Rubber-like synthetics known as elastomers are normally able to stretch to twice or three times their original length, so Stanford University researchers said they were surprised to find one new elastomer they developed could expand to more than 100 times its length.
Furthermore, a study on the elastomer published in Nature Chemistry revealed that subjecting it to an electric field causes it to grow and contract, which means it has potential as an artificial muscle.
Breaking barriers in robotics
Used in robotics, artificial muscles have shortcomings compared to real muscle. Small holes or defects in the materials used to make artificial muscle can deprive them of their resilience.
However, this new material has exceptional self-healing qualities. Damaged polymers normally call for a solvent or heat process to restore their properties, but the new material exhibited a remarkable capability to heal itself at room temperature, even if the affected pieces are aged for days. Indeed, scientists discovered that it could self-repair at temperatures as low as 4 degrees F, or about as cold as a walk-in freezer.
The team attributes the stretchability and self-healing ability of their new material to some vital improvements to a kind of chemical bonding referred to as crosslinking. This involves connecting linear chains of linked molecules in a kind of fishnet pattern, has prior produced a tenfold extension in polymers.
Pushing organic development
To develop the new elastomer, the team designed unique organic molecules to attach to the short polymer strands in their crosslink to produce a series of structure known as ligands. These ligands joined together to form longer polymer chains, essentially spring-like coils with inherent stretchiness.
Then, they added metal ions to their material. These ions have a chemical affinity for the ligands and when the polymer is 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,” study author Zhenan Bao, a chemical engineering professor at Stanford, said in a news release. “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.”
The team learned that they could adjust the polymer to be more elastic or recover faster by differing the amount or kind of metal ion included.
The new elastomer’s potential as an artificial muscle dovetails with Bao’s attempts to generate artificial skin designed to bring back some sensory function to individuals with prosthetic limbs. In previous research studies her team has generated flexible but fragile polymers, studded with stress sensors to recognize the difference between a handshake and a landing butterfly. This new, sturdy material could form portion of the physical structure of artificial skin, Bao said.
Image credit: Bao Research Group