Enlarge Image Cutting edge. After it was divided with a scalpel, a new polymer was able to heal itself, restoring most of its mechanical and electrical properties in 15 seconds. Credit: Benjamin Tee and Chao Wang Human skin is a special material: It needs to be flexible, so that it doesn't crack every time a user clenches his fist. It needs to be sensitive to stimuli like touch and pressure—which are measured as electrical signals, so it needs to conduct electricity. Crucially, if it's to survive the wear and tear it's put through every day, it needs to be...