Tech Briefs

Self-healing material could lead to artificial muscle.

Chemical engineers at Stanford University discovered that a new elastomer synthesized there had too much elasticity for the testing equipment the lab possessed. In fact, the clamping machine typically used to measure elasticity could only stretch about 45 inches from a one-inch sample of material. Similar materials can normally be stretched two or three times their original length and spring back to original size. However, the Stanford 1-inch polymer film sample was able to stretch to more than 100 inches. The researchers had to manually hold the material and walk across the room from each other to find the stopping point.

Fig. 1 – A new, extremely stretchable polymer film created by Stanford researchers can repair itself when punctured, a feature that is important in a material that has potential applications in artificial muscle. (Credit: Christoph Keplinger, University of Colorado at Boulder)

In addition to the super-stretchiness of the material, the researchers found that they could make this new elastomer twitch by exposing it to an electric field, causing it to expand and contract, making it potentially useful as an artificial muscle. (See Figure 1)

Artificial muscles currently have applications in robotics, small holes or defects in the materials can rob them of their resilience and they are not able to self-repair if punctured or scratched.

But, this stretchy new material also has self-healing characteristics 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 -4°F (-20°C), or about as cold as a commercial walk-in freezer.

How It Works

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. 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. 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.

The team discovered that they could “tune” the polymer to be stretchier or heal faster by varying the amount or type of metal ion included. The version that exceeded the measuring machine’s limits, for example, was created by decreasing the ratio of iron atoms to the polymers and organic molecules in the material.

The researchers also showed that this new polymer with the metal additives would twitch in response to an electric field. They plan to do more work to increase the degree to which the material expands and contracts and control it more precisely.

In addition to its long-term potential for use as artificial muscle, the team says that this material may be useful to create artificial skin that might be used to restore some sensory capabilities to people with prosthetic limbs. This work could also inspire the development of strong, flexible, electronically active polymers that could spawn a new generation of wearable electronics, or medical implants that would last a very long time without being repaired or replaced.

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