Publishing their results in the journal 2D Materials under the title "Integration of graphene sensor with electrochromic device on modulus-gradient polymer for instantaneous strain visualization", the researchers describe their bio-inspired colour-changing skin as the stretchable assembly of a highly sensitive resistive strain sensor made from graphene woven fabric (GWF) with a stretchable organic electrochromic device (ECD) using GWF as its stretchable electrodes.
The ECD itself consists of GWF as the electrode materials dip-coated with Polyaniline (PANI) as the organic electrochromic material which presents four redox states of different colours, yellow, green, blue and purple. Polyvinyl alcohol serves as the solid electrolyte.
What they found is that thanks to the highly strain sensitive nature of the graphene woven fabric, subtle strains from 0 to 10% were enough to drive clear color changes within a few seconds, which provided direct optical feedback on the applied strain.
What's more, in a proof-of-concept skin patch changing from green in a resting state, to yellow and blue on opposite electrodes, they demonstrated that these colors could be easily adjusted through a low driving potential (1V and 0V) thanks to a simple resistance-voltage conversion circuit. The colors remain stable once the power was switched off, which makes the electrochromic strain sensing skin particularly low power.
The researchers expect such user-interactive e-skin could find applications in wearables, robotics and prosthetics, but also in smart home furnishing, emotion recognition or camouflage.