Electronic skin, or “e-skin,” is a paper-thin, strong, flexible material designed to mimic the functions of human skin, such as sensitivity to pressure and temperature. Professor Zhenan Bao and her team of researchers at Stanford University have been investigating and improving on e-skin since 2008. Bao envisions a new generation of robots and prosthetics able to sense touch, with every square centimeter containing millions of tiny pressure sensors sensitive enough to detect the weight of a fly. Past efforts have focused on integrating solar cells, self-healing mechanisms, and adding lights, though this is the first time such materials have been synthesized with pigments that can change color.
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.
Dr. Ho-Hsiu Chou is a postdoctoral scholar of Zhenan Bao’s research group, and first author of the study. He explains that this is the first time that the three features of touch-sensitivity, stretchability, and color changing have been combined. This color change capability is a particular step up from light-up e-skin because the material can maintain a color change without consuming power.
The secret is in the layers. Past work has used special plastics integrated with carbon nanotubes—tiny molecular tubes made from rolled sheets of carbon atoms. This time, researchers were able to add a layer of “electrochromic polymer,” meaning that the color can change based on an electric signal.
Interesting things happen when the pressure-sensing and color-changing features are linked. To demonstrate this, Chou and his colleagues equipped a small white teddy bear with e-skin on its paw and tummy. A few seconds after a weak handshake caused the bear’s tummy to turn from red to gray. A stronger handshake then caused a change from red to blue.
Chou envisions this technology used “in smartphones and watches and clothing, because the material we use is soft and stretchable. 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.”
For now the electrochromic layer is only capable of turning two colors, though in one to three years’ time, Chou estimates they’ll be able make e-skin material turn any color. “It just depends on the [color change] material we can get,” he says.
Because the material is expected to be used in wearable devices, Chou and his colleagues are committed to addressing concerns about the safety of their choice of materials by using shorter carbon nanotubes that have been shown to cause less damage to DNA and less inflammation, as well as adding additional layers to e-skins to encapsulate the flexible electronics within. Chou also intends to explore options that increase the biodegradability of the material.
When the engineering worlds of chemistry, electronics, and material science collide, we get an explosion of color.