Color-shifting electronic skin could have wearable technology and prosthetic applications, according to the latest research from scientists at Tsinghua University in Beijing, China.

Numerous animals can change the color of their skin for camouflage, communication, temperature control and defense reasons. For instance, chameleons, octopi, and squid are all well known as color-shifting species. Materials scientists, often with an eye on the biomimetic solution, hope to find compounds that can emulate this color changing capacity of some animal skin. However, visible color changes in experimental skin-like layers are often seen only when a material is put under huge mechanical strain. Now, a team in China has developed a new type of user-interactive electronic skin, with a visible color change that requires a much-reduced level of strain for the change to take place. They suggest that their discovery might find uses in robotics, prosthetics and wearable technology.

The team has used that darling of modern materials science, flexible graphene, to make a highly-sensitive resistive strain sensor that can be combined with a stretchable organic electrochromic device. According to Tingting Yang, "We explored the substrate (underlying) effect on the electromechanical behavior of graphene. To obtain good performance with a simple process and reduced cost, we designed a modulus-gradient structure to use graphene as both the highly sensitive strain-sensing element and the insensitive stretchable electrode of the ECD layer." The researchers demonstrated a small strain of up to 10 percent was sufficient to cause an obvious color change. The measured RGB (red-green-blue) value in a digital photo of the color could be used to quantify the magnitude of the effect and equally the applied strain. [Tingting Yang et al., 2D Mater. (2017) 4 035020; DOI: 10.1088/2053-1583/aa78cc]

Team leader Hongwei Zhu adds that, "Graphene, with its high transparency, rapid carrier transport, flexibility and large specific surface area, shows application potential for flexible electronics, including stretchable electrodes, supercapacitors, sensors, and optical devices."

The researchers point out that earlier work in this area have tended to overlook the mechanical properties of the substrate but the current research shows that this is strongly correlated with the performance of such strain sensing materials. "We believe this should be closely considered in future studies of the electromechanical behavior of certain functional materials," adds Zhu.

This is the first time that a strong color change for such a small strain range has been reported, the team says.

David Bradley blogs at Sciencebase Science Blog and is on Twitter as @sciencebase.