Touch-sensitive plastic can ‘heal itself’

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Stanford University researchers have created the first synthetic material that is both sensitive to touch and capable of healing itself quickly and repeatedly at room temperature.

The advance, they believe, could lead to smarter prosthetics or resilient personal electronics that can repair themselves. The material was created by combining a plastic polymer with tiny particles of nickel. The polymer consisted of long chains of molecules joined by hydrogen bonds. According to researcher Chao Wang, these are what allow the material to 'self-heal'. He explained: "The molecules easily break apart, but then when they reconnect, the bonds reorganise themselves and restore the structure of the material after it gets damaged." The particles of nickel were added to the polymer to increase its mechanical strength and help make the material conductive. "This resulted in a polymer that was not only a good insulator, but an excellent conductor," Wang added. The next step for the researchers was to see how well the material could restore both its mechanical strength and its electrical conductivity after damage. The researchers 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, they found the material gained back 75% of its original strength and electrical conductivity. The material was restored close to 100% in about 30 minutes. "What's more," says Wang, "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." Because of the composite nature of the material, the team found that although nickel was key to making it strong and conductive, it also got in the way of the healing process by preventing the hydrogen bonds from reconnecting as well as they should. For future generations of the material, the team is looking to adjust the size and shape of the nanoparticles, or even the chemical properties of the polymer, to get around this trade-off. Lead researcher Professor Zhenan Bao believes the material is sensitive enough to detect the pressure of a handshake, opening up the possibility of it being used in prosthetics. Other potential applications include using the material as a coating for electrical devices so that they can repair themselves if damaged and get electricity flowing again. The team's goal now is to make the material stretchy and transparent, so that it can be used for wrapping and overlaying electronic devices or display screens.