Here you can see a self-healing polymer. The bright circles are actually microcapsules filled with a healing agent. The black diagonal line is a crack in the material and all of the microcapsules along that line have turned black because they have released the healing agent into the crack. For healing to occur, the healing agent needs to polymerize.

Here is a scanning electron microscope (SEM) image of the fracture surface of the self healing polymer. There are a lot of broken microcapsules and polymerized healing agent on th surface. It is difficult to see what is healing agent and what is the original material but with further evaluation it is possible to tell the difference.

This is another SEM image of the fracture surface of the self-healing polymer. This image has been colorized with the healed layer shown in blue. You can see the dendritic pattern that the polymerization of the healing agent created. Creating viable self-healing polymers requires the careful selection of host material, microcapsule type/size, healing agent, and catalyst (if it is required).