A self-healing plastic for astronauts

On Mars, if your spacesuit is punctured, you’ll almost certainly die – just ask Matt Damon, who stars in The Martian. But if future Mars colonists’ spacesuits were made from a material developed by Scott Zavada and his team at the University of Michigan, the terrifying whoosh of escaping oxygen might be just what saves their life.

Zavada’s self-healing material was published in ACS Macro Letters in July.

“The concept is impressive,” says Kei Saito, a polymer engineer at Melbourne’s Monash University.  “I’ve never heard of a material that self-heals this quickly.”

Humans on Mars will require a constant supply of oxygen and protection from the fatally thin atmospheric pressure and extreme cold (temperatures can reach -73°C at night near the equator). So in 2011 NASA put out a call for proposals to research materials that can mend their own rips.

The material Zavada and his team developed can repair a hole caused by a speeding bullet within a second. It could be used to line spacesuits and dwellings on Mars or the Moon.

“When you have a hole in a space habitat, you want that hole sealed up fast – within a second or two”

Self-healing materials have been in development for 15 years, but the are slow to fix a breach. Most are impregnated with small capsules of liquid polymers that burst when the material ruptures. The liquid polymer oozes into the hole then solidifies, sealing the gap within minutes. But “when you have a hole in a space habitat, you want that hole sealed up fast – within a second or two,” says lead author Zavada.  

When the NASA call came out, Zavada was working with the University of Michigan’s Timothy Scott to design surgical adhesives – liquid polymers that solidify when they’re exposed to oxygen in the air.

Zavada thought a similar kind of chemistry might be suitable for space suits, where oxygen would bleed from the puncture. NASA agreed. The organisation has funded his research ever since. After testing different polymer mixtures, Zavada and his team have now come up with a chemical cocktail that clots quickly. The cocktail’s special ingredient is tributylborane, a chemical that reacts instantaneously with oxygen to release free radicals, triggering a chain reaction that forms a solid polymer plug.

Zavada put his cocktail to the test at NASA’s Langley Research Centre, injecting the liquid between two transparent plastic sheets, then shooting it with a bolt-action rifle from 11 metres. Within a second, the liquid solidified and the bullet hole was sealed.

A bullet striking a regular polymer leaves a leaky hole, but Scott Zavada's self-healing polymer instantly re-seals itself after an identical strike.CREDIT: SCOTT ZAVADA / UNIVERSITY OF MICHIGAN.

Cold temperatures can dramatically slow chemical reactions – and it doesn’t get much colder than space. Thankfully, the clotting chemistry warms itself, Zavada says. The projectile strike delivers some heat, and when the polymers react with oxygen they release more heat – reaching 250°C.

The low pressures on Mars, and the near-vacuum of space, present more of a problem, as the team discovered when firing bullets at the self-healing polymer in a special near-vacuum ballistics box at NASA. When the bullet hit the material, the polymer liquid was sucked into the vacuum before any self-repair could take place.

But by adding particles and fibres to the liquid cocktail mix, Zavada has already managed to make it thicker and more resistant to a vacuum’s powerful tug. “Initial tests look promising,” he says.

So far Zavada has not tested bullets larger than five millimetres. For satellites and other spacecraft orbiting the junk-strewn low Earth orbit, debris between 5 millimetres and 10 centimetres is particularly dangerous. These fragments are too small to be tracked and avoided, but too big to be deflected by a protective shield. Zavada plans to test his polymer’s response to larger assailants later this year.