Stanford's touch-sensitive plastic skin heals itself

Nobody knows the remarkable properties of human skin like the researchers struggling to emulate it. Not only is our skin sensitive – sending the brain precise information about pressure and temperature – but it also heals efficiently to preserve a protective barrier against the world. Combining these two features in a single synthetic material presented an exciting challenge for Stanford chemical engineering Professor Zhenan Bao and her team.

Now, they have succeeded in making the first material that can both sense subtle pressure and heal itself when torn or cut. Their findings was published Nov. 11 in the journal Nature Nanotechnology.

In the last decade, there have been major advances in synthetic skin, but even the most effective self-healing materials had major drawbacks. Some had to be exposed to high temperatures, making them impractical for day-to-day use. Others could heal at room temperature, but repairing a cut changed their mechanical or chemical structure, so they could heal themselves only once. Most important, no self-healing material was a good bulk conductor of electricity, a crucial property.

The Stanford researchers succeeded by combining two ingredients to get "the best of both worlds" – the self-healing ability of a plastic polymer and the conductivity of a metal. They started with a plastic consisting of long chains of molecules joined by hydrogen bonds – the relatively weak attractions between the positively charged region of one atom and the negatively charged region of the next. The dynamic bonds allow the material to self-heal as even when the molecules easily break apart, when they reconnect, the bonds reorganize themselves and restore the structure of the material after it gets damaged. The result is a bendable material, which even at room temperature feels a bit like saltwater taffy left in the fridge.

To this resilient polymer, the researchers added tiny particles of nickel, which increased its mechanical strength. The nanoscale surfaces of the nickel particles are rough, which proved important in making the material conductive. The researchers then took a thin strip of the material and cut it in half with a scalpel. After gently pressing the pieces together for a few seconds, the researchers found the material gained back 75 percent of its original strength and electrical conductivity. The material was restored close to 100 percent in about 30 minutes – a lot faster than the speed in which human skin heals. What's more, the same sample could be cut repeatedly in the same place. After 50 cuts and repairs, a sample withstood bending and stretching just like the original.

The composite nature of the material created a new engineering challenge for the team. The team found that although nickel was key to making the material strong and conductive, it also got in the way of the healing process by preventing the hydrogenNike Jordan Melo Shoes