This article from New Scientist describes an innovation that seems, in retrospect, so intuitively obvious I'm surprised no one has thought of it before. (Has someone, actually? I don't think I've seen it in a sf story before, but then again I've hardly read everything.)

The researchers have taken inspiration from human skin, which heals a cut by exposing blood to air, which congeals to forms a protective scab. "The analogy is the vascular system of the human body," Bond told New Scientist. "The system needs to be completely autonomous."

The researchers came up with a similar idea for protecting spacecraft. They fabricated a composite laminate material containing hundreds of hollow glass filaments 60 microns (thousandths of a millimetre) wide, each with an inner chamber of 30 microns in diameter. Half of the filaments are filled with an epoxy polymer or resin and the other half filled with a chemical agent that reacts with the polymer to form a very strong and hard substance.