Super stretchy and self-healing polymer could be suitable for artificial skin and muscles

Original news release was published by Stanford University.

Testing the stretchiness of a newly synthesized rubberlike polymer in one of the Stanford University labs has yielded surprising results. Cheng-Hui Li, a member of a research group led by chemical engineering Professor Zhenan Bao, has developed a type of elastomer – the kind of plastic that can normally be stretched to two or three times its original length. One of the common tests includes stretching the material until it snaps. When the team tested the new one-inch sample (2.54 cm), they have reached the 45 inch limit (114.3 cm) of their clamping machine with no signs of wear on the new elastomer. Li and another team member have decided to pull on the two sides themselves, managing to stretch it to 100 inches (254 cm). Bao was stupefied.

They have presented their findings in yesterday’s Nature Chemistry, also showing that the new elastomer expands and contracts when exposed to an electric field, opening up options for future artificial skin and muscles. Artificial muscles currently used  in consumer technology and robotics are prone to puncturing and scratching, and need to be repaired by heat treatment, or with a solvent. This new material, however, has extraordinary self-healing properties even at room temperature. Researchers have found that it maintains the ability to heal itself at a temperature as low as -20 °C.

The extreme stretching and self-healing characteristics are attributed to improvements to a type of chemical bonding process known as crosslinking. This process, which involves connecting linear chains of linked molecules in a sort of fishnet pattern, has previously yielded a tenfold stretch in polymers.

First, the team designed special organic molecules to attach to the short polymer strands in their crosslink to create a series of structures called ligands. These ligands joined together to form longer polymer chains – spring-like coils with inherent stretchiness. Then they added metal ions, which have a chemical affinity for the ligands. When this combined material is strained, the knots loosen and allow the ligands to separate. But when relaxed, the affinity between the metal ions and the ligands pulls the fishnet taut. The result is a strong, stretchable and self-repairing elastomer.

“Basically the polymers become linked together like a big net through the metal ions and the ligands,” Bao explained. “Each metal ion binds to at least two ligands, so if one ligand breaks away on one side, the metal ion may still be connected to a ligand on the other side. And when the stress is released, the ion can readily reconnect with another ligand if it is close enough.”

They have found that they can adjust the properties of this elastomer by varying the amounts or type of metal ions used, resulting in a polymer that is even stretchier and that can heal faster. What follows is research into more controlled expansion and contraction of the material upon exposure to electricity. This would open doors to more promising applications, such as resilient artificial skin that provides tactile feedback, but also advances in wearable electronics or medical implants that would require lower maintenance.