How NASA and Michigan chemists teamed up to make a material that bleeds, and heals
In the recent Matt Damon film, The Martian, the astronauts repeatedly find themselves in situations where only a thin wall separates them and their breathable air from the cruel vacuum of space. If a small, high-speed projectile – say an undetectable meteor or a piece of space junk – were to puncture the hull of the spaceship Ares III, or the human habitat on Mars, explosive decompression could rapidly rob the chamber of its air, putting lives, missions, and billions of taxpayer dollars at risk.
The risk of such a catastrophe might have been greatly reduced if the designers of Damon’s habitats had read a recent paper by a team of chemists and engineers from University of Michigan and NASA. The paper, published by the American Chemical Society in July, details the design of a rapidly self-healing material. And yes, they tested it by shooting it with a pistol.
The team developed plates with three layers: an outer structural layer, a middle liquid layer, and an inner soft layer.
The inner layer is made of what’s known as an ionomer – a plastic laced with charged particles. Many ionomers melt at the high temperatures seen during ballistic penetration – this particular material melts just enough to seal the hole created by a 9mm bullet. The sealing happens fast enough to keep the inner liquid contained but slow enough to allow atmospheric oxygen into the hole; this turns out to be critical. Unfortunately, ionomers are rather weak – which is why you don’t already have a self-healing house. So for the other layers, the scientists needed a structural material – and a way to plug the holes in it.
The liquid layer in this self-healing material serves the same purpose as blood in wound healing. When you’re cut, you bleed. This is actually a good thing, since blood contains platelets to clot the wound and allow for healing to take place. Like blood, the liquid component of this material rushes out of the crack or hole and seals it upon exposure to oxygen.
The reason? It’s radical. Actually, radicals – the reactive molecules produced when certain “initiator” molecules are exposed to light, heat or, in this case, oxygen. Electrons like to exist in pairs. Radicals, however, have a single unpaired electron, which proceeds to grab another electron from a neighboring molecule’s pair. The new electron pair binds the two molecules together, but it leaves a new unpaired electron. This commandeers a third molecule’s electron, and so on. This chain reaction makes a literal chain of subunits until the molecules that were free-flowing in the liquid are all sucked into the fibers of a hard, plastic-like material.
This polymerization reaction happens so quickly that it prevents the liquid from splattering out of the hole formed by the bullet. It instead seals the hole entirely, milliseconds after impact. The sealed ionomer prevents air pressure on the inside from pushing out the resin “plug,” and the hard outer layer remains structurally sound.
And Matt Damon lives another day.