A new composite material can repeatedly restore its electrical and mechanical properties after being damaged. Its developers hope to use the material as electronic skin in robots or biomimetic prosthetic devices.
The material, developed by Zhenan Bao and coworkers at Stanford University, is made of a randomly branched, hydrogen-bonding polymeric network embedded with nickel microparticles that have nanostructured surfaces (Nat. Nanotechnol., DOI: 10.1038/nnano.2012.192).
Bao and coworkers started with a mixture of polyurethanes. To the polymer mixture, they added as much as 30% by volume nickel microparticles. The optimum amount of nickel is a trade-off between the material’s conductivity and flexibility.
The polymer network forms hydrogen bonds with itself and with oxides on the surface of the nickel particles. The hydrogen bonds are the weakest bonds in the system, and they preferentially break when the composite is damaged. When cut pieces of the material are brought together, the hydrogen bonds quickly re-form.
The healing properties are sensitive to moisture, confirming that hydrogen bonding is behind the process. “If water is adsorbed to the cut surface, it occupies the hydrogen bonding site and makes it difficult to repair,” Bao says.
The healing of the material is so rapid that the electrical conductivity is restored to more than 90% of its original value within 15 seconds. The mechanical properties take a little longer. Complete healing takes 30 minutes. And the material can heal itself repeatedly.
The ability to achieve both electrical and mechanical healing “is a tour de force,” says Ludwik Leibler, a researcher at the School of Industrial Physics & Chemistry, in Paris, whose group originally developed the mechanically self-healing polymers that Bao’s team used as its starting point. Incorporating the amount of nickel needed for conductivity while keeping mechanical self-healing is challenging, he adds.
The Stanford researchers used patches of the material as electronic skin on a doll-like mannequin. A touch sensor on the palm responds to pressure, whereas a flexion sensor at the elbow joint responds to bending.
The team now seeks to make the material stretchable as well as bendable so that it will be even more like natural skin.