Nary an engineering model exists like that of human skin, which makes it all the more difficult for researchers to replicate it. Skin is sensitive enough to send the brain precise information about pressure and temperature, but it can also heal efficiently enough to create a protective forcefield against the outside elements. The combination of these two features in a single synthetic material has presented an exciting challenge for Stanford Chemical Engineering Professor Zhenan Bao and her team.
Now, they've successfully created the first material that can sense subtle pressures and heal itself when torn or cut. Their findings will be published on November 11 in the journal Nature Nanotechnology.
The last decade has shown us major advances in synthetic skin, but even the most effective self-healing materials presented major drawbacks. Some needed to be exposed to high temperatures, rendering them impractical for day-to-day use. Others could heal effectively at room temperature, but repairing a cut or tear compromised their mechanical and chemical makeup, so they could only heal once. Most crucially, none of these self-healing synthetics could conduct electricity.
"To interface this kind of material with the digital world, ideally you want them to be conductive," said Benjamin Chee-Keong Tee, first author of the paper.
The success for the researchers lay in the fact that they combined two ingredients to get what Bao calls "the best of both worlds" – the self-healing ability of a plastic polymer and the conductivity of a metal.
The researchers began with a plastic comprised of long chains of molecules joined by hydrogen bonds. The weak attractions between the positively charged region of one atom and the negatively charged region of the next.
"These dynamic bonds allow the material to self-heal," said Chao Wang, a co-first author of the research. The molecules are easily broken, but when they reconnect, the bonds reorganize themselves in such a way that the structure is restored after it gets damage. The result is a flexible material, which maintains the consistency of saltwater taffy left in the fridge...even at room temperature.
To the polymer, the researchers introduced tiny particles of nickel, which helped to increase its mechanical strength. The nanoscale surfaces of the nickel particles are rough, an important part of making the material conductive. Tee compared these surface features to "mini-machetes," with each jutting edge concentrating an electrical field and making it easier for current to flow from one particle to the next.
The resulting polymer possessed uncommon characteristics. "Most plastics are good insulators, but this is an excellent conductor," Bao said.
The next step was to measure how well the material could restore both its mechanical strength and its electrical conductivity after damage. The researchers cut a thin strip of the material in half with a scalpel. After pressing the pieces together for a few seconds, they found the material regained 75 percent of its original strength and electrical conductivity. The material then regained close to 100% of its strength in about 30 minutes. "Even human skin takes days to heal. So I think this is quite cool," said Tee.
Additionally, the same sample could be cut repeatedly in the same place, and even after 50 cuts and repairs, it withstood bending and stretching with the same resilience as the original.
The composite property of the material was still an engineering hurdle for the team. Bao and her co-authors found that although the nickel was crucial for making the material strong and conductive, it could also impede the healing process, preventing the hydrogen bonds from reconnecting as well as they should.
Bao says that future incarnations of the material might involve adjusting the size and shape of the nanoparticles or even the chemical properties of the polymer.
Nonetheless, Wang said the extent of these self-healing properties came as a surprise: "Before our work, it was very hard to imagine that this kind of flexible, conductive material could also be self-healing."
The team also explored the material's capabilities as a sensor. For the electrons that make up an electrical current, trying to pass through this material is analogous to crossing a stream by hopping from stone to stones. The stones are the nickel particles and the distance between them indicates how much energy an electron will need to free itself from one stone and move to another.
Twisting or putting any amount of pressure on the synthetic skin changes the distance between the nickel particles and the easy with which the electrons can move. These subtle changes in electrical resistance can be translated into information about pressure and tension on the skin.
According to Tee, the material is sensitive enough to detect the pressure of a handshake. It's ideal use might be ideal in prosthetics. The material is sensitive not only to downward pressure, but also to flexion, so a prosthetic limb might someday be able to register the degree of bend in a joint.
Tee has also highlighted other commercial possibilities. Electrical devices and wires coated in the synthetic material could repair themselves and get electricity flowing through again without costly and difficult maintenance, especially in hard to reach places.
The team's next goal is to make the material stretchy and transparent. This will make it suitable for wrapping and overlaying electronic devices and display screens.
For more information, read the press release here: http://www.eurekalert.org/pub_releases/2012-11/ssoe-tps110812.php