In 1941, George de Mestral, a Swiss engineer, noticed his dog was covered in burrs after a long walk in the Alps. Fourteen years and many microscope slides later, de Mestral patented the hooked design for Velcro, inspired by the burrs. Velcro is probably the most well-known form of biomimicry, or human-made designs based on natural structures. Since then, slug mucus has inspired surgical glue, lotus leaves have helped create self-cleaning surfaces, and spider silk has been harnessed to create super-strong fibers.
“We have been always amazed by the power of nature to create sophisticated structures using [the] most elegant way,” Ming Yang, the senior author on this study and professor at the Harbin Institute of Technology in China, told me via email. Yang and his co-authors modeled the material they developed after human skin, setting out to create a polymer that is both self-healing and hard.
When the epidermis, the thick outermost layer of skin, is damaged, cells from the softer layer underneath migrate to the top to heal the injury, hardening and becoming dead cells to protect the live layers beneath.
Our skin is not very stiff, however. It is certainly not as hard as tooth enamel, but teeth cannot repair themselves, as anyone who has had a cavity filled knows. Yang and his co-authors created a material that represents the best of both worlds, with a multilayer structure similar to skin to mimic the self-healing process.
They created the layers using polyvinyl alcohol, a synthetic polymer that has been used in everything from fishing to eye drops, and tannic acid, which is used to stain wood and clarify beer. They are both environmentally friendly, Yang told me. He and his co-authors describe this as a “living” layer, and it acts like the live skin layered under your epidermis. The upper layers have high concentrations of graphene oxide, a hard substance also used in battery electrodes.
The researchers couldn’t just slap thick layers together, because it wouldn’t be self-healing. “You can simply stack two layers but you may end up with nothing than a thicker sheet,” Yang said.
The structure requires more nuance: layers with mixtures of the polymers and bonds between them so they would interact and respond to damage. The researchers created this effect with molecular assembly, a technique that positions molecules precisely to control chemical reactions. Instead of more cells dying and becoming harder as they reach the surface of the skin, the material has greater amounts of graphene oxide with each layer.
When the material is cut, the softer polyvinyl alcohol and tannic acid molecules can fill in to the damaged areas, pulled up by a network of hydrogen bonds. The hardness of the upper layers then traps these polymers underneath it.
The resulting material has record-high stiffness of 31.4 GPa, or gigapascals, equivalent to roughly 4.5 million pounds per square inch. This approaches the stiffness of artificial tooth enamel created using nanotechnology at 39.8 GPa, or 5.8 million pounds per square inch. (Real tooth enamel, by contrast, is 89 GPa.)
As for its self-healing abilities, Yang and his co-authors used a blade to make deep cuts, and sandpaper to damage large surface areas, but the material was able to make a “complete recovery” with every trial. It was even able to maintain that durable outer shell—the material was just as hard as it had been before it was damaged.
The material is also antibacterial. The researchers let a batch of E. coli run circles around the material on a culture dish, which showed it was just as protective as human skin. That could make it “a good choice for medical coatings,” Yang said. If combined with other polymer systems that have been developed in response to heat or light, he and the other authors envision its use in dentistry, which is where they’re going next with their research. This material could lower the risk of electronic failure and reduce maintenance costs, which may be especially important in the future considering the surging healthcare costs in the US.
I wonder if one day we will end up with self-healing iPhones as hard as our own teeth.
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