The colour-changing skin of chameleons, octopi and squid has long been a fascination for scientists.
However, their attempts to replicate it have failed to have the desired effect. Often, the colour change is only visible to the naked eye when the material is stretched and placed under mechanical strain.
But, researchers in China have developed a new type of colour-changing electronic skin, where the shifts in hue can be seen by humans, under much lower levels of strain than in previous attempts.
A study published today in the journal 2D Materials details the work, by researchers from Tsinghua University in Beijing. They used flexible electronics made from graphene, and a stretchable organic electrochromic device to create the effect. “We found subtle strain – between zero and 10 per cent – was enough to cause an obvious colour change, and the RGB value of the colour quantified the magnitude of the applied strain,” said lead author Tingting Yang.
In other words – the more they stretched the new material, the more its colour changed. “It is important to note that the capability we found for interactive colour changes with such a small strain range has been rarely reported before,” said Yang. “This user-interactive e-skin should be promising for applications in wearable devices, robots and prosthetics in the future.”
The researchers were excited by the potential of graphene for this kind of technology. “Graphene, with its high transparency, rapid carrier transport, flexibility and large specific surface area, shows application potential for flexible electronics, including stretchable electrodes, supercapacitor, sensors, and optical devices,” said senior author Hongwei Zhu.
In an email to Professional Engineering, Zhu said the next steps could include adding more colours to the electronic skin. “The current work provides the e-skin with a reversible color changing capability for detecting strain or stress load,” he wrote. “However, the change in colour range, just between yellow and blue, is limited. Next we hope to introduce and design more electrochromic materials to enable various colour switching, thus broadening the application scope.”
Ravinder Dahiya, a reader in electronic and nanoscale engineering at the University of Glasgow who was not involved in the research told Professional Engineering that it looked like the Chinese researchers had “come up with a solution that is more sensitive than in the past.”
However, he was doubtful about the applications of such technology for wearables and prosthetics. “They say it will be useful for wearables, prosthetics and robotics. How it will be useful they have not said,” said Dahiya. “I can only see one benefit when it comes to robotics and prosthetics – the only benefit I can see – for example if you press your limb very hard it changes colour – so that could reflect the amount of force that is being applied on the limb structure.
But he said there were potential benefits in other areas in manufacturing and construction. “For example if such a layer is present on electronic wafers where electronics is being fabricated a minor change in the colour would reflect a non-planar surface. It could be used for structural health monitoring,” he said.
Zhu said that his team’s technology could be used in clothes, smart phones and smart watches. “It should be promising to be used as an interactive decoration or emotional expression,” he told PE. “Besides, it also offers a feasible scheme for camouflage and may be used in military applications, prosthetics as well as intelligent robots.”