Researchers from the Chinese Harbin Institute of Technology have developed an intriguing material that combines a soft, self-healing polymer with a tough layer of graphene oxide and could one day form the basis of ultra-tough scratch resistant coatings.
The Harbin team sees the material as a combination of 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", say the researchers. "Unfortunately, this also means self-healing coatings are typically made of soft materials".
The new material features a soft self-healing layer, a mixture of tannic acid and polyvinylalcohol (PVA) which form a dynamic hydrogen-bonded network. But this part is reinforced by an outer PVA layer containing nanosheets of graphene oxide which gives greater scratch resistance. This is similar, the team 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," the team 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.
The researchers suggest that 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 some issues 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" the team explains. The group is also working on versions of the coating that self-heal in response to light or heat.