Dropping your mobile phone can ruin your whole day as you look down at the spiderweb of cracks surrounding a small hole in the once-pristine plastic case. Now imagine watching as those cracks and that hole seal up by themselves, leaving behind a completely healed case. That may sound like science fiction, but it may not be for long with a team of researchers at the University of Illinois having developed a new system that doesn't just repair minor cracks in plastic, but regenerates to heal large holes.

Self-repairing materials have been around since engineers came up with fuel tanks in the Second World War that were designed to seal themselves when punctured by bullets and shrapnel. Since then, there have been compounds for car engines designed to stop leaks, and in recent years, plastics have been created that have the ability to heal themselves whether it’s through resin held in microcapsules or various patterns of capillaries that burst in response to damage to release healing agents that bleed into cracks, sealing and strengthening the area.

Such self-repairing plastics work, but they only have limited application. They can handle microscopic cracks, but not outright punctures that leave holes in a plastic panel. When this happens, there isn't enough resin to do the job or, more likely, it will just pour out of the hole.

The idea behind the regenerative plastic developed at the University of Illinois is to produce a system that can handle actual holes rather than just cracks by filling them with regenerative materials in a way similar to how living organisms heal themselves. It builds on the team’s previous work in creating fiber-reinforced composite materials with a network of microscopic veins by lacing them with fibers that disintegrate, leaving behind a sort of artificial bloodstream. In this case, it’s two parallel systems; each carrying a different liquid. When separated, they flow freely, but if the plastic is damaged, the capillaries open and the liquids react with one another to form a polymer gel that fills the gaps and hardens.

"Vascular delivery lets us deliver a large volume of healing agents – which, in turn, enables restoration of large damage zones," says Nancy Sottos, a professor of materials science and engineering. "The vascular approach also enables multiple restorations if the material is damaged more than once."

According to the team, the tricky bit in all this is taking into account the various factors, such as gravity, that can affect the flow and hardening. If the liquid gels too quickly, it just seals up the capillaries without repairing any cracks or holes. If it flows too freely, then it leaks away uselessly like trying to apply epoxy to a boat hull underwater. The team designed the sealant as a gel because it remains in place and builds on itself, filling the hole.

Another factor is control. Not all damage is the same and what will repair one sort of problem will be largely useless for another, so the team made the system controllable. By adjusting temperature and flow rate, the right amount of gel forms in the right place to fill holes or cracks. In tests, the regenerative plastic could “heal” holes simulating a 9 mm bullet hole.

The team is currently working on how to adapt the system to industrial applications in different plastic materials, saying that it could be used in everything from car bumpers to aerospace applications.

The team’s findings were published in the journal Science.

The video below outlines how the regenerative plastic works.

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