Stretchy, self-healing polymer may lead to artificial muscles

Scientists have developed a polymer that can stretch to about hundred times its length and can even self-heal at room temperature, making it potentially useful as an artificial muscle.
The researchers at Stanford University in the US made a super-stretchy, rubber-like plastic known as an elastomer that expands and contracts when exposed to an electric field.
Elastomers can normally be stretched two or three times their original length and spring back to original size.
Researchers found that they were able to stretch a one inch piece of their new elastomer to about a 100 inches.
Small holes or defects in the materials currently used to make artificial muscle can rob them of their resilience. Nor are they able to self-repair if punctured or scratched.

However, the new material has remarkable self-healing characteristics.

Damaged polymers typically require a solvent or heat treatment to restore their properties, but the material showed a remarkable ability to heal itself at room temperature, even if the damaged pieces are aged for days.
Researchers found that it could self-repair at temperatures as low as minus 20 degrees Celsius, or about as cold as a commercial walk-in freezer.
The team attributes the extreme stretching and self-healing ability of their new material to some critical improvements to a type of chemical bonding process known as cross-linking.
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 they designed special organic molecules to attach to the short polymer strands in their crosslink to create a series of structure called ligands.
These ligands joined together to form longer polymer chains - spring-like coils with inherent stretchiness.
Then they added to the material 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.
When relaxed, the affinity between the metal ions and the ligands pulls the fishnet taut.
"Basically the polymers become linked together like a big net through the metal ions and the ligands," said Professor Zhenan Bao from Stanford.

The team found that they could tune the polymer to be stretchier or heal faster by varying the amount or type of metal ion included.
The researchers also showed that this new polymer with the metal additives would twitch in response to an electric field.
While more work is needed to control the material more precisely, the observation opens the door to promising applications, researchers said.
The study was published in the journal Nature Chemistry.