First pressure-sensitive, self-healing material

The material repairs itself in about 30 minutes after being sliced in half with a scalpel

Our largest bodily organ is also one of the most remarkable. Not only is our skin pressure sensitive, it is also able to efficiently heal itself to provide a protective barrier between our insides and the world around us. While we have covered synthetic materials that can repair themselves or are pressure sensitive, combining these properties in a single synthetic material has understandably proven more difficult. Now researchers at Stanford University have developed the first pressure-sensitive synthetic material that can heal itself when torn or cut, giving it potential for use in next-generation prostheses or self-healing electronic devices.

To create a material that combines the self-healing ability of a plastic polymer with the conductivity of a metal, the Stanford team in the lab of chemical engineering Professor Zhenan Bao, started with a plastic made of long chains of molecules joined by hydrogen bonds. It is these bonds, created by the relatively weak attractions between the positively charged region of one atom and the negatively charged region of the next, that team member Chao Wang says allows the molecules to break apart easily.

However, the bonds are able to reorganize themselves when reconnected so that the material’s structure can be restored after being damaged. The material is also bendable at room temperature, with a consistency that Wang says feels a bit like saltwater taffy that has been left in the fridge.

To provide the conductivity required to make the material pressure-sensitive, the researchers then added nickel nanoparticles. As well as increasing the mechanical strength of the material, the rough nanoscale surfaces of the nickel concentrate an electrical field to make it easier for a current to flow from one particle to the next. The result was a plastic with excellent conductive properties.

While the researchers knew the material could restore its mechanical strength after damage, they needed to check whether its electrical conductivity was also restored.

After cutting a thin strip of the material in half with a scalpel, they gently pressed the two pieces back together for a few seconds and found that 75 percent of its original strength and electrical conductivity was restored. After about 30 minutes, the restoration was close to 100 percent. Additionally, the same piece of material could be cut repeatedly in the same place, with a sample still able to repair itself and retain its flexibility after being cut 50 times.

Although the time the material takes to repair itself is impressive, the researchers say they may be able to improve upon it. The delay is caused by the nickel particles, which prevent the hydrogen bonds from reconnecting as well as they should. Bao said that adjusting the size and shape of the nickel nanoparticles or the chemical properties of the polymer may help speed up the healing process in future generations of the material.

Because twisting or putting pressure on the material alters its electrical resistance by changing the distance between the nickel particles, the team was able to use these subtle changes to provide information about pressure and tension on the material.

Researcher Benjamin Chee-Keong Tee said it was able to detect the pressure of a handshake. In addition to downward pressure, it is also able to detect flexing. So in addition to potentially being used as a touch-sensitive skin in prosthetic devices, the material may also be able to register the degree of bend in the joint of a prosthetic limb.

Other potential applications suggested by Tee include electrical devices and wires coated in the material that could self repair to get the electricity flowing without costly and time-consuming maintenance.

Not content with the material’s self-healing and pressure-sensitive properties, the researchers are aiming to make it stretchy and transparent. This would expand its potential applications to include protective coverings on electrical devices and displays.

The team’s research appears in the journal Nature Nanotechnology.

Source: Stanford University