Self-repairing plastic that can heal cuts and scratches on its own surface has been presented at a meeting of the American Chemistry Society.

Inspired by human skin, the new material could make for self-healing cars and smartphones, that change colour when damaged and can close over "wounds" of its own accord.

Professor Marek Urban demoed the self-healing plastic - sometimes described as "the Holy Grail of Material Science" - yesterday at the society's yearly conference. Not only is the plastic capable of healing over scratches and cracks, it also visually highlights where it is damaged by turning red, with the red colour disappearing as the plastic fixes itself.

The healing process is triggered by sunlight and variations in surrounding acidity or temperature.

A piece of plastic healing itself

Previous methods of making self-healing plastic have experimented with embedded capsules that release new repair material when cracked open. Prof Urban's approach is to make broken bonds in the cracked plastic reform when exposed to a stimulus, allowing the plastic to heal itself multiple times over instead of just once.

The secret lies in the small molecular links or "bridges" that span the long chains of chemicals that compose plastic. When plastic is scratched or cracked, these links break and change shape. Prof Urban tweaked them so that changes in shape produce a visible colour change — a red splotch that forms around the defect. In the presence of ordinary sunlight or visible light from a light bulb, or acidity and temperature changes, the bridges reform, healing the damage and erasing the red mark.

Prof Urban, of the University of Southern Mississippi, said:

Our new plastic tries to mimic nature, issuing a red signal when damaged and then renewing itself when exposed to visible light, temperature or pH changes.

The plastic research, part funded by the US Department of Defense, could help in many cases where fixing scratched or cracked plastic is impossible, with strong light being all that's required to smooth out a scratch. The ACS suggest it could have a use in battlefield weapons systems, in fixing simple things like scratched car fenders and in assessing damage in internal parts of anything from computers to aircraft. ®