the usefulness of the TENG by applying it to make self-powered optical wireless communication sheets, as well as gloves where blue light-emitting diodes (LEDs) were embedded in the TENG and wired using CNT films. Asst Prof Matsunaga explained, “For the communication sheets, we formed three separated electrodes and wires with the LEDs of different colours in the sheet. The sheet can send various optical signals depending on the type of touch — tap or swipe.

“To make the glove, we attached the TENG on the palm, and used a CNT wire to connect it to blue LEDs embedded on the back of the hand. The glove can drive LEDs by handclaps. The glove can be put on and taken off without affecting the functionality because of the excellent stretchability and durability of the CNT films.”

The technique has thus been shown to create relatively high power output, stretchable and flexible power generators, and could be the first step in decreasing our dependence on rechargeable batteries for wearable devices.

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