Stanford chemical engineering Professor ZhenanBao and one of her team members, Cheng-Hui Li, wanted to test the stretchiness and find the breaking point of a new elastomer that had just been synthesized. But the clamping machine typically used to measure elasticity could only stretch about 45 inches. So Li, a visiting scholar from China, and another team member had to hold opposing ends in their hands, standing further and further apart, eventually stretching a 1-inch polymer film to more than 100 inches. The new elastomer also twitches when exposed to an electric field, which causes it to expand and contract. This makes it potentially useful as an artificial muscle.
Artificial muscles are currently used in some consumer technology and robotics but Bao said that they have shortcomings compared to a real bicep. The materials currently used to make artificial muscles have small holes or defects that can take away their resilience. They are also unable to self-repair if punctured or scratched.
But the new material that the researchers have created is extraordinarily stretchy and has remarkable self-healing characteristics. . Damaged polymers typically require a solvent or heat treatment to restore their properties, but the new material showed a remarkable ability to heal itself at room temperature, even if the damaged pieces are aged for days. Indeed, researchers found that it could self-repair at temperatures as low as negative 4 degrees Fahrenheit (-20 C), or about as cold as a commercial walk-in freezer.
The extreme stretching and self-healing ability of their new material is attributed by the team to some critical 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. 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.”
In addition to its long-term potential for use as artificial muscle, this research dovetails with Bao’s efforts to create artificial skin that might be used to restore some sensory capabilities to people with prosthetic limbs. This new, durable material could form part of the physical structure of a fully developed artificial skin.
Even before artificial muscle and artificial skin become practical, this work in the development of strong, flexible, electronically active polymers could spawn a new generation of wearable electronics, or medical implants that would last a long time without being repaired or replaced.
This latest discovery is the result of two years of collaboration, overseen by Bao, involving visiting scholar Cheng-Hui Li, a Chinese organo-metallic chemist who designed the metal ligand bonding scheme; polymer chemist Chao Wang who had made previous iterations of self-healing elastomers; and artificial muscle expert Christoph Keplinger. Other contributors to the study, “A highly stretchable autonomous self-healing elastomer,” include Jing-Lin Zuo, LihuaJin, Yang Sun, Peng Zheng, Yi Cao, Christian Linder and Xiao-Zeng You.
Source: Stanford News