Researchers have made great progress in developing flexible electronics over the last decade, with devices such as flexible transistors and integrated circuits, stretchable luminescence structures, roll-up displays and smart sensor-integrated electronic skins. However, power devices that also boast these properties are still lagging behind.

A team led by Zhong Lin Wang has now created a soft skin-like, biocompatible triboelectric nanogenerator (STENG) that is both transparent and ultra-stretchable, and which is capable of converting mechanical or biomechanical motion into electrical energy. It is made by sandwiching a layer of ionic hydrogel (which acts as an electrode) between two elastomer films (which act as an electrification layer). Once the elastomer layers are electrified, static charges build up on their surface, driving the flow of current between the hydrogel and a grounding device (or a reference electrode).

Unlike previous STENGs, the new device employs an ionic rather than an electrical conductor as the electrode. This allows for greater stretch under high uniaxial strains (of 1160% in the present case). In tests, Wang and colleagues measured stretch ratios of over 100% and an ultra-high transparency of 96.2%. The energy harvester is also robust to temperatures of 30 °C and in humid conditions (of around 30%) – conditions that deteriorated such devices in the past.

Driving wearable electronics

The energy harvester is able to produce electricity with an instantaneous peak power density as high as 35 mW/m2 and can so be used to drive wearable electronics (for example, an electronic watch) by converting energy from mechanical motion such as hand tapping. And that's not all: it is also pressure-sensitive, which means that it might be employed in artificial electronic skin for sensing touch. Capacitors or batteries might be charged by this skin to, in turn, also power wearable electronics.

“As well as this, we expect that our device may find applications in soft robotics and foldable touchscreens,” team member and lead author of the study Xiong Pu told nanotechweb.org.

The researchers, reporting their work in Science Advances, say that they are now planning to increase the output of their device and integrate additional functions into it. They will do this by maximizing surface electrostatic charge density through surface treatments and optimizing the materials employed. They will also be looking at developing STENGs with other functional hydrogels and elastomers, and increasing the interface bonding between the two structures to further improve their mechanical performance.

Further reading

Paper generator harvests electricity from motion (May 2017)
Self-powered textile could be woven into smart clothes (Oct 2016)
Triboelectric nanogenerators power up (Oct 2015)