Human skin is a special material: It needs to be flexible, so that it doesn't crack every time a user clenches his fist. It needs to be sensitive to stimuli like touch and pressure—which are measured as electrical signals, so it needs to conduct electricity. Crucially, if it's to survive the wear and tear it's put through every day, it needs to be able to repair itself. Now, researchers in California may have designed a synthetic version—a flexible, electrically conductive, self-healing polymer.
The result is part of a decadelong 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 skinlike touch sensitivity to prosthetic limbs. The problem is that silicon, the base material of the electronics industry, is brittle. So various research groups have investigated different ways to produce flexible electronic sensors.
Chemists, meanwhile, have become increasingly interested in "self-healing" polymers. This sounds like science fiction, but several research groups have produced plastics that can join their cut edges together when scientists heat them, shine a light on them, or even just hold the cut edges together. In 2008, researchers at ESPCI ParisTech showed that a specially designed rubber compound could recover its mechanical properties after being broken and healed repeatedly.
Chemical engineer Zhenan Bao of Stanford University in Palo Alto, California, and her team combined these two concepts and explored the potential of self-healing polymers in epidermal electronics. However, all the self-healing polymers demonstrated to date had 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.
"I think it's kind of a breakthrough," says John J. Boland, a chemist at the CRANN nanoscience institute at Trinity College Dublin. "It's the first time that we've seen this combination of both mechanical and electrical self-healing." He is, however, skeptical about one point: "With a scalpel you can very precisely cut the material without inducing significant local mechanical deformation around the wound." Failure due to mechanical tension, however, could stretch the material, producing significant scarring and preventing complete self-healing, he suspects.
Now, Bao and her fellow researchers are working to make the polymer more like human skin. "I think it will be very interesting if we can make the self-healing skin elastic," she says, "because, while it's currently flexible, it's still not stretchable. That's definitely something we're moving towards for our next-generation self-healing skin."