A bio-inspired material that combines a soft, self-healing polymer with a tough layer of graphene oxide could one day form the basis of new, ultra-tough scratch resistant coatings.
The researchers who developed the material say it combines the best properties of skin – which can heal itself from the inside out – with tooth enamel, which is hard but cannot self-repair. ‘For a material to self-heal, it generally needs to be a highly dynamic polymer network,’ says Ming Yang from Harbin Institute of Technology in China. ‘Unfortunately, this also means self-healing coatings are typically made of soft materials.’
Their material does feature a soft self-healing layer, a mixture of tannic acid and polyvinylalcohol (PVA) which form a dynamic hydrogen-bonded network. But this is reinforced by an outer PVA layer containing nanosheets of graphene oxide – a tough material that gives greater scratch resistance. This is similar, Yang says, to the structure of the skin epidermis, where hardened outer cells protect the soft, living tissue beneath, which can repair damage.
‘The more dynamic sublayer provides additional polymers to the top layer, while the top hard layer behaves like a barrier to trap these diffusing polymers,’ he says. ‘Graphene oxide is, however, much harder than the plate-like cells in skin, so we get the hard surface.’
When tested in isolation, neither layer on its own was able to fully recover after scratching. But when the soft polymer and graphene oxide sheets were integrated correctly, the resulting skin-like hybrid could self-heal.
Yang suggests the material could be used to make anti-scratch coatings or even coatings for medical devices, as the components are biocompatible. But there are still hurdles to overcome. ‘The current coating system uses water as a stimulus to initiate the healing process. This may be acceptable for indoor use but is unwanted for outdoor applications or for electronic devices,’ Yang explains. He adds that the group are also working on versions of the coating that self-heal in response to light or heat.
Wayne Hayes, a polymer chemist at the University of Reading in the UK, says the use of relatively low-cost components such as PVA is ‘very impressive’. ‘It’s the way this material is processed, the careful layer-by-layer assembly approach,’ he says. ‘What will be interesting to see is how well they can actually use this method to produce very precise structures. I think that will be the next challenge.’