A team of chemists and engineers at Stanford has created the first touch-sensitive synthetic material capable of healing itself quickly and repeatedly at room temperature.

The advance has immense potential, from creating smarter prosthetics to developing more resilient devices that repair themselves. Researchers trying to emulate human skin have extensively researched its properties to find the optimum solution. The human skin is not only sensitive, sending precise information to the brain regarding pressure and temperature, but it is also capable of healing itself and preserving a protecting barrier.

Creating a synthetic material that combines these two remarkable properties has been a great challenge for Stanford Chemical Engineering Professor Zhenan Bao and colleagues. The scientists have now succeeded in developing the first material capable of sensing subtle pressure and healing itself when cut or torn.

Challenges & Drawbacks

According to Bao, the study's lead researcher, the last decade has seen major advances in synthetic skin, but even the most effective self-healing materials had notable drawbacks. While some worked only when exposed to high temperatures, which made them impractical for daily use, others were capable of healing at room temperature but could only heal themselves once, as repairing a cut changed their mechanical or chemical structure. In addition, none of those self-healing materials was a good bulk conductor of electricity, which is an essential property.

"To interface this kind of material with the digital world, ideally you want them to be conductive," said Benjamin Chee-Keong, the first author of the paper.

'The Best of Both Worlds'

With the new material, the scientists managed to combine two ingredients to achieve what Bao describes as "the best of both worlds" - the conductive properties of a metal and the self-healing ability of a plastic polymer.

The researchers started with a plastic comprising of long chains of molecules merged by hydrogen bonds - the somewhat 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," explained co-first author Chao Wang. While the molecules break apart easily, when they reconnect the bonds reorganize themselves and restore the structure of the material, thus healing the material after it gets damaged. This resulted in a bendable material.

The researchers added tiny particles of nickel to this resilient polymer, which increased its mechanical strength. Nickel particles have rough nanoscale surfaces, which played a major role in making the material conductive. The combination resulted in a polymer capable of serving as a great conductor.

Healing Quickly & Repeatedly

To see how well the synthetic material could restore its mechanical strength and its electrical conductivity after damage, the scientists took a thin strip and cut it in half with a scalpel. The researchers then pressed the pieces together for a few seconds, and found that the material gained back 75 percent of its original strength and electrical conductivity. In roughly half an hour, the material gained nearly 100 percent of its initial properties.

Not only did the material heal faster than human skin, but it also worked even after being cut repeatedly in the same place. A sample withstood bending and stretching just like the original material even after 50 cuts and repairs.

The composite nature of the material, however, posed a new engineering challenge for the scientists. Although nickel was the key to making the material strong and conductive, Bao and her colleagues discovered that it also impeded the healing process, preventing the hydrogen bonds from reconnecting as well as they should.

To deal with this trade-off, Bao said the team might adjust the size and shape of the nanoparticles, or even the polymer's chemical properties, for future generations of the synthetic material. The extent of these self-healing properties was amazing nonetheless, said Wang.

"Before our work, it was very hard to imagine that this kind of flexible, conductive material could also be self-healing," explained Wang.

Touch-Sensitive

The scientists also explored the possibility of using the material as a sensor. Electrons that make up an electrical current have to hop from one nickel particle to another to get through this material. The distance separating the nickel particles determines how much energy an electron will need to break free from one particle and move to another. Twisting or applying pressure on the synthetic skin changes the distance between the nickel particles, thus also changing how easily electrons can move.

Potential Applications

According to Tee, the material is sensitive enough to detect the pressure of a handshake, which, added Bao, would make it ideal for use in prosthetics. In addition to downward pressure, the material is also sensitive to flexion, which means that a prosthetic limb may one day be able to register the degree of bend in a joint.

Tee also pointed out other exciting commercial possibilities. Electrical devices and wires, for instance, could repair themselves and get electricity flowing again if coated in this material. This would eliminate expensive and difficult maintenance, and would be particularly helpful in hard to reach places.

The team's goal from now on is to make the material stretchy and transparent so that it can be used for coating and overlaying electronic devices or displays, added Bao.