An artificial skin attached to a person’s knee develops a purple “bruise” when hit forcefully against a metal cabinet. (Credit: Adapted from ACS Applied Materials & Interfaces 2021, DOI: 10.1021/ acsami.1c04911)

Because prosthetic limbs don’t have warning signs like bruising, they are susceptible to further injury. Now, researchers 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. The researchers 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.

Then, the team taped the I-skin to different body parts of volunteers, such as the finger, hand, and knee. Bending or stretching caused a change in the electrical signal but not bruising, just like human skin. However, forceful and repeated pressing, hitting, and pinching produced a color change. The I-skin, which responds like human skin in terms of electrical and optical signaling, opens up new opportunities for detecting damage in prosthetic devices.

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