Previously, we have talked about color-changing materials inspired by organic forms like squid and cuttlefish. Well, this time around, the scientists are inspired by the ‘king of camouflage’ – the chameleon. Researchers at the Stanford University have devised a new stretchable electronic skin (e-skin) that can automatically change its color depending on the varying degrees of pressure applied on its surface. To that end, the magnitude of pressure is responsible for changing (albeit indirectly) the chemical structure of this stretchable surface, which in turn affects the optical properties of the so-called ‘electrochromic’ material.

In terms of composition, the stretchable e-skin comprises a flexible microstructured polymer that has the ability to alter its voltage in accordance with the applied pressure. This is accompanied by a stretchable electrochromic polymer that changes its color in accordance with the applied voltage (that is changed by the aforementioned microstructured polymer). The color-changing effect was actually demonstrated on a teddy bear, with the microstructured polymer being attached to the toy’s paw. This in turn was connected to the electrochromic polymer mounted on the bear’s abdomen. Now, on closer inspection, one could see how the initially dark-red hue changes to blue-gray due to the effect of a light handshake (that entails around 50 kilopascals [kPa] of pressure). The red version is once again restored on removal of the pressure, while it completely alters to dark blue on re-applying the handshake pressure with greater magnitude (200 kPa).

Credit: Chou, et al. ©2015 Macmillan Publishers Limited.

Delving into the working scope of the setup, the pressure from the handshake is directly utilized by the microstructured polymer attached to the paw, which helps it to reduce its electrical resistance (since the polymer is already connected to a low-voltage power supply). This in turn increases the voltage to the electrochromic polymer mounted on the abdomen, which leads to the oxidization of the material, thus slightly changing its chemical structure. Finally, this affects the material’s light absorption spectrum by a larger scope – resulting in wholesome changes in colors. However, the entire color-changing effect can just as easily be reversed by releasing the pressure.

Credit: Chou, et al. ©2015 Macmillan Publishers Limited.

So, the question naturally arises – how is this e-skin different from the other color-changing materials devised up till now? Well, the greatest advantage of this combined-polymer material relates to its stretchable property, which is not found in comparable technologies. Moreover, this factor is complemented by the easy touch-sensitive nature of the ambit. Such aspects in turn pertains to a range of potential applications, including wearable devices and even futuristic camouflage for smart robots. As Ho-Hsiu Chou of Stanford University, the first author of the study, made it clear –

The e-skin can potentially be integrated into the things that we wear and carry, i.e., clothes, smart phones, smart watches, and any other kind of wearable devices. By integrating with this color-changeable e-skin, you can imagine that all the colors can be integrated into one device, and the user can change it interactively for decoration or to express emotion.

He further added –

Because the e-skin’s color change can also be in turn utilized to distinguish and quantify the magnitude of pressure we applied, the other potential application is that we can integrate the system into any surface where we want to know the magnitude of pressure applied on it. Also, the e-skin can provide the camouflage function for prosthetics and smart robots. In addition, the stretchable system allows it to attach on curvilinear or dynamic surfaces well, while conventional rigid devices cannot. This advantage can reduce the interface between the device and human body.

Lastly, the researchers have also incorporated another advantage into their newly devised e-skin by making use of carbon nanotubes that are comparatively small in diameter. This special design mode avoids the larger carbon nanotubes (shaped like asbestos) that potentially pose health risks due to their toxicity and carcinogenicity. Furthermore, the scientists have also made it clear that they are looking forth to more advancement in this stretchable color-changing material technology – by integrating safety features like silicone encapsulation and biodegradable elements.

The study was originally published in the Nature Communications journal.

Source: Phys