Saw this article, linked below… a neat development for car bodies…
"They have developed a material, which contains millions of liquid-filled microcapsules. When a crack forms, the shells of the microcapsules (which measures roughly 0.1 of a millimetre, releasing the liquid “healing agent” dicyclopentadiene (DCPD).
Capillary action draws the liquid into the crack, where it comes into contact with embedded catalyst particles, causing the liquid to harden in a matter of minutes."