From Terminator to airplane wings: self-repairing materials

Airplane wings, offshore wind turbines, sports equipment may all be repairing themselves thanks to some ‘what if’ thinking by chemists and engineers

Beyond the brutal comic-book destruction and havoc of the first Terminator movie, it’s the moment when this metal-machine from the future unpeels his skin and begins to fix his mechanical arm that delivers it’s a psychological punch.  The Terminator’s capacity for self-repairing is an unsettling exercise of intelligence, pulling together tools from its environment, transforming this brutal killing machine into something living, something more human – to misquote Descartes, ‘I self-repair, therefore I am.’ The ability of non-human things to interact with the environment in the process of repairing themselves remains a source of wonder, and it’s perhaps one reason for the extraordinary media interest around the research being done by Professor in the Duncan Wass and his team of researchers at the University of Bristol who have developed a self-healing material that could be used to fix the wings and fuselage of airplanes. 

The origins of this began as innovation often does – in a casual conversation. Professor Wass, from the University’s School of Chemistry, was chatting to his colleagues in Aerospace Engineering. ‘I’m a chemist and the engineers were saying we have this focus on composites,’ he explains, ‘but one of the particular problems we have is that if these composite materials get damaged – that could be debris from a runway flying up, or a bird strike – it can be very difficult to detect the damage and repair it.’ They call it barely visible damage. 

Over the last ten years composite materials, such as carbon-fibre reinforced composite materials have become highly valued materials in aircraft building – they are strong and light, providing better fuel-efficiency, saving money and the environment. The Boeing 787 Dreamliner achieved its fuel-efficiency partly through its fuselage being constructed through composite materials rather than aluminium. The energy industry also use composite materials, in constructing wind turbine blades.   

Professor Wass and his team sat down with the engineers, and found inspiration in the processes of the human body. ‘If we get damaged, say if you cut your finger, there are mechanisms that repair that damage, it bleeds then scabs and eventually it heals. We thought, “these self-healing functions that we have, can we put them in an airplane wing?” If you do that, you’re not going to be able to repair a huge great hole in the airplane wing, but what we should be able to repair are the tiny cracks that lead to the problems later on.’ 

Their starting point was research into other work in the field, as Wass notes, ‘you realize that lot’s of people have been looking at self-healing in all sorts of systems.’ But argues Wass, ‘what I would say is that a lot of what’s gone on before works really well in the lab – if you want to actually get it to work in real life you’d see some pretty fundamental flaws.’ 

Cosmetics, drug delivery and catalysts

While it’s just one research area in the group, it pulls together knowledge from many different areas. The team need to develop the healing agent to glue the cracks, what chemists call a monomer –small molecules stitched together to create the long molecules which will glue. Then they need to find a delivery system, ‘we encapsulate it within tiny microspheres, these things are a few microns across. You’d get tens of these across the width of a human hair.’ 

They’ve researched other areas where such a mechanism has been developed, in drug delivery and in even in formulations for cosmetics. So when the healing agent is developed they will need a ‘trigger’ that when the damage occurs and the capsules are burst, the liquid healing agent can become solid, gluing the cracks together.  ‘That’s probably where my core expertise is,’ says Professor Wass, ‘we’re going to need that trigger. They’re what we call catalysts, a lot of the science behind it is getting the right catalyst to trigger the healing event at the right time. You can see it’s bringing together things from lots of different areas to get this to work, in that sense it is closer to engineering than it is to science.’ 

Easier said than done. Wass explains, ‘the devil is in the detail to get this to work, that’s why it’s taken us three years to do this, because there is lots of hard graft in the lab to get something that really works.’ Equally crucial is getting the funding. The project started with funding linked to the UK Ministry of Defence, then they had some industry funding and now it’s being funded by the EPSRC (Engineering and Physical Sciences Research Unit). 

So when an excited media ask when is the work going to roll-out in an airplane, it really depends on commercial issues. ‘I can have something that is ready to go, but that doesn’t address the commercial challenges and the will within industry to actually apply this as well,’ says Professor Wass. In any case because of the safety issues, airplanes are the most challenging application of the technology, says Wass. Offshore wind turbines are another possible application as they are difficult to get to and repair. He also lists other carbon-fibre composite materials, ‘bicycle frames or sports equipment generally where you can imagine this being used.’ But excitable media have highlighted other game-changing potential applications – smart-phone screens and self-healing nail polish. ‘If you trawl round the web you will see people claiming we are saying we will repair those things’, says Professor Wass.  ‘That’s the press making up their own stories, I didn’t realize it was such an issue! We’d need to find a slightly different way to do it, but it would be great one if we could solve that.’