The human skin is a special material: It needs to be flexible, to prevent it from cracking every time a user clenches his fist, as well as be sensitive to stimuli like touch and pressure - which are measured as electrical signals, so it needs to conduct electricity. Crucially, if it is to survive the wear and tear it is put through every day, it needs to be able to repair itself. Researchers may have designed a synthetic version - a flexible, electrically conductive, self-healing polymer. The result is part of a decade long miniboom in "epidermal electronics"- the production of circuits thin and flexible enough to be attached to skin (for use as wearable heart rate monitors, for example) or to provide skin like touch sensitivity to prosthetic limbs. The problem is that silicon, the base material of the electronics industry, is brittle. Several research groups have investigated different ways to produce flexible electronic sensors. A research team has developed a fake skin that can heal itself in 30 minutes if torn or cut - Synthetic material from plastic that consists of a long chain of molecules joined by relatively weak hydrogen bonds, resulting in the molecules breaking easily. Tiny particles of nickel were mixed in to give the plastic conductivity and strength. As a result, the molecules easily break apart, but 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.

Chemical engineer Zhenan Bao of Stanford University in Palo Alto, California, and her team explored the potential of self-healing polymers in epidermal electronics. However, all the self-healing polymers demonstrated to date had very low bulk electrical conductivities and would have been little use in electrical sensors. Writing in Nature Nanotechnology, the researchers detail how they increased the conductivity of a self-healing polymer by incorporating nickel atoms, allowing electrons to "jump" between the metal atoms. The polymer is sensitive to applied forces like pressure and torsion (twisting) because such forces alter the distance between the nickel atoms, affecting the difficulty the electrons have jumping from one to the other and changing the electrical resistance of the polymer. To demonstrate that both the mechanical and the electrical properties of the material could be repeatedly restored to their original values after the material had been damaged and healed, the researchers cut the polymer completely through with a scalpel. After pressing the cut edges together gently for 15 seconds, the researchers found the sample went on to regain 98% of its original conductivity. And crucially, just like the ESPCI group's rubber compound, the Stanford team's polymer could be cut and healed over and over again. This is the first time that a combination of both mechanical and electrical self-healing has been successful. The team is working to make the polymer more like human skin.