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When a person bumps an elbow against a wall, they not only feel pain but can also experience bruising. But robots and prosthetic limbs don’t have these warning signs, and that could lead to further injury. Researchers reporting in ACS Applied Materials & Interfaces have developed an artificial skin that senses force through ionic signals and changes color from yellow to a bruise-like purple, providing a visual cue that damage has occurred.
Scientists have previously developed electronic skins (e-skins) that can sense stimuli through electron transmission; however, these electrical conductors are not always biocompatible, which could limit their use in some types of prosthetics. Ionic skins (I-skins) use ions as charge carriers, which is similar to human skin. When compared to e-skins, the ionically conductive hydrogels have superior transparency, stretchability and biocompatibility.
In order to develop an I-skin that could change color, Qi Zhang, Shiping Zhu and colleagues at the Chinese University of Hong Kong, Shenzhen, made an ionic organohydrogel that contained a molecule called spiropyran that changes color from pale yellow to bluish-purple under mechanical stress. In testing, the gel showed changes in color and electrical conductivity when stretched or compressed, and the purple color remained for 2–5 hours before fading back to yellow. In tests, bending or stretching caused a change in the electrical signal but not bruising, just like human skin. However, more forceful and repeated impacts produced a color change.
The researchers believe the new I-skin opens up the possibility of detecting damage in prosthetic devices and robotics.