Self-healing materials

Scientists at the New York University in Abu Dhabi are attempting to challenge the fundamental rule of nature that all materials follow a one-way path to deterioration with time by developing self-healing crystals. 

Self-healing had previously been reported in soft materials and organic polymers, but not a lot of research has focused on solids and crystals assuming that their high molecular order and low diffusion would preclude the possibility.

Capitalizing on dynamic covalent chemistry, however, a team of researchers now show that self-healing in crystals is not only possible, but can be demonstrated at an unprecedented rate. 

"That's one small step for a molecular crystal, one giant leap for the crystal engineering," says corresponding author Panče Naumov of NYUAD. "These findings are posed to change the way that the organic crystals have been percieved in the past -- as stiff, brittle, and irrecoverably fragile."

The team chose a compound called dipyrazolethiuram, whose crystals are held together by physically strong but chemically reversible disulfide bonds — covalent single bonds between two sulfur atoms. When crystals fissure, disulfide bonds break equally, leaving each sulfur atom with a single additional electron — a radical. 

Radicals are notoriously unstable and short-lived, but the high double bond nature of the rest of the compound can stabilize the radical by transferring the free electron from one bond to the next, giving disulfide bonds enough time to reconstruct when the radical sulfur pairs are brought back into close proximity — a concept known as “radical shuffling”.

To test their theory, the team synthesized crystals of dipyrazolethiuram, made an incision on the surface and then applied a pulling force to the edges, causing it to break along the incision. They then brought the two halves back together and applied a compression force. 

The crystal was considered healed if one half was able to support the weight of the second half. 

"For a project that started off as a pie in the sky, cocktail napkin idea, I'm wonderfully surprised that we were actually able to discover a self-healing crystal," comments first author Patrick Commins. "Now that the effect has been made known, we may find that the phenomenon is much more common than we initially perceived."

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