Researchers have already developed composites that are able to heal microscopic cracks, but it’s hard to include enough repair material to close larger cracks in this way. Li realised that, like human wounds, such large gashes must be closed before healing. He wanted to do this by embedding threads through the composite that would contract on demand. Li knew this was possible because spider silk contracts in response to humidity. But rather than farm spiders for their silk, Li’s team span polyurethane shape memory polymers into threads.
Spider silk protein, fibroin, features soft domains containing repeated alanine blocks embedded into harder domains containing repeated glycine blocks. The interplay between these blocks gives spider silk both humidity response and remarkable strength. To make their threads Li, his Louisiana State colleague Harper Meng, and Jinlian Hu from the Hong Kong Polytechnic University in Kowloon used polymers that also feature hard and soft regions.
David Haddleton, who researches self-healing materials at the University of Warwick, UK, calls the fact that the self-healing mechanism is so close to human wound healing ‘fascinating’. ‘The mechanical strength of these self-healing materials is quite remarkable, especially when compared to materials such as Kevlar and carbon fibre,’ he adds.
- G Li, H Meng and J Hu, J. R. Soc. Interface, 2012, DOI: 10.1098/rsif.2012.0409