Self-healing materials are still waiting to hit the shelves but the latest discovery needs nothing more than a bit of sunlight to get it going. This is the work of US-based scientist, Marek Urban. He’s taken a chemical called which is related to the substance chitin which forms the shells of animals like crabs and lobsters, and he’s linked that to a square-shaped molecule called an oxetane ring.
This mixture is blended into polyurethane, the same chemical that’s used to make paints and surface coatings at the moment, but when the surface is scratched, the damage causes that oxetane ring to break open which activates the chemicals and makes them much more reactive, and if UV light is then shone onto the injured area, it triggers the activated molecules to link together, pulling the wound closed and repairing the damage.
Marek Urban: What we’ve done, we’ve essentially picked up a few components of known chemistries and put these together in a thoughtful way that allows us to essentially scratch the surface, and upon exposure of the surface to sunlight, or to ultraviolet light, that scratch that was mechanically created will be able to repair itself.
Chris Smith: What’s in the mixture? What’s the chemistry that’s going on here?
Marek: It’s a very simple approach where we’ve taken known polyurethanes, which are utilised in a variety of applications ranging from automotive paints all the way to biomedical devices to polyurethane forms, and we’ve incorporated into those networks certain chemical entities which are able to break apart upon mechanical damage, and one of those chemicals is chitosan. It’s a derivative of chitin, and that chitin can be found in crab shells and shrimp shells.
That chitosan was incorporated into a polyurethane network, which was prior modified with highly reactive oxetane, which has like four bonds forming a square with oxygen in one corner and the other corners attached to the chitosan; that modification allows opening those highly strained bonds of that square to break apart and consequently create reactive groups that, upon exposure to the sunlight or ultraviolet light, are able to react back and consequently heal the wound.
Chris: So what you’re saying is that when the surface, this material, is damaged, the damage physically breaks open this ring of this square structure which is under strain or tension, and because it breaks open, it then becomes chemically active and can then begin to react with the other constituents of the polyurethanes which will effectively cause the wound that’s been created, the piece of damage, to heal again?
Marek: That is correct. Of course, we still need to learn a lot about specific chemistries and mechanisms responsible for that healing process, but it’s a really fascinating process because without those components, chitosan and oxetane, polyurethanes won’t be able to self-repair.
Chris: So if you watch the damage repairing itself, say down a microscope, how seamless is the repair of the surface and how long does it take to happen?
Marek: First of all, let me say this, that the concentration levels of those covalently attached additives is relatively small, that’s one requirement, but they are very sufficient to be able to repair the network within let’s say 30, 40 minutes upon exposure to ultraviolet sources.
Chris: And what does the ultraviolet do to make the repair happen?
Marek: Ultraviolet light essentially generates another reactive site, which in turn reacts with those that opened up as a result of damage, and consequently reacts with other constituents of the network, therefore self-repairing the entire network.
Chris: And is the amount of ultraviolet that you need to trigger this repair reaction, is there sufficient of it in sunlight for this to happen naturally?
Marek: It will just take a little longer. Typical experiments we’ve done are really going from zero to about 30 minutes, exposing to ultraviolet light, and that specific portion of the ultraviolet light that we utilise, which was about 300 nanometres, the energy density of that source corresponds to the energy density of the sunlight. So we are confident this will take a little longer, but it will obviously self-repair.
Chris: And how would you see this actually being used? Could you just work this into standard paints and surface coatings that are used on, say, cars now so that you could have a car that would, if it got scratched, repair itself?
Marek: There is no reason why it shouldn’t work. If you look at a typical example of, let’s say, a four-layer automotive paint, we tested damages in the range of about 50 microns which is about the thickness of two layers on the top. But there are other applications; I could envisage even electronic devices. Let’s say you take an iPod, you scratch it, or laptop computer, and that scratch is kind of permanent, you expose it to sunlight and that will self-repair. Of course, it may take a little longer but, as a user, I would be perfectly happy if that repair takes even a week, as long as it disappears.
This is an extract of an edition of Breaking Science. You can listen to the whole programme, originally broadcast on BBC Radio Five Live, March 2009.